API 598 Valve Inspection and Testing: Shell, Seat and Backseat Tests
Quick Answer
API 598 specifies production pressure-testing of metal valves after manufacture: a shell (body) hydrostatic test, a seat test (hydrostatic or pneumatic), and a backseat test for valves with a backseat. Test pressures scale with the valve rated class, and the accepted leakage allowance is stated per valve type and size.
What Gets Tested
| Test | Purpose | Typical pressure |
|---|---|---|
| Shell | Verify body/bonnet integrity | 1.5x rated |
| Seat | Verify closure tightness | 1.1x rated |
| Backseat | Verify stem seal when open | 1.1x rated (if applicable) |
Shell Test
The shell test is hydrostatic at 1.5 times the valve rated pressure. The body and bonnet must show no visible leakage; the test confirms the pressure boundary, not the seat.
Seat Test
The seat test checks closure. Depending on the valve and customer spec it is hydrostatic or pneumatic at about 1.1x rated. Soft-seated valves are normally required to be bubble-tight; metal-seated valves are allowed a small, graded leakage.
Backseat Test
Valves with a backseat (a seating surface in the bonnet that seals the stem when fully open) are tested with the valve open so the stem seal can be inspected. A good backseat lets packing be repacked while the valve is under pressure.
How It Relates to Leakage Classes
API 598 gives in-house production acceptance; field leakage expectations are often expressed per valve leakage classes. For the Chinese pressure-test calc see GB/T 13927. More on codes in the standards hub.
Frequently Asked Questions
What is the shell test in API 598?
A hydrostatic test at 1.5 times the rated pressure that confirms the body and bonnet pressure boundary has no visible leakage.
Why is a backseat test performed?
To verify the stem seal when the valve is fully open, which is what allows packing to be repacked while the valve remains under pressure.
Are soft-seated valves allowed to leak under API 598?
Typically no - soft-seated valves are required to be bubble-tight at the seat, whereas metal-seated valves may show a small graded leakage.