What is API 598 valve testing?

API 598 is the standard for valve inspection and testing published by the American Petroleum Institute. It covers hydrostatic shell (body) testing, hydrostatic backseat testing, hydrostatic seat (closure) testing, and pneumatic low-pressure seat testing for gate, globe, check, ball, plug, and butterfly valves. It specifies test pressures, test durations, and acceptance criteria based on valve class and size.

What is the difference between shell test and seat test?

Shell test (body test) checks the pressure integrity of the valve body and bonnet. The valve is partially open and pressurized to 1.5× the rated pressure at ambient temperature. No leakage through body, bonnet, or gland is acceptable. Seat test (closure test) checks the sealing ability of the valve in the closed position. The valve is fully closed and pressure is applied to one side. The allowable leakage rate depends on the valve type and size per API 598 tables.

What are the test pressures for different valve classes?

For WCB carbon steel material at ambient temperature: Class 150: Body 30 kg/cm² (427 PSI), Seat 22 kg/cm² (313 PSI). Class 300: Body 78 kg/cm² (1109 PSI), Seat 57 kg/cm² (811 PSI). Class 600: Body 156 kg/cm² (2219 PSI), Seat 115 kg/cm² (1636 PSI). Class 900: Body 234 kg/cm² (3328 PSI). Class 1500: Body 390 kg/cm² (5547 PSI). Class 2500: Body 651 kg/cm² (9259 PSI). Air test for all classes: 7 kg/cm² (100 PSI).

How long should each pressure test be held?

Per API 598, the minimum test duration depends on valve size. For NPS ≤ 2: Shell test 15 seconds, Seat test 15 seconds. For NPS 2.5 to 6: Shell test 60 seconds, Seat test 60 seconds. For NPS 8 to 12: Shell test 120 seconds, Seat test 120 seconds. For NPS 14 and above: Shell test 120 seconds, Seat test 120 seconds. Pneumatic test: minimum 15 seconds for all sizes.

Valve Pressure Testing: API 598 Complete Guide – Methods, Pressures & Acceptance Criteria

February 11, 2026 10 min read Valve Standards

Every industrial valve must be pressure tested before it leaves the factory. API 598 (Valve Inspection and Testing) is the globally accepted standard that defines how valves should be tested, at what pressures, for how long, and what leakage is acceptable. This guide provides the complete reference — drawn from actual valve manufacturer testing data — that engineers and procurement managers need when specifying or witnessing valve tests.

1. Types of Valve Tests per API 598

A. Hydrostatic Shell Test (Body Test)

Tests the pressure integrity of the valve body, bonnet, and gland area. The valve is placed in a partially open position (to prevent pressure acting on the seat) and pressurized with water to 1.5× the rated cold working pressure (CWP).

Acceptance: Zero visible leakage through body wall, body-bonnet joint, gland, or end connections
Medium: Clean water at ambient temperature
Valve position: Partially open

B. Hydrostatic Seat Test (Closure Test)

Tests the sealing ability of the valve seat in the closed position. Pressure is applied to one side of the closed valve at 1.1× the rated CWP. For gate valves, the test is performed from each side separately.

Acceptance: Zero visible leakage for metal-seated valves (API 598 latest edition). Soft-seated valves: zero leakage
Medium: Clean water at ambient temperature
Valve position: Fully closed

C. Pneumatic Low-Pressure Seat Test (Air Test)

Tests seat tightness at low pressure using air or nitrogen. Pressure: typically 6–7 kg/cm² (100 PSI) for all classes. Leakage is detected by submerging the valve in water and checking for bubbles, or by using leak detection fluid.

Acceptance: Per API 598 leakage rate tables (based on valve size and type)
Medium: Air or nitrogen at 6–7 kg/cm² (100 PSI)

2. Test Pressures by ASME Class (API 598)

The following table shows hydrostatic test pressures for WCB (carbon steel) material per API 598:

Class	Body Test (kg/cm²)	Body Test (PSI)	Seat Test (kg/cm²)	Seat Test (PSI)	Air Test (kg/cm²)
150#	30	427	22	313	7
300#	78	1,109	57	811	7
600#	156	2,219	115	1,636	7
800# (Std)	–	–	–	–	7
900#	234	3,328	172	2,446	7
1500#	390	5,547	286	4,068	7
2500#	651	9,259	477	6,785	7

Note: PN-rated valves (DIN/EN standards) use different test pressures. PN-10: Body 15, Seat 11 kg/cm². PN-16: Body 24, Seat 17.6 kg/cm². PN-25: Body 37.5, Seat 27.5 kg/cm². PN-40: Body 60, Seat 44 kg/cm². Per EN 12266.

3. Pressure-Temperature Rating (ASME B16.34)

Valves are rated for maximum allowable working pressure at specific temperatures. As temperature increases, the allowable working pressure decreases. This data is essential for selecting the right valve class:

Class	@ –29 to 38°C	@ 200°C	@ 300°C	@ 350°C	@ 425°C
150#	20.0	14.0	10.4	8.5	5.6
300#	52.1	44.6	40.6	38.3	23.2
600#	104.1	89.3	81.1	76.6	58.6
900#	138.8	119.1	108.3	102.1	78.2
1500#	260.3	223.0	203.0	191.5	146.6
2500#	433.8	372.0	338.3	319.0	244.4

Unit: kg/cm². Material: WCB (carbon steel) per ASME B16.34.

4. Test Duration Requirements

API 598 specifies minimum test hold times based on valve size:

NPS (Inch)	Shell Test Duration	Seat Test Duration
≤ 2"	15 seconds	15 seconds
2½" – 6"	60 seconds	60 seconds
8" – 12"	120 seconds	120 seconds
14" and above	120 seconds	120 seconds

5. Related Testing Standards

Standard	Scope	Region
API 598	Valve inspection and testing	International (API)
EN 12266	Industrial valve testing	Europe (CEN)
BS 5146	Inspection and testing of steel valves	UK (BSI)
DIN 3230	Valve testing and leakage rates	Germany (DIN)
API 607	Fire test for ball valves	International
API 6FA	Fire test for valves (general)	International

6. Common Testing Issues & Solutions

Body leakage at bonnet joint: Check gasket condition, re-torque bonnet bolts evenly
Seat leakage (metal-seated): Lap disc and seat faces, check for scratches or foreign particles
Gland leakage during shell test: Adjust gland packing compression, replace packing if hardened
Pressure drop during test: Check test setup for external leaks, verify pressure gauge accuracy
Air bubbles during pneumatic test: Apply leak detection fluid to suspect areas, mark and repair

Conclusion

Pressure testing per API 598 is the final quality gate before a valve enters service. Understanding the test types, pressures, durations, and acceptance criteria ensures that your valves meet the required integrity standards. Always request certified test reports (EN 10204 Type 3.1) with your valve orders — and consider third-party inspection for critical applications.

All Supreme Valves Products Are API 598 Tested

EN 10204 3.1 material test certificates. Third-party inspection available. Complete test documentation with every order.

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