Blowdown valve (BDV)

ISA 5.1-2009/2022 (final control element actuators) / PIP PIC001 / API 521

Drawn as

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.

Meaning

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