Control valves are tested to the ANSI/FCI 70-2 leakage classes: Class IV allows 0.01 percent of rated capacity with metal seats, Class V allows a tiny volume of water per inch of port diameter per psi of differential, and Class VI sets a count of air bubbles per minute based on port size and is typical of soft seats. Specifying a tighter class than the process needs adds seat load, actuator size and cost.
Datasheet calls for Class VI shutoff on the fuel gas valve, the vendor quoted Class IV, send it back.
Also heard as
- leakage class
- seat leakage class
Related terms
Seat
The surface inside a valve that the closure member presses against to stop flow.
Bubble-tight
Shut off with no visible seat leakage when tested with air or gas under water.
Control valve
A power-operated valve that modulates flow on a controller signal to hold pressure, level, flow or temperature at setpoint.
Passing
Leaking through the seat while closed, so fluid still gets past a valve that is supposed to be shut.