6 pcs of 38" triple-offset butterfly valves are passing (more than 50% of number installed). The valves worked fine for about 12 months. Increasing the torque from 75% to 100% didn't help.
Service: isolation valves for mercury removal filters (dry natural gas)
Class 600# Pop = 66 barg DP = 76 barg Top = 25 C DT = 82 C Materials:
body = A351 CF8M seat = 316SS / Graphite
Plant is 1 year old, there is a possibility of large quantity of dust / scale in the piping. In theory, these valves should be somewhat OK with solids in process fluid.When 'something' is going wrong on several valves after a certain periode of operating time, and nothing 'unregular' has happened (no pressure peaks, no un-normal operating conditions, no service (opening of pipeline, foreign objects introduced) etc., no wear or mechanical slip on actuators, no wear or slip on adjustment and mechanical transmissions on outside contraptions and no unnormal temperature variations, this will, as you suggest, be inside problems.The inner problem(s) is most likely caused the same thing for all valves.
Guesstimate is most likely wear on seats again caused by abration, and/or increased by cavitation caused by valves not beeing able to close 100% because of inital abrational wear or dust particles on seats (tiny opening increasing velocity of gas).
If this is the case you have to look at the seat construction, materials and exposure of seat sealings and sealing rings to evaluate improvements.
An idea could be to evaluate softer seat sealings.
Note: Some operators may tend to hide 'un-normal' incidents, initial wear problems could be caused started by 'false operation', giving condensation or other irregular happenings as mentioned above.

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