Valve Seat Leakage Classes: ANSI/FCI 70-2 and IEC 60534-4 Explained

Valve seat leakage classification provides a standardized way to specify and verify the maximum allowable leakage through a closed valve. Without a clear leakage standard, engineers and vendors would have no common language for discussing seal quality, and testing results from different manufacturers would be incomparable. The two most widely used leakage classification systems are ANSI/FCI 70-2, developed by the Fluid Controls Institute for control valves, and IEC 60534-4, the international equivalent used for industrial process control valves.

ANSI/FCI 70-2 Leakage Classes

ANSI/FCI 70-2 defines six leakage classes for control valve seats, designated Class I through Class VI. Each class specifies a maximum allowable leakage rate expressed as a percentage of rated valve capacity or as an absolute flow rate at defined test conditions. Class I has no specified test requirement and is acceptable for applications where some leakage is tolerable. Classes II through V specify progressively tighter leakage limits tested with air or water at defined pressure and temperature conditions. Class VI, the tightest classification, requires bubble-tight closure with only a defined number of air bubbles permitted per minute.

• Class I: no specified leakage test, suitable for non-critical isolation

• Class II: 0.5% of rated Cv capacity with 50 psi air or water test

• Class III: 0.1% of rated Cv capacity

• Class IV: 0.01% of rated Cv capacity, standard for most control valve applications

• Class V: 0.0005 ml/min per inch of port diameter per psi differential, metal-seated

• Class VI: bubble-tight, specified number of bubbles per minute, resilient (soft) seats only

IEC 60534-4 Leakage Classes

IEC 60534-4 defines leakage classes designated A through F, where F represents the tightest seal. The classification system is numerically different from ANSI/FCI 70-2 but covers a similar range of performance levels. IEC 60534-4 Class A corresponds roughly to ANSI Class II, while IEC Class F corresponds to ANSI Class VI. For international projects and European installations, IEC 60534-4 is the preferred reference standard. Many modern valve data sheets list both ANSI and IEC classifications to facilitate global procurement.

Specifying the Correct Leakage Class

Selecting an appropriate leakage class requires balancing performance requirements against cost. Tighter leakage classes require more precise seating surfaces, higher-quality seat materials, and more careful manufacturing, all of which increase cost. For most continuous modulating control valve applications, ANSI Class IV is adequate and represents a good balance of cost and performance. Safety shutdown valves and isolation valves in services where leakage past the seat would allow hazardous fluid to enter a protected area require Class V or Class VI. Specifying a class tighter than necessary adds cost without benefit and may reduce service life by requiring softer seating materials that wear more rapidly.

Testing Methods

Seat leakage testing is performed at the factory before shipment and may be repeated in the field after maintenance. The test involves applying a specified pressure to the inlet side of the closed valve while the outlet is open to atmosphere or to a collection vessel. The leakage through the seat is measured by collecting and measuring the outflow liquid, or by counting bubbles from a submerged outlet tube for Class VI air testing. Water is the standard test fluid for most classes, while Class VI testing uses air or nitrogen at 50 psi (3.4 bar) gauge pressure.