Seat leakage classifications

By Dave Harrold, CONTROL ENGINEERING September 1, 1999
Seat Leakage Classifications
(In accordance with ANSI B16.104-1976)
Leakage class designation Maximum leakage allowable Test medium Test pressure Testing procedures required for establishing rating
I No test required provided user and supplier agree.
II 0.5% of rated capacity Air or water at 50 to 125 45 to 60 psi (3.1 to 4.1 bar) or maximum operating differential whichever is lower Pressure is applied to valve inlet, with outlet open to atmosphere or connected to a low head-loss measuring device, full normal closing thrust provided by actuator.
III 0.1% of rated capacity As above As above As above
IV 0.01% of rated capacity As above As above As above
V 0.0005 liters per minute of water per inch (mm) of port diameter per psi (bar) differential. Water at 50 to 125 Maximum service pressure drop across valve plug, not to exceed ANSI body rating. 100 psi (6.9 bar) pressure drop minimum. Pressure applied to valve inlet after filling entire body cavity and connected piping with water and stroking valve plug closed. Use net specified maximum actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilize.
VI Not to exceed amounts shown in Class VI Seat Leakage Allowance table based on port (orifice) diameter. Air or nitrogen at 50 to 125 50 psig (3.4 bar) or maximum rated differential pressure across valve plug, whichever is lower. Actuator should be adjusted to operating conditions specified with full normal closing thrust applied to valve plug set. Allow time for leakage flow to stabilize and use suitable measuring devices.
Source: Control Engineering with data from Fisher Controls.

Class VI Seat Leakage Allowance
Nominal port diameter Leak Rate
Inches Millimeters ml per minute Bubbles per minute (1)
1.5 38 0.30 2
2 51 0.45 3
2.5 64 0.60 4
3 76 0.90 6
4 102 1.70 11
6 152 4.00 27
8 203 6.75 45
1. Bubbles per minute as tabulated are an easily measured suggested alternative based on a suitable calibrated measuring device such as aary from those shown, as long as they correctly indicate the flow in ml per minute.
Source: Control Engineering with data from Fisher Controls