And the control valves are manufactured based on API 6D standards. The leakage tests required in API 6D are mostly hydrostatic tests which are shell test(2200psi) and closure test(1650 psi). However, we request additional air closure test at the pressure of 77kg/cm2(1095psi) based on the reason that the operation pressure of our natural gas will reach as high as 60kg/cm2(853 psi). My question is,"Does the passing of hydrostatic tests will ensure the same air tightness for our high pressure service gas." Our vendor said the 77kg-air-test is apparently deviated from API standard and also unnecessary.
I work for a valve manufacturer and see nothing wrong with your request however.  One question is always the leakage rate that will be allowed.
Zero leak does not exist and high differential pressure seat tests must mention a maximum allowed leak rate.  Especially since mass flow meters and other tools can measure leakage rates that cannot be detected with a rotameter.

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