The API 6D specifies a double block and bleed valve (DBB) as a singular valve with two seating surfaces which, when closed, provide a pressure seal from both valve ends by either venting or bleeding the capacity between the surfaces. This kind of valve is used for primary and secondary isolation, especially where bleeding the valve is needed. Here’s more on its applications.
Application of DBBs
Double block and Bleed plug valves are quarter-turn motion valves that stop or start the flow of fluids. They isolate the flow of fluids from the upstream from mingling with fluids downstream. They offer on-off, diverting, and moderate throttle fluid moderating services. Using a DBB is akin to having three valves in one. It saves installation time and reduces the piping system’s weight and the space needed to install it. In addition, it cuts down leaks and reduces the risk of hazards when transmitting fluids harmful to people or the environment.
Double block and bleed valves are often utilized where bleeding the valve capacity is needed, critical isolation is required to prevent leakages, or for maintenance. This includes LNG transmission, petrochemical transmission, and storage in mainline and manifold plug valves in liquid pipelines.
In meter calibration, DBBs help prevent errors by ensuring tight seals. Any leak can result in errors that can be quite costly if left unresolved. Other scenarios where DBBs are used include:
• Liquid service points near waterways
• Meter calibration
• To prevent product contamination
• For maintenance
• In chemical injection and sampling points
• To isolate instruments such as lever gauges or pressure indicators
DBBs have numerous applications, but when picking one factor in the fluid properties, temperature and pressure. This will help determine the type of DBB to use. However, more importantly, how you use the valve will determine if it holds up to the application you use it for.
