Double isolation and bleed is an API 6D valve configuration in which at least one seat seals against pressure from both directions, so the body cavity stays isolated from both line sides simultaneously. It provides a higher isolation integrity than standard DBB, at the cost of requiring external cavity overpressure relief.
The difference from DBB is subtle and critical. A standard DBB trunnion valve uses self-relieving seats: if trapped liquid in the cavity warms and expands, the seat lifts momentarily and vents the excess into the line. That self-relief is exactly what DIB forbids — a bidirectional seat cannot lift toward the line, so cavity pressure must be managed with an external relief valve or routine manual bleeding instead.
DIB comes in two grades: DIB-1 with both seats bidirectional, and DIB-2 with one bidirectional and one self-relieving seat. Operators specify DIB where the cavity vent is used as a verified safe barrier during hot work or confined-space entry, and where even momentary seat lift toward the isolated side is unacceptable — hydrocarbon storage, plant battery limits and pig trap isolation are typical.
Because the seat arrangement changes thermal-relief behaviour, always state DBB, DIB-1 or DIB-2 on the purchase order together with the cavity relief method. VTXFLOW supplies all three seat configurations on trunnion ball valves to API 6D with test certificates documenting per-seat direction of sealing.