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While selecting a control valve for specific fluid (liquid or gas) medium, to which it is required to control or regulate, we must be ensure the proper sizing and flow capacity of control valve. This is expressed by Control valve sizing calculator | Cv Calculator and to define flow capacity of a control valve, there is two terms used Cv and/ or Kv. A diagram showing hoe differential pressure across valve happens, is shown below-
What is Cv ?
The term Cv is the flow coefficient in imperial units and used by United States of America based control valves.
“The flow coefficient of a control valve (Cv) is a numerical value expressing the number of US Gallons per minute flow of water at 600F the valve will pass with a constant pressure drop of 1 PSI across valve. This rating is usually given for the valve in its wide-open state.”
Cv calculation formula for liquid-
- Specific Gravity of liquid is taken with respect to water.
- Specific Gravity of water is 1.
- Temperature usually is ignored in liquid flow calculations because of too small effect.
Cv calculation formula for Gas-
The formulas for Cv calculation for gas is more complex that for liquid. Suppose P1 is upstream pressure and P2 is downstream pressure. There are two possibilities may occur-
When ∆P < (P1) /2 or (P2) > (P1) /2, the condition is called as non-chocked flow or sub-critical flow and calculated Cv will be –
When ∆P ≥ (P1) /2 or (P2) ≤ (P1) /2, the condition is called as chocked flow or critical flow and calculated Cv will be –
What is Kv ?
The term Kv is the flow coefficient in metric units and used by valve manufacturer in rest of the world.
“Kv is defined as a numeric value of Cubic meter per hour (Nm3/hr) of water at a temperature of 160c that will flow through a valve with 1 bar of pressure drop.”
Kv calculation formula-
It should be obvious that any control valve must be sized large enough (i.e. possess sufficient maximum Cv capacity) to flow the greatest expected flow rate in any given process installation. A valve that is too small for an application will not be able to pass enough process fluid through it when needed. It is done by Control valve sizing calculator | Cv Calculator
Relation between Cv and Kv –
The Relationship between Cv and Kv is given as follows-
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