FCI 70-2 Valve Seat Leakage Class
General / GeneralDefinition
FCI 70-2 (also published as IEC 60534-4) defines six classes of valve seat leakage, from Class I (no test required) through Class VI (virtually zero leakage). These classes provide a standardized, measurable way to specify how much leakage is acceptable when a valve is in the closed position. Without a specified leakage class, 'tight shutoff' means nothing — it is subjective and unenforceable. Specifying the correct leakage class is one of the most important technical decisions in valve procurement.
Working Principle
Each leakage class defines a maximum allowable leakage rate at a specified test pressure differential (typically the valve's rated pressure or a standard test pressure). The test procedure is defined in the standard: the valve is closed using the manufacturer's specified actuator force, pressure is applied to the inlet, and leakage past the seat is collected and measured. Class I requires no test. Class II allows up to 0.5% of rated Cv. Class III allows 0.1% of rated Cv. Class IV allows 0.01% of rated Cv. Class V allows 0.0005 mL per inch of seat diameter per psi differential. Class VI allows only a specified number of bubbles per minute in a bubble test — effectively zero visible leakage.
Types & Variants
| Leakage Class | Maximum Allowable Leakage | Test Method | Typical Valve Type |
|---|---|---|---|
| Class I | No test required | N/A | Non-critical isolation |
| Class II | 0.5% of rated Cv | Water or air test | Rough throttling, non-critical |
| Class III | 0.1% of rated Cv | Water or air test | Moderate tightness |
| Class IV | 0.01% of rated Cv | Water or air test | Metal-seated control valves |
| Class V | 0.0005 mL/in/psi | Water test only | High-integrity metal seats |
| Class VI | Specified bubbles/min | Air/nitrogen bubble test | Soft seats, zero-leakage requirement |
Materials
The achievable leakage class depends primarily on the seat material and design. Soft seats (PTFE, RPTFE, PEEK, elastomers) can achieve Class VI. Metal seats typically achieve Class IV or V; Class VI with metal seats requires premium lapping and is available from limited manufacturers at higher cost. Fire-tested valves specify Class IV post-fire leakage (soft seat destroyed, metal backup seat remains).
Applicable Standards
FCI 70-2-2021 (Fluid Controls Institute) is the American standard. IEC 60534-4 is the international equivalent with identical classes and test methods. API 598 defines valve inspection and testing including seat leak rates — API 598 allowable leakage for metal-seated gate and globe valves corresponds roughly to FCI Class IV. API 6D specifies zero leakage for pipeline ball valves, which corresponds to FCI Class VI. MSS SP-61 defines seat leakage for general-purpose steel valves.
Applications
Control valve specification: Class IV is standard for metal-seated control valves; Class V for critical services (minimum flow recirculation, high-pressure letdown); Class VI for soft-seated control valves on toxic or flammable service. Isolation valve specification: Class VI (zero leakage) for positive isolation before maintenance entry on hazardous lines. Class IV for non-critical water and utility isolation. Pipeline valves: Class VI per API 6D for all pipeline isolation. Safety relief valve inlet block valves: Class VI to ensure no leakage reduces the pressure relief capacity. Any application where the consequence of seat leakage (environmental, safety, product quality) justifies specifying a tighter class.
Advantages
- Objective, measurable criteria replace subjective 'tight shutoff' language
- Standardized test methods ensure consistent evaluation across manufacturers
- Classes cover the full range from rough isolation to zero-leakage
- International recognition (FCI 70-2 = IEC 60534-4)
- Allows fair comparison between different valve types and manufacturers
Disadvantages
- Higher classes require more expensive seats and manufacturing precisio