We are having really very tough time while producing small metal seated ball valves in the range of 3/4" class 150(Floaters) to 2" class 300(trunnion).we have done all permutation and combination but unable to achieve zero leakage as per API598.
these valves are for produced water package with a design temperature of 100 deg centigrade max.
even we replaced stellite  coating with tunsten carbide coating on ball and seat every efforts is in vain. we disassembled and did some manual lapping that worked a bit on trunnion but floaters are still unresolved mystery.Can some one throw light and provide some useful tips.DM

these are basically carbon steel cast valves with SS trim.

gerhardl is right, too many factors to pinpoint the cause of your problems.
My suggestion would be "as a starter" to check exactly where the leakage occurs, if it is always the same point/component that gives leakage.
Are the components visibly damaged/deformed when valve is disassembled for repair (again are they damaged in same point/way).
Once this is established & if it is always the same problem, I would look at design against materials received & used.
You have to introduce some kind of logic/repeatability to your trouble shooting, your R&D team along with your Q.A. should put down a test, disassembly, component check & report procedure to follow repetitively.
You wont get an immediate answer, but if you test batches (maybe from different sub-contracted machine shops), you will narrow the problem down to material specification, material received, design fault....Just good old trouble-shooting.
Hope this can assist.

seemingly its a quality problem as you stated at some point as out of around 50 valves just 16 passed in one go (with zero leakage for sizes 2" and smaller and logically to me if the one valves is passing there is no point we can attributre this leakge as a design failure. undoubtedly its a quality issue thanks a lot i know it a slow process however to achieve zero leakage is a project requirement and the project is on SOS level at this stage with all the piping skid badly waiting for these valves to be in place.
On the other hand our factory is having problem producing bidirectional flaoting ball valves(the seals are unidirectional)on valves of 3/4" unlike trunnion.can any one throw light on this what is the general trend in the valve industry or there are very few top rated company who can procuce birectional metal seated floaters on small size.

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