Understanding Seat Leakage Standards
Control valves cannot achieve absolute bubble-tight shutoff without soft resilient seating materials (like Teflon or EPDM). Metal-to-metal seating always allows a microscopic amount of fluid to pass under pressure.
The **ANSI/FCI 70-2** standard (harmonized with **IEC 60534-4**) establishes six specific seat leakage classes, matching control valve designs to acceptable leak limits:
Summary of Leakage Classes:
- Class I: No standard test is specified. It is a modification of Class II, III, or IV by agreement between buyer and manufacturer.
- Class II: Allows up to 0.5% of rated valve capacity. Uses air or water at 45–60 psig test pressure.
- Class III: Allows up to 0.1% of rated valve capacity. Uses air or water at 45–60 psig test pressure.
- Class IV: Allows up to 0.01% of rated valve capacity. This is the industry-standard benchmark for general metal-to-metal seated control valves.
- Class V: Allows up to \(5 \times 10^{-4}\) ml per minute of water per inch of port diameter per psi differential pressure. Used for critical high-pressure steam or chemical isolation.
- Class VI: Represents "bubble-tight" shutoff. Restricts leakage to a specific number of bubbles per minute using air or nitrogen gas at 50 psig or max service differential. Soft-seated valves (PTFE, Viton) are expected to meet Class VI.