A Custumer ask me a Self Venting Plug on a valve equipped with low emission packing.This Plug help to measure the lakage rate of the valve packing.I think it means a relief valve that reduces the build up overpressure in the body cavity - not into the open but to the low pressure side of the valve when closed so that the upstream pressure will act with full force on the valve seats.I believe that for ball valves you choose between single pistion or double pistion seats (for tunion mounted balls).Sometimes a valve in certain service needs to relieve with temperature change e.g a ball valve on liquid chlorine service, I quote "Ball valve design must ensure that excess pressure, resulting from expansion of liquid trapped in the ball and body cavity when the valve is closed, will relieve spontaneously in the direction of the higher line pressure. This relief may be accomplished by providing a relief hole in the ball, providing a passagein the body to the high pressure side, or by using cavity pressure self-relieving seats"If your liquid has unusual thermal expansion properties perhaps thats what the client is aluding to.

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