API 598 vs ISO 5208: Valve Testing Acceptance — Comparison Guide

This guide helps you understand the practical differences between API 598 and ISO 5208, and how each flows into your ITPs, vendor dossiers, and acceptance reporting (see [Vendor Inspection basics] (W1.1)).

📌 Last updated: September 2025 — References: API 598 (2023), ISO 5208 (2015), EN 12266-1 (2012), ANSI/FCI 70-2 (2021).
📂 Downloads: [Comparative Acceptance Table (PDF)] • [Seat/Hydro Test Form (Excel)]

 

API 598 vs ISO 5208 — Quick Guide

Philosophy

• API 598 defines valve acceptance as inspection + pressure testing with binary pass/fail outcomes.
• ISO 5208 relies on graded leakage Classes A–G, chosen in the contract, to tune acceptance against risk.

Scope of Tests

• API 598 covers shell, seat, and backseat (if designed).
• ISO 5208 is limited to shell and seat but adds quantified leakage values (see [Hydrostatic vs Seat Leak Tests] (W2.2)).

Control Valves
Both standards defer to ANSI/FCI 70-2 for control valve seat tightness (e.g., Class VI). This route must be documented per tag in your ITP (use the [Vendor Inspection ITP Template] (W1.4)).

Regional Alignment

• API 598 has no formal EN route.
• ISO 5208 is aligned with EN 12266-1 (2012), making it more common in European projects.

Documentation

• API 598 requires recording edition, medium, pressure, and hold time.
• ISO 5208 requires the same, plus leakage Class and units.

Quick Pick

• API 598 → Preferred for API-centric packages and on/off valves where a binary decision is enough.
• ISO 5208 → Common in EN/ISO projects or when leakage classes must be risk-tuned and contractually declared.
• Always make sure your choice is closed out properly in the vendor dossier (see [IR/NCR & Final Dossier] (W2.1)).

 

One-Line Definitions

• API 598: Factory inspection and pressure testing of metallic valves (shell/seat/backseat where applicable) with go/no-go acceptance.
• ISO 5208: Pressure testing of metallic valves (shell/seat) with measured leakage Classes A–G in defined units, allowing tailored acceptance.

📌 If tight acceptance is driven by chronic failure mechanisms, consult [Valve Failure Modes] (W3.4) to understand real-world failure patterns before fixing criteria.

 

Test Scope & Media (What Actually Happens on the Bench)

API 598. Shell/closure/backseat pressures and minimum hold times are tabled by edition; medium is typically water (air/gas by agreement and safety). Backseat is tested only if the design includes it—validate preconditions with your checklist.

ISO 5208. For seat tests you must declare the Class (A–G), the test medium, and the measurement units (e.g., bubbles/min for gas, mL/min for liquid). The form records the measured value against the class limit—see sequencing guidance in Hydrostatic vs Seat procedures.

EN 12266-1. EU-centric route often used alongside or instead of ISO 5208; cite it explicitly when the spec is EN-based, and lock the reference in your ITP.

 

Acceptance & Measurement (What to Record—No Ambiguity)

• API 598 (Seat): record PASS/FAIL plus medium, pressure, hold time; file the signed sheet into IR/NCR & dossier.
• ISO 5208 (Seat): record the selected Class, medium, test conditions, and measured leakage in the correct units for that Class.
• Shell (both): No visible leakage through the pressure boundary.

Engineer’s tip: High-containment or SIL-relevant services → API 598 or tight ISO Classes A/B. Moderate services → ISO mid-classes C–E after risk review; reflect the decision in the ITP.

 

Control Valves (Don’t Mix Standards)

ANSI/FCI 70-2 is the authoritative leakage test for control valves (incl. Class VI for very tight shutoff). In mixed packages, route on/off valves to API/ISO and controls to FCI 70-2; document per tag in the ITP and close evidence in the dossier.

 

Selection Guide (3 Steps)

1. Valve type? On/off → API 598 or ISO 5208 | Control → ANSI/FCI 70-2.
2. Contract route? API → API 598 | ISO/EN → ISO 5208 / EN 12266-1.
3. Service severity? High containment → no visible leakage / Class A–B | Moderate → Class C–E.

Always lock the edition, state medium/pressure/hold, and for ISO, Class + units (the ITP template helps enforce this consistently).

Mini examples.

• 2″, Class 300 ball, on/off, API supply → API 598 shell & seat (no backseat test).
• DN100 control valve, Class VI required → ANSI/FCI 70-2 Class VI.

 

Documentation & Common Pitfalls (Save Your Dossier)

Use a structured form: Tag, NPS/DN, Class/Rating, Material, Standard+Edition, Medium, Pressure, Hold, Temp, Seat direction, Measured leakage + units, Instrument IDs & cals, Operator/Witness. Standardize this across vendors with the [Master Inspection Templates Pack] (W2.5).

Frequent errors (+ quick fixes).

• Edition drift: table values change across editions → state the edition in PO/ITP and on the form.
• Medium mismatch: air used where water required (or vice versa) → invalidates results; align with spec and record the medium explicitly.
• Units confusion (ISO classes): bubbles/min vs mL/min → copy the exact units for the chosen Class.
• Backseat demanded on a valve without it: mark N/A; API backseat applies only if the feature exists—screen during checklist review.

 

Frequently Asked Questions

1) Is API 598 stricter than ISO 5208 for seat leakage?
API 598 is typically go/no-go; ISO 5208 is graded—tight A/B classes can approach near-zero when specified.

2) Which standard covers “Class VI” tight shutoff?
ANSI/FCI 70-2 (control valves). Do not use API 598/ISO 5208 for control-valve leakage classes.

3) Do I need both API 598 and ISO 5208 on the same valve?
Generally, no—choose one route per on/off valve; control valves → FCI 70-2. Mixed packages can be assigned per tag in the ITP.

4) Where does EN 12266-1 fit vs ISO 5208?
It’s the EU testing framework broadly aligned with ISO 5208; use it when the project specification is EN-centric.

5) What must be on the acceptance record to avoid disputes?
Standard + Edition, medium/pressure/hold, and for ISO 5208 the Class + units; attach calibration evidence and photos.

6) When is a backseat test required?
Under API 598, only if the valve has a backseat feature (e.g., gate/globe). ISO 5208 doesn’t include a backseat test.

 

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