How to Calculate Valve Test Pressure: GB/T 13927 vs API 598

The Three Tests Every Valve Must Pass

A valve leaves the factory only after three hydrostatic or pneumatic checks. Under GB/T 13927-2008 "Industrial Valves - Pressure Testing" (equivalent to ISO 5208), and mirrored by API 598, they are:

1. Shell test (壳体试验) - pressurises the closed body to confirm the casting or forging will not burst or deform.
2. Seat / closure test (密封试验) - checks the shut-off between the disc and the seat in the closed position.
3. Backseat test (上密封试验) - only for valves with a backseat; confirms the stem area seals when the valve is fully open.

The Formula (PN-Rated Valves)

For PN-rated valves the calculation is direct:

• Shell test pressure = 1.5 x PN (liquid medium) or 1.1 x PN (gas, special cases only)
• Seat test pressure = 1.1 x PN
• Backseat test pressure = 1.1 x PN

So the pressure scales straight off the nominal pressure. Worked values:

These numbers are exactly what a Chinese test report should show for a valve marked PN16, PN40 and so on.

The Low-Pressure Air-Seal Test

A subtle but important step: the seat test is often run twice - once at the high pressure above, and once at a low-pressure air test. Under GB/T 13927 the low-pressure seat test is 0.6 MPa (air or nitrogen); API 598 uses 0.4-0.7 MPa. The gas test is more sensitive than water and reveals micro-leaks the hydrostatic test can miss. Soft-seated valves are expected to be bubble-tight; metal-seated valves allow a small specified leakage rate.

The Class-Valve Trap

Here is the mistake that costs buyers: for a Class-rated valve, the test pressure is not 1.5 x the pound number. It is 1.5 x the material group's maximum working pressure at 38 deg C (100 deg F) - the same basis API 598 uses. So:

A Class 150 valve is therefore shell-tested at about 3.0 MPa, not at 1.5 x 150 psi (which would be meaningless). The rating already folds in the temperature correction, so the test pressure references the 38 deg C rating, not the class number. This is the same logic explained in our PN vs ASME Class guide.

What to Check on a Test Report

When a supplier says "tested per GB/T 13927" or "tested per API 598", treat them as the same hydrostatic basis. On the report, confirm:

1. The test medium (water is standard; gas only under controlled conditions).
2. The actual shell and seat pressures in MPa or bar, matching the formula above.
3. The hold time (typically 60 s for DN100 and below, longer for larger sizes).
4. The acceptance - no visible body leakage, and seat leakage within the standard's allowed rate.

Our Chinese-to-international valve standards cross-reference places GB/T 13927 and JB/T 9092 in the broader GB-to-API/ISO map. Test-pressure data and material grades for more than ninety manufacturers are catalogued in the ValveSpecs Pro database, which makes it straightforward to sanity-check a supplier's quoted test values before acceptance.