Control valves are comprised of different styles, including globe, ball, and butterfly. The style of control valve is usually determined by the application, past experiences, or plant preference. Valve selection can be a tricky process, but sizing the valve can be even trickier.
So, how do you size a control valve anyway?
When sizing a control valve, the rule of thumb is to size it so that it operates somewhere between 20-80% open at maximum required flow rate and whenever possible, not much less than 20% open at the minimum required flow rate. The idea is to use as much of the valve’s control range as possible while maintaining a reasonable, but not excessive, safety factor.
Cashco Pneumatic Control ValveWhy is control valve sizing so important? An undersized control valve doesn’t have the capacity to pass the required flow. But, more often than not, a control valve is too large for the application. An oversized valve is very sensitive to operating conditions. Even the smallest changes in valve position will cause significant changes in flow. This makes it difficult or even impossible for the valve to exactly adjust to the required flow.
According to an article in Valve Magazine, “selecting a properly sized control valve is essential to achieving the highest degree of process control for the liquid, gas or multi-phase fluid”.
For accurate control valve sizing (and selection), follow these guidelines:
If a set of loop tuning parameters only works at one end of the control range and not the other, the valve’s flow characteristic is most likely the wrong one.
An "equal percentage" control valve should be used when the pressure differential across the valve decreases as flow rate increases, and should be used in control loops where the process gain decreases as flow rate increases. If none of these conditions apply, a "linear" control valve is recommended.
For best control, size the control valve to operate 20% to 80% open at the maximum required flow, and not much less than 20% open at the minimum required flow. **Please note: This is not true for ALL types of control valves, however, and is also dependent on the valves flow vs. open characteristics
Properly sized full ball, segment ball and high performance butterfly valves are usually two sizes smaller than the line.
Properly sized globe valves are usually one size smaller than the line.
Most people consider it poor piping practice to use a control valve that is less than ½ the line size or larger than the line size.