The FCI 70-2 (Fluid controls institute Inc. intially ASME B16.104) Leakage classes are specified for an ESD ball valve and a Pipeline MOV Gate valve having composite seats. The composite seats are a metal + soft sealing combination. Class VI and V are specified for the ESD and MOV valves respectively. These leakage classes actually apply to control valves. Class VI is further valve nominal size depended and is stated upto 12". None of the classes are TSO.

The design standard API 6D / ISO 14313 also has leakage acceptance criteria referred to ISO 5208. "Leakage for soft-seated valves and lubricated plug valves shall not exceed ISO 5208 Rate A (no visible leakage)."

What may be the reason of specifing the test leakages by the FCI Control valve classes? API 598 resilient seated valves by default require 0 leakage rates. If testing is required to API 598 or API 6D, is it again required to state a test leakage rate?

FCI applies to control valves.  Control valves leak and FCI classifies the rate of the leakage.

Many instrumentation and control systems engineers who specify control valves also specify shutdown valves without ever reading piping standards.  API 598 and 6D are more applicable to isolation valves.

It could be difficult to train some of those control systems engineers who may think that they are superior to piping engineers.

Piping engineers often specify leakage testing per API 598.  Some may not realize the damage done by dirty hydrostatic fluid during testing thus don't know that those zero leakage valves leak lots when placed in service.

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