How Does A Double Block and Bleed Trunnion Ball Valve Work?

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.

trunnion ball valve double block
Trunnion ball valve double block & bleed

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).

ball valve bleeding
Trunnion Ball Valve Bleeding

How to Test?

To test the double block function of a trunnion ball valve.

Make the ball in half open posion, and then injection the water into the ball. Close the ball valve when the valve ball is full of water. Then inject the valve with pressure at 1.1 times of design pressure from the upstream to check if then is any leakage from the relief valve (open the relief valve before the test). If the blister is found at the relif valve, then there is leakage; if no blister, then there is no leakage from upstream. For the downstream, it is the same way.

To test the venting/bleeding function a trunnion ball valve

Make the ball in half open posion, and then injection the water into the ball. Close the ball valve when the valve ball is full of water. Injection the pressure to the valve ball through the relief valve at the same time at the pressure of 1.33 times to see if there is any leakage, if the leakage is found, then the venting/bleeding is Ok.

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