What are Control Valves seat leakage classification standard?

In the spection and testing of control valve, we usually as per the standard of ANSI FCI 70-2. In ANSI/FCI 70-2 standard, the seat leakage class of control valves is classificated to six class based on the size of the seat leakage, calss I, II, III, IV, V, and VI , as shown in the table1 below:

Table 1: Control Valves seat leakage classification

Description of Class IV in section 4.2.4 of ANSI/FCI 70-2

Class IV. This class establishes the maximum permissible leakage generally associated with commercial unbalanced single-seat control valves and balanced single-seat control valves with extra tight piston rings or other sealing means and metal-to-metal seats. Use test procedure Type A.

Description of Class V in section 4.2.5 of ANSI/FCI 70-2

Class V. This class establishes the maximum permissible leakage generally associated with metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness. Use test procedure Type B using water at the maximum operating differential pressure or Type B1 by using air at the specified conditions.

Description of Class VI in section 4.2.6 of ANSI/FCI 70-2

Class VI. This class establishes the maximum permissible seat leakage generally associated with resilient seating control valves either unbalanced or balanced single-seat with O-rings or similar gapless seals. Use test procedure Type C.

Table 2:Class VI maximum leakage rate of control valve seat

Table 2:Note:*The number of bubbles per minute listed in the table was measured using a properly calibrated measuring device and is for reference only. Immerse a pipe with an outer diameter of 6mm (0.25in.) and a wall thickness of 1mm (0.032in.) into water at a depth of 3-6mm (0.125-0.25in.), cut the pipe end flat and smooth (without chamfers or burrs), and keep the pipe axis perpendicular to the horizontal plane. Other structures of devices can also be used, and the number of bubbles per minute may differ from the numbers listed in the table, as long as the flow rate can be displayed correctly (in ml/min).
**If the valve seat diameter differs from the values listed in the table by more than 2mm (0.08in.), the leakage rate will be obtained using interpolation, assuming that the leakage rate varies with the square of the valve seat diameter.