The classic setup is two block valves with a bleed valve in between: shut both, open the bleed, and if it stays dead, the upstream seat is holding. Single valves can also offer it, such as trunnion ball valves that seal on both seats and have a body cavity bleed. The stricter double isolation and bleed requires each seat to seal on its own against pressure from the same side, so the second seat still holds if the first leaks into the cavity.
Double block and bleed on the instrument line, shut both roots, crack the bleeder and make sure it goes dead.
Also heard as
- DBB
Do not mix it up with
- Spectacle blind: Double block and bleed relies on valve seats; a blind is a solid plate and does not depend on any seat holding.
Related terms
Bleeder
A small vent or drain valve used to release pressure or fluid, especially between two block valves.
Trunnion-mounted ball valve
A ball valve whose ball is anchored top and bottom on trunnions while spring-loaded seats press against it.
Spade
A solid blank plate with a handle, slipped between flanges to positively isolate a line.
Passing
Leaking through the seat while closed, so fluid still gets past a valve that is supposed to be shut.