Point gas detector (flammable)

Offshore F&G layout practice (ISA 5.1-style letter codes) / ISO 7240 / NFPA 72

Drawn as

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.

Meaning

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