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What are the control valve Seat Leakage standards?

It is the kind of leakage rate you can expect from a control valve with a metal to metal or sofe seat. Here are the standard of control valve seat leakage for your reference.

The seat leakage classification of control valves complies with standard ANSI FCI 70-2 superseding ANSI B16.104.

Leakage
Class
Designation
Maximum Leakage
Allowable
Test MediumTest PressureTesting Procedures Required for Establishing Rating
CLASS INo test required provided user and supplier so agree
CLASS II0.5% of rated capacityAir or water at 50-125 F (10-52C)45-60 psig or max. operating differential whichever is lowerPressure 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.
CLASS III0.1% of rated capacityAir or water at 50-125 F (10-52C)45-60 psig or max. operating differential whichever is lowerPressure 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.
CLASS IV0.01% of rated capacityAir or water at 50-125 F (10-52C)45-60 psig or max. operating differential whichever is lowerPressure 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.
CLASS V0.0005 ml per minute of water per inch of port diameter per psi differentialWater at 50-125F (10-52C)Max service pressure drop across valve plug, not to exceed ANSI body rating.Pressure applied to valve inlet after filling entire body cavity and connected piping with water and stroking valve plug closed. Use net specified max actuator thrust, but no more, even if available during test. Allow time for leakage flow to stabilize.
CLASS VINot to exceed amounts shown in following table based on port diameter.Air or nitrogen at 50-125 F (10-52C)50 psig or max 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 seat. Allow time for leakage flow to stabilize and use suitable measuring device.

Control Valve Seat Leakage Classifications

NOMINAL PORT DIAMETER (INCHES)NOMINAL PORT DIAMETER (MILLIMETERS)LEAK RATE (ML PER MINUTE)LEAK RATE (BUBBLES / MINUTE*)
3760.96
41021.711
6152427
82036.7545
10254963
1230511.581

*Bubbles per minute as tabulated are a suggested alternative based on a suitable calibrated measuring device, in this case, a 0.25-inch OD X 0.032-inch wall tube submerged in water to a depth of from 1/8 to 1/4 inch. The tube end shall be cut square and smooth with no chamfers or burrs. The tube axis shall be perpendicular to the surface of the water. Other measuring devices may be constructed and the number of bubbles per minute may differ from those shown as long as they correctly indicate the flow in milliliters per minute.

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