International standards for control valve leakage categorization include ANSI (American National Standards Institute)/FCI 70-2(Fluid Controls Institute) & IEC 60534-4.
These standards classify leakage levels depending on allowable leakage rates for the different valve types & sealing mechanisms.
Leakage Classifications
1). Class I – No Test Required
No specified leakage restrictions. The valve is not tested for the seat leakage.
2). Class II – Low Leakage (0.5% of the Valve Capacity)
Allowable leakage - 0.5% of the valve’s rated capacity.
3). Class III - Medium Leakage (0.1% of the Valve Capacity)
Allowable leakage - 0.1% of the valve’s rated capacity.
4). Class IV - Tight Shutoff (0.01% of the Valve Capacity)
Allowable leakage - 0.01% of the valve’s rated capacity. Metal-seated control valves are often used.
5). Class V - Extra Tight Shutoff (0.0005 ml/min per inch of the seat diameter at a rated pressure drop)
Designed for metal-seated valves with a very low leakage. Utilized in high-pressure applications.
6). Class VI – Bubble Tight (Soft-Seated Valves)
Maximum permitted leakage - bubbles per minute utilizing air (or) nitrogen at 50 psi differential pressure. Designed for soft-seated control valves needing minimal apparent leakage.
Selection Considerations
- Metal-seated valves primarily meet Class IV (or) V.
- Soft-seated valves often satisfy the Class VI (bubble tight).
- Essential applications (such as nuclear or toxic fluids): Require greater tightness levels, such as Class V (or) VI.