Double block and bleed is a valve configuration — defined in API 6D — with two seating surfaces that each block flow from line pressure, plus a means of venting (bleeding) the cavity between them. It lets operators prove positive isolation with a single valve instead of installing two valves and a spool.
In practice, DBB is delivered by a trunnion-mounted ball valve whose two spring-energised seats each seal against their own side of the line. Closing the valve and opening the small body vent proves the isolation: if product keeps flowing from the vent, a seat is leaking and the line is not safe to open. Maintenance crews use that vented cavity as the physical proof-of-isolation that permits breaking containment downstream — meter runs, launcher/receiver traps, instrument take-offs and tank manifolds are classic applications.
The API 6D definition matters commercially: each DBB seat blocks flow from the pressure source, but the standard does not require each seat to seal in both directions. When the process cannot tolerate any cavity communication with either side — hydraulic isolation for hot work, for example — specify the stricter double isolation and bleed (DIB) arrangement instead.
When ordering, state DBB explicitly, the vent/drain sizes and orientation, and whether a monoflange or needle valve should be fitted to the bleed. VTXFLOW builds DBB trunnion ball valves from 2" to 40", Class 150–2500, with body vents and drains per API 6D as standard.