Valves are sized according to their Cv value. For liquid service, the equation for Cv is:
The required flow and pressure drop information used to size a valve is based on the process operations and equipment.
Once the Cv value is known, the rated* Cv can be determined from the manufacturer’s data books.
A general guideline is that valves should be sized so that maximum flow is obtained at about 90% valve open.
Valves should be able to provide normal flow condition at around 60% to 70% of the travel. Valve should provide minimum flow when about 10% open.
The control valve need not be of the same size as the pipe. It is better to make an error in under sizing a control valve than to oversize it.