API 598 vs ISO 5208

API 598 and ISO 5208 are the two most widely referenced standards for production (shop) testing of industrial valves. Both define shell (pressure) tests and closure (seat) tests, but they differ in test pressures, hold times, and leakage acceptance criteria. The choice between them depends on the project specification, regional preference, and the valve product standard referenced.

When Each Standard Applies

API 598 is referenced by API valve product standards (API 600, 602, 608, 609, 6D) and is standard practice for oil and gas projects worldwide. Most international EPC contractors specify API 598 by default.

ISO 5208 is the international equivalent, referenced by EN and ISO valve standards (EN ISO 10434, EN ISO 17292). It is commonly specified on European-origin projects and by clients who prefer ISO-based specifications.

Specifications and Comparison

Feature	API 598	ISO 5208
Scope	Production testing of metallic valves	Production testing of metallic valves
Shell test pressure	1.5 x CWP (cold working pressure at 38 deg C)	1.5 x PN (at 20 deg C)
Shell test medium	Water (or other specified liquid)	Water (or other specified liquid)
Shell test duration	Size-dependent: 2-5 minutes	Size-dependent: per Table 3
Seat test (high pressure)	1.1 x CWP	1.1 x PN
Seat test (low pressure)	0.4-0.7 bar (air or gas)	0.5-0.7 bar (air or gas)
Seat test medium	Water (HP), air/gas (LP)	Water (HP), air/gas (LP)
Leakage classification	Zero leakage (metal seat: rate A, B, C, D)	Rate A through G (graduated)
Metal seat acceptance	Rate per valve product standard	Rate per specification (A through D typical)
Soft seat acceptance	Zero visible leakage (Rate A)	Rate A (zero leakage)
Backseat test	Required (where applicable)	Not always required
Test certificate	API 598 test report	ISO 5208 test report
Referenced by	API 600, 602, 608, 6D, 594	EN ISO 10434, EN ISO 17292, EN 12516

Comparison: Leakage Rates

Rate	API 598 Description	ISO 5208 Description	Acceptance
A	Zero leakage	Zero leakage	Soft-seated valves
B	Drops per minute (size-dependent)	cm3/min per DN (small rate)	Metal-seated valves (tight shut-off)
C	Higher rate (size-dependent)	cm3/min per DN (moderate rate)	Standard metal-seated valves
D	Per API 598 table	cm3/min per DN (higher rate)	Less critical services
E-G	Not defined in API 598	Progressively higher rates	ISO only; industrial/non-critical

Common Mistake

Assuming API 598 and ISO 5208 are identical. While both test at 1.5x rated pressure for the shell test, the leakage acceptance criteria differ, particularly for metal-seated valves. A valve that passes ISO 5208 Rate D may not pass API 598’s more restrictive leakage requirements. Always verify which standard and leakage rate the project specification requires before approving valve test results.

Testing Sequence

Both standards follow the same general sequence:

1. Shell (hydrostatic) test: Valve open, pressurized to 1.5x rated pressure, hold for specified duration. No leakage through body, bonnet, or end connections.
2. High-pressure closure (seat) test: Valve closed, pressurized to 1.1x rated pressure from one side. Measure leakage past the seat.
3. Low-pressure closure test: Valve closed, pressurized with air/gas at 0.4-0.7 bar. Check for bubble leakage past the seat.
4. Backseat test (API 598 only, where applicable): Stem packing removed, valve fully open, pressurized. No leakage past backseat.

All test results must be documented on the mill test certificate or a separate test report. NDT of the valve body (RT, UT, MT, PT) is performed before the pressure test per the applicable valve product standard.

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