While a thorough dynamic analysis of the system can be performed to guide the selection of a particular valve characteristic, the use of established rules of thumb generally results in effective control and satisfactory performance. Refer to the guidelines that are included in Work Aid 1G.
Pressure – The guidelines for pressure applications are based on the assumption that in most pressure processes, the valve pressure drop decreases as the flow rate increases; therefore, an equal-percentage characteristic is generally appropriate. An exception is a large-volume application in which the ratio of the maximum to minimum pressure drop is less than approximately 5:1. In this application, a linear characteristic is recommended.
Temperature – Most temperature processes perform well with an equal- percentage characteristic.
Flow – The majority of flow loops work well with equal percentage valve characteristics. One exception is a liquid process with a wide range of setpoints.
Liquid level – The guidelines for liquid level systems are based on the assumption that, in most applications, the pressure drop decreases with increasing load or flow. Again, the exception to the equal percentage guideline is when the ratio of the maximum to minimum pressure drop is less than approximately 5:1. For such applications, a linear characteristic is recommended.