Selecting A Characteristic – To provide linear system gain over the entire operating range of the system, an equal percentage valve characteristic is selected. This valve characteristic compensates for the characteristics of the process. Figure 45 illustrates the concept of compensation. Note that the valve characteristic, or gain, is defined by the percent change in output (Cv) for a given change in input (stem position). Note also that the process characteristic, or gain, is defined by the percent change in output (flow) for a given change in input (control valve Cv). Theoretically, the combination of the valve and process characteristics would result in a linear characteristic, which is ideal for many applications.
Combined Gain – In reality, the combined gain is not perfectly linear (refer to Figure 46); however, the combined gain more nearly approximates a linear characteristic and the system will be much easier to control.
Additional Considerations – Even though flow characterization results in more linear gain, note that the gain is still quite high at low percentages of valve travel. The high gain at low percentages of valve travel is a common occurrence, and it explains why many loops become unstable at low flows.
Gain And Sizing Issues – The high valve gain that is experienced at low percentages of valve travel is one reason why valves should not be allowed to throttle below specific travel limits. As a general guideline, most valves should not be allowed to throttle below 10 percent travel. Throttling below 10 percent travel can result in control instability. In addition, seat wear can result from high velocity flow, and from the plug bouncing in and out of the valve seat.