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

Valve seat leakage classification provides a standardized framework for specifying and verifying the maximum allowable leakage rate through a closed valve. Without standardized leakage classes, it would be impossible to compare valve offerings from different manufacturers or to verify that a purchased valve meets the shutoff performance required by the application. ANSI/FCI 70-2 (American standard) and IEC 60534-4 (international standard) define the leakage test procedures and acceptance criteria for control valve seat leakage. Understanding these standards allows engineers to specify the appropriate leakage class for each application and to interpret seat leakage test certificates provided by valve suppliers.

ANSI/FCI 70-2 Leakage Classes

ANSI/FCI 70-2 defines six leakage classes designated Class I through Class VI, with increasing stringency. Class I applies when no seat leakage test is required. Class II allows a maximum leakage of 0.5 percent of the valve's rated Cv capacity, tested with air or water at 45 to 60 PSI. Class III allows 0.1 percent of rated Cv. Class IV (the most commonly specified class) allows 0.01 percent of rated Cv and is the standard for metal-seated globe control valves in most process applications. Class V applies to metal-seated valves in critical applications and requires leakage not exceeding 0.0005 milliliter per minute per inch of seat diameter per PSI of differential pressure. Class VI (bubble-tight) requires that soft-seated valves allow no more than a defined number of bubbles per minute based on valve size.

• Class I: no seat leakage test required

• Class II: 0.5% of rated Cv maximum leakage

• Class III: 0.1% of rated Cv maximum leakage

• Class IV: 0.01% of rated Cv maximum leakage (standard for metal seat globe valves)

• Class V: 0.0005 ml/min/inch/PSI (critical applications, metal seat)

• Class VI: bubble-tight (soft seat valves, specific bubble counts by valve size)

IEC 60534-4 Leakage Classification

IEC 60534-4 defines four leakage classes for control valves. Class A is equivalent to ANSI/FCI Class II (0.5% Cv). Class B corresponds to ANSI Class III (0.1% Cv). Class C corresponds to ANSI Class IV (0.01% Cv). Class D corresponds to ANSI Class VI for soft-seated valves (bubble-tight per specific bubble count criteria). The IEC standard also specifies test procedures and acceptance criteria for each class, using water or air as the test fluid at defined test pressures. Most international valve specifications now reference IEC 60534-4, while North American specifications more commonly reference ANSI/FCI 70-2, but the practical performance levels are equivalent between the two systems.

Selecting the Appropriate Leakage Class

The leakage class specified for a control valve should be matched to the actual leakage performance required by the application. Over-specifying a high leakage class (such as Class V or VI) when it is not required increases valve cost, may require softer seat materials that are less durable, and imposes tighter dimensional tolerances on seat and plug surfaces that increase manufacturing cost. Class IV is appropriate for the majority of process control valve applications where some degree of leakage through a closed valve is acceptable and the downstream process can tolerate a small bypass flow. Class V or VI should be specified only when no leakage is acceptable, such as for tight shutoff of hazardous materials or where leakage would cause safety or environmental problems.