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.
Why 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.
When sizing and selecting a control valve for your application, always consult a qualified valve engineer capable of analyzing the application to ensure the right valve is selected and sized appropriately.
And, for more information about control valves, sizing and selection, download the guide below!
Traditionally, refineries heavily favor globe valves whereas pulp and paper plants favor Segmented ball valves and V Port ball valves. Although there are instances when one valve type is more appropriate than another, most control valve installations can be built around many types of control valves. Planning is the most important aspect including selecting materials and trim. The final selection of the type of control valve and trim should ultimately be dictated by the flow conditions required after carefully analyzing all the variables. The best way to accomplish this is by working closely with your salesperson and a Bray applications engineer. Bray Rotary Control valves have distinct advantages over linear control valves including; Higher Cv for the same size globe valve, resulting in higher control valve flow, allowing the use of smaller valve and actuation packages that reduce cost, have a smaller form factor, better packing performance and life, and Superior solids handling capabilities.
An undersized control valve will cause several problems including flow restriction (choked flow) - where the control valve is unable to deliver the required flow rate even when fully opened. Restricted flow can cause a momentary pressure drop resulting in cavitation, a phenomenon where the formation and then sudden implosion of bubbles, causes erosion. Another common control valve issue is Flashing. Flashing occurs when a liquid vaporizes due to a pressure drop across the vena contracta and is highly erosive. An oversized control valve sacrifices control resolution compared to a properly sized control valve, resulting in a loss of fidelity in the desired control range. The general rule is to size a control valve to utilize as much of the range as possible with the desired setpoint occurring at approximately 60%-80% of maximum being the ideal setpoint condition of the process.
Bray Sizing Program (BII Sizing) software is powerful control valve sizing software that performs control valve sizing calculations automatically and can help immensely with proper control valve sizing. BII sizing software produces an ISA specification sheet, based on your control valve flow requirements and can predict cavitation, noise and Cv at various openings ensuring accuracy. Although this software is provided free of charge and is generally considered to be reliable and accurate, it should not be considered a substitute for the assistance of an application engineer from the Bray team. Please consult your sales rep and or the factory for assistance with your control valve application needs.