Trim & Internals

Leakage Class (FCI 70-2 / IEC 60534-4)

Seat leakage classes quantify how much a valve leaks when fully closed, under a standard test. The classification used industry-wide comes from ANSI/FCI 70-2, harmonized as IEC 60534-4:

Class I — nominal, no test required.
Class II — 0.5 % of rated capacity, dual-ring graphite or PTFE seats, balanced trim.
Class III — 0.1 % of rated capacity.
Class IV — 0.01 % ("metal-to-metal" globe standard).
Class V — 5×10⁻⁴ mL/s per inch of seat diameter per psi drop — requires lapped-in, matched seats; specified for tight shutoff with metal seats.
Class VI — bubble-tight with resilient (soft) seats; leak rates are counted in bubbles per minute.

Selection: don't buy Class VI where Class IV serves — bubble-tight requires soft seats with their temperature/erosion limits, and over-specified tightness costs money twice (purchase and maintenance). Also note leakage is defined at rated shutoff ΔP with the specified actuator; a weak actuator quietly converts a Class IV valve into a leaker in the field.

Related glossary terms

More like this: browse the full control valve glossary, or see how the terms fit together in How a Control Valve Works.

Editorial note. Definitions reflect common industry usage and the conventions of IEC 60534 and ISA-75. Engineering content is educational; always verify against the applicable standard and manufacturer data.