Mark Nord, control valve solution architect for Emerson’s Flow Controls Products, recently published an article in the December 2020 issue of Flow Control. The article helps the reader determine the proper trim characteristic for a particular process application. The article is titled Overcoming Control Valve Trim Selection Challenges and is summarized below.
Specifying a control valve correctly can be a challenging task. Configuration assistant programs help designers perform the sizing calculations, but selecting the proper valve flow characteristic is not always an obvious choice. Mark explains the purpose of his article:
An incorrect trim selection can make control difficult or even impossible, so choosing the proper characteristic is critical to valve performance. This article describes the different control valve flow characteristic options and explains how to pick the best one to provide precise and consistent control across a variety of process conditions.
Quick opening, linear, or equal percentage
Most control valve inherent flow characteristics fall into three major categories: quick opening, linear, or equal percentage. The rated travel percentage versus the rated flow coefficient percentage for the three categories are shown in Figure 1.
Figure 1: The three major categories of control valve trim characteristics are: quick opening, linear, and equal percentage. Note the significant differences in flow capacity versus valve travel for each type.
As the graph shows, the three trim profiles are very different. The linear flow characteristic draws a flat line as its name suggests, with valve travel and flow capacity changing at roughly the same rate. Globe valves and some butterfly valves typically exhibit a linear inherent characteristic.
The quick opening profile is well named, with flow rising quickly as valve first opens. A quick opening trim will usually generate 50% of flow with the valve only 25% open, and 70% or more of flow at 50% opening. After that point, the flow rises little as the valve strokes the last 50%.
Equal percentage trim is the opposite of quick opening. Flow is minimal as the valve begins to open and the valve flows only 25% when it is 50% open. However, as the valve moves from 50% to 100% the flow increases quickly, adding an additional 50% of flow in the last 25% of travel. Ball valves, some high-performance butterfly valves, and globe valves have equal-percentage characteristics.
Which to choose?
The first step to choosing the right characteristic for an application is to realize the automation system expects a linear response when it moves a control valve in a loop. Whether the PID controller moves the valve from 10% to 15% or from 85% to 90%, the process variable (PV) should change the same amount. Ideally, the percent increase in PV should match the valve change, so the PV should change about 5% in either case.
Mark describes why this is important:
When the system response is not linear, a PID controller can experience significant stability problems. In a non-linear system, if the controller is tuned for one valve position, for example 15% open, it will perform well at that point. However, if the valve has to open to 85% to provide significantly more flow, the non-linear system response at that valve position will be very different, and the loop may oscillate badly. In fact the loop may not be able to control in automatic mode at all.
But a linear installed response is determined by the inherent characteristic of the valve AND by the differential pressure across the valve, which can vary greatly with flow. If the differential pressure is consistent with flow, then a linear control valve characteristic provides a linear installed response.
However if the differential pressure falls as the flow increases, the control valve needs to offset this effect. Figures 2 and 3 describe a typical process where the DP acts in a quick opening fashion (very high at low flows, very low at high flows). In this case the valve needs to have an equal percentage characteristic to reverse effect of variable DP and create an overall linear response.
Figure 2: Typical piping arrangement with a centrifugal pump moving liquid through a control valve and a long length of pipe to feed a downstream vessel.
Figure 3: The curves above show how pump pressure falls with increasing discharge pressure, while the downstream piping pressure increases as the flow rate increases. The resulting pressure drop across the control valve (in gray) falls very quickly as the system flow rate increases.
Trim selection rules of thumb
Mark offers the following suggestions when selecting a control valve flow characteristic.
Linear control valves are usually chosen for pressure control with near-constant valve DP, near- constant inlet pressure with critical pressure drop, high inlet pressure with low outlet pressure (such as steam pressure letdown valves), positive displacement compressor bypass valves, distillation/fractionation column pressure control applications, and others. Linear valves are also chosen for level control applications with a near constant valve DP, temperature control applications throttling a liquid to a heat exchanger, or the use of two valves or a three-way valve in heat exchanger bypass application.
Equal-percentage characteristics are usually selected for pressure control applications where valve DP is inversely proportional to flow, such as maintaining gas pressure to a burner or providing constant steam flow to a reboiler. They are also used for flow control with valve DP less than one-third of piping circuit pressure drop—and for temperature, flow, or pressure control of steam or other condensing/heat transfer processes. (See Figure 4.)
Figure 4: This Fisher easy-e globe valve assembly utilizes an equal percentage trim.
Mark notes that quick opening valves do not usually work well as control valves but can be utilized for drain/vent applications to provide high flows quickly.
Conclusion
Mark offered these suggestions as he closed his article:
Choosing the correct control valve inherent flow characteristic can make the difference between a successful control valve installation and an erratic automated loop control nightmare. The idea of achieving a linear installed response is simple enough, but to accomplish this goal the end user must possess a good understanding of their process. Discussing the flow dynamics with process experts can yield big dividends in terms of controllability and process stability.
Figures all courtesy of Emerson
About the Author
Mark Nord is the control valve solution architect for Emerson’s Flow Controls Products in Marshalltown, Iowa. He is responsible for solving the most challenging control valve applications. Mark has 30 years of experience in plant operation, control valve and steam conditioning applications, and holds a BSME from the University of North Dakota.