The Feedback Control Loop: Valve Characteristics

Control valves have unique characteristics of their own which can significantly affect the performance of a loop. The steady-state gain of the valve relates controller output to a process flow. How this flow affects the controlled variable of the process defines the range of control. For servo control, the range of control would be defined as the range of setpoints achievable at a given load. For regulator control, it would be defined as the range of loads for which the given setpoint could be maintained. Attempting to operate outside the range of control will always result in the valve being either fully open or closed and the controlled variable offset from setpoint.

The steady-state gain of a control valve is determined at its operating point, since its gain may vary somewhat throughout its stroke. Valves have internal trim that provide a specified gain as a function of position, such as Linear, Equal Percentage, or Quick Opening inherent characteristics. Typically, the trim is chosen such that the installed characteristics provide an approximately linear flow response. Thus for a valve operating with critical gas flow, Linear trim would provide an approximately linear flow response. An Equal Percentage trim may be used to provide a more linear response for gas or liquid flow where line pressure drop is equal or greater than the valve pressure drop. The Quick Opening trim is usually not chosen for linear response in continuous control applications, however, it provides a high gain near the closed position, which is useful for fast responding pressure relief applications.

One common non-linear characteristic of control valves is hysteresis, which results in two possible flows at a given valve position, depending upon whether the valve is opening or closing. In the steady-state, hysteresis limits resolution in achieving a specific flow with its desired effect on the process. Dynamically, hysteresis also creates pre-stoke deadtime, which contributes to total loop deadtime, thus degrading the performance of the loop. Prestroke deadtime is the time that elapses as the controller output slowly traverses across the dead band before achieving any change in actual valve position or flow.

The use of a valve positioner can significantly reduce both hysteresis and thus pre-stroke deadtime. A valve positioner is recommended for all control loops requiring good performance. Typical hysteresis may be 2–5% for a valve without a positioner, 0.5–2% for a valve with an analog positioner, and 0.2–0.5% for a valve with a
digital positioner.

On some control loops, a variable-speed drive on a pump, fan or blower may be used as the final element connecting the controller output to the process. Variable-speed drives provide fast and linear response with little or no hysteresis and therefore are an excellent choice with respect to control performance. As the initial cost of variable-speed drives continues to decrease, their use should become a more widespread practice.