Choosing the correct valve size will probably have the greatest impact on the performance of the control valve in the process environment. There are two major considerations when sizing control valves:

1. Size of the connecting pipe.
2. Cv to meet the flow requirements.

Unfortunately, these two requirements rarely coincide. In many cases, it is desirable to choose the valve body size to mate with the piping, and install restricted trim to meet the Cv. A classic example is an application for a process that is designed with large enough capacity to meet future growth expectations.

The basic flow equation is solved for Cv by using the Robertshaw Flo-rule or Valve-sizing program. Refer to the engineering section of the sales catalog for a detailed explanation of Cv calculations.

Generally speaking, the valve is chosen to have a Cv ranging from 1.1 to 1.3 times the calculated Cv for maximum flow. Refer to the Selection Guide Matrix for Cv for each valve type and size. After choosing a valve based on maximum conditions, refer to the characteristic curve for that valve and determine what the position will be at normal flowing conditions. It may then become necessary to compromise the selection to more closely fit the normal flowing conditions.