We are facing large seat leakage over metal seated dual plate check valves during commissioning.
Valves are new and certificates show acceptable high pressure seat test hydrostatic (1,1*pd) and low pressure seat test pneumatic at 6 barg.
Valves will be installed in gas application.

At the moment I'm looking for detailed info regarding choices in selecting the test fluid, either hydrostatic or pneumatic at 1.1*pd as per API 598.

Anyone having detailed experience in assessment of pressure testing methods?

Is it possible to demount one or several valves to inspect visually for seat damage, particles and dirt and then test?

Best test is of course always with the valve out of pipeline and testing with blind flanges.

If this is not possible I imagine you would have to give more spesific details to get further advice.

    Maybe a "large leakage" is a subjective concept, also depending on the type of valve and the plan requirements... (...usually check valves are not intended to provide seat tightness as their primary function, are they?)
    
    What is, more or less, the maximum seat leak rate your process can tolerate?

Any acceptable test fluid which will give you more better test result?, otherwise you are just making a question for test fluid as per API Specification 598?

I'm sorry to send a question to question, but I can not define either of above or else you're looking for.

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