Control Valve Seat Leakage Calculation Class 2,3,4 and 5

Control Valve Leakage –

This is basically the fluid which passes through the valve when the valve is fully closed. This value however should not be considered as the valve Cv at NIL Opening.

So this leakage shall depend on the contact of valve plug & seat with the seating force applied for holding the plug over the seat.

ANSI/FCI 70-2-2005 Valve Leakage Classification

There are six seat leakage classifications defined by ANSI/FCI 70-2 (supersedes ANSI B16.104). The six valve leakage classifications are as follows:

Class I   

  •  Identical to Class II, III, and IV in construction and design intent, but no actual shop test is made. Cass I is also known as dust tight and can refer to metal or resilient seated valves.

Class II      

  • Intended for double-port or balanced single-port valves with a metal piston ring seal and metal-to-metal seats. Air or water at 45 to 60 psig is the test fluid. Allowable leakage is 0.5% of the rated full open capacity.

Class III      

  • Intended for the same type of valves as in Class II. Allowable leakage is limited to 0.1% of rated valve capacity.

Class IV

  • Intended for single-port and balanced single-port valves with extra-tight piston seals and metal-to-metal seats. Leakage rate is limited to 0.01% of rated valve capacity (known as metal to metal). Test fluid is air or water, pressure 45 and 50 psig or the operating pressure if lower.

Class V        

  • Intended for the same types of valves as Class IV. The test fluid is water at 100 psig or operating pressure. Leakage allowed is limited to 5 x 10-4 ml (= .0005ml) per minute per inch of orifice diameter per psi differential.

Class VI. (EN60534-4)

Intended for resilient-seating valves. The test fluid is air or nitrogen. Pressure is the lesser of 50 psig or operating pressure. The leakage limit depends on valve size and range from 0.15 to 11.5 ml per minute for valve sizes 1 through to 12 inches (known as soft seat classification). Class VI (EN60534-4) is also frequently used as a leakage bench mark for metal seated control and isolation valves where a tight degree of shut off is required.



Type – A , Test Procedure seat Leakage for Class II , III and IV :- 

  1. Test medium shall be clean air or water at 10-51°C (50-125°F).
  2. Pressure of test medium shall be 3-4 bar (45-60 psig) or within +/-5 percent of the maximum operating differential pressure, whichever is less.
  3. Leakage flow and pressure data shall be accurate to +/-10 percent of reading.
  4. The test fluid shall be applied to the normal or specified valve body inlet. The valve body outlet may be open to atmosphere or connected to a low headloss measuring device.
  5. The actuator shall be adjusted to meet the operating conditions specified. The full normal closing thrust as applied by air pressure, a spring, or other means shall then be applied. No allowance or adjustment shall be made to compensate for any increase in seat load obtained when the test differential is less than the maximum valve operating differential pressure.
  6. On valve body assemblies made for stock, tested without the actuator, a test fixture should be utilized which applies a net seat load not exceeding the manufacturer’s normal expected load under maximum service conditions.
  7. On water test, care shall be taken to eliminate air pockets in the valve body and piping.
  8. The leakage rate thus obtained can then be compared to the calculated values for Classes II, III and IV. See Table 1.

Type – B , Test Procedure seat Leakage for Class V :-

  1. Test fluid shall be clean water at 10-52 °C ( 50-125°F).
  2. The water test differential pressure shall be within +/-5 percent of the maximum service pressure drop across the valve plug, not exceeding the maximum operating pressure at room temperature as determined by ANSI BI6.1, BI6.5, or BI6.34, or some lesser pressure by individual agreement. Pressure measurement accuracy IS to be 111 accordances with paragraph 3 of the Type A test procedure.
  3. The fluid shall be applied to the normal specified inlet of the valve body. The valve plug shall be opened and the valve body assembly filled completely with water, including outlet portion and any downstream connecting piping, and then stroked closed.
  4. The water test differential pressure as specified in point no. 2 is then applied with the actuator adjusted to meet the operating conditions specified. The net actuator thrust shall be the specified maximum. Net actuator thrust above the specified maximum is not to be used.
  5. When leakage flow is stabilized, the quantity should be observed over a period of time sufficient to obtain the accuracy under paragraph 3 of the Type A test procedure.
  6. The leakage rate thus obtained shall not be greater than the value calculated from the definition of maximum seat leakage for Class V as shown in Table I. The nominal seat diameter is understood to be the diameter at the point of seating contact to the nearest 2mm (1116 inch).

Type – C , Test Procedure seat Leakage for Class VI :-

  • Test medium shall be air or nitrogen gas at 10-52°C (50-125°F).
  • Pressure of the test medium shall be the maximum rated differential pressure across the valve plug or 3.5 bar (50 psig) whichever is the least.
  • The test fluid shall be applied to the normal or specified valve body inlet, and the outlet connected to a suitable measuring device.
  • The actuator shall be adjusted to meet the operating conditions specified. The full normal closing thrust as applied by air pressure, a spring, or other means shall then be applied. No allowance or adjustment shall be made to compensate for any increase in seat load obtained when the test differential is less than the maximum valve operating differential pressure.
  • On valve body assemblies made for stock, tested without the actuator, a test fixture should be utilized which applies a net seat load not exceeding the manufacturer’s normal expected load under maximum service conditions.
  • With the control valve adjusted to meet the operating conditions specified and with sufficient time allowance for stabilizing flow, the leak rate shall not exceed the values as below table.








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