Control Valve · Glossary

What is Seat Leakage Class?

ANSI/FCI 70-2 classesControl Valve

Seat leakage class is the maximum allowed flow through a closed valve, defined by ANSI/FCI 70-2 and IEC 60534-4 as classes I to VI.

Seat Leakage Class Explained

Metal seated control valves are usually class IV; tight shut off metal seats class V; soft seats class VI.

Higher classes need more actuator force and better seats. Specify the class needed by the process, not the tightest possible.

Class II	0.5 % of rated capacity
Class III	0.1 % of rated capacity
Class IV	0.01 % of rated capacity
Class V	5 × 10⁻⁴ ml/min of water per inch of port diameter per psi
Class VI	bubbles per minute by port size (soft seats)

What is Seat Leakage Class?

Seat leakage class is the maximum allowed flow through a closed valve, defined by ANSI/FCI 70-2 and IEC 60534-4 as classes I to VI.

Why does a new class IV valve pass?

Class IV allows small leakage by definition; if you need tight shut off, specify class V or VI or a separate isolation valve.

What causes leakage to increase?

Seat erosion, trapped debris, insufficient actuator thrust or wrong bench set.

Control valve passing causes

A practical guide on instrumentationblog.in with more detail on seat leakage class and how it is applied on real plants.

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