What is control valve?
Process throttling device
What is Cv?
Definition: is the number of U.S. gallons per minute of water at 60 Deg. F that will pass through a given flow restriction with a pressure drop of one psi. In simple sense Cv is the Capacity of the valve.
Example: a control valve that has a maximum flow coefficient, Cv, of 12 has an effective port area in the full open position such that it passes 12 gpm of water with one psi pressure drop
Effect of Specific gravity on valve capacity
In Cv formulas specific gravity is a square root function, so small differences in gravity have a minor effect on valve capacity. If you don't know specific gravity assume 0.9. So even the actual specific gravity is 0.8, the error in Cv calculation will be less than 5%.
What are standards for sizing control valve?
ISA standard S75.01.01
IEC standard 60534-2-1
Pressure drop across the valve
Pressure differential absorbed by the control valve in actual operation will be the difference between the total available head and that required to maintain the desired flow through the valve. It is determined by the the system characteristics rather than by the theoretical assumptions of the engineer.
A valve can only regulate flow by absorbing and giving up pressure drop to the system. As the proportion of the system drop across the valve is reduced, its ability to further increase flow rapidly disappears.
