# P&ID legend: every common symbol

Generated from `registry/pid/library.json` (293 symbols). Each category is also one image: `https://deck-twin-library.demoareeb.workers.dev/symbols/legend-<category>.svg`, which you can view side by side with the drawing. Symbols marked *logical* are not hardware: read them, then skip them in the 3D model. Where a symbol has a component, that is the registry name to use.

## Valves

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-valves.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Gate valve** `valve-gate` (HV) | Two open triangles meeting tip to tip on the pipe (a bow-tie), with nothing in the middle. A handwheel stem (a T on top) is optional. | Manual isolation valve, normally fully open or fully closed, not for throttling. On PIP/ISA-style legends the plain bow-tie is the gate valve; on ISO 10628 drawings the plain bow-tie is a generic valve, so check the legend. Variants: ISO 10628-2: the plain bow-tie means "valve, general"; Filled (black) bow-tie: normally closed; Some legends add a short vertical bar at the tips to mean gate specifically; Size/class appear only in the line number, not in the symbol | `gate-valve` |
| **Globe valve** `valve-globe` (HV) | Bow-tie with a solid black dot where the triangle tips meet. | Manual throttling valve: regulating flow, control-valve bypasses, and vents and drains that are opened slowly. A globe body with an actuator on top is a control valve (see act-diaphragm). Variants: Some legends draw the dot as a small filled circle with a stem and handwheel; Bypass globe valves around a control valve are normally closed: the body or dot may be filled; Look-alikes: open circle = ball valve, small rectangle = plug valve | `globe-valve` |
| **Ball valve** `valve-ball` (HV) | Bow-tie with an open (white) circle at the centre, where the tips meet. | Quarter-turn isolation valve: full-bore isolation, pigging lines, ESD and on/off duty when actuated. Small sizes have a lever, large ones a gear. Variants: Full-bore vs reduced-bore is usually written as FB / RB next to the symbol; Trunnion vs floating is not shown, only on the valve datasheet; Look-alikes: filled dot = globe valve, small rectangle = plug valve | `ball-valve` |
| **Ball valve, encircled style** `valve-ball-encircled` (HV) | A circle large enough to enclose the bow-tie, so it reads as a circle with an X in it, usually between two flange bars on the line. | Ball valve in legends that draw the ball as a full circle around the bow-tie (common on European and NORSOK-style drawings). Becomes ball-valve; a filled bow-tie or NC beside it means normally closed. Variants: Check the sheet legend: a few legends use the encircled X for a general or special valve; Look-alike: a circle with an X on an electrical or lighting drawing is a lamp, not a valve | `ball-valve` |
| **Butterfly valve** `valve-butterfly` (HV) | Two short vertical lines across the pipe (wafer body between flanges) with a diagonal line through the middle (the disc) and a dot at its centre (the shaft). | Quarter-turn disc valve used for large low-pressure lines: seawater, cooling water, firewater and HVAC. Can isolate or throttle. Variants: Very common alternative (PIP-style): a bow-tie with a vertical line through the centre and a dot; Some legends draw the disc as a thin ellipse instead of a line; Triple-offset (metal-seated) types are only identified on the datasheet | `butterfly-valve` |
| **Plug valve** `valve-plug` (HV) | Bow-tie with a small upright rectangle (the plug) at the centre. | Quarter-turn valve with a tapered or cylindrical plug, often lubricated. Used for isolation in sandy or dirty service and on wellhead / manifold duties. Variants: Some legends fill the rectangle; Multi-port plug valves are drawn with three or four triangles (see the 3-way valve); Look-alikes: open circle = ball valve, filled dot = globe valve | `plug-valve` |
| **Needle valve** `valve-needle` (HV) | Small bow-tie with a stem carrying a solid triangle pointing down onto the body (the needle), topped by a handwheel bar. | Fine-throttling instrument valve, ½" and below: instrument roots, gauge isolation, sample and purge lines. Usually not modelled at plant scale. Variants: Often drawn without the needle as a plain small bow-tie on an instrument impulse line; Instrument manifolds (2-, 3-, 5-valve) are drawn as boxes or groups of small valves | `needle-valve` |
| **Diaphragm valve** `valve-diaphragm` (HV) | Bow-tie with a curved arc (the diaphragm bonnet) spanning the top of the body where the tips meet, and a stem above. | Valve that closes by pressing an elastomer diaphragm onto a weir. Used for corrosive, slurry or chemical-injection service. No dedicated 3D component; use globe-valve and note the real type. Variants: Some legends draw a small dome or a horizontal bar above the tips; Weir type vs straight-through is only on the datasheet | – |
| **Pinch valve** `valve-pinch` (HV) | A flexible sleeve: two curved lines bowing in towards the centre between short end bars, with two solid arrowheads pressing in from above and below. | Valve that closes by squeezing a rubber sleeve. Used for slurry, sand and abrasive service. Rare offshore. No dedicated component; use a generic valve and note it. Variants: Legends vary widely: some show a bow-tie with "P" or "PINCH" beside it; Air-operated pinch valves show the air connection to the body | – |
| **Knife gate valve** `valve-knife-gate` (HV) | Bow-tie with a thin upright rectangle (the knife blade) through the centre that rises above the body, then a stem and handwheel bar. | Gate valve with a sharpened blade that cuts through solids. Used for slurry, sand, drains and low-pressure bulk lines. Variants: Some legends draw only a vertical line through the tips; Look-alike: needle valve has a solid triangle on the stem, not a blade | `knife-gate-valve` |
| **Angle valve** `valve-angle` (HV) | Two triangles at right angles: one pointing in from the side (inlet) and one pointing up from below (outlet), tips meeting, with a stem and handwheel on top. | Globe-type valve whose outlet is at 90° to the inlet. Common for drains, chokes and blowdown. A spring on top makes it a relief valve, not an angle valve. Variants: A dot at the tips makes it an angle globe valve; With an actuator it is an angle control valve; With a spring or a pilot box it is a PSV (see relief) | – |
| **Three-way valve** `valve-3way` (HV, TV, XV) | Three triangles meeting tip to tip: left, right and one pointing up from below, each with a pipe stub. Usually carries an actuator or handwheel. | Diverting or mixing valve, e.g. a cooling-medium bypass (TCV) or a PSV changeover. With an actuator, arrows on the ports show the failed flow path. No dedicated component; use ball-valve or control-valve and note it. Variants: A filled triangle marks the port that is normally closed; PSV changeover (interlocked) valves are often drawn as a pair of 3-way valves joined by a dashed mechanical link; Plug-type 3-way bodies show a small rectangle at the centre | – |
| **Four-way valve** `valve-4way` (HV, XV) | Four triangles meeting at one point, like two bow-ties crossed, with a pipe stub on every side. | Four-port diverter: flow reversal (e.g. backflushing a filter or a meter prover diverter). No dedicated component. Variants: Filled triangles mark closed ports; Prover four-way diverters are often drawn as a box labelled "4-way" | – |
| **Normally closed valve** `valve-normally-closed` (HV) | Bow-tie with both triangles filled solid black. | The valve is closed in normal operation: drains, bypasses, crossovers, spare connections. Read the body type from the legend; the fill only means closed. Set the valve normally closed in the model. Variants: Some legends fill only the inner part, or write NC beside an open bow-tie; Filled globe, ball or plug valves keep their centre mark; A few old legends use filled bodies for check valves instead, so check the legend | `gate-valve` |
| **Locked open valve** `valve-locked-open` (HV) | A valve body with the letters "LO" written beside or above it. | The valve is locked in the open position, for example the isolation valves under a PSV or on a vent, so it cannot be shut by mistake. Model it as normally open and note "LO". The lock or seal is a padlock or chain; the body type is read from the bow-tie, and gate-valve is only the default. Variants: The letters can be below the body or at the end of a leader; Some legends add a small padlock or a "C" hook; Ball valves with LO/LC have an open circle in the bow-tie | `gate-valve` |
| **Locked closed valve** `valve-locked-closed` (HV) | A valve body with the letters "LC" written beside or above it. The body is often drawn filled because the valve is closed. | The valve is locked closed, for example a drain or a spool to a flare, so it cannot be opened by mistake. Model it as normally closed and note "LC". The lock or seal is a padlock or chain; the body type is read from the bow-tie, and gate-valve is only the default. Variants: The letters can be below the body or at the end of a leader; Some legends add a small padlock or a "C" hook; Ball valves with LO/LC have an open circle in the bow-tie | `gate-valve` |
| **Car-sealed open valve** `valve-car-sealed-open` (HV) | A valve body with the letters "CSO" written beside or above it. | The valve is sealed open with a car seal. Breaking the seal needs a permit, and seals are audited. Typical on PSV inlet and outlet isolations. Model it as normally open and note "CSO". The lock or seal is a numbered car seal (wire tag); the body type is read from the bow-tie, and gate-valve is only the default. Variants: The letters can be below the body or at the end of a leader; Some legends add a small padlock or a "C" hook; Ball valves with LO/LC have an open circle in the bow-tie | `gate-valve` |
| **Car-sealed closed valve** `valve-car-sealed-closed` (HV) | A valve body with the letters "CSC" written beside or above it. The body is often drawn filled because the valve is closed. | The valve is sealed closed with a car seal. Model it as normally closed and note "CSC". The lock or seal is a numbered car seal (wire tag); the body type is read from the bow-tie, and gate-valve is only the default. Variants: The letters can be below the body or at the end of a leader; Some legends add a small padlock or a "C" hook; Ball valves with LO/LC have an open circle in the bow-tie | `gate-valve` |
| **Float-operated valve** `valve-float` (LCV, LV) | Bow-tie with a stem rising to a horizontal lever that ends in a circle (the float). | Self-acting level valve (ball-cock): tank make-up water, diesel day tank fill, drain pots. No instrument loop; the float moves the valve directly. Variants: Sometimes drawn with a dashed mechanical link from the valve to a float inside the tank; Tagged LCV (self-actuated level control valve) | – |
| **Double block and bleed** `valve-double-block-bleed` (HV) | Two isolation valves in series on the pipe with a small bleed valve on a branch between them, often inside a dashed box when it is a single-body DBB valve. | Positive isolation for maintenance: both blocks are shut and the cavity between them is bled to a safe place. Required on high-pressure hydrocarbon isolations and instrument connections. Model the block valves as ball-valve or gate-valve and list the bleed as not modelled. Variants: Single-body DBB valves (two balls in one body) are drawn as one box with two bow-ties; The bleed may go to a closed drain or a vent line; Instrument DBB manifolds are often shown as a single rectangle labelled DBB | – |
| **Damper (louvre)** `valve-damper` (HD, FD, MD) | A rectangle across the duct with several parallel slanted blades inside. | Duct damper on HVAC, fired-heater stack or fan systems. A fire damper (FD) closes on fire; a gas-tight damper isolates enclosures on gas detection. No dedicated component. Variants: Single-blade dampers look like a butterfly valve on a duct; Fire dampers may be marked FD or carry a fusible-link note; Motorised dampers carry an M actuator | – |
| **Fixed (positive) choke** `choke-fixed` (CH, FO) | An angle body (inlet triangle from the side, outlet triangle below) with a plain cap on top instead of a handwheel, and an orifice (a split bar) across the outlet. | Wellhead or manifold choke with a fixed-size bean (orifice). Changing rate means changing the bean, so there is no handwheel. Sits on the christmas-tree wing outlet. Variants: Some legends draw a bow-tie with "CH" or a bean (small black rectangle) inside; Bean size is written as a fraction of an inch in 64ths, e.g. 32/64" | `choke-valve` |
| **Adjustable choke** `choke-adjustable` (HCV, CH, PCV) | An angle body with a handwheel stem and a long diagonal arrow through the body (the ISO mark for "adjustable"). | Wellhead choke whose opening is set by hand or by an actuator (with an actuator it is a choke control valve, often HCV or PCV). Controls well rate and drops pressure from wellhead to manifold. Variants: Actuated chokes carry a stepping actuator or an M motor; Some legends draw a bow-tie with a diagonal arrow; Choke position may be shown as % or 64ths | `choke-valve` |

## Check valves

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-check-valves.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Check valve (general)** `check-general` (NRV, CV) | Bow-tie with the downstream (outlet) triangle filled black, and a flow arrow above it. | Non-return valve: flow only in the arrow direction. Found on pump discharges, compressor discharges and anywhere back-flow must be stopped. The filled side points the way flow goes. Swing type is the default when the legend does not say. Variants: ISA/US style: an open triangle against a vertical bar (the seat), with or without an arrow; Some legends draw "N" or "NRV" beside a plain bow-tie; Check the arrow, not the fill, for direction if the drawing is mirrored | `swing-check-valve` |
| **Swing check valve** `check-swing` (NRV) | Two vertical end bars across the pipe with a disc hanging diagonally from a pivot dot at the top (the hinged clapper), and a flow arrow above. | Check valve with a hinged disc that swings open with flow. The usual type for horizontal liquid lines and large pipe. Variants: Often drawn simply as the general check (bow-tie with one side filled); Tilting-disc check valves are drawn the same way | `swing-check-valve` |
| **Lift (piston) check valve** `check-lift` (NRV) | Bow-tie with the outlet triangle filled and an open circle (the piston or disc) where the tips meet, with a flow arrow above. | Globe-pattern check valve with a piston or disc that lifts off its seat. Good for high pressure, small bore and pulsating flow (reciprocating pumps and compressors). Variants: Some legends draw a globe-valve body with an arrow; Look-alike: ball check (ball in front of a seat bar), stop-check (adds a handwheel) | `check-valve-piston` |
| **Ball check valve** `check-ball` (NRV) | An open triangle pointing downstream at a small black ball, which sits against a vertical bar (the seat). | Check valve with a free ball. Used on small lines, metering-pump discharges and chemical injection. No dedicated component; use check-valve-piston if it must be modelled. Variants: Some legends draw a bow-tie with a filled circle in the outlet side; Injection check valves at the quill are often drawn as this symbol | – |
| **Dual-plate check valve** `check-dual-plate` (NRV) | Two vertical bars (a wafer between flanges) with two short plates hinged at the centre and opening downstream in a "<" shape, and a flow arrow above. | Wafer check valve with two spring-loaded half-discs on a central hinge pin. Short and light, so common on compressor and pump discharges and on large water lines. Variants: Many legends just use the general check symbol and write "dual plate" or "DPC"; Some draw a single vertical bar with a filled triangle | `dual-plate-check-valve` |
| **Stop-check valve** `check-stop` (NRV) | A lift check (bow-tie, outlet filled, open circle at the centre) with a stem and handwheel on top, and a flow arrow beside it. | Screw-down non-return valve: acts as a check valve, but the handwheel can also hold it shut. Used on boiler feeds and parallel pump discharges. No dedicated component; use check-valve-piston and note the handwheel. Variants: Some legends draw a globe valve with an arrow and a handwheel | – |

## Relief, regulators and safety devices

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-relief.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Pressure safety valve (spring loaded)** `psv-spring` (PSV, PRV, TSV, SV, PSV, PRV, TSV) | An angle body with the inlet from below and the outlet to the side, and a zig-zag spring on top capped by a short bar. A PSV bubble with the loop number usually sits beside it (not drawn here), with the set pressure and the orifice size (e.g. "3L4"). | Last-line overpressure protection. It lifts at the set pressure and relieves to flare, vent or atmosphere. Inlet size, orifice letter and outlet size read "3L4" = 3" inlet, L orifice, 4" outlet. TSVs are small thermal-relief valves on blocked-in liquid lines. Variants: Bellows-balanced PSVs have a small bellows drawn under the spring; Some legends draw a straight-through body with a spring; PSVs often sit between LO/CSO isolation valves, with a spare PSV on a changeover valve | `pressure-safety-valve` |
| **Pilot-operated relief valve** `psv-pilot` (PSV, PSV) | An angle relief body with a piston chamber (a box) on top instead of a spring, and a small spring-loaded pilot beside it with a sensing line back to the inlet. | Relief valve whose main piston is held shut by inlet pressure and released by a small pilot. Tight up to the set point, so used where operating pressure is close to set pressure (gas export, large flare loads). Variants: The pilot can be drawn as a small PSV symbol or a box with "P"; Remote-sensing pilots have the sensing line run to the vessel, not the valve inlet | `pressure-safety-valve` |
| **Pressure-vacuum (breather) valve** `breather-valve` (PVRV, PSV) | A housing on a tank nozzle with two arrows inside: one pointing up (pressure relief out) and one pointing down (vacuum relief in), each over a weighted pallet. | Tank breather valve on atmospheric tanks (API 650/2000). It vents vapour when the tank fills or heats and lets air in when it empties. No dedicated component; note it on the storage-tank. Variants: Vacuum breakers alone show only the down arrow; Often combined with a flame arrester below it; Some legends draw a PSV symbol with "P/V" beside it | – |
| **Rupture disc (bursting disc)** `rupture-disc` (PSE, RD, BD, PSE) | Two short parallel bars (the holder) across a vertical pipe, with a curved arc between them bulging towards the outlet. | Non-reclosing relief device that bursts at a set pressure. Used alone, or under a PSV to protect it from corrosion (then a pressure gauge or PI is drawn between them). ISA tag is PSE. Becomes rupture-disc. Variants: Some legends draw a vertical pipe with a dashed or zig-zag line across it; Buckling-pin relief valves are drawn as a body with a pin and tagged PSE too | `rupture-disc` |
| **Flame arrester** `flame-arrester` (FA) | An in-line rectangle on the pipe with zig-zag lines inside (the crimped-metal element). | Passive device that stops a flame front passing down a vent line or into a tank. Found on tank vents, drain-tank vents and on vent stacks. No dedicated component. Variants: Detonation arresters are drawn the same and marked "DA"; Some legends use a rectangle with crossed diagonal hatching | – |
| **Self-actuated pressure regulator (PCV)** `regulator-pressure` (PCV, PCV) | A bow-tie body with a diaphragm dome on its stem, a spring above the dome, and a thin sensing line from the dome to the pipe downstream of the valve. | Pressure-reducing regulator with no instrument loop: downstream pressure acts on the diaphragm directly. Fuel gas, instrument air, nitrogen and blanket gas. Tag PCV. No dedicated component; control-valve is the closest stand-in. Variants: Internal sensing: no external line is drawn; Some legends draw the sensing line as a dashed or plain thin line; Blanketing regulators sit on a tank roof | – |
| **Back-pressure regulator** `regulator-back-pressure` (PCV, BPR, PCV) | Like the pressure regulator, but the thin sensing line runs from the dome to the pipe upstream of the valve. | Holds upstream pressure: it opens as upstream pressure rises. Used on separator gas outlets to flare, pump minimum-flow and seal systems. No dedicated component. Variants: Some legends write "BPR" beside it; Look at which side the sensing line goes to; that is the controlled side | – |
| **Self-actuated temperature regulator** `regulator-temperature` (TCV, TCV) | A bow-tie body with a diaphragm dome, and a capillary (line with small × marks) running from the dome to a sensing bulb. | Thermostatic valve driven by a filled bulb: lube-oil coolers, tracing and jacket water. No instrument loop and no dedicated component. Variants: The bulb may be drawn as a thermowell in the process line; Wax-element 3-way thermostatic valves are drawn as a 3-way valve with TCV | – |
| **Excess-flow valve** `excess-flow-valve` (EFV) | A bow-tie with a spring drawn inside the inlet triangle and a flow arrow above. | Closes automatically when flow exceeds a set rate (a line break downstream). Found on instrument connections, LPG and chemical lines. No dedicated component. Variants: Usually just a check-valve-like symbol tagged EFV; Some legends write "XS" or "EFV" next to a plain bow-tie | – |

## Actuators, operators and valve accessories

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-actuators.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Handwheel operator** `op-handwheel` (HV) | A valve body with a stem and a short horizontal bar on top (the handwheel), a "T". | Manual valve operated by a handwheel. The body below tells the valve type; the handwheel just confirms it is manual. Tag HV if it has one. Variants: Most legends omit the handwheel on manual valves altogether; Hand-operated control valves are tagged HCV | `gate-valve` |
| **Lever operator** `op-lever` (HV) | A ball-valve body with a short stem and a lever drawn as a slanted line from the top of the stem. | Quarter-turn valve with a lever handle: small ball, plug and butterfly valves (typically 4" and below). Variants: Some legends draw the lever horizontally; A padlock or "LO" beside the lever means locked open | `ball-valve` |
| **Gear operator** `op-gear` (HV) | A valve body with a small square box (gearbox) on its stem, and a handwheel shaft coming out of the side of the box. | Manual valve with a gearbox, for large or high-pressure quarter-turn valves (ball, plug, butterfly) where a lever would be too hard to move. Variants: Some legends draw a handwheel T with "G" or "GO" beside it; Bevel-gear operators on gate valves are drawn the same | `ball-valve` |
| **Manual operator with position indicator** `op-handwheel-indicator` (HV, ZI) | A handwheel valve with a small half-round dial with a pointer beside the stem, or a ZI bubble linked to the stem. | Manual valve with a local open/closed position indicator, often required on firewater and deluge isolation valves so the position can be checked at a glance. Variants: ISA style: a ZI bubble joined to the stem by a short line; Rising-stem gate valves are sometimes noted "OS&Y" (outside screw and yoke) instead | – |
| **Diaphragm actuator (control valve)** `act-diaphragm` (FV, LV, PV, TV, FCV, LCV, PCV, TCV, FV, LV, PV, TV, HV) | A bow-tie (often with a globe dot) with a stem and a dome-shaped head on top (a half circle sitting on a flat bar). | Pneumatic spring-diaphragm actuator: the classic modulating control valve. Its tag (FV/LV/PV/TV) names the loop, and FO/FC beside it gives the fail position. Becomes control-valve. Variants: The dome may be drawn flat-topped or as a full ellipse; A positioner box on the stem is common; The body may be a butterfly or ball with the same dome for rotary control valves | `control-valve` |
| **Valve positioner** `act-positioner` (ZC, FY, LY, PY, ZC) | A control valve with a small box on the stem below the dome (the positioner), joined to the stem, with a signal line entering the box. | Positioner on a control valve: compares the controller signal with stem position and drives the actuator. An accessory of the control valve; it is not modelled separately. Variants: Smart (HART/fieldbus) positioners may be shown as a bubble with ZC or as a box with "P"; Many P&IDs omit the positioner and show it only in the instrument index | `control-valve` |
| **Piston actuator, single acting (spring return)** `act-piston-spring` (ESDV, SDV, XV, XV, SDV) | A valve with a rectangle (the cylinder) on its stem; inside it a horizontal line (the piston) with a zig-zag spring above it. | Air or hydraulic cylinder with spring return. On/off shutdown valves (ESDV, SDV, XV) use it so the spring drives the valve safe on loss of air. With an ESDV/SDV/XV tag it becomes esd-valve; with BDV it becomes blowdown-valve. Variants: Spring and piston are often omitted: a plain rectangle on the stem; Scotch-yoke quarter-turn actuators are drawn the same on a ball valve; ISA 5.1-2009 draws the piston actuator as a rectangle with the stem through it | `esd-valve` |
| **Piston actuator, double acting** `act-piston-double` (XV, HV, XV) | A rectangle (cylinder) on the valve stem with a piston line across the middle and a signal port into each end, with no spring. | Cylinder powered in both directions. It stays put on loss of air (fail last), so it is used where the valve must not move on air failure, or with an accumulator. On/off valves (XV) mostly; use esd-valve only if the tag says ESDV/SDV. Variants: Ports may be drawn as short stubs with pneumatic (//) or hydraulic (L) marks; Often noted "FL" or "fail last" | – |
| **Electric motor actuator (MOV)** `act-motor` (MOV, XV, HV, XV, HV) | A valve with a circle containing "M" on top of its stem. | Electric motor actuator: large isolation valves, pipeline and firewater valves, and tank valves where air is not available. It fails in place (last position) on power loss. Variants: Some legends draw a square with M; An integral control unit may be shown as a small box beside the M | `motor-operated-valve` |
| **Solenoid actuator / solenoid valve (SOV)** `act-solenoid` (SOV, XY, XV, XY, XV) | A square box containing "S" on the stem of a small valve, or a small S box on the air supply to a larger actuator. | Solenoid. On its own small valve it is a solenoid valve; on the air line of an ESDV actuator it is the SOV that vents the actuator to trip the valve. An accessory, not modelled separately; the main valve is what becomes esd-valve or blowdown-valve. Variants: Manual-reset solenoids add "R" in the box or beside it; De-energise-to-trip vs energise-to-trip is written on the C&E, not the symbol | – |
| **Electro-hydraulic actuator** `act-electro-hydraulic` (XV, ESDV, SDV, XV) | A rectangle on the valve stem marked "EH". | Self-contained actuator with its own electric pump and hydraulic cylinder. Used for remote, large or subsea-tie-in valves where there is no air. ESD duty makes it an esd-valve in the model. Variants: Some legends draw a hydraulic cylinder with an M beside it | – |
| **Hydraulic actuator** `act-hydraulic` (XV, SSV, SCSSV, XV) | A cylinder rectangle on the stem with a piston line and "H", fed by a hydraulic signal line (a line with "L" marks). | Hydraulically powered cylinder, typical on wellhead valves (SSV/USV, wing valves) supplied from the wellhead HPU through the wellhead control panel. Variants: Wellhead surface safety valves are often drawn as a gate with a hydraulic piston and spring, tagged SSV or USV; The hydraulic supply line may be drawn without marks and just labelled HPU | – |
| **Emergency shutdown valve (ESDV)** `valve-esdv` (ESDV, SDV, XV, SSIV, XV, UV) | A ball (or gate) valve with a spring-return cylinder on the stem, a small "S" box on its air supply, and "FC" written under the body. | Shutdown valve tripped by the ESD/PSD system. It fails closed: the spring shuts it when the SOV vents the air. Inlet, outlet, riser and fuel-gas isolations. Becomes esd-valve. Variants: Some companies tag it SDV or XV; riser valves are often ESDV-R…; The body may be drawn filled when normally closed; Partial-stroke test devices and limit switches (ZSO/ZSC) are often drawn beside it | `esd-valve` |
| **Blowdown valve (BDV)** `valve-bdv` (BDV, BDV) | A valve body drawn filled (normally closed) with a spring-return actuator, an "S" box, and "FO" under the body. Its outlet goes to flare. | Depressuring valve. Closed in operation and opens on ESD or fire to blow inventory down to the flare (API 521: typically to 50 % or 6.9 barg within 15 min). Fails open. Becomes blowdown-valve. Variants: Usually followed by a restriction orifice (RO) to limit the flow; Tagged BDV, or XV with an FO mark | `blowdown-valve` |
| **Fail open (FO), arrow form** `fail-fo-arrow` (FO) | A control valve with an upward arrowhead drawn on the stem between body and actuator. | On loss of air, power or signal the valve goes fully open. Typical for blowdown valves, cooling-water valves and anti-surge recycle valves. It is a property of the valve (set its fail action to FO), not a separate item. Variants: The letter form writes the letters below the body instead; The bar forms for FL/FI come from secondary sources describing ISA 5.1 and are less widely used than the letters; check the legend; On 3-way valves, arrows on the ports show the failed flow path | *logical* |
| **Fail closed (FC), arrow form** `fail-fc-arrow` (FC) | A control valve with a downward arrowhead drawn on the stem between body and actuator. | On loss of air, power or signal the valve goes fully closed. Typical for ESD valves, feed and fuel valves. It is a property of the valve (set its fail action to FC), not a separate item. Variants: The letter form writes the letters below the body instead; The bar forms for FL/FI come from secondary sources describing ISA 5.1 and are less widely used than the letters; check the legend; On 3-way valves, arrows on the ports show the failed flow path | *logical* |
| **Fail locked (last position) (FL), arrow form** `fail-fl-arrow` (FL) | A control valve with two short horizontal bars across the stem between body and actuator. | On loss of motive power the valve stays where it was. Typical for motor-operated and double-acting valves. It is a property of the valve (set its fail action to FL), not a separate item. Variants: The letter form writes the letters below the body instead; The bar forms for FL/FI come from secondary sources describing ISA 5.1 and are less widely used than the letters; check the legend; On 3-way valves, arrows on the ports show the failed flow path | *logical* |
| **Fail indeterminate (FI), arrow form** `fail-fi-arrow` (FI) | A control valve with one short horizontal bar across the stem between body and actuator. | The fail position is not defined: the valve may drift. Seen on double-acting actuators without a lock-up. It is a property of the valve (set its fail action to FI), not a separate item. Variants: The letter form writes the letters below the body instead; The bar forms for FL/FI come from secondary sources describing ISA 5.1 and are less widely used than the letters; check the legend; On 3-way valves, arrows on the ports show the failed flow path | *logical* |
| **Fail open (FO), letter form** `fail-fo-letters` (FO) | The letters "FO" written just below (sometimes beside) a valve body. | On loss of air, power or signal the valve goes fully open. Typical for blowdown valves, cooling-water valves and anti-surge recycle valves. Set the valve's fail action to FO. Variants: Some legends write "AO/AC" (air to open / air to close) instead. Air-to-open means fail closed; The fail position is the shelf (no-air) position, not the normal operating position | *logical* |
| **Fail closed (FC), letter form** `fail-fc-letters` (FC) | The letters "FC" written just below (sometimes beside) a valve body. | On loss of air, power or signal the valve goes fully closed. Typical for ESD valves, feed and fuel valves. Set the valve's fail action to FC. Variants: Some legends write "AO/AC" (air to open / air to close) instead. Air-to-open means fail closed; The fail position is the shelf (no-air) position, not the normal operating position | *logical* |
| **Fail locked (last position) (FL), letter form** `fail-fl-letters` (FL) | The letters "FL" written just below (sometimes beside) a valve body. | On loss of motive power the valve stays where it was. Typical for motor-operated and double-acting valves. Set the valve's fail action to FL. Variants: Also written "FLP" or "FAIL LAST"; The fail position is the shelf (no-air) position, not the normal operating position | *logical* |
| **Fail indeterminate (FI), letter form** `fail-fi-letters` (FI) | The letters "FI" written just below (sometimes beside) a valve body. | The fail position is not defined: the valve may drift. Seen on double-acting actuators without a lock-up. Set the valve's fail action to FI. Variants: Some legends write "AO/AC" (air to open / air to close) instead. Air-to-open means fail closed; The fail position is the shelf (no-air) position, not the normal operating position | *logical* |
| **Valve limit switches (ZSO / ZSC)** `limit-switches` (ZSO, ZSC, ZSH, ZSL, ZSO, ZSC, ZSH, ZSL, ZS) | Two small bubbles marked ZSO and ZSC on either side of an actuated valve, joined to the stem by a short line. | Open and closed position switches on an actuated valve, wired to the DCS/SIS for valve status and ESD verification. Valve accessories: keep them as notes on the valve, not as separate items. Variants: Older drawings use ZSH (open) / ZSL (closed); Sometimes one bubble "ZS" with "O/C" beside it; Transmitters of valve position are ZT | – |
| **I/P converter** `ip-converter` (FY, LY, PY, TY, FY, LY, PY, TY) | A bubble with "Y" as the second letter (e.g. FY) with a small square beside it containing "I/P". An electric (dashed) signal goes in and a pneumatic (//) signal comes out. | Converts the 4–20 mA controller output to a 0.2–1 barg air signal for the actuator. Often built into the positioner. Accessory of the control valve; not modelled separately. Variants: Function-block letters may be E/P or I/P; Behind-panel Y functions have a dashed bar in the bubble | – |
| **Hand switch (HS)** `hand-switch` (HS, HS, HSS, HZ) | A bubble with "HS" and a number, with an electric (dashed) signal line to the device it operates. | Push-button or selector: local start/stop for a motor, open/close for an MOV, a manual ESD push-button (often HS with a solid bar = control room). Field HS buttons are usually on a local control station (local-control-station). Variants: A solid bar in the bubble means panel/console mounted; ESD push-buttons are sometimes HZ or ESD-PB | – |

## Piping fittings and specials

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-fittings.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Concentric reducer** `reducer-concentric` | A symmetric trapezoid in the line: tall at the large end and short at the small end, with both pipe centrelines in line. | Size change on vertical lines, or where the centreline must stay put. The sizes are written beside it (e.g. 8"x6"). In plant.json the assembler inserts reducers automatically when port sizes differ. Variants: Some legends use a plain triangle; Swages (small bore, threaded or socket-weld) are drawn the same | `reducer` |
| **Eccentric reducer, flat on top (FOT)** `reducer-eccentric-fot` | A trapezoid with a flat top edge and a sloped bottom edge, so the two pipe centrelines are offset. Often marked "FOT". | Eccentric reducer with the flat side up. Standard on pump suctions so no vapour pocket can form. Sizes are written beside it. Variants: The flat side may be marked only by the text FOT, with a concentric-looking symbol | `reducer` |
| **Eccentric reducer, flat on bottom (FOB)** `reducer-eccentric-fob` | A trapezoid with a flat bottom edge and a sloped top, centrelines offset. Often marked "FOB". | Eccentric reducer with the flat side down. Used on pipe racks to keep the bottom of pipe level, and on lines that must drain. Variants: Marked only by the text FOB on some legends | `reducer` |
| **Spectacle blind, open (spacer in line)** `spectacle-blind-open` (SB) | A figure-8 between two flange bars: the open ring sits on the pipe, and the solid (filled) blank sits off the line. | Spectacle blind in the open position: flow passes through the ring. It is turned to the closed position for positive isolation during maintenance. Becomes spectacle-blind. Variants: Some legends write "NO" (normally open) beside it; Spectacle blinds in plant isolations are often marked NO / NC; The offset form (figure-8 beside the flange pair on a short stem) means the same thing: see spectacle-blind-offset | `spectacle-blind` |
| **Spectacle blind, closed (blank in line)** `spectacle-blind-closed` (SB) | A figure-8 between two flange bars: the solid (filled) blank sits on the pipe, and the open ring is off the line. | Spectacle blind in the closed position: positive isolation, nothing passes. Typical on spare connections, start-up lines and vessel isolations. Variants: Some legends write "NC" beside it; A filled circle without a ring is a spade (line blank); The offset form (figure-8 beside the flange pair on a short stem) means the same thing: see spectacle-blind-offset | `spectacle-blind` |
| **Spectacle blind, offset figure-8 (drawn beside the flanges)** `spectacle-blind-offset` (SB) | A flange pair (two short bars) on the line, with a figure-8 drawn off to one side and joined to the flanges by a short stem. The circle next to the stem is the half installed in the line; the far circle is the spare half. The blank half may be filled, hatched or drawn as concentric circles; the ring half is a plain open circle. | Spectacle blind between the flanges, drawn beside the line so the pipe stays unbroken. Open ring next to the stem = open (NO); blank next to the stem = closed (NC). The line size (e.g. 4") is usually written beside it. Becomes spectacle-blind with state set from the half next to the stem. Variants: Some legends draw both halves as plain circles and write NO / NC beside it; Look-alike: a plain flange pair has no figure-8; do not drop the blind; Look-alike: a single open circle on a stem at a flange pair is a spacer ring; a single solid paddle is a spade | `spectacle-blind` |
| **Spade (line blank)** `spade` | A single solid blank between two flange bars with a short handle tab sticking out, and no ring. | Removable paddle blank: positive isolation for large sizes, where a spectacle blind would be too heavy. No dedicated component; spectacle-blind is the closest. Variants: Often drawn the same as a closed spectacle blind and labelled "SPADE" | – |
| **Spacer (ring spacer)** `spacer` | A single open ring between two flange bars with a short handle tab, and no blank. | The ring that is fitted in place of a spade when the line is in service. No dedicated component. Variants: Usually drawn together with a spade, one of them in the line | – |
| **Blind flange** `blind-flange` | The pipe ends at a flange bar followed by a thicker solid bar. | Bolted blind on the end of a line or a spare nozzle. Becomes blind-flange. Variants: Some legends draw two parallel bars with the outer one thicker; Look-alike: pipe cap (a small half-round at the end, welded) | `blind-flange` |
| **Flanged joint** `flange-pair` | Two short parallel bars across the pipe, close together. | Bolted flange connection, usually shown only where it matters: equipment nozzles, spool breaks, spec breaks and maintenance breaks. Becomes flanged-joint. Variants: A single bar is a flange face at an equipment nozzle; Look-alike: union has a shorter third bar in the middle; Look-alike: a flange pair with a figure-8 drawn beside it on a short stem is a spectacle blind (spectacle-blind-offset), not a plain joint | `flanged-joint` |
| **Pipe cap** `pipe-cap` | The pipe ends in a small half-round (a "D"). | Welded or threaded cap closing a pipe end: dead legs, future connections, header ends. No dedicated component. Variants: Threaded plugs are drawn as a small filled triangle or square at the end; Look-alike: blind flange (bars, not a half-round) | – |
| **Union** `union` | Two short bars across the pipe with a shorter third bar between them. | Threaded union, used on small-bore lines (instrument air, chemical injection, tubing) to allow disconnection. No dedicated component. Variants: Legends differ: some show "//" with a small gap, others write "U"; Hammer (wing) unions on well-test and temporary lines become hammer-union; threaded small-bore unions are part of small-bore-fitting work | – |
| **Flexible hose** `flexible-hose` (FH) | A wavy line between two short end bars. | Flexible hose or hose assembly: loading hoses, temporary connections, vibrating equipment hook-ups, and HPU or chemical hoses. Becomes flexible-hose. Variants: Some legends draw a loop or a zig-zag; Bunkering and loading hoses may carry a breakaway coupling mark | `flexible-hose` |
| **Expansion joint (bellows)** `expansion-joint` (EJ) | A zig-zag bellows drawn above and below the pipe between two flange bars. | Metallic or rubber bellows that absorbs thermal movement or vibration: exhaust ducts, seawater pump discharges and flare headers. Becomes expansion-joint. Variants: Rubber expansion joints may be drawn as a bulged section | `expansion-joint` |
| **Y-strainer** `strainer-y` (S, STR) | A leg slanting down from the pipe at about 45°, with a dashed screen line inside and a cap at its end. | Permanent in-line strainer protecting pumps, control valves and meters. Often has a DP gauge across it. Becomes y-strainer. Variants: Some legends draw a plain diagonal line with a dashed screen; A blow-off valve may be drawn on the cap | `y-strainer` |
| **Basket (T-type) strainer** `strainer-basket` (S, STR) | A rectangle hanging below the line with a cover bar on top and a dashed U (the basket) inside. | Large-capacity strainer for pump suctions and seawater systems. Duplex units have two baskets and a changeover. T-type strainers are drawn the same way. Becomes basket-strainer. Variants: Duplex: two baskets side by side with a 3-way changeover valve; Some legends draw a circle with a dashed screen | `basket-strainer` |
| **Temporary (cone) strainer** `strainer-temporary` (TS) | A dashed cone (triangle) pointing downstream inside the pipe, just downstream of a flange pair. | Start-up strainer fitted between flanges for commissioning and removed afterwards. Usually marked "temporary" or "TS". No dedicated component; normally not modelled. Variants: Some legends use a dashed rectangle; Witch-hat strainers point the cone upstream | – |
| **Steam trap** `steam-trap` (ST, TR) | A small circle on the line with "T" inside. | Lets condensate out of a steam line or tracer while holding steam back. Seen at the end of steam tracers and on steam-line drip legs. Becomes steam-trap. Variants: ISO-style legends draw a square on its corner with a filled dot; Some write "ST" next to a small square | `steam-trap` |
| **Sight glass (sight flow indicator)** `sight-glass` (SG, FG, FG) | Two concentric circles on the line (a viewing window). | In-line window to see that liquid is flowing: pump seal flush, lube oil, chemical injection. ISA calls it FG (flow glass). Becomes sight-glass. Variants: Some legends draw a circle with two short vertical bars; ISA style: a bubble with FG; A level gauge glass on a vessel is LG, not this | `sight-glass` |
| **Restriction orifice** `restriction-orifice` (RO, FO, FO, RO) | An orifice plate between two flange bars (a bar across the pipe with a gap in the middle), labelled "RO" or with an FO bubble. | Fixed plate that limits flow: after blowdown valves, on minimum-flow lines and on purge connections. It is not a meter, so there are no dp taps. Becomes restriction-orifice. Variants: Multi-stage ROs are drawn as several plates in series; Look-alike: orifice flow element (FE) has dp tap lines to a transmitter | `restriction-orifice` |
| **Insulating joint (monolithic)** `insulating-joint` (IJ) | A short rectangle on the line with diagonal hatching inside, sometimes labelled "IJ". | Electrically isolates a pipeline (cathodic protection) from the plant. Seen on risers, export lines and at battery limits. Becomes insulating-joint. Variants: Insulating flange kits: a flange pair with a thick bar between them; Some legends draw two flange bars with a small gap | `insulating-joint` |
| **Drain funnel (tundish)** `drain-funnel` (OD, TD) | A drain line ending in the open, just above a V-shaped funnel that leads down to the open drain. | Open drain hub with a visible air break. It collects hydrocarbon-free or open drains into the open-drain system. Becomes drain-hub. Variants: Some legends draw a cup or a small triangle; Hazardous open drains must be sealed; look for a seal loop or a P-trap under it | `drain-hub` |
| **Hose connection** `hose-connection` (HC, UC) | A small isolation valve on a branch that ends in a coupling (a short block with ridges), often marked "HC". | Utility hose connection (utility station: air, nitrogen, water or steam) or a flushing / steam-out connection. A utility station (several hose drops on one post) becomes utility-station; a lone hose connection is listed, not modelled. Variants: Quick-connect couplings may be drawn as a small triangle; Utility stations are often one symbol with a list of services | `utility-station` |
| **Vent with cap** `vent-capped` (VT) | A short branch rising from the line with a small isolation valve and a cap (half-round) on top. | High-point vent for hydrotest and commissioning: valve plus cap or blind. Usually ¾" and not modelled. Variants: The end may carry a blind flange or a plug instead of a cap; Vents to atmosphere on hydrocarbon lines need a cap or plug, so check the line class | – |
| **Drain with plug** `drain-plugged` (DR) | A short branch going down from the line with a small isolation valve and a solid plug (filled square) at the end. | Low-point drain for hydrotest and maintenance: valve plus plug. Usually ¾" and not modelled. Variants: Drains to the closed drain system are drawn connected to the CD header instead | – |
| **Sample connection** `sample-point` (SC, SP, AX, SP) | A small branch from the line with a needle or isolation valve, ending in an open end over a small bottle; sometimes a box labelled "SC" or a sample cooler. | Manual sample point for lab analysis (oil-in-water, BS&W, H2S, glycol). Closed-loop samplers return to the process. Listed, not modelled. Variants: ISA style: a bubble with AX or SP; Closed sample systems are drawn as a box with in and return lines | – |
| **Piping specification break** `spec-break` | A short bar across the line with the two piping class codes written on either side of it (e.g. A1A / D6A). | Where the piping class (material and pressure rating) changes, usually at a valve or a flange. Not a physical item, but it tells you the rating on each side. Use it for each line's spec. Variants: Some legends use a small diamond or a "Z" across the line; Often sits right next to the last valve rated for the higher class | *logical* |
| **Insulation mark** `mark-insulation` | A process line with thin dashed lines running parallel above and below it over the insulated length. | The line is insulated (heat conservation, personnel protection or cold). Most drawings show it only as the insulation code at the end of the line number, e.g. "-H" or "-PP". Set the line as insulated. Variants: Insulation codes: H heat conservation, C cold, PP personnel protection, AC acoustic, N none; Some legends hatch a short stretch of the line instead | *logical* |
| **Electric heat tracing mark** `mark-tracing-electric` (ET, EHT) | A process line with a thin zig-zag (resistor-like) line running beside it, marked "ET". | Electrically traced line to stop freezing, waxing or hydrates. A tracing circuit and controller exist. Shown in the insulation code as "ET". Note it on the line; it is not modelled as geometry. Variants: Many legends draw a plain dashed line alongside with "ET"; Self-regulating vs constant-wattage cable is only on the heat-tracing isometrics | *logical* |
| **Steam tracing mark** `mark-tracing-steam` (ST) | A process line with a thin dashed tracer line beside it, marked "ST", usually ending at a steam trap. | Steam- or hot-oil-traced line. Mostly onshore and in refineries; rare offshore. Note it on the line. Variants: Hot-water or glycol tracing: same, marked "WT" or "GT"; Jacketed lines are drawn differently (see the jacketed line) | *logical* |
| **Slope (fall) mark** `slope-arrow` | The word "SLOPE" or a ratio (e.g. 1:100) with an arrow along the line pointing downhill. | The line must fall continuously in the arrow direction: drains, flare headers and relief lines (no pockets). Affects routing, not components. Keep runs level or falling towards the drum. Variants: Some legends use a small right triangle above the line; "No pockets" or "free draining" notes mean the same | *logical* |
| **Flow direction arrow** `flow-arrow` | A solid arrowhead on the line. | Direction of normal flow in the line. Use it to orient check valves and to decide which equipment is "from" and which is "to". Variants: Open-headed arrows are used for signal and relief flow on some legends | *logical* |
| **Line crossing (no connection)** `line-crossing` | Two lines crossing, with one broken (a small gap either side of the other line), or with a small hop (semicircle) over it. | The lines cross on paper only and are not connected. Don't make a tee here. Variants: Convention: the vertical line is usually the one broken; Older drawings use a hop (semicircle); A plain cross with no dot is ambiguous: treat it as no connection unless there is a dot | *logical* |
| **Line junction (tee)** `line-junction` | A branch line meeting a run line in a T, with a solid dot at the junction. | Branch connection: a tee, or a welded branch fitting (olet). A welded branch outlet (weldolet, sockolet) becomes olet. In plant.json, model a tee with the tee component and connect the run ends and the branch port. Variants: The dot is often omitted on T junctions (a T cannot be a crossing); Branch size may differ: reducing tee or olet | `tee` |
| **Injection quill / point** `injection-quill` (IP, CI) | A small branch entering the pipe (often after a check valve), with its end bent inside the pipe and pointing downstream. | Chemical injection point: corrosion inhibitor, scale inhibitor, methanol, demulsifier, biocide. The supply comes from the chemical injection package. Listed, not modelled; the package is chemical-injection-skid. Variants: Often drawn just as a small line ending in a dot or an arrow on the main line, with the chemical name; A check valve and a double block on the branch are typical | – |
| **Corrosion probe / coupon** `corrosion-probe` (CP, CC, CE) | A short access fitting on top of the line with a small box or bubble marked "CP" (probe) or "CC" (coupon), and a probe tip into the flow. | Corrosion monitoring point: an ER/LPR probe or a weight-loss coupon in a high-pressure access fitting. Listed, not modelled. Variants: Legends vary: a diamond, a box or a circle with CC/CP/CE; Sand probes are drawn similarly, marked SP or SE | – |
| **Vent to atmosphere (gooseneck)** `vent-atmosphere` (VA, TV) | A line rising and turning back down at the top (a gooseneck or bent open end), sometimes with "ATM" written beside it. | Open vent to a safe location: instrument-air and nitrogen relief, tank vents and utility PSV outlets. For hydrocarbons, cold vents go to a vent stack instead. Variants: A plain open end with "TO ATM"; A mesh or bird screen may be drawn at the end | – |

## Lines and signals

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-lines.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Main process line** `line-process-main` | A heavy (thick) solid line, with a line number written along it (e.g. 8"-PG-1101-D6A-N). | Main process pipe. Each one becomes a plant.json line between equipment ports or battery limits. Read size, service, sequence number, class and insulation from the line number. Variants: Line weight is the only difference from secondary lines; some CAD exports lose it; Line-number order differs by company (size-service-number-class-insulation is common) | – |
| **Secondary / minor process or utility line** `line-process-secondary` | A medium-weight solid line, thinner than main process lines, with its own line number. | Utility, minor process or small-bore line: drains, vents, chemical injection, fuel gas, air. Model it if it is a numbered line between modelled items. Variants: Some companies draw utilities in a medium weight and instrument process connections thin | – |
| **Future line** `line-future` | A heavy line made of long dashes, often marked "FUTURE". | Planned but not part of this scope. Do not model it; at most, leave a blinded tie-in. Variants: Some legends use a dash-dot line; Future equipment is drawn dashed too | *logical* |
| **Existing line** `line-existing` | A screened (grey or lighter) line, sometimes labelled "EXISTING" or with a note, on brownfield drawings. | Existing pipe that this project does not change. Model it only if the existing plant is in scope. Variants: Monochrome prints may show existing lines as thin, or hatched, or with an "E" in the line number; Check the legend: some brownfield sets reverse this and screen the new work | – |
| **Demolished / to be removed line** `line-demolished` | A line with short diagonal hatch marks (or crosses) along it, often labelled "TO BE REMOVED" or "DEMOLISH". | Pipe to be removed in this project. Do not model it. Variants: Some legends use × crosses (do not confuse with the thin capillary line); Removed equipment is cross-hatched or crossed out | *logical* |
| **Instrument supply / process connection** `line-instrument-supply` | A thin solid line from a process line or vessel to an instrument bubble (or from a supply header). | Impulse line from the process to an instrument, or an instrument supply (air, power). It shows where the instrument is mounted. Not a numbered process line. Variants: ISA 5.1-2009 also uses a plain thin line for "undefined signal"; Supply type may be written on it: AS (air supply), ES (electric), NS (nitrogen), HS (hydraulic) | – |
| **Pneumatic signal** `line-pneumatic` | A thin line with pairs of short diagonal slashes (//) at intervals. | Air signal, e.g. from an I/P or positioner to an actuator, or an instrument-air signal to a pneumatic controller. Not modelled. Variants: Older drawings (ISA 1984) used the same // marks; Some legends use three slashes for electric (pre-1984) | *logical* |
| **Hydraulic signal** `line-hydraulic` | A thin line with small "L" marks at intervals. | Hydraulic signal or power: wellhead control panel to SSV and SCSSV actuators, HPU lines. Not modelled, but it identifies hydraulic actuators. Variants: Hydraulic supply lines may also be drawn as process lines with the HYD service code | *logical* |
| **Electric signal** `line-electric` | A thin dashed line. | Analog (4-20 mA) or discrete electrical signal between instruments, DCS, PLC and SIS. It shows the loop, not a cable route. Not modelled. Variants: ISA 1984 drew electric signals as a dashed line too; very old drawings use ///; Some legends draw binary (on/off) electric signals with a different dash pattern | *logical* |
| **Capillary tube (filled system)** `line-capillary` | A thin line with small × marks at intervals. | Filled capillary: a thermal system bulb to its gauge or regulator, or a diaphragm seal to a transmitter. Part of the instrument; not modelled separately. Variants: Do not confuse with demolished lines (heavy, hatched) on brownfield drawings | – |
| **Guided electromagnetic or sonic signal** `line-guided-em` | A thin solid line with small sine-wave marks on it at intervals. | Signal carried along a guide: fibre optic, or a guided-wave radar probe (signal down the rod). Not modelled. Variants: Some legends draw guided signals as a solid line with small loops; The exact mark varies between editions and company legends; check the legend | *logical* |
| **Unguided electromagnetic, radio or sonic signal** `line-unguided-em` | A free sine wave with no straight line under it. | Wireless or through-the-air signal: radio and WirelessHART transmitters, non-contact radar, nuclear level gauges, light and sound. Not modelled. Variants: Wireless instruments may also be marked "W" or have an antenna mark on the bubble | *logical* |
| **Data link / software link** `line-data-link` | A thin line with small open circles on it at intervals. | Internal system or communication link: software connections inside the DCS (e.g. a cascade setpoint) or a bus between systems (DCS to PLC, Modbus, fieldbus). Not modelled. Variants: ISA 5.1-2009 separates shared-system links from fieldbus links with different marks; many legends use filled dots or diamonds for fieldbus; Very old drawings: o-o-o meant "software or data link" only | *logical* |
| **Mechanical link** `line-mechanical-link` | A thin line with a small open circle at each end (pivots). | Mechanical connection between devices: a float to a valve, interlocked PSV changeover valves, or a key interlock. Not modelled; note the interlock. Variants: Legends differ: some draw a plain line with "MECH. LINK", others a dashed line with circles; Key interlocks are often shown as a dashed line with a key symbol or "KI" | *logical* |
| **Heat-traced line** `line-heat-traced` | A heavy process line with a thin dashed line running alongside it. | The process line is traced (electric or steam). The tracer type is in the insulation code (ET/ST) or a tag on the tracer. Set the line as insulated. Variants: See the electric and steam tracing marks for the lettered forms | – |
| **Jacketed line** `line-jacketed` | A heavy process line with a thin outer line on both sides, closed at the ends (a pipe inside a pipe). | Jacketed pipe: a heating medium flows in the outer jacket (sulphur, wax and heavy oil). Rare offshore. Model only the inner pipe and note the jacket. Variants: Some legends draw a double line only over the jacketed length | – |

## Connectors, boundaries and annotations

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-connectors.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Off-page connector, outgoing** `offpage-to` | The line ends at the flat side of a pentagon (an arrow-shaped box) that points away in the flow direction. The destination drawing number and the service or equipment, e.g. "TO PID-102 / V-2100", are written inside or beside it (left blank here). | The line continues on another sheet. If that sheet is in the job, join the two halves into one line (same line number). Otherwise end the line at a battery limit in plant.json ({ at, label: "TO …" }). Variants: Some legends use an arrow with the text above, or a rectangle with an arrowhead; Instrument signals use smaller connectors, often a circle with a letter or a hexagon | *logical* |
| **Off-page connector, incoming** `offpage-from` | A pentagon at the start of the line, pointing into the line in the flow direction, with "FROM PID-…" and the source text inside or beside it (left blank here). | The line comes from another sheet. Join it to the matching outgoing connector if that sheet is in scope; otherwise start the line at a battery limit ({ at, label: "FROM …" }). Variants: Some legends point both connector types to the right regardless of direction; read the TO/FROM text | *logical* |
| **Utility connector** `utility-connector` | A small rounded box at a line end with the utility code inside (e.g. N2, IA, UA, PW, FG). | Utility supply or return from the utility distribution sheets, without the full off-page reference. End the line at a battery limit labelled with the utility. Variants: Some legends use an ellipse, a flag or a hexagon; Utility stations (hose connections) carry a list of codes | *logical* |
| **On-sheet continuation** `connector-on-sheet` | A line ending in an arrow at a small circle with a letter; the same letter appears where the line resumes on the same sheet. | The line jumps across the same drawing to avoid a long route. Treat both ends as one continuous line. Variants: A small circle with a letter can look like an instrument bubble; continuation letters are single letters and have no number | *logical* |
| **Tie-in point** `tie-in` (TP, TI) | A bar across the line where new pipe joins existing pipe, with a leader to a hexagon or flag holding the tie-in number (e.g. TP-01). | Where new work connects to existing plant (hot-tap, cold cut, or flange break). In plant.json a tie-in is a battery limit: end the line with { at, label: "TIE-IN TP-01" }; it renders as a blinded end with a yellow band. Variants: Tie-in marks vary: hexagon, flag, circle with TI, or a diamond; Hot taps are often noted "HT" | *logical* |
| **Battery limit** `battery-limit` (BL) | A long dash-dot boundary line crossing the pipe, labelled "B.L." or "BATTERY LIMIT", often with the scope owner written on each side. | The edge of the modelled scope. In plant.json the line's from/to becomes a BatteryLimit object { at: [x, y, z], direction?, label? } instead of "TAG.port". It renders as a blinded end with a yellow band and the label. No component. Variants: Some legends draw a heavy dashed line with a flag; Package (vendor) boundaries look similar but enclose equipment | *logical* |
| **Package / vendor scope boundary** `package-boundary` | A dash-dot rectangle drawn around a group of equipment and piping, with the package tag and vendor written in a corner, and connections crossing its edge. | Everything inside is supplied as one package (skid), and its details may be on the vendor's P&IDs. Map the package to a package component (e.g. chemical-injection-skid, instrument-air-package) and the crossing lines to its ports. Variants: Some legends use long dashes or a double-dash-dot; Interface points on the boundary may carry small flags or "VI" (vendor interface) marks | *logical* |
| **Interlock / logic diamond** `interlock` (I) | A diamond with "I" (or an interlock number) inside, with signal lines in and out. | Interlock or logic function, defined on the logic diagrams or the cause & effect chart: e.g. LAHH trips a pump and closes an ESDV. Not physical; use it to understand the shutdowns. Variants: The number inside refers to the logic or C&E sheet; ISA logic gates (AND/OR) may be drawn as boxes on the P&ID; SIS trips are often a diamond with "S" or "SIS" | *logical* |
| **Note reference** `note-marker` | A number inside a small hexagon (or a flag or box) with a leader to the item; the note itself is in the notes block. | Points to a drawing note: slopes, no pockets, vendor scope, minimum distances or hold items. Read the note; it often explains things the symbols cannot. Variants: Some companies use a flag or a circle; a hexagon with a number only is a note, while an ISA computer-function hexagon has tag letters | *logical* |
| **Hold / revision cloud** `hold-cloud` | A scalloped (cloud) outline around part of the drawing, often with "HOLD" or a revision triangle. | The clouded part is on hold (unconfirmed) or changed in this revision. Treat the data inside as provisional and list it as an assumption. Variants: Holds may be written "HOLD 3" with a hold list; Revision clouds are paired with a revision triangle | *logical* |
| **Revision marker** `revision-triangle` | A small triangle with a revision number or letter inside, next to a change (often inside a cloud). | Marks what changed in that revision. Check the revision block to see whether the change is approved. Variants: Some companies use a circle or hexagon for revisions | *logical* |

## Instrument bubbles and functions (ISA 5.1)

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-instruments.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Field-mounted instrument (discrete)** `isa-field` (PI, PT, TT, LT, FT) | A plain circle with no horizontal line, containing the tag letters and usually the loop number. | A discrete instrument mounted in the field, on or next to the pipe or vessel: transmitters, gauges, switches and local controllers. These are the bubbles that become physical instruments; read the letters to choose the component (PT → pressure-transmitter, PI → pressure-gauge, etc.). Variants: Bubble size varies; ISA suggests about 11 mm (7/16"); Wireless field instruments may carry a small antenna mark; Two touching circles = one device with two functions | – |
| **Instrument in main control room (primary location)** `isa-panel-primary` (FIC, PIC, LIC, TIC, HS) | A circle with one solid horizontal line across the middle; letters above the line, loop number below. | A discrete (hardwired) instrument or function in the main control room, normally accessible to the operator: a panel indicator, controller or push-button. It is not on the plot; don't model it, but it tells you which field instrument exists. Variants: On ISA 5.1-2009 drawings, solid bar = central/main control room; Circle-in-square with a bar = the same location but in the DCS | *logical* |
| **Instrument on local / auxiliary panel** `isa-panel-auxiliary` (PIC, HS, XA) | A circle with two parallel horizontal lines across the middle (a double bar). | Instrument or function on a secondary or local panel accessible to the operator: a local control panel, package panel or LER panel. Not on the plot; the package or panel may be modelled as a building or skid. Variants: ISA 5.1-2009: double solid bar = secondary/local panel, normally accessible; double dashed bar = local, not normally accessible; Older drawings (1984): double bar = auxiliary location | *logical* |
| **Instrument behind the panel (not accessible)** `isa-behind-panel` (FY, PY, TY) | A circle with a dashed horizontal line across the middle. | Function not normally accessible to the operator: behind the panel, in a marshalling cabinet or rack room (relays, converters, signal isolators). Not modelled. Variants: Circle-in-square with a dashed bar = a DCS function not shown to the operator (internal calculation) | *logical* |
| **Shared display / shared control (DCS)** `isa-dcs` (FIC, PIC, LIC, TIC, FI, PI, LI, TI) | A circle inside a square, with a solid horizontal line across, letters above and loop number below. | A function in the basic process control system (BPCS/DCS) shown on operator screens: indication, control, alarms. Software, not hardware on the plot. A PIC-101 in the DCS implies a field PT-101. Variants: No bar = a field-mounted shared display (e.g. a local HMI); Dashed bar = DCS function the operator cannot see; ISA 5.1-2009 redefined circle-in-square as the BPCS whatever the hardware | *logical* |
| **PLC / alternate control system function** `isa-plc` (YIC, XC, HS) | A diamond inside a square, with a horizontal line across when operator-accessible. | A function in a PLC or other non-BPCS control system: package PLCs, sequence logic, compressor UCP. Software, not modelled. Variants: ISA 5.1-2009 lets the user assign diamond-in-square to either "alternate control system" or SIS; the drawing legend says which; Some old sources describe a triangle-in-square for PLCs; the standard shape is the diamond | *logical* |
| **Safety instrumented system (SIS) function** `isa-sis` (LSHH, PSHH, LZ, PZ, XS) | A diamond inside a square (with a bar when operator-accessible), carrying a trip function such as LSHH or PSHH. Many legends add "SIS" or a distinct colour or letter. | A trip or permissive in the safety instrumented system: PSD/ESD logic on independent SIL-rated hardware. Not physical. Each one implies dedicated field sensors (often LZT/PZT, or a second transmitter) and final elements (ESDV, BDV, motor trips). Variants: ISA 5.1-2009 introduced the diamond-in-square option for SIS. ISA describes the 2022 edition as a corrections release, and no change to this symbol was reported (the 2022 text was not checked); Many operators mark SIS loops with Z (e.g. PZT, LZT, XZV) or a "S" prefix; this is company practice, not ISA; Some legends use a hexagon or a double-bordered diamond for the SIS and keep diamond-in-square for PLCs; A note such as CMS beside it puts the trip in the machinery protection / condition monitoring system (API 670 rack), not the process SIS: see cms-system | *logical* |
| **Computer function** `isa-computer` (FQ, UY, KY) | A hexagon (pointed left and right), with a horizontal line across when operator-accessible. | A function on a separate computer (supervisory or optimisation system, advanced process control, flow computer software, historian calculation). Not modelled. Variants: A hexagon with only a number and no letters is a note marker, not a computer function; Flow computers on metering skids are often drawn this way, with FQ | *logical* |
| **Multi-function instrument (touching bubbles)** `isa-multifunction` (PIT, PSH, TT, TSH) | Two bubbles drawn touching side by side, each with its own letters, e.g. PIT and PSHH. | One physical device performing two functions (e.g. a transmitter with a built-in switch), or two functions in one housing. Model one instrument. Variants: Bubbles of different shapes may touch (a field device with its DCS function) | – |
| **Function block (computing / signal conversion)** `isa-function-block` (FY, TY, PY) | A small square drawn beside a bubble (usually a Y relay or compute bubble) containing a math or conversion symbol: √, Σ, ×, ÷, I/P, f(x), >, <. | What the Y function does: square-root extraction (√), totalising (Σ), high or low select (> <), conversion (I/P). Not physical. Variants: Some legends put the symbol inside the bubble instead | *logical* |
| **Alarm and trip annotation** `isa-alarm-annotation` (H, HH, L, LL) | Small letters next to a bubble: H, HH, L, LL (or AH, AHH, AL, ALL), sometimes with a setpoint value. | The loop has alarms (H/L) or trips (HH/LL) at those points. HH and LL usually drive the cause & effect matrix. Not physical; record them as instrument alarm limits. Variants: ISA succeeding letters: A = alarm, S = switch/trip; H, L, M = high, low, middle; Trips are often in the SIS bubble (e.g. LSHH) instead of annotations | *logical* |
| **ISA 5.1 identification letters** `isa-letters` | Tag letters inside any bubble, e.g. "FIC" over "101": first letter = measured variable, following letters = functions. | FIRST LETTER (measured or initiating variable, with optional modifier): A analysis, B burner/combustion, C user's choice (often conductivity), D user's choice (often density), E voltage, F flow (modifier F ratio), G user's choice (often gauging/position), H hand, I current, J power, K time/schedule, L level, M user's choice (often moisture), N user's choice, O user's choice, P pressure/vacuum, Q quantity (modifier Q integrate/totalise), R radiation, S speed/frequency (modifier: S safety), T temperature, U multivariable, V vibration/machinery analysis, W weight/force, X unclassified, Y event/state/presence, Z position/dimension. First-letter modifiers: D differential, F ratio, J scan, K time rate of change, Q integrate/totalise, S safety, X/Y/Z axis. SUCCEEDING LETTERS: A alarm, B user's choice, C control, D differential (modifier), E sensor/primary element, G glass/gauge/viewing device, H high, I indicate, K control station, L light or low, M middle/momentary, N user's choice, O orifice/restriction, P point (test connection), Q integrate, R record, S switch, T transmit, U multifunction, V valve/damper/louvre, W well (thermowell), X unclassified, Y relay/compute/convert, Z driver/actuator/final control element. Examples: PT pressure transmitter, PDT differential-pressure transmitter, FE flow element, FIC flow indicating controller, LSHH level switch high-high, TW thermowell, PSV pressure safety valve, XV on/off valve, HS hand switch, ZSO position switch open, AT analyser transmitter, FQ totaliser, LG level gauge. Variants: Company standards fill the user's-choice letters differently (C, D, G, M, N, O); Some companies put Z after the first letter for SIS loops (PZT); that is not ISA; Loop numbers may include an area or unit prefix, e.g. 20-PT-1101 | *logical* |
| **Voting (MooN) annotation** `voting-annotation` | Text such as 1oo2, 2oo2 or 2oo3 written beside the signal lines where two or more measurements join into one alarm, trip or SIS function. | M-out-of-N voting: 1oo2 trips when either of two sensors trips (safe, more spurious trips); 2oo2 needs both; 2oo3 needs any two of three. It tells you how many field sensors exist: model N of them. Logic only, nothing physical beyond the sensors. Variants: Some legends put the voting inside the logic box (e.g. UZ 2oo3); F&G uses the same notation for detector voting (2ooN confirmed gas or fire) | *logical* |
| **Condition monitoring / machinery protection system (CMS)** `cms-system` | A rectangle labelled CMS (or MPS, VMS), or the letters CMS written beside vibration and bearing-temperature alarm functions. | The machinery protection and condition monitoring system: an API 670 rack that receives the VE / ZE / bearing TE signals, raises alarms and trips (often voted 1oo2 or 2oo2), and passes trends to analysis software. Not physical on the P&ID; in a twin the rack sits in an equipment-room system-cabinet. Variants: Some legends draw it as a DCS-style square with the CMS tag; The machine trip from the CMS usually goes to the motor MCC or the SIS | *logical* |

## Primary elements and field instruments

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-primary-elements.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Orifice plate flow element with dp transmitter** `fe-orifice-plate` (FE, FT, FE, FT, FIT) | An orifice plate between a flange pair on the line (a bar across the pipe with a gap in the middle for the bore), with two impulse lines from the flange taps up to an FT bubble. | Differential-pressure flow measurement. FE is the plate in its orifice-flange pair and FT is the dp cell. Fiscal and allocation meters, gas flow to flare and fuel. Model one item (orifice-flowmeter) for the FE+FT pair. Variants: Simplified form: two short bars across the line with an FE bubble; A restriction orifice (RO) looks the same but has no taps; Orifice fittings (senior / dual-chamber) are drawn as a box labelled with the type | `orifice-flowmeter` |
| **Venturi tube** `fe-venturi` (FE, FE) | The line narrows through a short throat and widens again (converging and diverging lines either side of the pipe), with an FE bubble above. | Low-loss dp flow element for large water and gas lines and multiphase meters (venturi + gamma). No dedicated component; orifice-flowmeter is the nearest dp meter. Variants: Some legends draw it as a box with "V"; Multiphase flow meters (MPFM) combine a venturi with a gamma densitometer box | – |
| **Flow nozzle** `fe-flow-nozzle` (FE, FE) | A flange pair with two curved lines inside the pipe converging downstream (the nozzle profile), and an FE bubble. | Dp flow element for high-velocity steam and gas. No dedicated component. Variants: Often just a box with "FN" or the same symbol as an orifice with "NOZZLE" | – |
| **Pitot tube** `fe-pitot` (FE, FE) | A probe from an FE bubble into the pipe, bent into an L whose open end faces upstream. | Point-velocity flow probe. Rare on process lines, used for testing and ducts. No dedicated component. Variants: Some legends draw a small open-ended hook symbol without a bubble | – |
| **Averaging pitot (multiport probe)** `fe-averaging-pitot` (FE, FE) | A bar across the full pipe bore with several small holes (dots) on its upstream face, under an FE bubble. | Multi-port averaging pitot (Annubar type). Used on large gas lines and flare headers because the pressure loss is low. No dedicated component. Variants: Often drawn like a pitot tube with "AVG" or the vendor name | – |
| **Electromagnetic flowmeter** `fe-magnetic` (FE, FT, FE, FIT) | An in-line rectangle on the pipe with "M" inside, under an FT/FIT bubble. | Magnetic flowmeter for conductive liquids only: seawater, produced water, water injection, chemicals. Not for hydrocarbons or gas. Becomes magnetic-flowmeter. Variants: Some legends write "MAG" or draw two coil arcs either side of the pipe | `magnetic-flowmeter` |
| **Vortex flowmeter** `fe-vortex` (FE, FT, FE, FIT) | An in-line rectangle with a small triangle (the bluff body) and a wavy vortex trail behind it, under an FT bubble. | Vortex-shedding meter for steam, gas and clean liquids: utilities, fuel gas, instrument air. Becomes vortex-flowmeter. Variants: Some legends write "VOR" or "VORTEX" in the box | `vortex-flowmeter` |
| **Turbine flowmeter** `fe-turbine` (FE, FT, FE, FQ) | An in-line rectangle with a small rotor (a hub with blades) inside, under an FT or FQ bubble. | Turbine meter for clean light liquids: fiscal oil metering (with a prover), diesel, chemicals. No single-meter component; fiscal runs are metering-skid. Variants: Some legends write "T" or "TUR" in the box; Helical turbine meters on fiscal skids are drawn the same | – |
| **Positive displacement flowmeter** `fe-positive-displacement` (FE, FQ, FQ, FQI) | An in-line rectangle with two small touching ovals or circles (meshing rotors) inside, often with an FQ (totaliser) bubble. | Oval-gear or rotary PD meter for viscous liquids, chemical dosing and diesel transfer. Counts volume directly. No dedicated component. Variants: Some legends write "PD" in the box | – |
| **Ultrasonic flowmeter** `fe-ultrasonic` (FE, FT, FE, FIT, FQ) | Two small transducers on opposite sides of the pipe, offset along it, with a dashed diagonal sound path between them, under an FT bubble. | Transit-time ultrasonic meter: fiscal gas metering (multipath), flare gas and large liquid lines. Becomes ultrasonic-flowmeter. Variants: Some legends draw a box with "US" or "UT"; Clamp-on meters are drawn outside the pipe wall | `ultrasonic-flowmeter` |
| **Coriolis mass flowmeter** `fe-coriolis` (FE, FT, FE, FIT, FQ) | An in-line box on the pipe with a looped (U-shaped) tube inside, under an FT bubble. | Coriolis mass meter: condensate, oil allocation, chemicals and LACT/fiscal liquid service. It measures density too. Becomes coriolis-flowmeter. Variants: Some legends write "C" or "CORIOLIS" or "MASS" in the box | `coriolis-flowmeter` |
| **Variable-area flowmeter (rotameter)** `fe-variable-area` (FI, FG, FI, FG) | A vertical tapered tube, wider at the top, with a small float inside; the flow enters at the bottom and leaves at the top. | Local flow indicator for purge gas, small utilities, seal flush and chemical injection. No dedicated component; usually not modelled. Variants: ISA style: an FI bubble on the line with "VA" or "ROTA"; Metal-tube rotameters with transmitters are tagged FIT | – |
| **Thermowell with temperature element / transmitter** `te-thermowell` (TW, TE, TT, TW, TE, TT, TIT) | A U-shaped pocket dipping into the line (the thermowell), with a line up to a TT or TE bubble. | Temperature measurement: a thermowell (TW), an element (TE, an RTD or thermocouple) and a transmitter (TT). Model one temperature-transmitter for the loop. Variants: Some legends draw the well as a short line into the pipe with a dot; A test thermowell (TW only, no TE) is for a portable probe; skip it; Some legends show a flanged thermowell connection as two short bars (a flange pair) on the branch to the bubble, with the connection size written beside it | `temperature-transmitter` |
| **Local temperature gauge** `tg-temperature-gauge` (TI, TG, TI) | A thermowell (U-shaped pocket in the line) with a TI bubble on it. | Bimetallic dial thermometer in a thermowell, read locally. Becomes temperature-gauge. Variants: Tagged TG on some legends; A dial (circle with a pointer) is sometimes drawn instead of a bubble | `temperature-gauge` |
| **Local pressure gauge** `pi-pressure-gauge` (PI, PG, PI) | A PI (or PG) bubble on a thin impulse line from the pipe, often with a small root or isolation valve on the impulse line. | Bourdon-tube dial gauge read locally: pump discharges, filters and vessels. Becomes pressure-gauge. Variants: ISA tag is PI; many companies use PG; A gauge with a seal or snubber shows extra small symbols on the impulse line; A small box with a letter on the gauge branch (F, S, D, P) is a project-specific accessory such as a snubber, seal, filter or pulsation damper: read the drawing legend, then model the gauge only | `pressure-gauge` |
| **Pressure transmitter** `pt-pressure-transmitter` (PT, PIT, PT, PIT, PZT) | A PT (or PIT) bubble on a thin impulse line from the pipe or vessel, with an electric (dashed) signal leaving to the DCS. | Electronic pressure transmitter feeding the DCS or SIS. Becomes pressure-transmitter. Variants: SIS transmitters are often PZT or carry an "S" suffix; Multivariable transmitters on flow loops may be tagged FT with P and T inputs | `pressure-transmitter` |
| **Differential pressure transmitter** `pdt-dp-transmitter` (PDT, PDIT, PDT, PDIT, PDI) | A PDT bubble with two impulse lines: one to each side of a filter, strainer, exchanger or column section. | Differential pressure across equipment: filter or strainer fouling, scrubber dp, column flooding. Becomes dp-transmitter. Variants: A local dp gauge is PDI or PDG; Dp level transmitters are tagged LT (see the dp level entry) | `dp-transmitter` |
| **Pressure switch** `ps-pressure-switch` (PS, PSH, PSL, PSHH, PSLL, PSH, PSL, PSHH, PSLL) | A bubble with PSH, PSL, PSHH or PSLL on an impulse line from the process. | Pressure switch giving a discrete alarm or trip. Old-style or package protection. Becomes pressure-switch. Variants: On modern plants the same letters often mean a trip point inside the DCS/SIS from a transmitter; then the bubble has a bar or a square; Packaged machines (lube oil, seal gas) still use real switches | `pressure-switch` |
| **Level gauge glass** `lg-gauge-glass` (LG, LG) | A tall narrow rectangle beside the vessel, joined to it by two horizontal nozzles (top and bottom), with an LG bubble. | Reflex or transparent sight glass showing the level directly. Common on older plant; many operators now use magnetic gauges, which are tagged LG too. No separate component; magnetic-level-gauge is the closest. Variants: Stacked or overlapping glasses are drawn as two or three offset rectangles; Look-alike: magnetic level gauge (chamber with a float and a flag indicator) | – |
| **Magnetic level gauge** `lg-magnetic` (LG, LI, LG, LI) | A chamber beside the vessel on two nozzles, with a float inside and a column of indicator flaps (half-filled marks) on its face, and an LG bubble. | Float-and-magnet level indicator on a bypass chamber, readable from a distance. Often carries a level transmitter too. Becomes magnetic-level-gauge. Variants: Often drawn exactly like a gauge glass and tagged LG; the datasheet tells them apart; A clamp-on magnetostrictive LT on the chamber is drawn as a separate LT bubble | `magnetic-level-gauge` |
| **Displacer level transmitter** `lt-displacer` (LT, LT, LIT, LDT) | An external cage beside the vessel on two nozzles, with a long slender displacer hanging inside from a torque tube at the top, and an LT bubble. | Buoyancy (displacer) level or interface transmitter. Classic on separators for oil/water interface. No dedicated component; model it as a level transmitter note on the vessel. Variants: Internal displacers (inside the vessel) are drawn as the displacer inside the vessel outline; Interface service may be tagged LDT or LT with "INTERFACE" | – |
| **Differential pressure level transmitter** `lt-dp-level` (LT, LT, LIT) | An LT bubble with two impulse lines to the vessel: one from a low nozzle (high side) and one from a high nozzle (low side). Remote seals are drawn as capillaries (line with × marks). | Level from hydrostatic head: the most common level transmitter on separators and drums. Becomes dp-transmitter; the bubble letters (LT) give the service. Variants: With diaphragm seals the lines carry × marks (capillaries); Bubble-tube (purge) level measurement looks similar with a purge supply | `dp-transmitter` |
| **Guided wave radar level transmitter** `lt-gwr` (LT, LT, LIT) | An LT bubble on a vessel-top (or chamber-top) nozzle, with a straight rod or cable probe running down into the liquid. | Guided wave radar: a pulse travels down the probe and reflects at the surface or interface. Separators, drums and bridles. Becomes guided-wave-radar. Variants: Often mounted in an external chamber (bridle), drawn like a displacer cage with a probe; Some legends mark it "GWR" beside the bubble | `guided-wave-radar` |
| **Non-contact radar level transmitter** `lt-radar` (LT, LT, LIT) | An LT bubble on a vessel-top nozzle, with a cone (horn antenna) pointing down and wave arcs (unguided signal) towards the liquid. | Free-space radar for tanks and sumps: storage tanks, open drains and caissons. No dedicated component; guided-wave-radar is the closest stand-in. Variants: Ultrasonic level transmitters are drawn the same and marked "US" | – |
| **Nuclear (gamma) level transmitter** `lt-nuclear` (LT, RE, LT, RE) | A source holder (small box) on one side of the vessel and a detector rod on the other, joined by a wavy (radiation) line across the vessel, with an LT bubble on the detector. | Gamma level or density measurement through the vessel wall: slug catchers, desanders, profilers in separators. Needs radiation controls. No dedicated component. Variants: The source holder may be marked with the radiation trefoil or "RS"; Density profilers have many detectors in a vertical row | – |
| **Vibrating fork level switch** `ls-vibrating-fork` (LS, LSH, LSL, LSHH, LSLL, LSH, LSL, LSHH, LSLL) | A short side nozzle into the vessel with two small prongs (the fork) inside, and an LS bubble (LSH, LSHH…) outside. | Point level switch for high or low alarms and trips, independent of the transmitter. Becomes level-switch. Variants: Many legends draw just the LSHH bubble on the vessel without the fork; Capacitance and float switches use the same bubble letters | `level-switch` |
| **Float level switch** `ls-float` (LS, LSH, LSL, LSH, LSL) | A float (circle) on a short arm inside the vessel, pivoted at the wall, with an LS bubble outside. | Mechanical float switch: sumps, drain drums, day tanks and lube-oil reservoirs. No dedicated component; level-switch is the nearest stand-in. Variants: Cage-mounted float switches sit in an external chamber like a displacer | – |
| **Analyser (with sample probe)** `at-analyser` (AT, AE, AIT, AT, AE, AIT, AI) | An AT (or AE) bubble linked to a probe that dips into the line, with the measured component written beside the bubble (O2, H2S, H2O, OIW, BS&W, CO2…). | Online analyser: water dew point, H2S, oxygen, oil-in-water, BS&W or water-cut. In-line probes sit on the pipe; extractive analysers have a sample line to an analyser shelter (analyser-shelter). No single component for the probe. Variants: The analyte is written beside the bubble, not as part of the tag; Extractive systems show a sample line, conditioning box and return; Gas chromatographs on metering skids are AT with "GC" | – |
| **Diaphragm (remote) seal with capillary** `diaphragm-seal` (PT, LT) | A small flat seal on the pipe or vessel nozzle (a short rectangle with a curved diaphragm line), joined to a transmitter bubble by a capillary (a line with × marks). | Remote seal protecting a transmitter from corrosive, viscous or hot process fluid. Part of the transmitter; model the transmitter only. Variants: Some legends write "DS" or "SEAL" beside the instrument | – |
| **Vibration element / transmitter on a machine** `ve-vibration-element` (VE, VT, VE, VT, VXE, VYE, VI, VSHH) | A VE (or VT) bubble joined by a short line to a small square or dot on the outline of a motor, pump, compressor or bearing. Often in pairs (A / B or X / Y), one per bearing or per axis, with VI / VA / VSHH functions downstream. | Machinery vibration sensor: proximity probes (shaft) or accelerometers (casing) on the bearings, wired to a proximitor junction box and a protection rack. V as the first letter is vibration in ISA 5.1. Becomes vibration-probe on each monitored bearing; set pair for X-Y probes. Variants: Some legends draw only the bubble beside the machine with a note such as NOTE / TYP. for every bearing; VXE / VYE mark the X and Y probes of a pair; ZE on a machine is an axial position (thrust) probe and also becomes vibration-probe; Look-alike: V on a valve tag (XV, HV) is a valve, not vibration; check the first letter | `vibration-probe` |

## Vessels, columns and tanks

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-vessels.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Vertical vessel** `vessel-vertical` (V, D) | A tall rectangle with rounded (dished) top and bottom heads, with nozzles: inlet on the side, gas at the top, liquid at the bottom. Equipment tag and design data written beside it. | Vertical pressure vessel: scrubber, KO drum, flash drum or receiver. Gas/liquid duty with small liquid volume becomes vertical-separator; an air or nitrogen receiver becomes air-receiver. Read the title and service. Variants: ISO 10628-2 shows support skirt, legs or brackets as extra marks under the vessel; Internals (mesh pad, vane pack, inlet device) are often drawn inside; Jacketed or insulated vessels have hatching or a second outline | `vertical-separator` |
| **Vertical scrubber with mist eliminator** `vessel-scrubber` (V) | A vertical vessel with a band near the top filled with a zig-zag or cross-hatch (the mesh pad or vane pack) and an inlet on the side below it. | Gas scrubber or KO drum: compressor suction scrubbers, fuel-gas scrubbers, inlet KO drums. Becomes vertical-separator with mesh or vane internals. Variants: Vane packs may be drawn as vertical zig-zags; Cyclonic internals are drawn as small cones; Filter-separators are horizontal with cartridges, see separation | `vertical-separator` |
| **Horizontal vessel** `vessel-horizontal` (V, D) | A long horizontal rectangle with rounded heads at both ends, with nozzles on the top and bottom. | Horizontal drum: two-phase separator, surge drum, KO drum or drain drum. Becomes horizontal-separator (set the service: 2-phase, 3-phase or KO). Variants: Saddle supports are drawn as small trapezoids underneath; A horizontal vessel on saddles holding LPG is a bullet (see tank-bullet) | `horizontal-separator` |
| **Three-phase separator (with boot and weir)** `separator-three-phase` (V) | A horizontal vessel with a small vertical boot under it (a short rectangle), and an upright weir plate inside towards the outlet end. Separate oil and water outlets. | Oil/gas/water separator: HP and LP production separators, test separators. Water settles into the boot or before the weir, and oil spills over the weir into the oil bucket. Becomes horizontal-separator with service 3-phase. Variants: Some have no boot: interface level is held with a weir and separate oil and water outlets; Bucket-and-weir layouts show two plates; Inlet device, mesh pad and vortex breakers may also be drawn | `horizontal-separator` |
| **Trayed column** `column-trayed` (T, C) | A tall vertical vessel with dished heads and short horizontal lines inside that alternate from left to right (the trays). | Tray column: stabilisers, TEG contactors, amine contactors and fractionators. Becomes column (trayed). Variants: ISO draws trays as dashed horizontal lines; Tray numbers are written at feed and draw-off points; Bubble-cap and valve trays have small caps or marks on the tray lines | `column` |
| **Packed column** `column-packed` (T, C) | A tall vertical vessel with one or more rectangles crossed corner to corner (an X), the packed beds, each with a short distributor line above it. | Packed column: structured or random packing for TEG contactors, strippers and scrubbers. Becomes column (packed). Variants: Beds may be drawn as dotted or hatched areas; An X box is also the ISO symbol for a fixed catalyst or adsorbent bed; read the service | `column` |
| **Cone-roof storage tank** `tank-cone-roof` (T, TK) | A wide flat-bottomed rectangle with a shallow pointed (conical) roof. | Atmospheric storage tank (API 650): diesel, water, produced water, slop or crude storage. Becomes storage-tank. Variants: A breather valve and flame arrester are usually drawn on the roof; Insulated or heated tanks show hatching or coils | `storage-tank` |
| **Dome-roof storage tank** `tank-dome-roof` (T, TK) | A wide flat-bottomed rectangle with a curved (domed) roof. | Atmospheric or low-pressure tank with a self-supporting dome roof. Becomes storage-tank. Variants: Geodesic aluminium domes over floating-roof tanks are drawn as a dome with the floating roof inside | `storage-tank` |
| **Floating-roof tank** `tank-floating-roof` (T, TK) | A flat-bottomed tank with an open top and a horizontal deck floating inside below the rim, often with a rolling ladder drawn diagonally from the rim to the deck. | External floating-roof tank (API 650 Annex C) for crude and volatile products: the roof rides on the liquid to cut vapour losses. Becomes storage-tank. Variants: Internal floating roofs are drawn inside a cone or dome roof; The roof drain (articulated) is drawn as a pipe from the deck to the shell | `storage-tank` |
| **Spherical tank (sphere)** `tank-sphere` (T, TK, V) | A circle standing on vertical legs. | Pressurised sphere for LPG, butane or NGL storage. Becomes spherical-tank. Variants: ISO draws a plain circle for "spherical vessel" and adds legs for "on legs" | `spherical-tank` |
| **Bullet tank (horizontal pressure storage)** `tank-bullet` (T, TK, V) | A horizontal vessel with rounded heads standing on two saddles, sometimes inside a mound (a hump of earth). | Horizontal pressure storage for LPG, propane or NGL. Becomes bullet-tank. It looks like a horizontal drum; tell them apart by the tag and service. Variants: Mounded bullets are drawn under a curved earth line | `bullet-tank` |
| **Caisson (sump / pump caisson)** `caisson` (CS, X) | A long vertical tube hanging down from the deck through the sea surface (a wavy line), open at the bottom. | Deck-hung caisson: seawater lift and firewater pump caissons, open drain and produced-water disposal caissons. Becomes caisson. Variants: Pump caissons show a vertical turbine or submersible pump inside; Disposal caissons show the inlet near the top and an open end | `caisson` |
| **Slug catcher (finger type)** `slug-catcher` (V, SC) | Several long parallel pipes (fingers) joined at the inlet end by a manifold, with gas risers at the inlet end and a liquid header at the far end. | Pipeline slug catcher at an onshore terminal: the fingers store liquid slugs and the gas leaves from the top. Becomes slug-catcher. Variants: Vessel-type slug catchers are drawn as a large horizontal drum | `slug-catcher` |

## Heat transfer equipment

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-exchangers.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Heat exchanger (general)** `hx-general` (E, HX) | A circle with a zig-zag line passing through it (one fluid) and pipe stubs on the circle (the other fluid). | Any heat exchanger, drawn generically (common on PFDs and simplified P&IDs). Usually a shell-and-tube unit, so map it to shell-and-tube-exchanger unless the title, tag or datasheet says plate, air-cooled or hairpin. Variants: ISO 10628-2 also uses it for "condenser"; Some legends draw two concentric circles or a circle with a horizontal coil | `shell-and-tube-exchanger` |
| **Shell-and-tube exchanger, fixed tubesheet (BEM)** `hx-fixed-tubesheet` (E) | A horizontal shell with a rounded bonnet at each end and straight tube lines running the full length between two tubesheets. | Fixed-tubesheet exchanger (TEMA BEM/NEN): cheap and tight, but the tube bundle can't be pulled, so it is used for clean shell-side fluid. Becomes shell-and-tube-exchanger (temaType BEM). Variants: Simplified legends draw just a rectangle with a zig-zag tube line; An expansion bellows on the shell means a large thermal difference | `shell-and-tube-exchanger` |
| **Shell-and-tube exchanger, U-tube (BEU / AEU)** `hx-u-tube` (E) | A horizontal shell with a channel head at one end only, a rounded shell cover at the other, and tube lines drawn as a U bending back. | U-tube exchanger: one tubesheet, removable bundle, takes thermal expansion. High-pressure gas coolers and heaters. Becomes shell-and-tube-exchanger (temaType AEU/BEU). Variants: The U may be drawn as one hairpin line | `shell-and-tube-exchanger` |
| **Shell-and-tube exchanger, floating head (AES)** `hx-floating-head` (E) | A horizontal shell with a flat channel cover at one end and a larger domed shell cover at the other; inside the cover, a small floating-head arc closes the straight tubes. | Floating-head exchanger (TEMA AES/AET): removable bundle and free expansion, for dirty or fouling service such as crude coolers. Becomes shell-and-tube-exchanger (temaType AES). Variants: Often simplified to the fixed-tubesheet symbol with "AES" written beside it | `shell-and-tube-exchanger` |
| **Kettle reboiler** `hx-kettle` (E) | A small channel and tube bundle at one end, with the shell swelling into a much larger diameter over the bundle (a kettle), a weir plate at the far end, and a vapour outlet on top. | Kettle (TEMA K) reboiler or vaporiser: liquid boils around the bundle, vapour leaves the top and liquid spills over the weir. Glycol regeneration, propane chillers. Map to shell-and-tube-exchanger and note the kettle shell. Variants: Thermosyphon reboilers are drawn as vertical shell-and-tube exchangers beside the column | `shell-and-tube-exchanger` |
| **Condenser** `hx-condenser` (E) | A shell-and-tube exchanger with a large vapour inlet on top, a small condensate outlet (often from a boot or hotwell) underneath, and cooling medium through the tube side. | Shell-side condenser: overhead condensers, regeneration still condensers. Same hardware as a shell-and-tube unit, so map it to shell-and-tube-exchanger (or air-cooled-exchanger if drawn with a fan). Variants: ISO 10628-2 uses the general exchanger symbol for condensers; Air-cooled condensers are drawn as an air cooler | `shell-and-tube-exchanger` |
| **Plate heat exchanger** `hx-plate` (E) | A rectangle (frame) with many thin vertical lines inside (the plates) and four connections, usually all on one end plate. | Gasketed or welded plate exchanger: compact cooling-medium/seawater coolers, TEG coolers. Becomes plate-heat-exchanger. Variants: Some legends draw a rectangle with a diagonal zig-zag; Printed-circuit (diffusion-bonded) exchangers are drawn as a solid block, which is a different component (printed-circuit-heat-exchanger) | `plate-heat-exchanger` |
| **Air-cooled exchanger (fin-fan)** `hx-air-cooler` (E, AC) | A long horizontal bundle (a rectangle with a zig-zag or straight tube passes) with header boxes at the ends and a fan (a circle with blades) beside or under it, often with an M motor. | Fin-fan cooler: compressor discharge coolers, lube-oil and condensate coolers onshore. Becomes air-cooled-exchanger. Variants: Forced draught: fan below; induced draught: fan above; Louvres and VFD drives are drawn as extra marks; Some legends draw a rectangle with an X and a fan circle | `air-cooled-exchanger` |
| **Double-pipe (hairpin) exchanger** `hx-double-pipe` (E) | A U-shaped (hairpin) pipe drawn as a pipe within a pipe: an inner line running through an outer annulus around the bend. | Hairpin or double-pipe exchanger: small duties at high pressure, sample coolers, small gas coolers. Becomes hairpin-exchanger. Variants: Simplified: two concentric lines with in and out arrows | `hairpin-exchanger` |
| **Fired heater** `heater-fired` (H, F) | A tall box (radiant section) with burners (small flames or triangles) along the floor, a narrower convection box above, and a tapered stack on top. A coil (zig-zag) runs through. | Direct-fired process heater: hot-oil heaters, glycol reboilers, crude heaters (mostly onshore). Becomes fired-heater. Variants: Simplified: a rectangle with a flame triangle under it; Box vs cylindrical heaters look similar on P&IDs | `fired-heater` |
| **Electric heater** `heater-electric` (E, H, EH) | A horizontal vessel with a heating element drawn as a zig-zag (resistor) inside, and a terminal box at one end. | Electric process heater: fuel-gas superheaters, regeneration-gas heaters, start-up heaters. Becomes electric-heater. Variants: ISO 10628-2 shows a small resistor rectangle on the vessel side; Some legends write "EH" or "ELECTRIC" inside | `electric-heater` |
| **Cooling tower** `cooling-tower` (CT) | A tapering box with a fan on top (induced draught), water spray or fill inside, and a basin at the bottom. | Evaporative cooling tower for the onshore cooling-water system. Becomes cooling-tower. Variants: Natural-draught towers are drawn as a hyperbolic outline; Fan below = forced draught | `cooling-tower` |

## Pumps

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-pumps.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Centrifugal pump** `pump-centrifugal` (P) | A circle with the suction at the centre of one side and the discharge leaving tangentially from the top, standing on a small base. Often a small triangle inside points towards the discharge (drawn here). | Horizontal centrifugal pump: the default process and utility pump. Becomes centrifugal-pump (API 610 OH2); use between-bearings-pump for multistage BB units and vertical-inline-pump for OH3 when the datasheet says so. Variants: Some legends omit the inner triangle; ISO-style legends may draw a "V" from the suction side instead; The driver (M) may be drawn joined by a shaft line; Spared pumps A/B are drawn twice | `centrifugal-pump` |
| **Vertical in-line pump** `pump-vertical-inline` (P) | A pump circle with suction and discharge on the same horizontal line (in-line), and a motor box (M) mounted directly above it. | Close-coupled vertical in-line pump (API 610 OH3/OH4): cooling medium, chemicals, small duties. Becomes vertical-inline-pump. Variants: Often drawn as an ordinary centrifugal pump with a note "vertical in-line" | `vertical-inline-pump` |
| **Vertical turbine (caisson) pump** `pump-vertical-turbine` (P) | A motor on top of a discharge head on the deck, with a long column running down into a caisson or sump and a stack of bowls (stages) and a suction bell at the bottom, below the water line. | Line-shaft vertical turbine pump (API 610 VS1): seawater lift and firewater pumps on caissons. Becomes vertical-turbine-pump. Variants: Diesel-driven firewater versions have DE instead of M (see the fire pump); Simplified: a centrifugal pump symbol with a long dip pipe | `vertical-turbine-pump` |
| **Submersible pump** `pump-submersible` (P) | A capsule (pump and motor in one) at the bottom of a caisson or sump, below the water line, hanging from its discharge pipe; the power cable runs up beside it. | Electric submersible pump: seawater lift caissons, open-drain and sump pumps. Becomes submersible-pump. Variants: Some legends draw a centrifugal pump circle inside a sump outline with "SUB" | `submersible-pump` |
| **Reciprocating (plunger) pump** `pump-reciprocating` (P) | A cylinder (rectangle) with a piston or plunger (T shape) inside, connected to a crank circle, with suction and discharge connections on the cylinder head. | Positive displacement plunger pump (API 674): high-pressure injection, glycol circulation, mud pumps and pigging. Becomes reciprocating-pump (use mud-pump for drilling mud pumps). Variants: ISO draws a circle with a piston mark inside; Pulsation dampeners and a relief valve on the discharge are usually drawn nearby | `reciprocating-pump` |
| **Gear pump** `pump-gear` (P) | A rounded casing with two meshing gears (two touching circles) inside, suction on one side and discharge on the other. | Rotary positive displacement pump: lube oil, fuel oil and chemical transfer. No dedicated component; usually part of a package. Variants: Some legends draw a pump circle with two small circles inside; Look-alike: PD flowmeter (smaller, on the line, with an FQ bubble) | – |
| **Screw pump** `pump-screw` (P) | A long casing with a helical screw (a zig-zag along the axis) inside, suction underneath and discharge on top, with a drive shaft out of one end. | Twin or triple screw pump: crude transfer, lube oil, multiphase boosting. No dedicated component. Variants: Multiphase twin-screw pumps carry an "MPP" note | – |
| **Metering (dosing) pump** `pump-metering` (P) | A pump circle with an internal triangle and a long diagonal arrow through it (adjustable capacity); diaphragm heads may be drawn as a curved line inside. | Diaphragm or plunger metering pump (API 675): chemical injection, methanol, corrosion inhibitor. Usually part of the chemical injection package. Becomes metering-pump. Variants: Some legends write "MP" or draw a diaphragm arc on the head; Multi-head pumps are drawn as several circles on one driver | `metering-pump` |
| **Progressive cavity pump** `pump-progressive-cavity` (P) | A long horizontal casing with a single sine-shaped rotor inside, suction at one end on top and discharge at the other end, often with a motor box. | Single-screw (Moineau) pump: sludge, oily water, drain sump and polymer duties. Becomes progressive-cavity-pump. Variants: Some legends draw a box with a helix and "PCP" | `progressive-cavity-pump` |

## Compressors, blowers and drivers

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-machines.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Centrifugal compressor** `compressor-centrifugal` (K, C) | A trapezoid lying on its side: wide at the suction end, narrow at the discharge end, with a shaft line to the driver. | Centrifugal (turbo) compressor (API 617): export, booster, flash-gas and recompression. Becomes centrifugal-compressor; a gas-turbine-driven train is gas-turbine-compressor. Variants: Multi-stage casings may show several trapezoids on one shaft; Anti-surge recycle (FV with UIC/FIC) and dry gas seals are drawn around it; Look-alike: a turbine trapezoid widens in the flow direction | `centrifugal-compressor` |
| **Reciprocating compressor** `compressor-reciprocating` (K, C) | A cylinder rectangle with a double-acting piston inside, joined by a rod to a crank circle, with suction on top and discharge underneath each cylinder. | Reciprocating compressor (API 618): instrument air, fuel-gas boosting, high-pressure small flows. Becomes reciprocating-compressor (instrument-air packages become instrument-air-package). Variants: Each stage cylinder may be drawn separately with pulsation bottles; It looks like a reciprocating pump; the tag (K) and the service decide | `reciprocating-compressor` |
| **Screw compressor** `compressor-screw` (K, C) | A casing containing two meshing helical rotors (two zig-zags side by side), with suction and discharge connections and a shaft to the driver. | Rotary screw compressor (API 619): instrument and utility air, flash gas and vapour recovery. Becomes screw-compressor. Variants: Some legends draw the centrifugal trapezoid with "SCREW" written beside it; Oil-injected packages show an oil separator vessel downstream | `screw-compressor` |
| **Axial compressor** `compressor-axial` (K, C) | A long trapezoid narrowing in the flow direction with rows of short vertical lines inside (the blade rows), and a shaft. | Axial-flow compressor: large air or gas flows, mainly as the compressor section of a gas turbine. As a separate machine it is rare offshore. No dedicated component. Variants: Usually seen only as part of the gas-turbine symbol | – |
| **Blower / centrifugal fan** `blower-fan` (K, B, F, FN) | A circle (the scroll) with the outlet leaving tangentially as a short rectangular duct, and curved blades (or a small fan wheel) drawn inside. | Centrifugal fan or blower: HVAC supply and extract, combustion air, purge air. Becomes centrifugal-fan. Variants: Axial and propeller fans are drawn differently: see fan-axial; Positive displacement (roots) blowers are drawn as a circle with two lobes | `centrifugal-fan` |
| **Axial / propeller fan** `fan-axial` (F, FN, EF) | A two-blade propeller (two teardrop or figure-8 lobes) on the end of a shaft line, often drawn on top of a motor circle, or a propeller inside a circle or duct outline. | Axial-flow or propeller fan: motor-driven cooling and ventilation fans, HVAC extract, cooler fans. No dedicated component: air-cooled-exchanger and the E-house and analyser-shelter HVAC include their own fans, and centrifugal-fan is the closest standalone fan. Variants: Look-alike: a spectacle blind is a figure-8 at a flange pair on a pipe, not on a motor shaft; A propeller inside a short duct section is a duct (tube-axial) fan | – |
| **Ejector / eductor** `ejector` (J, EJ, X) | A converging nozzle inside a body that narrows to a throat and then widens (diffuser): the motive fluid enters at the nozzle end, suction comes in from the side, and the mixture leaves at the wide end. | Jet pump with no moving parts: gas ejectors for low-pressure flash gas recovery, water eductors for sand jetting and drains. No dedicated component. Variants: Some legends draw two triangles (nozzle and diffuser) tip to tip with a side branch | – |
| **Turbo-expander (expander-compressor)** `turbo-expander` (K, KT, EX) | Two trapezoids on one shaft: the expander widens in the flow direction and the compressor narrows, back to back. | Expander drives a compressor from the energy of a gas pressure drop (NGL recovery, dew-pointing). No dedicated component. Variants: A generator-loaded expander has a G circle instead of the compressor | – |
| **Gas turbine** `driver-gas-turbine` (GT, KT) | A compressor trapezoid (narrowing), a small combustor circle, and a turbine trapezoid (widening), on a common shaft, with air in and exhaust out. | Gas turbine driver (API 616). Not modelled alone: with a generator it becomes gas-turbine-generator, and with a compressor it becomes gas-turbine-compressor. Variants: Many legends draw a box with "GT"; Two-shaft turbines show a separate power turbine trapezoid | – |
| **Gas turbine generator set** `gas-turbine-generator` (GT, G, TG) | A gas turbine (two trapezoids with a combustor circle, or a box marked GT) on a shaft to a circle marked "G". | Main power generation: a gas turbine driving a generator, running on fuel gas with diesel backup. Becomes gas-turbine-generator. Variants: Electrical drawings show only the G circle on the single-line diagram | `gas-turbine-generator` |
| **Gas-turbine-driven compressor train** `gas-turbine-compressor` (K, GT) | A gas turbine symbol on a shaft to a compressor trapezoid. | Gas-turbine-driven centrifugal compressor (export or injection). Becomes gas-turbine-compressor. Variants: Electric-motor-driven compressors show an M circle instead (then centrifugal-compressor) | `gas-turbine-compressor` |
| **Steam turbine** `driver-steam-turbine` (ST, KT, T) | A trapezoid widening in the flow direction, with steam in at the narrow end and exhaust at the wide end, and a shaft to the driven machine. | Steam turbine driver: onshore and refinery pumps, compressors and generators. Not used offshore. No dedicated component. Variants: A turbine trapezoid widens in the flow direction; a compressor trapezoid narrows | – |
| **Electric motor (driver)** `driver-motor` (PM, KM, M) | A circle containing "M" joined by a shaft line to the driven pump, fan or compressor. | Electric motor driver. Usually part of the pump or compressor package; becomes electric-motor if modelled separately. Variants: Variable-speed motors are marked VSD/VFD or have a diagonal arrow; Electrical drawings use "M" with "3~" | `electric-motor` |
| **Diesel engine (driver)** `driver-diesel-engine` (DE, D) | A rectangle marked "DE" (or "D"), joined by a shaft to the driven machine, with fuel and exhaust connections. | Diesel engine driver: firewater pumps, emergency and essential generators, crane engines. With a pump it is diesel-firewater-pump; with a generator it is diesel-generator. Variants: ISO draws an engine as a rectangle with a cross or a piston mark | – |
| **Diesel generator set** `diesel-generator-set` (DG, EG, G) | A "DE" rectangle on a shaft to a "G" circle. | Emergency or essential diesel generator: starts on main power failure and feeds the emergency switchboard. Becomes diesel-generator. Variants: On the single-line diagram only the G circle is shown | `diesel-generator` |

## Filters, separators and mixers

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-separation.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Filter (general)** `filter-general` (F, S) | A vessel outline divided by a dashed line (the filter medium), with inlet on one side and outlet on the other. | Any filter: liquid filter, gas filter, bag filter. If cartridges are drawn it is cartridge-filter; with a coalescing and vane section it is filter-separator. Variants: Some legends draw a rectangle with a dashed diagonal; Duplex filters are two such vessels with changeover valves | – |
| **Cartridge filter** `filter-cartridge` (F, S) | A vertical vessel with a tubesheet line across it and several narrow rectangles (cartridges) hanging from it. | Cartridge filter or coalescer housing: fuel-gas filters, injection-water polishing, chemical filters. Becomes cartridge-filter. Variants: Bag filters are drawn with U-shaped bags; A dp gauge (PDI) is almost always drawn across it | `cartridge-filter` |
| **Media filter (multimedia / nutshell)** `filter-media` (F) | A vertical vessel partly filled with a dotted area (the filter bed), with an inlet distributor on top and a collector at the bottom. | Deep-bed filter: seawater injection filtration, produced-water nutshell filters. Usually several in parallel with backwash. Becomes water-filter-package. Variants: Layers may be shown with different hatching (anthracite, sand, garnet) | `water-filter-package` |
| **Filter-separator / coalescer** `filter-separator` (V, F) | A horizontal vessel with two sections: filter or coalescer elements (dashed rectangles) in the first, a vane pack (zig-zag) in the second, and a liquid sump under each. | Two-stage gas filter-separator: fuel-gas and seal-gas conditioning, sales-gas polishing. Becomes filter-separator. Variants: Vertical coalescers are drawn as a vertical vessel with cartridges; Liquid-liquid coalescers show a hatched coalescing pack | `filter-separator` |
| **Cyclone (gas-solid or gas-liquid)** `cyclone` (V, CY) | A cylinder on top of a cone pointing down, with a tangential inlet at the side of the cylinder, a gas outlet tube dipping in from the top and a solids or liquid outlet at the cone tip. | Centrifugal separator. In oil and gas, cyclonic internals and cyclone separators remove liquids or sand from gas. No dedicated component; desanders and hydrocyclones have their own. Variants: Multicyclone banks are drawn as several small cones in one vessel | – |
| **Hydrocyclone (de-oiling liner)** `hydrocyclone` (X, Z) | A long narrow cone lying on its side: tangential inlet at the wide end, a small reject (oil) outlet on the axis at the wide end, and the clean water outlet at the narrow tip. Packages show a vessel holding many liners. | De-oiling hydrocyclone for produced water: removes oil droplets by spinning. Becomes hydrocyclone-package (the vessel with liners and its reject and water lines). Variants: Package P&IDs draw one vessel with "liners" written inside; Desanding hydrocyclones are the same shape, mounted vertically, with sand out at the tip | `hydrocyclone-package` |
| **Desander** `desander` (V, DS) | A vertical vessel with an inverted cone (cyclone liner) inside and a sand accumulator chamber at the bottom with a sand dump valve. | Wellhead or produced-water desander: removes sand before the separators. Becomes desander. Variants: Multiple liners in one vessel are drawn as several cones; The sand outlet goes to a sand-handling or jetting system (SD lines) | `desander` |
| **Agitator / mixer in vessel** `agitator` (A, AG, MX) | A vessel or tank with a motor (M) on top, a shaft running down into the liquid, and an impeller (two short blades or a propeller) at the bottom. | Stirred vessel: chemical make-up tanks, mud tanks, polymer and flocculant mixing. Model the tank; note the agitator. Variants: ISO shows different impeller marks (propeller, anchor, turbine); Side-entry mixers come in through the shell | – |
| **Static (in-line) mixer** `static-mixer` (MX, X) | A short in-line rectangle on the pipe with alternating diagonal or helical elements inside. | Motionless mixer: mixes injected chemicals, wash water or demulsifier into the main flow. No dedicated component. Variants: Mixing valves (globe-type) and mixing tees are drawn as valves and tees | – |
| **Silencer** `silencer` (SL, X) | A short fat cylinder (rounded rectangle) on a vent, exhaust or blow-off line, with baffles or a perforated tube inside. | Reduces noise: gas-turbine exhausts, PSV and blowdown discharges to atmosphere, steam vents. No dedicated component; vent-stack may include one. Variants: Often written simply as "SILENCER" on the line | – |

## Flare, wellhead, pipeline and packages

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-oilfield.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Flare stack / tip** `flare-stack` (FS, FL, X) | A tall vertical stack with a flame shape at the tip, often with a molecular seal (a bulge) below the tip and pilot and ignition lines. | Elevated flare: burns relief and blowdown gas. Becomes flare-stack (offshore booms are flare-boom). Variants: Ground or enclosed flares are drawn as a box with a flame; HP and LP tips may be drawn side by side | `flare-stack` |
| **Flare boom** `flare-boom` (FB, X) | A long lattice boom (two parallel lines with a zig-zag between them) angled up and out from the platform edge, with the flare header inside it and a flame at the tip. | Offshore flare boom carrying the flare header out from the platform. Becomes flare-boom. Variants: P&IDs often draw it as a flare stack with "BOOM" noted | `flare-boom` |
| **Flare knock-out drum** `flare-ko-drum` (V) | A horizontal drum with flare inlets at one or both ends on top, the gas outlet to the flare from the top centre, and a liquid outlet underneath (often to pumps). | Flare KO drum: catches liquid droplets before the flare. Becomes horizontal-separator with service KO, fed by the flare header. Variants: Vertical KO drums are drawn as a vertical vessel; Liquid seal drums (water seal) are drawn with a dip pipe | `horizontal-separator` |
| **Cold vent stack** `vent-stack` (VS, X) | A vertical stack with an open top and no flame, sometimes with a small rain cap or dispersion arrows at the top. | Atmospheric vent for low-pressure hydrocarbon vents (TEG still, flash, riser blowdown). Becomes vent-stack. Variants: A flame arrester or snuffing connection may be drawn at the tip | `vent-stack` |
| **Pig launcher** `pig-launcher` (L, PL) | An enlarged barrel (a fat pipe section) with a quick-opening closure door (thick bar) at the back end, a reducer to the pipeline at the front, a kicker line from the back of the barrel through a valve to the main line, and a flow arrow away from the door. | Launches pigs into an export pipeline or flowline. Becomes pig-launcher. A pig signaller (XI) is often drawn on the outgoing line. Variants: The pig itself may be drawn as a dashed bullet inside the barrel; Pressure gauges, vents and drains on the barrel are typical | `pig-launcher` |
| **Pig receiver** `pig-receiver` (L, PR) | The mirror of a launcher: the pipeline enters through a reducer into the enlarged barrel, which ends at a closure door, with a bypass line and a flow arrow towards the door. | Receives pigs at the end of a pipeline. Becomes pig-launcher (the same component serves launcher and receiver). Variants: A pig basket or bar tee at the barrel outlet may be drawn | `pig-launcher` |
| **Christmas tree (wellhead valves)** `christmas-tree` (XT, WH) | A vertical stack of valves on the wellhead: lower and upper master valves, a cross with a production wing valve (actuated) and a kill wing to the side, a swab valve and tree cap on top, and a choke on the production wing outlet. | Surface christmas tree on a well. The upper master (SSV) and production wing valves are usually actuated and tripped by the wellhead control panel. Becomes christmas-tree; the wing outlet line goes to the manifold through the choke. Variants: Each valve may carry its tag (LMV, UMV/SSV, PWV, KWV, SV); The SCSSV downhole is drawn as a valve in the tubing below the wellhead; Annulus valves (A/B annulus) appear on the wellhead below the tree | `christmas-tree` |
| **Production / test manifold** `production-manifold` (M, MF) | Two parallel headers (production and test) with several incoming well lines; each well line has a valve to each header, so one well at a time can be routed to test. | Gathers well flowlines into the production header and diverts one well to the test separator. Becomes production-manifold (slots = number of well inlets). Variants: Three-header manifolds add an HP/LP or a gas-lift header; Multi-port selector valves (MSV) replace the valve pairs on newer designs | `production-manifold` |
| **BOP stack** `bop-stack` (BOP) | A vertical stack: an annular preventer on top (a rounded block), then two or three ram blocks (rectangles with rams drawn as opposing filled triangles), with choke and kill side outlets and valves. | Blowout preventer stack on the wellhead during drilling and workover. Becomes bop-stack; the side outlets go to the choke & kill manifold (choke-manifold). Variants: Ram types are written beside each block (pipe, blind, shear) | `bop-stack` |
| **Riser** `riser` (R) | A line running from the topsides down through the sea surface (a wavy line) to the seabed (a hatched line), where it turns into the pipeline; often with an ESDV at the top and "RISER" or a pipeline number written on it. | Pipeline riser: import or export riser on the jacket. Becomes riser; the topsides riser ESDV is a separate esd-valve. Variants: Subsea isolation valves (SSIV) are drawn on the pipeline near the riser base; Flexible risers are drawn wavy | `riser` |
| **Package / skid (generic)** `package-skid` (X, A, PK) | A rectangle (often inside a dash-dot boundary) labelled with the package tag and name, standing on skid rails, with connections crossing its edge. | Vendor package whose internals are on the vendor P&IDs. Map it by name to the matching package component: chemical-injection-skid, instrument-air-package, teg-regeneration, nitrogen-generator, hydraulic-power-unit, hydrocyclone-package, water-filter-package, hypochlorite-generator or metering-skid. Variants: On overview P&IDs, packages are often a single labelled box | – |
| **Chemical injection package** `chemical-injection-package` (C, X, CI) | A dash-dot package boundary holding a chemical tank (or tote) and one or more metering pumps (pump circles with diagonal arrows), with injection lines leaving to the process. | Chemical injection skid: corrosion and scale inhibitor, demulsifier, methanol, biocide, antifoam. Becomes chemical-injection-skid (outlets = number of injection lines). Variants: Methanol packages are drawn the same; Each injection line often has a flow indicator and a check valve at the injection quill | `chemical-injection-skid` |
| **Fiscal metering skid** `fiscal-metering-skid` (X, FQ) | A dash-dot boundary with an inlet header, two or more parallel meter runs (each with isolation valves and a meter box or FE), an outlet header, and often a prover connection and a sampler. | Fiscal (custody transfer) metering for oil or gas export. Becomes metering-skid; a compact prover (compact-prover) is usually connected. Variants: Gas skids use ultrasonic or orifice runs; oil skids use turbine, Coriolis or ultrasonic runs; Flow computers are drawn as hexagons (computer functions) | `metering-skid` |
| **Meter prover (compact piston prover)** `compact-prover` (X) | A cylinder with a piston (a short vertical bar) inside and detector marks, with inlet and outlet connections and a 4-way or double-block valves to the meter runs. | Calibrates the fiscal meters in place. Becomes compact-prover. Variants: Pipe (ball) provers are drawn as a long U-loop with launching chambers | `compact-prover` |
| **Hydraulic power unit (HPU)** `hydraulic-power-unit` (X, HPU) | A dash-dot package with an oil reservoir (open-top tank), a pump with a motor, and an accumulator (a capsule split by a line), with hydraulic supply and return lines leaving. | Wellhead HPU supplying hydraulic pressure to the SSVs, SCSSVs and wing valves through the wellhead control panel. Becomes hydraulic-power-unit. Variants: Accumulators alone (ISO 1219) are a capsule with a horizontal dividing line | `hydraulic-power-unit` |
| **Gas cylinder bank** `gas-cylinder-bank` (X, N2) | A row of tall narrow cylinders with domed tops (gas bottles), each with a valve, joined to a common manifold. | Bottle rack or quad of compressed gas: nitrogen backup, calibration gas. Becomes gas-cylinder-quad. CO2 or inert-gas fire-suppression banks are drawn the same but belong to fire & gas. Variants: A single bottle is one cylinder symbol; Quads are often drawn as one box labelled "N2 QUAD" | `gas-cylinder-quad` |

## Fire & gas and fire protection

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-fire-gas.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Point gas detector (flammable)** `fg-gas-detector` (GD, GT, AT, AT, AE) | A small bubble (or hexagon or triangle, by legend) with "GD", often with the gas noted (HC, CH4) and the zone or voting group beside it. | Catalytic or infrared point detector for flammable gas, alarming at about 20 % LEL and confirming at about 60 % LEL with 2ooN voting. Becomes point-gas-detector. Variants: F&G layouts differ a lot: circles, triangles or hexagons with GD, IR, HC or "G"; P&IDs may draw it as an ISA bubble AT/AE with "LEL"; Duct (HVAC inlet) detectors are drawn on the duct | `point-gas-detector` |
| **Toxic gas detector (H2S, CO)** `fg-toxic-gas-detector` (GD, TD, AT, AT) | A detector bubble marked with the toxic gas, usually "H2S" (or "CO"). | Electrochemical toxic gas detector, usually H2S with alarms at about 5 and 10 ppm. Becomes toxic-gas-detector. Hydrogen detectors (battery rooms) are marked "H2" and become hydrogen-detector. Variants: Some legends use a triangle with T or TOX | `toxic-gas-detector` |
| **Open-path (line-of-sight) gas detector** `fg-open-path` (OGD, OPGD, LOS) | A transmitter box and a receiver box joined by a dashed beam line, marked OGD or Tx/Rx. | Infrared beam detector covering a path (LEL·m), for perimeters and modules. Becomes open-path-gas-detector. Variants: Some legends draw a single box with a long dashed arrow | `open-path-gas-detector` |
| **Acoustic (ultrasonic) gas leak detector** `fg-acoustic-leak-detector` (UGD, AGD, UGLD) | A detector bubble with "UGD" and curved sound arcs beside it. | Ultrasonic leak detector that hears high-pressure gas leaks, independent of wind dilution. Becomes acoustic-gas-leak-detector. Variants: Some legends mark it "AGLD" or draw a microphone symbol | `acoustic-gas-leak-detector` |
| **Flame detector** `fg-flame-detector` (FD, BS, IR3, BE, BS) | A detector bubble with "FD" and a dashed cone showing its field of view. | Optical flame detector (IR3, UV/IR), voting 2ooN for confirmed fire. Becomes flame-detector. Variants: ISA letter B (burner, combustion) is used on some P&IDs: BE or BS; The cone shows the aiming direction on F&G layouts | `flame-detector` |
| **Heat detector** `fg-heat-detector` (HD, TS, TSH) | A detector bubble with "HD" (sometimes a TSH bubble, or a square in building legends). | Fixed-temperature or rate-of-rise heat detector in enclosures, turbine hoods and accommodation. Becomes heat-detector. Variants: Building fire-alarm legends (NFPA 170, BS 5839) use their own forms; check the legend; Linear heat-detection cable is drawn as a dashed line with LHD | `heat-detector` |
| **Smoke detector** `fg-smoke-detector` (SD) | A detector bubble with "SD". | Point smoke detector (optical or ionisation) in control rooms, LERs, E-houses and quarters. Becomes smoke-detector. Beam detectors (beam-smoke-detector) and aspirating systems (aspirating-smoke-detector) are separate components. Variants: Aspirating (VESDA) systems are a box "ASD" with a sampling pipe with holes; Beam smoke detectors are drawn as a Tx/reflector pair; In-duct smoke detectors are drawn on the HVAC duct | `smoke-detector` |
| **Manual call point (MAC)** `fg-manual-call-point` (MAC, MCP, HS, HS) | A square with a smaller filled circle inside (the break-glass button), sometimes marked MAC or MCP. | Manual alarm call point. Pressing it raises a fire alarm (and on many platforms initiates ESD or GA). Becomes manual-call-point. ESD push-buttons look similar but are marked ESD. Variants: Yellow (ESD) vs red (fire) call points are only told apart by text or colour; NFPA-style legends: a small rectangle with "F" or "MS" (manual station) | `manual-call-point` |
| **Alarm beacon (visual)** `fg-beacon` (XL, BCN, XL) | A circle with an X inside (the IEC lamp symbol) and short rays around it, often with the colour noted (red, yellow, blue). | Flashing beacon for general alarm, gas or fire alarm, CO2 release or prepare-to-abandon. Colour gives the meaning. Becomes alarm-beacon. Variants: Some legends draw a dome with rays or write "B" in a circle; Often combined with a sounder as one device | `alarm-beacon` |
| **Alarm sounder / bell** `fg-sounder` (XA, SNDR, XA) | A half-circle dome sitting on a flat base line (the IEC bell or sounder), with its connection below. | Electronic sounder or bell for fire, gas and general alarm. Usually paired with a beacon; becomes alarm-beacon (beacon and sounder). Large-area PA/GA horns are the loudspeaker symbol. Variants: Horn symbols (a trumpet shape) are also used for sounders | `alarm-beacon` |
| **PA/GA loudspeaker (horn)** `fg-horn-speaker` (LS, PA) | A small rectangle with a flared trumpet (trapezoid) opening out of it, the IEC loudspeaker or horn. | PA/GA horn loudspeaker carrying alarm tones and voice messages across the plant. Becomes pa-loudspeaker. Variants: Ceiling speakers in buildings are drawn as a circle with a small square | `pa-loudspeaker` |
| **Deluge valve** `fg-deluge-valve` (DV, XV, XV) | A valve drawn filled (normally closed) on the firewater supply with a trip actuator on top (often a solenoid "S" box or hydraulic latch), and "DV" or the deluge zone written beside it. Open (dry) pipework leads to the nozzles downstream. | Deluge valve: opens on confirmed fire (or manually) to flood a zone through open nozzles. Becomes deluge-valve-skid (valve set with isolations, trim and test line). Variants: Alarm check valves for wet sprinkler systems look similar and are marked "ACV"; The skid usually shows LO isolation valves either side | `deluge-valve-skid` |
| **Sprinkler head (closed)** `fg-sprinkler` (SPK) | A small drop from the pipe ending in a closed head: a small filled circle (the glass bulb) with a short deflector bar. | Automatic sprinkler: opens on its own when its bulb bursts. Used in quarters, workshops and stores. Becomes deluge-nozzle, which also covers sprinkler heads. Variants: Building legends draw a small circle with a cross, or a filled dot; Upright vs pendent is written beside it | `deluge-nozzle` |
| **Deluge (open) spray nozzle** `fg-deluge-nozzle` (NZ) | A small drop from the pipe ending in an open nozzle (a small triangle), with dashed spray lines fanning out. | Open water-spray nozzle on deluge pipework. All nozzles in the zone discharge together when the deluge valve opens. Becomes deluge-nozzle. Variants: Foam-water nozzles are drawn the same with "F"; Nozzle layouts are usually on separate fire-protection drawings | `deluge-nozzle` |
| **Water mist nozzle / system** `fg-water-mist` (WM) | A nozzle on a small-bore line with a fine dotted spray cone, often inside an enclosure outline and marked "WM". | High-pressure water mist for turbine hoods, generator enclosures and machinery spaces. Usually a vendor package with its own pump and cylinders. No dedicated component. Variants: Mist skids are drawn as a package box with cylinders or a pump | – |
| **Fire hydrant** `fg-hydrant` (HYD, FH) | A pillar on the firewater main with a cap on top and two hose outlets on the sides. | Hydrant on the firewater ring main for hoses. Mostly onshore; offshore uses hose reels and monitors. Becomes fire-hydrant. Variants: Plot-plan legends draw a circle with a cross or a filled half-circle | `fire-hydrant` |
| **Fire monitor** `fg-fire-monitor` (FM, MON) | A riser from the firewater main with a swivel joint (circle) on top and a nozzle barrel angled out of it, sometimes with an arc showing its sweep. | Fixed water or foam monitor covering helidecks, wellbays and tank areas. Becomes fire-monitor. Variants: Remote-controlled monitors carry an M actuator; Foam-water monitors are marked "F" | `fire-monitor` |
| **Fire hose reel** `fg-hose-reel` (HR) | A drum (two concentric circles) fed from the firewater line through a valve, with a hose ending in a nozzle. | Fire hose reel at escape routes and process areas. Becomes hose-reel. Variants: Foam hose reels are marked "F"; Layout legends draw a circle with "HR" | `hose-reel` |
| **Portable fire extinguisher** `fg-extinguisher` (EXT, FE) | A canister outline with a handle and short hose, marked with the agent: DP (dry powder), CO2, F (foam) or W (water). | Portable or wheeled extinguisher. Shown on safety layouts more than P&IDs. Becomes fire-extinguisher. Variants: Layouts often use a triangle or circle with the agent letters; Wheeled units are drawn with wheels | `fire-extinguisher` |
| **Firewater pump (diesel driven)** `fg-fire-pump` (P, FWP) | A pump (centrifugal circle, or a vertical turbine pump into a caisson) driven by a "DE" diesel engine box, discharging into the firewater ring main. | Diesel firewater pump that starts on low ring-main pressure or a fire signal. Becomes diesel-firewater-pump. Electric firewater and jockey pumps are drawn with M. Variants: Offshore pumps are usually vertical caisson pumps with a right-angle gearbox; Jockey pumps keep the ring main pressurised and are drawn small with M | `diesel-firewater-pump` |
| **Foam system (concentrate tank and proportioner)** `fg-foam` (C, X) | A small vertical tank of foam concentrate (marked AFFF or F) joined to an in-line proportioner (a venturi-like body) on the firewater line. | AFFF foam skid: proportions foam concentrate into firewater for helideck and wellbay monitors and deluge. No dedicated component. Variants: Bladder tanks are drawn with an internal bladder line; Some legends write "FOAM" in a box | – |
| **CO2 / inert gas extinguishing system** `fg-inert-gas` (X, CO2) | A bank of cylinders on a manifold with a release valve, piped to discharge nozzles inside an enclosure outline, marked CO2, IG-541 or N2. | Total-flooding gas system for turbine hoods, switch rooms and generator enclosures. The release is interlocked with a beacon and sounder and the enclosure dampers. No dedicated component (gas-cylinder-quad is for nitrogen bottles). Variants: Clean-agent (FM-200, Novec) systems are drawn the same | – |
| **Safety shower and eyewash** `fg-safety-shower` (SS, EW) | A shower head (an inverted cone) on a pipe stand, often with a small eyewash bowl on the side, fed from potable water. | Emergency shower and eyewash station near chemical injection, battery rooms and laboratories. Becomes safety-shower. Variants: Layout legends use the ISO 7010 E012 / E011 pictograms | `safety-shower` |
| **Fusible plug loop** `fg-fusible-plug` (FP, TSE, TSE) | An instrument-air (pneumatic //) loop with small fusible plug elements (small filled squares marked FP) placed around the wellhead or equipment. | Pneumatic fire detection: a plug melts in a fire, the loop pressure falls, and the wellhead or package shuts in. Classic on wellhead platforms. Not modelled; note it on the trees. Variants: API RP 14C tags them TSE; Some legends draw a circle with "FP" on the loop | – |

## Electrical (single-line)

Sheet: https://deck-twin-library.demoareeb.workers.dev/symbols/legend-electrical.svg

| Symbol | Drawn as | Meaning | Component |
|---|---|---|---|
| **Two-winding transformer** `el-transformer-2w` (TR, T, TX) | Two overlapping circles (one per winding) on the single-line diagram, with the rating and vector group written beside them. | Power or distribution transformer, e.g. 11/0.69 kV. Becomes transformer. ANSI drawings show two facing coils instead. Variants: A diagonal arrow through the circles = on-load tap changer; Winding symbols (Δ, Y, Yn) may be drawn inside the circles | `transformer` |
| **Three-winding transformer** `el-transformer-3w` (TR, T) | Three overlapping circles in a triangle: one primary on top, two secondaries below. | Transformer with two secondary windings, e.g. feeding two switchboard sections or a 12-pulse VFD. Becomes transformer. Variants: Autotransformers are a single circle with a tap | `transformer` |
| **Circuit breaker** `el-circuit-breaker` (CB, Q) | A switch (a lead with a diagonal blade) with a small × on the fixed contact. | Circuit breaker that makes, carries and interrupts fault current: incomers, feeders and bus ties. Inside a switchboard it is part of switchgear. In an outdoor substation (air-insulated bay) it becomes hv-circuit-breaker. Variants: ANSI legends draw a square box or two arcs; draw-out breakers show chevrons (» «) above and below; A filled square is often used for a closed breaker on SLD overviews | `hv-circuit-breaker` |
| **Disconnector (isolator)** `el-disconnector` (DS, Q) | A switch blade with a short bar across the fixed contact. | Off-load isolator giving a visible break for maintenance. Inside a switchboard it is part of switchgear. In an outdoor substation it becomes disconnector. Variants: Switch-disconnectors (load-break) add a small circle on the contact as well | `disconnector` |
| **Earthing switch** `el-earthing-switch` (ES) | A disconnector blade whose lower side goes straight to an earth symbol, branching off a main circuit. | Earths a circuit or busbar for safe maintenance, interlocked with the breaker. Inside a switchboard it is part of switchgear. In an outdoor substation it is the earthing blade of a disconnector (disconnector). Variants: Often drawn beside a cable feeder below the breaker | `disconnector` |
| **Fuse** `el-fuse` (F, FU) | A small rectangle with the conductor line passing straight through it. | Fuse protecting a circuit, VT or small motor feeder. Part of switchgear or distribution boards; not modelled. Variants: A fuse-switch combines a switch blade with the fuse rectangle | – |
| **Contactor** `el-contactor` (K, KM) | A switch blade with a small half-circle on the fixed contact. | Motor starter contactor in an MCC bucket: switches motors on and off, often with a thermal overload. Part of the MCC (switchgear); not modelled. Variants: Starters may be drawn as a box with "DOL", "Y/Δ" or "SS" (soft starter) | – |
| **Motor (electrical)** `el-motor` (M, PM) | A circle containing "M" and "3~" at the end of a feeder. | Three-phase induction motor load with its kW rating. On the SLD it is the motor of a pump, fan or compressor; becomes electric-motor if modelled separately. Variants: "M" with "1~" = single-phase; Synchronous motors are marked "MS" | `electric-motor` |
| **Generator** `el-generator` (G) | A circle containing "G" (often with a "~"), at the top of a feeder to the switchboard. | AC generator. On the SLD its driver is written beside it: GT means gas-turbine-generator, DG means diesel-generator. Variants: Some legends draw a circle with a sine wave only | – |
| **Variable frequency drive (VFD)** `el-vfd` (VFD, VSD, U) | A square split by a diagonal, with "~" in both halves (an AC/AC converter), feeding a motor. | Variable-speed drive for large pumps, compressors and fans. Becomes vfd-cabinet. Variants: Some legends draw two boxes: rectifier (~/=) then inverter (=/~); Often written VSD | `vfd-cabinet` |
| **Uninterruptible power supply (UPS)** `el-ups` (UPS) | A rectifier box (~/=) and an inverter box (=/~) in series with a battery between them, often inside a dashed frame with a static bypass. | UPS that keeps control, F&G, ESD and telecom systems running on mains failure. Becomes ups-battery-cabinet. Variants: DC UPS systems show only the rectifier (charger) and the battery | `ups-battery-cabinet` |
| **Battery** `el-battery` (BAT, G) | Pairs of parallel plates: a long thin line (positive) and a short thick line (negative), repeated. | Battery bank for a UPS, a diesel engine start or switchgear tripping. Becomes ups-battery-cabinet when it is the UPS or DC battery room. Variants: A dashed line between two cells means many cells in series | `ups-battery-cabinet` |
| **Busbar** `el-busbar` (BB) | A thick horizontal line with feeders dropping from it, labelled with the board name and voltage. | Switchboard busbar section: main, essential and emergency boards. The busbar belongs to the switchboard (switchgear); it is not modelled separately. Variants: Bus couplers (ties) are drawn as a breaker linking two bars; Double busbars are two parallel thick lines | *logical* |
| **Current transformer (CT)** `el-current-transformer` (CT, T) | A small circle around the conductor line, with secondary leads to a meter or relay. | Steps down current for protection relays and meters. Inside a switchboard it is part of switchgear. A free-standing outdoor CT becomes instrument-transformer. Variants: Ratio (e.g. 400/1 A) and class are written beside it; Some legends draw a small loop on the conductor | `instrument-transformer` |
| **Voltage transformer (VT)** `el-voltage-transformer` (VT, PT) | Two small overlapping circles (like a power transformer, but smaller), on a fused tap from the busbar, with the secondary earthed. | Steps down voltage for metering, protection and synchronising. Inside a switchboard it is part of switchgear. A free-standing outdoor VT/CVT becomes instrument-transformer. Don't confuse it with a power transformer (bigger, in the main feeder path). Variants: ANSI legends call it a PT (potential transformer) | `instrument-transformer` |
| **Surge arrester** `el-surge-arrester` (SA, F) | A rectangle with a downward arrow inside, connected between the conductor and earth. | Protects transformers and cables from switching and lightning surges. Becomes surge-arrester in an outdoor substation or at a transformer; inside switchgear it is not modelled separately. Variants: Older legends show a spark gap (two arrows tip to tip) | `surge-arrester` |
| **Earth (ground)** `el-earth` (E, PE) | A vertical lead ending in three horizontal lines that get shorter downward. | Connection to earth: neutral earthing, equipment and instrument earthing. Not modelled. Variants: Protective earth may be drawn in a circle; Clean (instrument) earth is sometimes marked IE | *logical* |
| **Capacitor** `el-capacitor` (C) | Two short parallel plates with a lead on each side. | Power-factor correction capacitor bank or harmonic filter. Part of switchgear; not modelled. Variants: Capacitor banks are often a single capacitor symbol with the kvar rating | – |
| **Neutral earthing resistor (NER)** `el-neutral-earthing-resistor` (NER, R) | A rectangle (resistor) between a transformer or generator neutral point and an earth symbol. | Limits earth-fault current on an MV system. Becomes neutral-earthing-resistor. Variants: ANSI resistors are drawn as a zig-zag | `neutral-earthing-resistor` |
| **Switchboard / MCC** `el-switchboard` (SWB, SWG, MCC) | A dashed frame around a busbar with an incoming breaker and outgoing feeders (breakers or contactors), labelled with the board tag and voltage. | HV/LV switchboard or motor control centre. Becomes switchgear (set the kind: MV, LV or MCC), usually inside an E-house (e-house). Variants: MCC buckets are drawn as contactor + overload + motor; Overview SLDs show each board as a single bar | `switchgear` |
| **Junction box** `el-junction-box` (JB) | A small square marked "JB" with cables entering it. | Field junction box (Ex e) where field cables meet multicore cables. Usually on loop and layout drawings, not P&IDs. Becomes junction-box. Variants: Instrument JBs and power JBs are told apart by the tag (IJB, EJB) | `junction-box` |
| **Local control station (LCS)** `el-local-control-station` (LCS, HS, HS) | A small box beside a motor marked "LCS", with start/stop push-buttons (small circles) or HS bubbles, and a dashed signal line to the MCC. | Field push-button station for local start/stop and emergency stop of a motor. Becomes local-control-station. Variants: P&IDs often show it as an HS bubble joined to the motor by a dashed line | `local-control-station` |
| **Protection relay (ANSI device number)** `el-protection-relay` (R) | A small circle or box with an ANSI device number (e.g. 50/51 overcurrent, 87 differential, 27 undervoltage), joined to a CT or VT by a dashed line. | Protection function inside the switchgear relay. Not on the plot; not modelled. Variants: IEC drawings may use symbolic forms (I>, I>>) instead of numbers | *logical* |