The precise adjustment of process feedstock flow is often necessary to maintain the processing conditions required to meet product specifications. To meet the target plant production rates, a high feedstock flow rate is often required. If a single valve is used to regulate the feedstock flow, then the precision with which the flow rate may be regulated is determined by the valve position resolution. The position resolution of a large valve may be the same as that of a small valve in terms of percent of maximum flow. However, as the valve gets bigger, that percent of scale represents a much larger flow rate. Precise metering requirements may be met with a small valve, but a small valve may not have the flow capacity needed to maintain the plant product rate. An ideal solution would be to use a small valve to make fine changes, and use a larger valve to address the need for flow capacity. Split-range control cannot be used to satisfy this requirement since the valves are sequenced one at a time, that is, one valve is used and then the other valve. To achieve precise flow rate control over a large flow range, it is necessary to use both valves at the same time. As fine adjustments in flow are made with the smaller valve, the large valve is automatically adjusted to keep the small valve within its operation range of 0-100%. Valve position control may be used to achieve this type of regulation of the small and large valve.
Valve position control is a straightforward concept that may be easily implemented in most modern control systems. The basic components of a valve position control strategy are illustrated as shown below.
In this example, the feedback controller directly adjusts the small valve input to the process. The output of the feedback controller–the implied valve position of the smaller valve–is also the control parameter of an integral-only (I-only) controller. In this example, the setpoint of the I-only is set to 50%. If the feedback controller moves the small valve away from 50% open for a sufficiently long period of time, then the I-only controller will gradually adjust the large valve in a manner that drives the small valve back to its normal operating point of 50%. The tuning of the I-only controller is very slow and so a change in the large valve appears as a load disturbance to the feedback controller and is quickly corrected through adjustment of the small valve.
When implementing valve position control, both the feedback controller and the I-only controller may be implemented using a PID block. As indicated above, the output of the controller used for feedback control would be connected to the small valve and would also be the input of the I-only controller. The blocks and connections that would be required for valve position control are illustrated below.
Several valve position control application examples are detailed in chapter 13 of Control Loop Foundation – Batch and Continuous Processes. Also, the valve position control workshop included in that chapter provides several exercises that may be used to further explore valve position control. By accessing the book’s web site, you may complete this valve position control workshop using your web browser. The viewer below may be used to see the solution to this exercise.