The valve seat leak test is performed after successful completion of valve shell test (Body Test). The test shall be performed based on the valve manufacturer approved valve seat leak test procedure.
The most of these procedures are driven by the requirements of the API Std 598 and ASME B16.34
The valve seat leak test is addressed in the I4I Academy API Source Inspection Training course. The API 598 is included in this course. Another related course is the API 570 piping inspector course. The API RP 574 Inspection Practices for Piping System Components is part of the API 570 course and has sessions about various types of valves and inspection and testing requirements.
The valve is mounted on the test bench, and valve shall be completely closed. One side (valve inlet or outlet) shall be subjected to the hydrostatic pressure and amount of leak shall be measured on the opposite side of the valve. It means if you are pressurizing valve inlet, then you have to measure the amount of leakage in the outlet.
The API STD 598 requires the valve being tested at 110% of the maximum allowable pressure at 38 °C (100 °F). The ASME B16.34 requires the test be performed not less than 110% of the 38°C (100°F) pressure rating which almost is the same.
If you have a valve with 2500 pressure rating and with cast steel material such as A216 Gr. WCB, then your maximum working pressure as per ASME B16.34 table 2-1.1 will be 425.5 bar (6254.85 psi). The 110% of this value will be 6880.3 psi. So you have to perform seat leak test with 6880.30 psi.
ASME b16.34 require the seal leak test be performed 15 seconds for valves with size less than 2 inch and 30 seconds for valves between 2 1/2 to 8 inch, 60 seconds for valves between 10 and 18 inch and 120 seconds for valves greater than 20 inch. The API 598 timing requirement is similar.
During seat leak test valve shall not leak from the stem and packing, and the leak will not be permissible. The certain amount of leakage from sealing surface of disc and seat is permissible.
The API Std 598 provides the allowable amount of leakage in Table 6, and it is measured based on drops per minutes. The amount of leakage also depends on the valve size.
In above example, the valve shall be tested at 6880.3 psi and the pressure shall be held for 120 seconds (we assume the valve is 24 inch). During this 120 seconds, the amount of leakage shall not be more than 48 drops /min. (as per table 6 in API 598)
A graduated lab test tube is used for measurement of the leakage. The other side of the valve connected to this tube and amount of leakage is measured.
You do not need to count the drops; you just need to keep the time (120 seconds in above example). 1 milliliter is equal to 16 drops. you can convert milliliter to drops after completion of the test and determine the test result.
The ASME B16.34 does not provide the permissible leakage values and refers that to the agreement between purchaser and manufacturer.