Valve Leakage Classification and Seat Tightness Standards

Valve leakage classification defines the acceptable leak rate through a closed valve under specified test conditions. Industrial valves are tested per ANSI/FCI 70-2 for control valves or API 598 for isolation valves, and assigned a leakage class that determines their suitability for specific applications. Understanding leakage classifications helps engineers specify valves correctly for tight shutoff or standard isolation service.

ANSI/FCI 70-2 Leakage Classes

ANSI/FCI 70-2 defines six leakage classes for control valves. Class I allows visible leakage during testing and is rarely specified. Class II allows 0.5% of full-open valve capacity. Class III allows 0.1% of rated capacity. Class IV (the most common standard) allows 0.01% of capacity with metal seats. Class V (special class) allows 5 x 10^-4 ml/min per inch of port diameter per psi differential, tested with water at full shutoff pressure. Class VI (soft-seat) specifies leakage in bubble counts per minute based on port size, typically requiring elastomeric or PTFE seating surfaces.

• Class I: no test required, visible leakage acceptable

• Class II: 0.5% of rated Cv, metal seats

• Class III: 0.1% of rated Cv, metal seats

• Class IV: 0.01% of rated Cv, standard metal seat specification

• Class V: 5e-4 ml/min per inch per psi, water test at full pressure

• Class VI: bubble-tight, elastomeric or PTFE seats, per port size bubble count

API 598 Leakage Testing for Isolation Valves

API 598 covers shell testing, seat testing, and back-seat testing for isolation valves including gate, globe, plug, ball, and butterfly valves. Shell testing verifies body pressure integrity at 1.5x design pressure. Seat testing is performed at 1.1x rated pressure. Allowed leakage rates depend on valve size and class: metal-seated valves 6 inches and below have allowable leakage of 0 ml for liquid and 0 bubbles for gas; larger valves allow minor measurable leakage. Soft-seated valves must achieve zero leakage in all sizes.

Bubble-Tight vs. Standard Shutoff

Bubble-tight shutoff (zero leakage) is required for applications involving toxic fluids, hydrogen service, high-pressure gas isolation, or safety-critical isolation functions. Standard metal-seated shutoff (Class IV) is acceptable for most process control applications where minor leakage does not compromise safety or process efficiency. Specifying bubble-tight when not required increases valve cost, complexity, and maintenance frequency because elastomeric seats are more susceptible to damage from particulates and high temperatures.