Regarding the pump backflow pressure control circuit
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Hey everyone, when designing the automatic control system for pump outlets, have you come across the \"backflow pressure control circuit\"? The pumps involved in the project are arranged in a two-in-one-backup configuration, primarily to meet the varying flow rate requirements of the pumps under different operating conditions. There are five operating conditions in total; the normal flow rate is only 1/10 of the normal flow rate of a single pump, while the maximum flow rate is the sum of the flow rates of the two pumps. The remaining three operating conditions fall between the two. Moreover, the pressures under different operating conditions vary slightly. In this case, it is necessary to regulate the outlet flow rate of the pump through recirculation; the flow meter at the pump outlet is used to ensure the opening degree of the loop control valve, thereby maintaining the flow rate downstream. I would like to ask everyone the following: 1. How can export flow meters and loop control valves be properly controlled to meet the requirements of different operating conditions? 2. How can downstream pressure be maintained stable while regulating flow rate through backflow? 3. Additionally, could someone please explain how the backflow pressure control circuit is implemented? Thank you so much!
In fact, controlling the outlet flow is equivalent to controlling the outlet pressure, as pressure is proportional to flow. In this case, it is necessary to adjust the pump’s outlet pressure through recirculation; the pressure at the pump outlet is used to determine the opening degree of the control valve in the circuit, thereby maintaining the flow rate downstream. A pressure transmitter is installed at the pump outlet, and a control valve is placed between the fluid outlet and the pump outlet to regulate the outlet flow rate. (For further clarification, refer to \"Automatic Control in Petrochemical Industries: Control Schemes for Typical Production Units – Control of Fluid Transfer Equipment\".)
Let me share my understanding; it can be considered a approach: 1. Firstly, it’s best to use frequency control for this purpose, rather than control valves. In this way, there will be no sudden changes in pressure or flow when increasing or decreasing the pump speed. 2. The two frequency converters are controlled using a stepped approach: when the flow rate is within a certain range, only one pump is operated for regulation. When the flow rate approaches the full load capacity of a single pump and more flow is needed, the backup pump is started at low speed first, after which the flow rate is adjusted to operate the backup pump as well. 3. To ensure stable pressure as well, add a cascade control circuit for the pressure auxiliary loop. However, this means modifying the process again, but it seems that it should be easier to control than the current process.
I agree with what was said above, but the third one isn’t necessary. It’s sufficient to choose just one primary variable for control; having too many makes things confusing.:handshake