Technical Guide June 28, 2026 9 min read

Valve Testing per API 598: Hydrostatic & Pneumatic Tests Explained

API 598 is the most widely referenced standard for factory testing of industrial valves. Every valve leaving our facility undergoes shell (body) and seat (closure) testing per API 598 requirements. This guide explains each test type, test pressures, hold times, and acceptance criteria so you can verify valve test certificates with confidence.

Overview of API 598 Tests

API 598 "Valve Inspection and Testing" defines four types of factory tests:

Shell (body) test: Tests the integrity of the pressure-containing body, bonnet, and all joints at 1.5× rated pressure
Low-pressure closure (seat) test: Tests seat tightness at low pressure (typically air at 0.4-0.7 bar) to detect gross leakage
High-pressure closure (seat) test: Tests seat tightness at 1.1× rated pressure with water
Backseat test: Tests stem backseat integrity (gate and globe valves with backseat feature)

Shell (Body/Hydrostatic) Test

The shell test verifies the structural integrity of the valve body, bonnet, and body-bonnet joint under pressure. The valve is tested with the disc/ball in the partially open position so that both upstream and downstream body cavities are pressurized simultaneously.

Parameter	Requirement
Test medium	Water (ambient temperature)
Test pressure	1.5 × rated pressure at 38°C (100°F)
Minimum duration	DN ≤ 50: 15 sec \| DN 65-150: 60 sec \| DN 200-300: 120 sec \| DN ≥ 350: 300 sec
Acceptance	Zero visible leakage through body wall, body-bonnet joint, and all pressure-boundary joints

High-Pressure Closure (Seat) Test

Tests the seat sealing ability under high pressure. Water is applied to one side of the closed valve at 1.1× rated pressure, and any leakage past the seat is measured on the downstream side.

Parameter	Requirement
Test medium	Water (ambient temperature)
Test pressure	1.1 × rated pressure at 38°C
Minimum duration	Same as shell test (size-dependent)
Acceptance (soft seat)	Zero leakage (no visible drip)

Low-Pressure Closure (Pneumatic Seat) Test

Tests seat sealing at low pressure using air or nitrogen. This test is more sensitive than the hydrostatic seat test because gas is more likely to leak through small imperfections than water. Leakage is measured by bubble counting underwater or with a calibrated flow meter.

Parameter	Requirement
Test medium	Air or nitrogen at 0.4 to 0.7 barg (6-10 psig)
Duration	Same as shell test durations
Acceptance (soft seat)	Zero bubbles
Acceptance (metal seat)	Allowable leak rate per API 598 Table 6 (bubbles per minute based on valve size)

Test Pressures by Pressure Class (WCB)

Class	Rated Pressure (bar)	Shell Test 1.5× (bar)	Seat Test 1.1× (bar)
150	19.6	29.4	21.6
300	51.1	76.7	56.2
600	102.1	153.2	112.3
900	153.2	229.8	168.5
1500	255.3	383.0	280.8

Key Points for Valve Buyers

Every valve from Supreme Valves is tested per API 598 with documented test certificates
Soft-seated valves (PTFE, RPTFE) must achieve zero leakage on both seat tests
Metal-seated valves have allowable leak rates per API 598 Table 6
Test certificates show actual test pressures, hold times, and results — verify these against the standard
For enhanced testing (e.g., API 6D Appendix F, zero-leakage metal seat), specify in the purchase order
Third-party witnessed testing (TPI) can be arranged with Lloyd's, Bureau Veritas, TUV, or SGS

Frequently Asked Questions

What is the difference between API 598 and API 6D testing?

API 598 covers factory testing for general industrial valves (gate, globe, check, ball, butterfly, plug). API 6D covers pipeline valves specifically and includes additional closure tests and more stringent requirements. API 6D requires extended test durations, multiple seat tests (in each flow direction), and optional Appendix F testing for enhanced seat tightness (zero gas leakage). For pipeline ball valves, API 6D testing supersedes API 598.

Why is the pneumatic seat test more stringent than the hydrostatic seat test?

Gas (air/nitrogen) has much lower viscosity than water and can leak through imperfections that would seal against water. A valve that passes a hydrostatic seat test may fail a pneumatic seat test. This is why many specifications require both tests — the hydrostatic test at high pressure verifies the seat can hold process pressure, while the pneumatic test at low pressure verifies the seat is truly leak-tight for gas service or fugitive emission applications.

Can I request zero-leakage testing on metal-seated valves?

Standard API 598 allows a defined leak rate for metal-seated valves (bubbles per minute based on valve size). However, zero-leakage (bubble-tight) metal seat testing can be specified as a supplementary requirement — this is common for triple-offset butterfly valves and metal-seated ball valves used in critical isolation service. Specify "API 6D Annex F Category CC" or "zero gas leakage" in the purchase order for bubble-tight metal seat requirements.

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