“Control valve” and “regulating valve” are simply different names for what is essentially the same control valve. Therefore, what matters for stabilizing PID control is not the difference in terminology but selecting the right combination of three factors—rangeability (the ratio of controllable flow rates), flow characteristic, and responsiveness—to match the actual operating flow range. In particular, whether you have grasped not only the maximum flow rate but also the minimum flow range is what determines success or failure in stabilization. The following organizes the key concepts for selection.
Key Points of This Article
- “Control valve” and “regulating valve” are synonymous; the choice should be based on control performance, not on which term is preferable.
- The keys to PID stabilization lie in three factors: rangeability, flow characteristic, and responsiveness.
- Grasping the operating conditions, including the minimum flow range, helps prevent hunting caused by an oversized valve.
The Selection Approach for Stabilizing PID Control
Control Valves and Regulating Valves Are the Same Thing
The Japanese term “調節弁” (chosetsu-ben) is a translation of the English “control valve,” and standards such as JIS use “調節弁” as well. Depending on the catalog or manufacturer, terms like “control valve” or “regulating valve” may be used interchangeably, but they refer to the same valve used for control. For this reason, there is no need to compare the two as if one were superior; in practice, it is effective to focus on the control performance itself rather than on the name during selection.
An Oversized Valve Tends to Cause Hunting
If you determine the valve size based on the maximum flow rate alone, the normal operating point tends to fall at a low opening of around 10 to 20%, where a slight change in opening causes a large change in flow rate. This makes the control gain excessive and becomes a cause of hunting. Therefore, in addition to the maximum flow rate, it contributes to stabilization to grasp the minimum flow range and to select a valve size and model that can cover that ratio (the rangeability, generally taken as several tens to 1 as a guide). As a basic rule, the valve size should be determined based on the Cv value derived from the required flow rate, not matched to the pipe size.
Flow Characteristic and Actuator Response
Flow characteristics include the equal-percentage characteristic and the linear characteristic. In systems where the effect of pipe resistance is large, the equal-percentage characteristic is generally said to make the change in flow rate relative to opening more uniform and therefore easier to stabilize (this varies depending on the conditions). Along with this, the responsiveness, dead time, and hysteresis of the actuator and positioner also affect stability. It is advisable to confirm, together with the operating conditions, whether the time constant of the controlled object and the valve response are balanced.
Frequently Asked Questions
Q1. Are a control valve and a regulating valve different things?
A1. They are basically different names for the same thing. It is a difference in translation or notation, and there is no need to regard it as a functional distinction. In selection, please focus on control performance rather than on the name.
Q2. Should the valve size simply be matched to the pipe size?
A2. The pipe size and the valve size do not necessarily match. The basic approach is to select based on the Cv value derived from the required flow rate; note that matching the valve to the pipe tends to result in an oversized valve.
Q3. Hunting is occurring. Is it a problem with the valve?
A3. Several factors can be considered, such as low-opening operation due to an oversized valve, a mismatch in flow characteristic, responsiveness, and PID parameters. It is effective to check the operating flow range and opening, and to isolate the cause rather than attributing it to a single factor.
Q4. Which should I choose, equal-percentage or linear?
A4. In systems where pipe resistance is dominant, equal-percentage is generally said to be easier to stabilize, but this varies depending on the system conditions. We recommend making an individual judgment based on the operating conditions.
Q5. I want to replace an existing valve with an equivalent product.
A5. It is advisable to check not only the valve size, Cv value, and connection standard, but also the flow characteristic and actuator specifications. If the specifications are unknown, please consult us based on the model number and operating conditions of the existing valve.
Q6. Can’t it be solved just by adjusting the PID parameters?
A6. There are cases where parameter adjustment brings improvement, but if the valve is oversized or undersized relative to the operating range, a review of the mechanical selection may be necessary. Checking from both sides is the reliable approach.
Summary
A control valve and a regulating valve are the same thing, and in stabilizing PID control it is important to select based on rangeability, flow characteristic, and responsiveness to match the operating conditions, rather than on the name. In particular, grasping the operating conditions including the minimum flow range helps prevent hunting caused by an oversized valve.
If you encounter any of the following difficulties during selection, please make use of Yoshitake’s technical and selection consultation services. We can assist with concerns such as “I can’t determine whether the valve size is appropriate,” “I know the maximum flow rate but not the minimum flow rate,” “I want to replace an existing valve but don’t know how to choose an equivalent product,” or “I want to confirm whether the product I selected myself is appropriate,” working from your operating conditions. Even if you don’t have the specifications or drawings on hand, you are welcome to consult us based on the conditions you do know.