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
Testing procedure required for establishing rating
Valve Seat Leakage Classification - Table