Seat leakage Classification of Control Valve

• In many process and industrial applications, the fundamental requirement is that a control valve must be able to control the flow rate or stop the complete flow of fluid in the fully closed position.
• In practice, most control valves still allow for the flow of fluid through the passage even when the valve is in the fully closed position.
• To overcome this problem of fluid flow in a closed condition.
• A control valve installation must have a block upstream and downstream for providing a tight shut-off of fluid flow.
• There are several classifications of control valves depending upon the rating of seat leakage or ability to tight shut off the flow of process fluid

ANSI: Abbreviation for American National Standards Institute.

FCI: Abbreviation for Fluid Controls Institute.

Both ANSI and FCI provide standards and reference materials to help purchasers and users in understanding and use fluid control and conditioning equipment.

IEC 60534-4:  It specifies the requirements for accurate inspecting and testing of control valves. It is applicable for control valves that don’t exceed the pressure rating of class 2500, and for actuators that apply only for pneumatic actuators.

Facts about seat leakage of control valve standard:

1. Seat leakage rates of the control valve are defined in the ANSI or FCI standard as
   • A fraction of the rated valve coefficient,
   • As a function of the total perimeter of the orifice plate.
   • A seat length of a valve for a particular range of closure pressure from the actuator.
2. Control valve testing is carried out by using Air, Water, or Nitrogen as a test media.
3. Control valve tests are carried out using suitable test media like water, air, or nitrogen at a rated differential pressure of 3.5bar or 50 PSI, except for class V, because these tests are carried out at the maximum operating pressure they may be above 50 PSI.

What is Seat Leakage in a control valve?

It is defined as the quantity of fluid flowing through a control valve in the fully closed position at 100% of opening, by applying maximum available seat load with specified differential pressure and temperature.

Seat leakage classification of control valves as per ANSI/FCI 70-2 and IEC 60534-4:

The ANSI classification of a control valve according to seat leakage class is shown below:

ANSI/FCI Class  I:

• Designated Leakage Class:  Class I
• Maximum Allowable Leakage: ——
• Test Media: ——-
• Test Pressures:  ——–
• Testing Procedure Required Establishing Valve Rating: No test procedures are provided.
• Remarks : ———

ANSI/FCI Class II:

• Designated Leakage Class:  Class II
• Maximum Allowable Leakage: 0.5% of the rated flow capacity of the valve, Cv, air, or water as the test medium.
• Test Media: Air or water at 10-52°C.
• Test Pressures: Maximum rated differential pressure of 3.5 bar or 50 PSIG, or Pressure across valve plug, whichever is lower.
• Testing Procedure Required Establishing Valve Rating:  Pressure is applied to the inlet side of the valve, with an outlet port vented to the atmosphere or connected to a low head loss measuring device, and full normal and closing thrust provided by the actuator.
• Remarks: Used for pressure-balanced trims with metal seals.

ANSI/FCI Class III:

• Designated Leakage Class:   Class III.
• Maximum Allowable Leakage: 0.1% of the rated flow capacity of the valve, Cv, air, or water as the test medium.
• Test Media: Air or water at 10- 52°C
• Test Pressures: Maximum rated differential pressure of 3.5 bar or 50 PSIG, or Pressure across valve plug, whichever is lower.
• Testing Procedure Required Establishing Valve Rating:  Pressure is applied to the inlet side of the valve, with an outlet port vented to the atmosphere or connected to a low head loss measuring device, and full normal and closing thrust provided by the actuator.
• Remarks: —

ANSI/FCI Class IV:

• Designated Leakage Class:  Class IV
• Maximum Allowable Leakage: 0.01% of the rated flow capacity of the valve, Cv, air, or water as the test medium.
• Test Media: Air or water at 10- 52°C
• Test Pressures: Maximum rated differential pressure of 3.5 bar or 50 PSIG, or Pressure across valve plug, whichever is lower.
• Testing Procedure Required Establishing Valve Rating: Pressure is applied to the inlet side of the valve, with an outlet port vented to the atmosphere or connected to a low head loss measuring device, and full normal and closing thrust provided by the actuator.
• Remarks: Used for all standard valves with metal seals

ANSI/FCI Class V:

• Designated Leakage Class:  Class V
• Maximum Allowable Leakage: 0.0005 ml/min water per inch of orifice size per PSI of differential pressure drop. (5 X 10⁻¹² m³/s water per mm of orifice diameter per bar differential).
• Test Media: Water at 10-52°C.
• Test Pressures:  Maximum service pressure drop across valve plug must not exceed or reduce as per ANSI body rating by agreement.
• Testing Procedure Required Establishing Valve Rating: Pressure is applied to the inlet side of the valve after the entire body cavity is filled, connected to a pipeline with water as a medium, and the stroking valve plug is kept closed. Use maximum required actuator thrust, even if available during the test. Allow time to balance for leakage flow.
• Remarks:  ——-

ANSI/FCI Class VI:

• Designated Leakage Class:  Class VI
• Maximum Allowable Leakage: Bubble Tight, air, or Nitrogen as a test medium, and must not cross or exceed limit based on port diameter.
• Test Media: Air or Nitrogen at 10-52°C
• Test Pressures: Maximum rated differential pressure of 3.5 bar or 50 PSIg, or Pressure across valve plug, whichever is lower.
• Testing Procedure Required Establishing Valve Rating: Pressure is applied to the inlet side of the valve. A valve actuator must be adjusted to specified operating conditions with full closing thrust and normal closing thrust applied to the seat and valve plug. Allow some time to stabilize for leakage flow to use a suitable measuring device.
• Remarks: Used for soft-seated valves where tight shutoff is required.