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What is The API 598 Leak testing of Valves

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API is American Petroleum Institute

The API standard 598: Valve Inspection and Testing, covers the testing and inspection requirements for gate, globe, check, ball, plug and butterfly valves. It has acceptable leakage rates for liquid as well as gas testing. All valves built to the various API standards are required to meet API-598 leakage criteria prior to shipment from the manufacturer or supplier.

API 598 states for shell and BACK SEAT tests, no visible leakage is permitted. If the fluid is a liquid, there shall be no visible evidence of drops or wetting of the external surfaces (no visible leakage through the body, body liner, if any, and body-to-bonnet joint and no structural damage).

If the test fluid is air or gas, no leakage shall be revealed by the established detection method. For both the low-pressure closure test and the high-pressure closure test, visual evidence of leakage through the disk, behind the seat rings, or past the shaft seals (of valves that have this feature) is not permitted (Plastic deformation of resilient seats and seals is not considered structural damage). The allowable rate for leakage of test fluid past the seats, for the duration of the tests, is listed in the following table:

Valve Size
NPS		 All Resilient
Seated
Valves		 All Metal Seated Valves
(except Check Valves)
Liquid Test
(drops
min.)		 Gas Test
(bubbles
min.)
< 2 0 0 (1) 0 (1)
2 – 6 0 12 24
8 – 12 0 20 40
> 12 0 28 56
Valve Size
NPS		 All Resilient
Seated
Valves		 Metal Seated
Check Valves
Liquid Test
(drops
min.)		 Gas Test
(bubbles
min.)
< 2 0 (2) (3)
2 – 6 0 (2) (3)
8 – 12 0 (2) (3)
> 12 0 (2) (3)

General notes:

  • 1 milliliter is considered equivalent to 16 drops.
  • For check valves larger than NPS 24, the allowable leakage rate
    shall be per agreement between purchaser and manufacturer.

Notes:

  1. There shall be no leakage for the minimum specified test duration. For liquid test, 0 drop means no visible leakage per minimum specified test duration. For gas test, 0 bubble means less then 1 bubble per minimum specified test duration.
  2. The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
  3. The maximum permissible leakage rates shall be 1.5 standard cubic feet (0.042 cubic meter) of gas per hour per inch of nominal pipe size.