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
