Control Valve Seat Leakage Classification ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Control Valve Seat Leakage Classification

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In many process applications, there is a fundamental requirement that a control valve should be able to completely stop fluid flow when in fully closed position. In practice, this is often not the case as many control valves still allow fluid passage even when in fully closed position. To overcome this problem, control valve installations usually have a block upstream and another downstream to provide tight shut off of fluid if desired.

We have several classifications for control valves, rating them in their ability to
fully shut off process fluid. The ANSI classification of control valve according to seat leakage tolerances is shown in the table below:

ANSI Class
Maximum Allowable Leakage Rate
Remarks
I
 No specification  No specification
II
 0.5% of rated flow capacity of valve,  Cv, air or water as test medium  Used for pressure balanced trims  with metal seals
III
 0.1% of rated flow capacity of valve,  Cv, air or water as test medium
IV
 0.01% of rated flow capacity of valve,  Cv, air or water as test medium  Used for all standard valves with  metal seals
V
 0.0005 ml/min water per inch orifice  size per PSI pressure drop
VI
 Bubble Tight, air or Nitrogen as test  medium  Used for soft-seated valves  where tight shut off is important.

Basic facts about the ANSI leakage standard for control valves:
  • Valve seat leakage rates are defined in the ANSI standard as a fraction of the rated Cv or as a function of the orifice perimeter or seat length for particular ranges of closure pressure from the actuator.
  • Valve testing is carried out with air, water or Nitrogen as appropriate.
  • All tests are carried out with air or water at 3.5bar, except for class V which is tested at the maximum operating pressure