API 598 vs ISO 5208: Valve Pressure Testing Standards Explained

Every industrial valve should leave the factory pressure-tested, but "tested" means different things under different standards. The two most frequently referenced documents in international valve purchasing are API 598 and ISO 5208. Understanding what each one requires helps buyers write clear purchase orders and audit test certificates with confidence.

What API 598 covers

API 598 (Valve Inspection and Testing) is the default test standard for valves built to API and ASME design codes — gate, globe, check, ball and butterfly valves alike. It defines three core tests:

• Shell test — the assembled body and bonnet are pressurised to 1.5× the 38°C rating with the valve partly open. No visible leakage through pressure-boundary walls or joints is allowed.
• High-pressure closure (seat) test — 1.1× the rated pressure applied across the closed seat. Resilient-seated valves must show zero leakage; metal-seated valves are allowed a small drop-per-minute rate that scales with size.
• Low-pressure air seat test — 4 to 7 bar air with the seat wetted or monitored by bubble count, an effective way to reveal fine seat defects that high-pressure water can mask.

What ISO 5208 covers

ISO 5208 (Industrial valves — Pressure testing of metallic valves) takes a different approach: instead of fixed pass/fail rules per valve type, it defines leakage rate classes — Rate A (no visually detectable leakage), then AA, B, C, CC, D, E, EE, F and G in increasing allowance. The purchaser selects the rate class; the manufacturer tests to it. European design standards (EN 1984, EN 13709, EN 593) and many DIN-heritage specifications call up ISO 5208 by default.

Which one should be on your order?

• Valves designed to API 600, API 602, API 608 or API 623 → test to API 598 (API 6D pipeline valves use API 6D Annex tests instead).
• Valves designed to EN / DIN standards → test to ISO 5208, and state the rate class — Rate A for soft-seated, Rate B or C typical for metal-seated.
• Mixed international projects → either is defensible; what matters is naming the standard and the acceptance class explicitly on the PO.

The practical differences are small for soft-seated valves — both effectively demand zero leakage. For metal-seated valves the allowances differ, so a valve that passes API 598 may not meet ISO 5208 Rate A. Specify the tighter class where the process demands it, and expect the price to reflect the extra lapping time.

How VTXFLOW tests

Every VTXFLOW valve is 100% shell and seat tested in our dedicated inspection centre to API 598 or ISO 5208 as ordered, with computerised test benches up to 42 MPa. Test reports and EN 10204 3.1 material certificates are supplied with each shipment, and third-party witness inspection (SGS, BV, TUV) is welcome. See our material-specific product pages or request a quotation with your required test standard.

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