Direct answer · Updated 25 October 2026
What is the low-pressure closure test in API 598?
The API 598 low-pressure closure test is a seat leakage test with air or inert gas at 4 to 7 bar (60 to 100 psi). The valve is closed, one side is pressurised and leakage is measured at the other side for a minimum time based on valve size. Soft-seated valves must show no visible leakage; metal-seated valves have a small allowance by size. It complements the high-pressure closure test with water at 1.1 times the rated pressure.
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API 598 Low-Pressure Closure Test Explained
How the gas seat test works, why it is often stricter than the hydro seat test, and what leakage is allowed.
Where the low-pressure closure test fits in API 598
API 598 specifies which closure tests are required and which are optional for each valve type, size and pressure class. Soft-seated valves (ball, butterfly, plug) normally require the low-pressure gas closure test. Purchasers can also ask for both tests on any valve.
Why test seats at low pressure?
For many valves, line pressure helps the seat seal: in a floating ball valve or a wedge gate valve, higher differential pressure pushes the closure member harder against the downstream seat. At 4–7 bar there is little of that help, so the low-pressure gas test shows whether the seat seals on its own geometry and finish. Gas also finds much smaller leak paths than water. Many valves that are tight in the high-pressure hydro test show leakage in the low-pressure gas test, which is why it is often the more demanding of the two for soft-seated and resilient-seated valves.
How the test is done
- Clean the seats and remove any test liquid from earlier hydro tests.
- Close the valve with normal operating force (no extra torque or wheel keys beyond design).
- Pressurise one side with air or nitrogen at 4–7 bar; the other side is open to atmosphere.
- Hold for the minimum duration in API 598 (from 15 seconds for small valves to several minutes for large valves) and measure leakage at the open side, usually by counting bubbles through a tube submerged in water.
- Repeat in the other direction for bidirectional valves.
Allowable leakage
For soft-seated valves and lubricated plug valves, API 598 allows no visible leakage during the test. Metal-seated valves have a small allowance that increases with size. Typical API 598 limits for metal-seated valves (other than check valves) are shown below; always check against the edition named in the purchase order.
If a specification asks for ISO 5208 Rate A or API 6D, that means no visible leakage, which is stricter than the API 598 metal-seat allowance. State the required leakage class in the enquiry.
What to send with your enquiry
- Valve type, size, class and quantity
- Test standard and edition (API 598, API 6D, ISO 5208, EN 12266)
- Required closure tests: low-pressure gas, high-pressure liquid, or both
- Leakage class (for example ISO 5208 Rate A)
- Witness or third-party inspection requirement
Send the line list or datasheet to info@supremevalves.in or use the quote builder. Technical clarifications and a priced offer follow within one working day for standard items.
Related products and guides
API 598 valve pressure testing guide
Full guide to API 598 tests.
Valve testing pressure chart
Shell and seat test pressures by class.
Hydrostatic vs pneumatic testing
Liquid and gas test methods.
Pressure rating chart
PN and Class in bar, kg/cm² and psi.
API 598 tested butterfly valves
Butterfly valves with test reports.
Fire-safe ball valve
Soft-seated ball valve tested to API 598.
Frequently asked questions
What pressure is used for the API 598 low-pressure closure test?
Air or inert gas at 4 to 7 bar (60 to 100 psi).
What is the difference between the low-pressure and high-pressure closure test?
The low-pressure test uses gas at 4–7 bar; the high-pressure test uses liquid at 1.1 times the rated pressure. The gas test finds smaller leak paths and does not get the sealing help of high line pressure.
What leakage is allowed in the low-pressure closure test?
None visible for soft-seated valves and lubricated plug valves. Metal-seated valves have an allowance that increases with size, for example 24 bubbles per minute for NPS 2½ to 6.
How long is the closure test held?
From 15 seconds for valves NPS 2 and smaller up to several minutes for large valves, as tabulated in API 598.
Is ISO 5208 Rate A the same as API 598?
No. ISO 5208 Rate A means no visible leakage for the test duration, which is stricter than the API 598 allowance for metal-seated valves.
Do you supply API 598 test reports with valves?
Yes. Every valve is shell and seat tested, and test reports are supplied with EN 10204 3.1 material certificates. Third-party witness testing is available.
About this page: Written by the Supreme Valves India engineering team, Ahmedabad (supplying industrial valves since 1986). Last reviewed 25 October 2026. Figures are typical values for selection; the governing values are those in the referenced standard and on the approved datasheet for each order. Entity facts: Supreme Valves brand facts.
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