There are 6 leakage classes.
Most common are class 4 and Class 6.
Class 4 is intended for single ported,metal to metal seats.Leakage rate is limited to 0.01 of rated valve flow capacity.
Class 6 is intended for resilient seated valves. Leakage rate is determined by valve size on a variable scale. Typical soft seat materials are teflon and elastomer rubber.
Control Valve Leakage Class Classification Following information are given in the table. Leakage Class Designation Maximum Leakage allowable Test Medium Test Pressure Testing procedure required for establishing rating
Valve Seat Leakage Classification - Table