ISO 5208 Seat Leakage Classes

ISO 5208 defines six leakage rate classes (A through F) for industrial valve testing. The permissible leak rate depends on valve size (DN) and test pressure differential (ΔP).

Class	Liquid Leak Rate	Gas Leak Rate	Description
A	Zero	Zero (no visible)	Zero leakage — no visible leak during test period
B	Zero at low P	Defined by spec	No visible leak at reduced test pressure
C	0.0006 × DN² × √ΔP	0.06 × DN² × √ΔP	Low leakage — stringent process valves
D	0.0018 × DN² × √ΔP	0.18 × DN² × √ΔP	Standard leakage — most common industrial class
E	0.018 × DN² × √ΔP	1.8 × DN² × √ΔP	Higher leakage — less critical applications
F	Special	Special	Manufacturer-defined or customer-specified

Formulas

For liquid testing (water), the permissible leak rate in ml/min:

For gas testing (air/N₂), the permissible leak rate in bubbles/min (or mm³/s):

Practical Example