What Is API 598? Valve Testing Standard

API 598 (Valve Inspection and Testing) defines the factory testing requirements for metallic valves, including the test types, test pressures, hold times, and acceptance criteria for shell integrity and seat tightness. It is the primary testing standard referenced in valve purchase orders for oil and gas projects, covering gate, globe, check, ball, butterfly, and plug valves.

Term	Definition
Shell test	Pressurize the body with valves partially open to check body and bonnet integrity
Backseat test	Test the stem backseat seal with packing gland loose
Closure test (seat test)	Pressurize one side with valve closed to check seat tightness
High-pressure closure	Seat test at 1.1x CWP (cold working pressure) with liquid
Low-pressure closure	Seat test at 0.4-0.7 bar (6-10 psi) with air or gas
CWP	Cold Working Pressure at ambient temperature per ASME B16.34

API 598 Test Types

Test	Medium	Pressure	Duration (per size)	Acceptance
Shell test	Water (liquid)	1.5x CWP	2-30 min (based on size)	No visible leakage through body or bonnet
Backseat test	Water (liquid)	1.1x CWP	2-30 min	No visible leakage past backseat
High-pressure closure	Water (liquid)	1.1x CWP	2-30 min	See leakage rates below
Low-pressure closure	Air or gas	0.4-0.7 bar (6-10 psi)	2-30 min	See leakage rates below

Test Duration by Valve Size

Nominal Size	Minimum Test Duration
Up to 2”	2 minutes
2-1/2” to 6”	5 minutes
8” to 12”	15 minutes
14” to 24”	30 minutes
Above 24”	Per agreement

Seat Leakage Acceptance

API 598 defines two categories of acceptable seat leakage for the high-pressure closure test:

Valve Type	Allowable Leakage (Liquid)
Gate, globe, plug, ball (metal seat)	Rate A: 0 drops for NPS <=2”, increasing with size
Gate, globe, plug, ball (soft seat)	Zero visible leakage
Check valve	Rate B: slightly higher allowance than Rate A
Butterfly valve	Per manufacturer specification or Rate A

For the low-pressure air closure test, leakage is measured as bubble rate at the downstream flange. Zero bubbles is required for soft-seated valves; a limited rate is acceptable for metal-seated valves.

Shell Test Pressure Calculation

The shell test pressure is 1.5 times the CWP (Cold Working Pressure) at the applicable pressure class:

Pressure Class	CWP at Ambient (psi)	Shell Test Pressure (psi)
Class 150	285	428
Class 300	740	1,110
Class 600	1,480	2,220
Class 900	2,220	3,330
Class 1500	3,705	5,558
Class 2500	6,170	9,255

API 598 vs ISO 5208

Feature	API 598	ISO 5208
Scope	Factory testing of metallic valves	Pressure testing of industrial valves
Shell test	1.5x CWP	1.5x maximum allowable pressure
Seat test	1.1x CWP (high pressure)	Multiple leakage rates (A through G)
Leakage classes	Rate A and Rate B	Rates A through G (more granular)
Primary use	North America, oil and gas projects	International, European projects

Pro Tip

When reviewing valve test reports, verify that the test duration matches the API 598 requirement for the valve size. Short test durations are a common shortcut by some manufacturers. A 14” valve tested for only 2 minutes instead of the required 30 minutes has not been properly tested per API 598.

Read the full guide to valve types