I have a problem with MOV motor operated valve paaing through. when I looked at the MOV data sheet, it was not speified that the valve as Tight Seal Closed but it is referred the seat leakage rate to API 598. So, i realized that the passing from the seat. Can any body help me to find a way to stop the passing?I would distinguish between a limit switch issue and a valve leakage class issue. What is the leakage class of your valve? If the valve is manufactured to allow a particular leakage, I don't think you can get improvements only playing with limit switches. You can start with what the differences are between valve position problem (valve does not reach desired position, hence does not seat/seal) and a seat leakage problem (seat damaged, poor surface finish, foreign bodies).As suggested by ione, check your paperwork & verify the original seat leakage class requested, and remember that there is no such thing as a "zero-leakage" valve, even though you may see the term mentioned, it basically tells you how much a "zero-leakage" valve can pass; Eg. class V allowable leakage rates (critical service) 5 x 10-4 ml per minute of water per inch of orifice diameter per psi differential (5 x 10-12 m3 per second of water per mm of orifice diameter per bar differential).Typically the term leakage class applies to control valves per IEC/FCI. I don't have a copy of the latest since FCI and IEC jointly published. Motor operated valves are typically isolation valves that require API STD 598 compliance. API STD 598 uses the terms "no bubbles" and "no drops" for resilient seated valves and metal seated valves NPS 2 and smaller for duration of the test. This applies to the factory test. Once the rusty hydrostatic testing fluid scratches the ball and seat then the factory testing is moot. Still, sometimes you can move the ball a degree or two and find a fresh seating spot.

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