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All control valve manufacturers will tell you that it is important to carefully calculate and select the valve type in control applications. This is to ensure that the process can be precisely controlled, minimizing variability and all inherent instability issues that come with it, and minimizing the impact on raw material and utility costs.

However, valves are usually chosen for reasons other than controllability. There are two main considerations when selecting a valve for a given control application. One is the total range of expected control of the valve, and the second is the mounting gain of the control valve within its operating range. Gain is the ratio of the rate of change of input to the rate of change of output. Thus, if a 10% increase in the input signal results in a 10% change in flow, the gain is treated as one. Installation gain depends on process conditions and inherent valve characteristics. When the valve stroke percentage is plotted as a percentage of maximum flow and the pressure drop is constant, the inherent characteristics are described by the shape of the graph. There are three main features, fast open, linear and equal percentage