Reliable cavity relief for DBB floating and trunnion ball valves

Learn all relevant information about cavity relief for DBB floating and trunnion ball valves in our short but very informative explanatory video.

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Pipelines that transport oil and gas over long distances often pass through rough environments.

They are exposed to temperature variations and other harsh weather influences.

For example, the transported media can expand extremely when heated. If the media cannot expand because it is trapped in a fixed space, the pressure will increase, and this can also happen inside an isolated ball valve.

However, it is important, that the body cavity is not released to atmosphere.

There are two types of ball valve designs for which you should provide reliable cavity relief in such cases.
- The floating ball valve design and
- the trunnion ball valve design.

Let's first look at the requirements of the floating ball valve design.

For Double Block and Bleed Ball Valves, there are two important technical specifications you should consider when you want to use a floating ball valve design:

The SHELL MESC 77/170 specification which stipulates a pressure equalizing hole of at least 3 mm in the ball.

And the EEMUA 182 specification which prohibits such a gap. In contrast, it states that valves must have an automatic way to relieve overpressure.

However, what is behind the floating ball valve design?

The ball seats are fixed in the valve body and in the closed position a lateral movement of the ball is possible.

When closing the valve, media is trapped in the cavity.

The pressure relief hole connects the valve body cavity to the high pressure side of the ball valve when this is in the closed position according to Shell MESC SPE 77/170.

Another solution is using self-venting ball seats according to EEMUA 182 specification.

The surplus pressure is released around the upstream ball seat.

Now let's check how cavity relief works with the trunnion ball valve design.

A trunnion mounted ball valve means that the ball is constrained by bearings and is only allowed to rotate but the ball seats can move laterally. In the closed and even open position of the valve, the media is trapped in the cavity and isolated from the pipeline.

Thus, the pressure caught in the body cavity might be different from that in the line. An example of this situation is when cold liquid like LNG is trapped and heated. That temperature change of the body cavity causes then an extreme pressure build-up. It pushes the ball seat away from the ball by rising pressure against the spring load. When this happens, the pressure is released.

There you have it!

Now you know how to ensure reliable cavity relief with a DBB floating or trunnion ball valve to ensure a smooth and safe distribution of liquids and gases and prevent leakages.

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