If the body cavity keeps venting fluid at a constant flow rate for a very long time,
it is a signal of bad sealing of the valve seats. For liquid service only a lower drain
valve will immediately indicate a leak.
8.5.2 Draining Procedure
The purpose of draining the body cavity is to bleed out all the liquid collected in the body cavity. Even in
gas service, some condensate, compressor oil and other residues may get trapped in the body cavity
after a while. We recommend to drain the body cavity once a year.
Drain the liquids into a suitable bucket to avoid releasing them to the environment.
Keep your working area clean. The cavity can be exposed to line or cavity pressure,
wear protective eye wear.
To drain the body cavity, proceed as follows:
1. Operate the valve to the fully open or fully closed position, whichever is more convenient for you.
2. Turn loose the head of the drain fitting (24) located at the bottom of the body, to bleed liquids trapped
in the body cavity. We recommend a special drain valve specified for this task.
It will take some time to bleed the body cavity. It depends on the size of the fitting, pressure in the body
cavity and compressibility of the fluid.
If there are hydrates in gas service, the drain fitting may freeze during high-
pressure draining. Work slowly and ensure that the pressure trapped in the body
cavity has been completely released, by opening and closing the drain fitting several
times.
9 . 0 O V E R P R E S S U R E R E L I E F
API6D trunnion mounted ball valves with single piston effect seats (SPE) are designed to automatically
release cavity overpressure into the pipeline automatically using “self relieving seats”. Since body cavity
and pipeline are isolated in the fully open and fully closed position, pressure trapped in the body cavity
may be different than pressure in the line. Examples of this situation are during pipeline de-pressurisation,
or heating of body cavity causing a sudden pressure build-up.
Overpressure in the body cavity will pop the seats off the ball surface, and excess pressure will be
relieved into the pipeline. After the pop-off action, the seats will return to their normal position, in
contact with the ball.
The pop-off action is triggered by differential pressure between body cavity and pipeline. The body cavity
Australian Pipeline Valve - Installation, Operation and Maintenance Manual
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SIDE ENTRY 3P TRUNNION BALL VALVES API6D STANDARD - BVF100 SERIES
