Valve Seat Leakage Rates as per API 598

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API 598 covers inspection and testing requirements of gate, globe, check, ball, plug and butterfly valves.

For shell and backseat test, no visually detectable leakage is permitted through body, body liner or joints. For liquid test, there should be no visible evidence of drops or wetting of the external surfaces. For gas test, 0 bubbles means less than 1 bubble per minimum specified test duration.

Note that backseat test applies only to gate and globe valves which have the backseat feature due to design of the obturator. 

Table below lists the allowable valve seat leakage rates as per API 598. Note that for all resilient seated (soft seated) valves, the permissible leakage rate is zero for the minimum specified test duration. 

DN
(mm)		 NPS
(in)		 All Resilient
Seated Valves		 All Metal-Seated Valves (except Check Valves) Metal-Seated Check Valves
Liquid Test (drops/min) Gas Test (bubbles/min) Liquid Test (cc/min) Gas Test (m3/hr)
≤ 50 ≤ 2 0 0 0 6 0.08
65 2-1/2 0 5 10 7.5 0.11
80 3 0 6 12 9 0.13
100 4 0 8 16 12 0.17
125 5 0 10 20 15 0.21
150 6 0 12 24 18 0.25
200 8 0 16 32 24 0.34
250 10 0 20 40 30 0.42
300 12 0 24 48 36 0.50
350 14 0 28 56 42 0.59
400 16 0 32 64 48 0.67
450 18 0 36 72 54 0.76
500 20 0 40 80 60 0.84
600 24 0 48 96 72 1.01
650 26 0 52 104 78 1.09
700 28 0 56 112 84 1.18
750 30 0 60 120 90 1.26
800 32 0 64 128 96 1.34
900 36 0 72 144 108 1.51
1000 40 0 80 160 120 1.68
1050 42 0 84 168 126 1.76
1200 48 0 96 192 144 2.02

From the above table it is noted that for liquid test on metal seated valves (except check valves), the permissible leakage rate for valve sizes above 2 inches is 2 drops per minute per inch of nominal pipe size (NPS) and for gas test the permissible leakage rate for valve sizes above 2 inches is 4 bubbles per minute per inch of NPS.

For metal seated check valves, the permissible leakage rate for liquid test is 3 drops per minute per inch of NPS.

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