API 508 defines leakage rate for "TSO".
My question:
1. What is the leakage rate defined for "TSO" class as per API508 / ISO 5208, Any one has this standard please let me know.
2. Is TSO leakage rate superior than ANSI Class VI.
I don't find any API standard, RP or other publication 508.  This is likely a typographical error intending to reference API STD 598.  API STD 598 addresses allowable seat leakage rates using terms like visible leakage or bubbles - not tight shutoff.
Consider using industry standards instead of blogs to define engineering requirements.MSS SP-67 uses the phrase tight shutoff for type I butterfly valves.  Test with liquid requires no visible evidence of drops or wetting through the disc or leakage past the seat. With air, no visible bubbles are permitted when the closure is covered with a soap solution.  The testing duration increases from 15-seconds for small valves to 60 seconds for large valves.

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