Control valve performance
The key to control valve performance is creating a measurable change in flow through the valve in response to small, input step changes (1% and less). A change in flow indicates that the valve's flow coefficient has actually changed in response to a change in the input signal. If the actual flow through the control valve is not available or is not measured, then the valve stem, shaft or actuator movement may be used to estimate the response of the valve. However, the movement of the valve stem, shaft or actuator may not be an accurate representation of the actual change in the valve flow coefficient for all changes of the input signal to the valve. For example, the inboard end of the shaft of a rotary valve might move in response to a change in the input to the control valve but the actual flow coefficient might not change because shaft windup occurs and the valve closure member does not move. In some cases the actuator position may be used to measure the control valve response and this adds yet another potential discrepancy between the input to the control valve and the actual change in the valve flow coefficient. However, it is important to note that the response of the valve flow coefficient can be no better than that of the stem, shaft or actuator position.
2012-08-30
