Double Block and Bleed

Double Block and Bleed

API 6D defines a double-block-and-bleed valve (DBB) as a “single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces.”


A Block and Bleed Valve System is a combination of one or more block/isolation valves, usually ball valves, and one or more bleed/vent valves, usually ball or needle valves. The purpose of the Block and Bleed Valve System is to isolate or block the flow of fluid in the system so the fluid from upstream does not reach other components of the system that are downstream. This enables the engineers to bleed off or vent or drain the remaining fluid from the system on the downstream side in order to execute some kind of work (maintenance/ repair/ replacement), sampling, flow diversion, chemical injections, integrity check for leakage etc.


Double Block and Bleed Valves operate on the principle that isolation can be achieved from both the upstream and downstream fluid flow / pressures. This is achieved by two isolation valves (may be ball, gate, globe, needle, etc.) placed back to back, with a third bleed valve in the center cavity. Once isolation has been achieved in one or more of the main process isolation valves, the cavity that is created between these isolation valves can be drained through the bleed valve. Bleed valves will either be vented directly to atmosphere locally or via a hose connection / piping to an appropriate closed disposal system.