Home › Guides › Shell, seat and backseat valve tests explained
Four production tests, four different questions about the valve. Here is what each one answers, the order we see them run, and a real test-bench case where one test passed and another did not.
Short answer: The shell test proves the body and bonnet hold pressure; the backseat test proves the stem backseat seals; the high-pressure seat test proves the closed valve seals under high liquid pressure; and the low-pressure air seat test proves it seals when pressure gives little help. Run them in that order, on bare, dry valves.
By Kevin Lyu (吕中国), ValveWitness Inspection, Ningbo · Published 2026-11-14
Which tests are mandatory depends on the standard, the valve type, size and class, and your PO. The descriptions below are general; read pressures, durations and acceptance from the licensed edition named in your PO.
| Test | What it proves | How it is typically run | Watch for |
|---|---|---|---|
| Shell strength test | The pressure-containing envelope (body, bonnet, body joints) holds pressure without leakage or permanent deformation. | Valve partly open, ends blanked, filled with water, raised to the shell test pressure of your edition and held. | Sweating or weeping through the casting, joint leaks, pressure drop during the hold. |
| Backseat test | The stem backseat seals so packing can be serviced with the valve open, on valves designed with a backseat. | Valve fully open on the backseat, with the packing arranged as the standard requires; pressure applied and held. | Leakage at the stem; only applies where a backseat exists. |
| High-pressure closure (seat) test | The closed valve seals against pressure on the seat at the high test pressure. | Valve closed, pressure applied on the side(s) the standard and valve type require, typically with water. | Drops past the seat measured against the leakage rate or allowance in the PO. |
| Low-pressure pneumatic seat test | The seat seals at low gas pressure, where the closing force from pressure is small. | Valve closed, low-pressure air or inert gas applied; outlet watched under water or with leak-detection fluid. | Bubbles; leaks that high water pressure “pushed shut”. |
Any re-test must be recorded with its reason. “Test until it passes” off the record is the single biggest risk in a test that nobody witnesses.
On one silent (noise-reduction) check valve design, valves were tight on the water seat test at 18 bar (1.1× the standard pressure), but showed micro-leakage on the 6 bar air seat test. The investigation traced it to a design change: the shaft-to-bracket clearance had been reduced by 0.10 mm to make closing quieter, so the disc slightly interfered as it closed. High water pressure still pushed the disc onto the seat; low air pressure could not. Restoring the original clearance tolerance removed the leak.
The lesson for buyers: a valve can pass one closure test and fail another. If your service is low-pressure gas, the low-pressure air seat test is the one that matters — so name it in the PO.
Only if neither the standard edition nor your PO requires it. Where it is optional, say in the PO whether you want it — it finds leaks the high-pressure water test can miss.
The shell test should be done on bare, dry valves unless your specification explicitly allows otherwise, because paint can seal porosity during the hold.
No. The backseat test applies only to valves designed with a backseat, typically gate and globe valves with rising stems.
Need the tests witnessed? See valve pressure test witnessing and who will be at the bench.
We report facts from the samples inspected. Standards named in these guides are public standards we work to, not certifications or accreditations we hold. We do not issue product certificates or accreditation marks. Test pressures, durations, leakage rates and acceptance numbers must be read from the licensed edition named in your purchase order; we do not reproduce copyrighted tables from the standards.