Trunnion Ball Valve Double Block and Bleed
Trunnion Mounted Ball Valve Double Block and Bleed, in the closed position, provides a seal against pressure from both ends of the valve with a means of vending/bleeding the cavity between the seating surfaces.
Key Specifications / Features
Usually, there are two types of double block and bleed trunnion mounted ball valves, according to the experience of customer request.
Type 1- a trunnion mounted ball valve in the closed position, provides a seal against pressure from both ends of the valve with a means of vending/bleeding the cavity between the seating surfaces.
According to API 6D “Specification for Pipeline Valves” standards, a double block-and-bleed (DBB) valve is 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.
As the definition, a double block and bleed ball valve has two seats, each of the seats provide a seal against pressure seperately. That is to say one seat is sealing the fluid from the upstream and the other seat is sealing the fluid from the downstream. For a trunnion ball valve with double block and bleed, when the valve is in closed position and the fluid comes from the upstream, it pushes the seat against the ball to realize the sealing. The other seat is sealing in the same way.
How does the function of venting/bleeding work? When the pipeline is with pressure, the trunnion ball valve is closed with the fluid trapped in the ball and then there is pressure in the ball. When the temperature in the ball becomes higher, the pressure in the ball will rise too. It is necessary to vent/bleed the pressure in case of risk when the pressure in the ball becomes high enough. For a trunnion ball valve with bleed function, the pressure in the ball pushes the seat back as the spring behind the seat compresses. Then the there is vent/bleeding between the sealing surfaces. Generally speaking, the venting/bleeding is required at the pressure of less than 1.33 times of the design pressure rating for the ball valves as API 6D specifications. Some critical project request to vent/bleed at the pressure of less than 1.1 times of the design pressure rating. (eg. For a PN20 trunnion ball valve, the pressure trapped in the ball shall vent/bleed at the pressure of less than 2.2 Mpa).
Type 2- a trunnion mounted ball valve with two balls and a bleed valves.
These valves have a single body design with double balls with a bleed valve (normally a needle valve or globe valves) installed between the two main line valves. Double block and bleed trunnion ball valves are available bi-directional in metal seated and soft seated style, in bubble tight shut off. Sometimes called twin ball valves. These DBB design valves replace existing traditional techniques employed by pipeline engineers to generate a double block and bleed configuration in the pipeline.
What to look for when selecting a double block and bleed valve?
As with any valve, during selection, one should consider the fluid properties, pressure, temperature. Perhaps most importantly, however, is how the valve will be used. With a number of configurations offered, consider which of these will fit your process and needs best.
If you’re unsure which double block and bleed valve is right for you process, talk to an engineer. They can help determine which one will achieve your objectives best.