When purchasing valves from a valve company is leak testing of valves by a supplier usual practice or a requirement of a standard? (Not pressure testing, just checking if there is any leakage over the valve seat at MAWP or MOP).If it is normal practice to leak test valves, would it be satisfactory to test a full batch or only a sample.I have used Zwick and have seen Bray. I am unaware of problems with either.It is a normal practice to require API STD 598 compliance certification for seat leakage. Some project require testing a sample. Some may test all critical valves. Testing costs time and money. Neither leak testing nor certification assure that the valve won't leak after plant hydrostatic testing.API 598 is a stand-alone document and has no relationship with ANSI/ASME B31.1 or B31.3 construction codes. API 598 is referenced as the testing standard for valves made to API downstream valve standards such as API 600 (gate valves) and others. The only ANSI/ASME standard that comes close to API 598 is B16.34, which specifies testing procedures but does not address leakage rates. API 598 does address leakage rates.