Double block and bleed

PIP PIC001 / API 6D (DBB / DIB)

Drawn as

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.

Meaning

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