TAT is done as per 'Shell' specification 77-300 in which valve is subjected to seat test, fugitive emission test and torque test under pressure, during and after mechanical and thermal cycling.
The qualification range is mentioned in spec 77/300B.
While TAT is a term frequwntly used in Shell specifications. It also appears in other standards besides Shell. It is testing that is done to a product to confirm the design is good. It is a product performance test which is different from FAT (Factory Acceptance Test), which is done to verify the quality of the product comming out from the factory.
In short, TAT = design verification testing
and FAT = Verification of production quality
In addition to the above valuable posts, I may add that EN 736-3 standard about "Valves - Terminology - Definition of terms" distinguishes among type test (similar to TAT mentioned above), production test and acceptance test.
