A pressure test is performed to ensure that the packing is sealed perfectly so that there cannot be any leakage through the valve.
For a control valve, a small amount of leakage is allowed as they move up and down, for a gate valve the sealing must be perfect at it’s rated pressure.
For every valve, there is maximum working pressure. The pressure test is done till this maximum pressure.
Here the pressure test for a gate valve is described:
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Seal the outlet of the valve using a blanking plate, so that fluid cannot flow through.
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Connect the pressure test rig at the inlet of the valve.
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Fully open the valve.
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Operate the hand pump of pressure test rig until the maximum pressure, water can be used as pressurising liquid.
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If the water didn’t leak out, then the seal is perfect. As the test is done in fully opened condition then the valve will work fine in fully closed condition.
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And if there is a leakage in the seal, open the depressurising valve and tight the packing.
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Then repeat the pressure test until the valve is leak free.