(Seeking help) I’m asking experts: can two single-loop controls be used as a substitute for proportional control?
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As the title suggests, I currently have one pressure transmitter (PV) and two control valves (OP), and I want to use this PV value to control the two control valves. There is discussion as to whether to use split-range control or two separate single-loop controls. The curves are drawn as shown: on the left is the curve for control valve 1, and on the right is the curve for control valve 2. There are two questions: First: Step control is generally used when the adjustable range of a single valve is insufficient to meet the process requirements; in such cases, multiple valves are connected in parallel and controlled by a single regulator, thereby increasing the adjustable range. But I think it’s also possible to use two single-loop controls: reference the PV value twice, feed it into two separate PID blocks, and then generate two separate OP values to control the two control valves. Is that an option? What are the advantages and disadvantages of this approach compared to split-range control? Second: If these two control valves have no connection with each other, that is, they are not connected in parallel nor are they used to increase the adjustable range; rather, they represent two separate control circuits on the same process equipment. Yet, these two control valves are controlled by the same PV value, and their performance curves match those shown in the diagram. In such a case, is it better to use two separate control loops or one proportional control system? Some people suggest using two separate loops, but I think proportional control can also work. I would appreciate it if experts could analyze the advantages and disadvantages of each approach. Thank you!
The original poster did not truly understand stepwise adjustment, and the diagram they provided is also incorrect.
I would like to ask: what should be the horizontal and vertical coordinates on the correct graph, or how should it be drawn? Also, could you provide a more in-depth explanation of range division? I hope experts will kindly offer their guidance
To put it simply, the nouns are too technical and not relatable. Staged control is necessary because there isn’t enough valves, or because different valve adjustments are required at various stages. For example, there is pressure regulation with a heater, involving a large valve and a small valve; the small valve is opened first for adjustment, and when that is not sufficient, the large valve is then opened. This is the same-direction splitting mentioned upstairs. Taking the example mentioned above, if the PV value referred to by the poster is used twice to control 2 PID controllers, a problem will arise: both control valves will act simultaneously. Therefore, the key to zoning is to divide the PV value into two segments, with each segment controlling one control valve respectively, in order to achieve the desired control effect. The heating effect is satisfactory when only small valves are used; however, if the heating effect is poor, larger valves are necessary to make adjustments. Additionally, regarding the second scenario mentioned by the original poster, it is important to note whether these two control valves require the PV values to be used separately. For example, if the PV value is between 0% and 50%, the first valve operates while the second one does not; in such a case, a multi-stage control approach is necessary. If both valves are adjusted across the full range of 0~100% of the PV value, then 2 single loops are required. Divide into stages – the key lies in the word “divide”.
According to the diagram you provided, it’s possible to use a pressure transmitter to control two valves
The two methods you mentioned can be represented using block diagrams. The biggest problem with the control method shown below is system coupling; it is possible that, when approaching the set value, if the parameter tuning is not optimal and one of the controllers adjusts in advance or experiences overshoot, this will affect the other controller. This ultimately leads to system oscillation and divergence, failing to achieve the desired control effect. In terms of the above segmented control, since control is performed in segments based on an output value, when one actuator is adjusting, the other actuator remains in a fixed state of either fully open or fully closed; as a result, disturbances are relatively small, which makes it easier to tune the system parameters.
Thank you, big bro. Can it be understood that the horizontal axis represents the OP of the controller, such as the OP of a PID block, while the vertical axis represents the opening degree of the on-site valve?
Thank you so much, expert! You even created a diagram, which makes things clear at a glance. Thanks!