A Double Block & Bleed or DBB valve is defined as a valve isolation that consists of two valves in series with an intermediate venting facility. The vent allows personnel to monitor the performance of the primary isolation valve for integrity and prevent an increase of pressure at the secondary isolation valve. Dbb valve are used for isolation
A Double Block & Bleed installation shall be implemented by means of two block valves in series with an interconnecting piping spool fitted with a bleed connection. Where practical, Double Block & Bleed (DBB Valve) sets shall be installed fitting to fitting to minimize the trapped volume between the two block valves. The bleed valves should be arranged to ensure that any discharge cannot harm personnel and are easily accessible for checking.
Bleed valves will either be vented directly to atmosphere locally or via a hose connection/hard piped to an appropriate disposal system. The decision as to whether an inventory is hosed to flare or vented locally will be addressed as part of the Task Risk Assessment carried out prior to the issue of the permit to work.
However, where the volume between the block valves is significant, i.e. the DBB set is not fitting to fitting and the fluid is hazardous, bleeds will be hard piped to the LP flare or closed drain system.
The requirement to hard pipe bleeds may be established case by case during detailed design as the piping design of the systems is developed. Bleed valves not connected to the flare system must be blinded off with a fully rated blind flange or plugged when not in use.
Emergency Shutdown (ESD) valves and Shutdown valves (SDV) may be used as block valves in Double Block & Bleed installation provided the valve itself is not in need of service and can be reliably immobilized and the actuator is in its de-energized fail closed (but not fail open) position.
The valve shall be rendered passive whilst closed for isolation purposes to ensure it cannot be inadvertently opened. When used in a Double Block & Bleed (DBB valve) arrangement, the actuated valve must be the secondary isolation valve.