Every control valve specification ends with the same line: "shut-off class per ANSI/FCI 70-2". Yet leakage class is frequently defaulted to the highest number on the sheet without considering what each class actually measures, what seat material it implies, and what it costs. Understanding the six classes prevents both overbuying and under-specifying.
ANSI/FCI 70-2 (harmonized as IEC 60534-4) defines standard leakage classifications for control valves — the allowable leakage through the valve in the closed position, measured at a stated test pressure with air or water. It is a seat-leakage standard, not an external-leakage standard, and it applies to control valves rather than on-off isolation valves.
Each step up the ladder narrows manufacturing tolerances and often changes seat materials. Jumping from Class IV to Class V on a metal-seated valve typically means lapped-in seats, tighter stem guidance and more exhaustive factory testing — measurable cost and lead-time impact. Class VI means elastomer or PTFE seats, which reintroduce the temperature and chemical limits that soft materials carry. On steam or hot hydrocarbon service, Class VI may simply be unachievable, and specifying it only surfaces at the factory acceptance test.
Start from the process consequence of leakage, not from the class table. If a bypass is always closed and leakage merely wastes pump head, Class IV metal seats are usually sufficient. If the valve isolates a catalyst bed from a relief header, the tighter class is justified. And when Class VI is required, confirm the soft-seat material against line temperature and duty cycle before the PO is issued — that is where Class VI schedules slip.