DBB ball valve

A DBB ball valve (Double Block and Bleed) is a high-integrity isolation valve designed for safety-critical fluid services. It integrates two independent sealing surfaces—an upstream seat and a downstream seat, or two balls in series—within a single compact body. When closed, both seats block line pressure from either end, while a dedicated bleed/vent port in the inter-seat cavity allows trapped pressure and fluid to be safely discharged and verifies seal integrity: if media continues to flow from the bleed after isolation, seat leakage is immediately indicated. Typical trunnion-mounted DBB valves use self-relieving single-piston-effect (SPE) seats that automatically relieve cavity overpressure caused by thermal expansion. Compared with the conventional “two block valves plus a bleed valve” arrangement, the DBB design reduces flange joints, footprint, and fugitive-emission paths. It is widely used in oil & gas pipelines, metering skids, refinery and petrochemical plants, tank farms, loading systems, and instrument isolation, in compliance with API 6D, API 608, and ISO 14313.