ABSTRACT
A control valve regulates the supply of material or energy to a process by adjusting an opening through which the material flows; it is a variable orifice in a line. The selection of the proper valve characteristic is one of the most important phases of designing a control loop. Flashing and cavitation must be considered to ensure proper valve sizing and to allow the selection of a valve that will resist their effects. If the presence of cavitation and flashing is neglected when valves are sized, undersized valves will be selected and rapid valve deterioration can take place. A misapplied equal percentage valve characteristic results in increasing valve sensitivity at high-flow rates and can cause instability unless the controller proportional band is adjusted at the high-flow rate. The control loop will then tend to be overdamped at the low-flow rates, with corresponding sluggish response.
