Control valve seat leakage class
In past articles, we introduce “What caused the leakage of valve” and “The leak rates standards of the industrial valve”, today here we will continue to discuss the valve leakage class and classification.
ANSI FCI 70-2 is an industry standard for control valve seat leakage, specified six leakage classes (Class I, II, III, IV, V, VI) for control valves and defines the test procedure, and superseding ANSI B16.104. The most commonly used are CLASS I, CLASS IV and CLASS Vl. Metal-elastic seal or metal seal should be selected in engineering design according to the characteristics of the medium and the opening frequency of the valve. Metal seated valve seal grades should be is stipulated in the order contract, the rates I, Ⅱ, Ⅲ is used less due to request a lower level, generally choose Ⅳat least and V or Ⅵ for higher requirements.
Classifications of control valve seat (ANSI/FCI 70-2 and IEC 60534-4)
| Leakage class | Maximum leakage allowable | Test medium | Test pressure | Test rating procedures | Valve type |
| Class I | / | / | / | No test required | Metal or resilient seated valves |
| Class II | 0.5% of rated capacity | Air or water at 50-125 F (10-52C) | 3.5 bar, operating differential whichever is lower | Lower of 45 to 60 psig or maximum operating differential | Commercial double-seat control valves or balanced single-seat control valves with a piston ring seal and metal-to-metal seats. |
| Class III | 0.1% of rated capacity | As above | As above | As above | Same as class II, but a higher degree of seat and seal tightness. |
| Class IV | 0.01% of rated capacity | As above | As above | As above | 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. |
| Class V | 0.0005 ml per minute of water per inch of port diameter per psi differential | Water at 50-125F (10-52C) | Max service pressure drop across valve plug, not to exceed ANSI body rating. | Maximum service pressure across valve plug not to exceed ANSI rating | Metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness |
| Class VI | Not to exceed amounts shown in following table based on port diameter. | Air or nitrogen at 50-125 F (10-52C) | 3.5 bar(50 psig) or max rated differential pressure across valve plug whichever is lower. | Maximum service pressure across valve plug not to exceed ANSI rating | Resilient seating control valves either unbalanced or balanced single-seat with “O” rings or similar gapless seals. |
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