Valve Leakage Classes: ANSI/FCI 70-2, IEC 60534-4, and How to Specify Shutoff Requirements

One of the most important — and often misunderstood — aspects of valve specification is leakage class. How tightly must a valve close? The answer has significant implications for safety, process efficiency, and cost. Understanding the international leakage classification standards helps engineers specify the right level of shutoff for every application.

Why Leakage Class Matters

Not every valve application requires bubble-tight shutoff. For isolation valves separating process systems from maintenance areas, near-zero leakage is critical. For control valves that cycle between positions, a small allowable leakage may be acceptable and actually reduces actuator force requirements and seat wear.

Specifying a tighter leakage class than necessary drives up cost through more precise machining tolerances, harder seat materials, and higher actuator forces. Conversely, under-specifying leakage can result in safety incidents, process contamination, and excessive product loss.

ANSI/FCI 70-2 Leakage Classes

ANSI/FCI 70-2 defines six leakage classes for control valves, widely used in the USA and by many international OEMs. Test conditions include air or water at specified differential pressures.

• Class I: No test required; manufacturer's own specification applies

• Class II: 0.5% of rated Cv (air test at 50 psi differential)

• Class III: 0.1% of rated Cv

• Class IV: 0.01% of rated Cv — the most common 'standard' class for process control valves

• Class V: 5×10⁻⁴ mL/min per inch of port diameter per psi differential (water test, metal seats)

• Class VI: Bubble-tight; ≤0.15 to 6.75 mL/min depending on port size (soft seats)

IEC 60534-4: The International Standard

IEC 60534-4 is the international equivalent and closely parallels ANSI/FCI 70-2. It defines seat leakage rates for control valves and is required for valves sold into European and many Asian markets. Classes A through F correspond broadly to FCI Classes I through VI.

A key difference: IEC 60534-4 Class D corresponds to FCI Class IV, Class E to FCI Class V, and Class F to FCI Class VI. Engineers working across jurisdictions should confirm which standard applies in their project specifications.

ISO 5208: Leakage Standards for Industrial Valves

While FCI 70-2 and IEC 60534-4 focus on control valves, ISO 5208 covers pressure testing of industrial valves more broadly, including gate, globe, ball, butterfly, and check valves. It defines permissible leakage rates for body, seat, and backseat testing under defined test conditions.

ISO 5208 Rate A requires zero visible leakage (bubble-tight), while Rates B through F allow increasing leakage quantities per unit of pipe diameter per unit of test duration. API 598 is a similar standard commonly used in the USA for production valve testing.

How to Specify Leakage Class

• Start with safety and operational requirements: Does any leakage cause safety hazard, product contamination, or significant loss?

• Consider valve type: Soft-seated ball and butterfly valves readily achieve Class VI; metal-seated valves typically achieve Class IV or V

• Specify test conditions: State whether air, water, or nitrogen; test pressure and duration

• Reference the applicable standard: ANSI/FCI 70-2, IEC 60534-4, or ISO 5208

• Balance cost vs. performance: Each step up in leakage class increases valve cost significantly

Common Leakage Class Assignments by Service

• Standard process control: Class IV (FCI) / Class D (IEC)

• Cryogenic or high-purity gas: Class V or VI

• Block and bleed isolation: Class VI / ISO 5208 Rate A

• Steam isolation: Class IV–V with metal seats

• Toxic or hazardous media: Class VI or better

Conclusion

Leakage class specification is a critical engineering decision. By understanding ANSI/FCI 70-2, IEC 60534-4, and ISO 5208, engineers can align valve selection with actual process requirements — achieving the appropriate shutoff performance without over-specifying and incurring unnecessary cost.