API 602 vs API 603: Two Small-Bore and Cast Valve Standards Explained

API 602 vs API 603: Two Small-Bore and Cast Valve Standards Explained

The Short Answer: API 602 covers compact, bolted bonnet steel gate, globe, and check valves in sizes typically up to 4 inches—usually forged or barstock construction—while API 603 covers flanged and butt-welded steel gate valves in sizes up to 24 inches, usually cast construction. Choose API 602 for small, high-integrity forged valves; choose API 603 for larger cast gate valves on flanged lines.

What Each Standard Covers

API 602

API 602 is the industry standard for compact steel gate, globe, and check valves with bolted bonnets, in nominal sizes up to and including NPS 4, with pressure classes up to 1500 in many configurations. The standard emphasizes body material integrity, stem design, and testing for small forged and barstock valves used in oil, gas, and process service.

API 603

API 603 applies to steel gate valves with flanged or butt-welded ends, in sizes NPS 1 through 24, in pressure classes 150 through 600. These valves are typically cast bodies and are used where larger line sizes need a standard-compliant gate valve with full bore flow and a recognized testing regime.

Key Differences

Feature	API 602	API 603
Valve types	Gate, globe, check	Gate only
Size range	Up to NPS 4	NPS 1 to 24
Construction	Forged / barstock typical	Cast typical
End connections	Threaded, socket weld, flanged	Flanged, butt-welded
Pressure class	Up to Class 1500 typical	Class 150–600
Typical duty	Small high-integrity lines	Large pipeline and process isolation

Which to Choose

Specify API 602 when you need a compact, high-pressure, small-bore valve with documented integrity—standard for 2-inch and smaller branches in oil and gas. Specify API 603 when the line is larger and you need a flanged or butt-welded cast gate valve with a recognized standard and testing regime. The two standards rarely compete; they serve different size ranges.

Common Pitfalls

• Calling a small imported valve “API 602” without the documentation—compliance is proven by the data sheet and test records, not the name.
• Using API 603 thinking it covers globe and check valves—it covers gate valves only.
• Selecting API 602 beyond NPS 4—the standard’s scope does not extend there.
• Ignoring pressure class and material limits when moving between the two standards on one line.

Practical Recommendation

Use the line size and valve type to pick the standard: small forged multi-type valves go API 602; larger cast gate valves go API 603. Always request the manufacturer’s compliance documentation and verify the pressure class, end connection, and body material against the project specification before purchase.

Documentation Considerations

Standards compliance is a documentation exercise as much as a design exercise. For both API 602 and API 603, request the manufacturer’s data sheet showing the standard designation, pressure class, body material, and test results, and keep it in the project file. Small forged valves are easy to mislabel, so verify the marking on the body matches the certificate.

Also confirm the end connections: API 602 valves come threaded, socket-weld, or flanged, and the connection must match the piping spec exactly. An API 603 gate valve is flanged or butt-welded, so check the flange facing and welding procedure against the project requirements before ordering.

A Field Example

On a gas plant’s small utility branches, a contractor offered “API 602” gate valves without certificates. The engineering team rejected the batch because the body markings did not match the quoted standard, and reordered from a manufacturer that supplied the full test documentation. The small cost difference was nothing compared to a compliance issue on a safety-critical line.

Conclusion

API 602 and API 603 split the field by size and construction: forged small-bore valves on one side, cast larger gate valves on the other. Matching the standard to the line avoids both over-specification and compliance gaps.

Sources & Further Reading

API, ASME

Tags: Valve Standards