Can two single-loop controls be used to replace split-range 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 a 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!
In a control system, the PV value is the controlled variable. A controlled variable is sent to two controllers; the outputs from these two controllers are respectively sent to two identical control valves, and both of these control valves affect the controlled variable. These two control systems are closely related (since they both control the same variable), and it depends on whether both systems can remain stable simultaneously. The two control valves shown in the diagram are of the opposite-action type (A closes when air is supplied, B opens when air is supplied). This configuration does not allow for an increase in the adjustable range, as it is not possible to create an equivalent control valve with identical characteristics. If a single PV value is supplied to these two control valves, it is necessary to ensure that negative feedback is established in both branches. In summary, segmented control is designed to meet specific control requirements in production, not merely to increase the adjustable range. The situation shown in the diagram is widely used for nitrogen sealing to isolate air and prevent oxidation.