Is the valve actually leaking? In most cases, the leaking is caused by an outside source affecting the valve.
A few troubleshooting questions that may help address this issue are:
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Is this a new installation or repair?
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Is the valve installed vertically?
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Is the installation piping (inlet and outlet) correct?
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Has the system been recently changed or modified (including the valve manufacture and model)?
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Is a proper operating gap being maintained?
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Have pressure-measuring devices been calibrated recently?
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Where is the pressure measurement being sampled in relationship with the valve and is there a correction required for pressure differences between the two areas?
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Does the valve exhaust into a single exhaust or a common header?
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Is the system seeing any vibration?
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Was the valve opened in an over-pressure condition? If so, how many times?