Automatic operation control method for variable-back-pressure series-parallel centrifugal pump set

Abstract

The invention relates to an automatic operation control method for a variable-back-pressure series-parallel centrifugal pump set. The automatic operation control method comprises the steps that (1) the hydraulic performance of centrifugal pumps at different rotating speeds is tested; (2) determining the stable operation condition range of the pump in series and parallel connection states; (3) measuring pipeline resistance characteristics; (4) the frequency converter collects inlet pressure and receives flow and back pressure input locally or sent by a superior system; (5) calculating pipeline resistance according to the flow and resistance characteristic curve; (6) calculating the required lift according to the pipeline resistance, the inlet pressure and the back pressure; (7) determining serial and parallel connection states and operation rotating speeds of the pump set according to the pump set performance curve and the efficiency; (8) switching serial and parallel connection states to adjust the rotating speed of the pump set; and (9) adjusting the running rotating speed in real time according to back pressure and flow change in the running process. The stable operation condition range of the series-parallel centrifugal pump is partitioned, the series-parallel state and the operation rotating speed of the pump set are determined according to the back pressure, the inlet pressure, the flow and the pipeline resistance, real-time regulation and control can be achieved, and automatic operation control over the variable-back-pressure series-parallel centrifugal pump is achieved.

Description

Technical Field

[0001] This invention patent belongs to the field of fluid machinery, and in particular relates to an automatic operation control method for a series-parallel centrifugal pump set with variable back pressure. Background Technology

[0002] With the development of automation and intelligence, research on the automated operation control of centrifugal pumps has gradually increased in recent years, resulting in numerous patents and technological achievements. The current state of this technology can be summarized as follows:

[0003] 1. Dynamic modeling and real-time optimization

[0004] The patent (CN113901710A) "Centrifugal Pump Operation Optimization Control Method" (2022) combines a theoretical mathematical model (offline database) with an improved BP neural network (online correction). It uses a binary tree search algorithm to quickly match the optimal speed, solving the model deviation problem caused by equipment aging and improving response efficiency in transient conditions (such as the startup phase). The brushless doubly-fed motor control patent proposes a multi-objective model based on the flow-head digital curve (lowest cost, fewest start-ups and shutdowns, highest reliability), and uses the Artificial Bee Colony Algorithm (MOABC) to solve for the optimal scheduling scheme, suitable for large pumping stations.

[0005] 2. Variable frequency and closed-loop control

[0006] Some patents utilize flow deviation analysis and pressure fluctuation constraint coefficients to dynamically optimize PID parameters, reducing frequency converter regulation lag and significantly improving head control accuracy. Other patents propose real-time identification of the high-efficiency operating range based on the QH characteristic curve, using coordinated adjustment of electric valve opening and servo drive to regulate displacement, achieving energy savings of over 15%.

[0007] 3. Condition monitoring and fault diagnosis

[0008] Some studies extract long-sequence global fault features for different operating conditions (multi-source domain and target domain), establish time-series correlation models, solve the problem of fault identification under unlabeled operating conditions, and reduce the false negative rate. There is a patented multi-parameter evaluation system based on rotor vibration vector, liquid mass ratio, and flow anomaly index, which calculates the anomaly index through resonance degree and support stiffness, and combines it with neural networks to predict stability. There are also some studies focusing on data twins and fault diagnosis.

[0009] 4. Regarding parallel operation, it lacks the ability to switch between series and parallel operation. For example, the patent document (CN115270358B, "Optimization and Control Method for Operation of Parallel Centrifugal Pump System") only focuses on the optimization of parallel centrifugal pumps. It achieves operation control by establishing a parallel pump characteristic model and multi-objective optimization (electricity cost, reliability, number of start-stop cycles), but it does not involve the series operation mode, let alone the series-parallel state switching mechanism. Moreover, it does not consider the impact of back pressure changes on head requirements, and cannot adapt to variable back pressure conditions. When the back pressure fluctuates, it is easy to cause insufficient head or energy waste.

[0010] 5. It only addresses series operation and lacks parallel and series-parallel coordinated control. For example, the patent document (CN120350731A, "An All-Terrain Mobile Pump Station Rapid Drainage Adaptive Adjustment System") only solves the pipeline resistance imbalance problem of series pump stations. It achieves adaptive adjustment by monitoring the difference between resistance and flow and predicting the critical time point, but it does not involve parallel operation and cannot meet the demand for large flow. Moreover, its control objective is only drainage efficiency, without considering the efficiency optimization of the pump set itself, and there is no head calculation model under variable back pressure, resulting in poor adaptability to operating conditions.

[0011] 6. Lack of transition zone processing mechanism, resulting in insufficient operational stability. Existing technologies (the aforementioned patent documents) do not divide the series-parallel transition zone, but rely solely on a single operating condition range to determine the operating mode. When parameters (back pressure, flow rate) fluctuate, the series-parallel state is prone to repeated switching, causing the pump unit to oscillate during operation. Furthermore, there is a lack of efficiency-based transition zone determination logic, making it impossible to balance stability and efficiency.

[0012] 7. Lack of adaptability to variable back pressure: As in the existing technologies (the aforementioned patent documents), no correlation calculation model has been established between back pressure and head. When the back pressure changes, it is impossible to accurately match the required head, which may result in insufficient delivery pressure or redundant energy consumption, and thus fail to achieve efficient operation under variable back pressure.

[0013] In summary, based on the above search and analysis, there are currently no research or patents specifically addressing the automatic operation control of series-parallel centrifugal pumps, nor are there any studies on the automatic operation of series-parallel centrifugal pump sets under varying back pressure. Furthermore, existing technologies cannot meet the requirements for automatic, efficient, and stable operation of series-parallel centrifugal pump sets under varying back pressure scenarios. Therefore, there is an urgent need for a control method that can achieve series-parallel switching, adapt to varying back pressure, and balance efficiency and stability. Summary of the Invention

[0014] This invention provides an automatic operation control method for a series-parallel centrifugal pump set with variable back pressure. By dividing the stable operating range of the series-parallel centrifugal pumps into zones, the series-parallel state and operating speed of the pump set are determined based on back pressure, inlet pressure, flow rate, pipeline resistance, etc. A frequency converter and an electric actuator are used to realize automatic switching and speed adjustment between series and parallel states. During operation, the system can adjust the pump set in real time according to changes in parameters such as back pressure and required flow rate, thus realizing automatic operation control of the series-parallel centrifugal pump with variable back pressure.

[0015] To achieve the above objectives, the technical solution of the present invention is: an automatic operation control method for a series-parallel centrifugal pump set with variable back pressure, comprising the following steps: S1: testing the hydraulic performance of the centrifugal pump at different speeds in series and parallel states, wherein the hydraulic performance includes flow rate, head, and efficiency; S2: determining the stable operating range of the pump in series and parallel states respectively, and dividing the stable operating range of series and parallel states into three regions: series region, parallel region, and transition region, wherein the transition region is a region in which both series and parallel states can operate stably; S3: measuring the pipeline resistance characteristics and establishing a flow-pipeline resistance relationship curve; S4: the frequency converter collects the inlet pressure P1 of the centrifugal pump and receives the target flow rate Q1 and back pressure Pb sent by the local input or the upper-level system; S5: calculating the pipeline resistance h based on the target flow rate Q1 and the flow-pipeline resistance relationship curve; S6: calculating the pipeline resistance h based on the pipeline resistance h, inlet pressure P1, and back pressure Pb. b S7: Calculate the required head H of the pump set; S8: Determine the series / parallel connection status and operating speed n of the pump set based on the pump set performance curve and efficiency; S9: Switch the series / parallel connection status of the pump set using an electric actuator, and adjust the pump set speed to the operating speed n using a frequency converter; S10: During operation, adjust the pump set speed according to the back pressure P. b In accordance with the changes in the target flow rate Q1, repeat steps S4-S8 to adjust the series and parallel status and operating speed of the pump set in real time.

[0016] Furthermore, in step S2, the specific determination of the stable operating range of the pump in series and parallel states is as follows: the stable operating range of the pump in series and parallel states is set according to 30% to 120% of the rated flow rate of the pump at each speed.

[0017] Furthermore, in step S2, the series region is the region that can operate stably only in the series state, and the parallel region is the region that can operate stably only in the parallel state.

[0018] Furthermore, in steps S7 and S9, to prevent back pressure P b Fluctuations in the inlet pressure P1 parameter cause repeated switching between series and parallel operation. By continuously collecting data multiple times, the series and parallel operation status can be determined, effectively improving the stability of the pump unit operation.

[0019] Furthermore, in steps S7 and S9, to prevent back pressure Pb Fluctuations in the inlet pressure P1 parameter cause repeated switching between series and parallel states. By introducing the efficiency span Δη, the series and parallel states in the transition region can be determined, effectively improving the stability of pump unit operation.

[0020] Furthermore, calculate the efficiency η1 corresponding to the series state and the efficiency η2 corresponding to the parallel state respectively. If η1-η2≥△η, then choose the series state; if η2-η1≥△η, then choose the parallel state.

[0021] Furthermore, in step S6, the required head H of the pump set is calculated using the formula: H = h + P b / ρg-P1 / ρg

[0022] , where ρ is the density of the transport medium and g is the acceleration due to gravity.

[0023] Furthermore, in step S7, the series or parallel state and operating speed n of the pump set are determined based on the pump set performance curve and efficiency. Specifically: if the target flow rate Q1 and the head H are in the series region, including the boundary line, the pump set is selected to operate in the series state; if the target flow rate Q1 and the head H are in the parallel region, including the boundary line but excluding points O and E, the pump set is selected to operate in the parallel state; if the target flow rate Q1 and the head H are in the transition region, excluding the boundary line, the pump set is selected to operate in the series or parallel state based on the efficiency.

[0024] Furthermore, in step S8, the series and parallel states of the pump set are switched by the electric actuator. Specifically, the frequency converter outputs a control signal to the electric actuator, and the electric actuator drives the switching valve to rotate to the corresponding position. If the switching valve is already in the target series or parallel state, the electric actuator does not operate. After the switching is completed, the frequency converter adjusts the pump set speed to achieve coordinated control of switching and speed regulation.

[0025] Furthermore, step S4 also includes alarm judgment: if P1 / ρg is less than the required net positive suction head of the pump, the frequency converter issues an alarm signal and suspends the pump group startup; in step S9, if the target flow rate Q1 is collected and calculated multiple times and there are points where the head H is not in the stable operating condition range, the frequency converter issues an alarm signal.

[0026] The beneficial effects of this invention are:

[0027] 1. The stable operating range of series-parallel centrifugal pumps is divided into three zones: series zone (OABECO zone), parallel zone (OEDO zone), and transition zone (OCEO zone). Based on back pressure, inlet pressure, flow rate, pipeline resistance, etc., the series-parallel status of the pump set and the operating speed are determined. A frequency converter and electric actuator are used to realize automatic switching and speed regulation between series and parallel status. During operation, the pumps can be adjusted in real time according to changes in parameters such as back pressure and required flow rate, thus realizing automatic operation control of variable back pressure series-parallel centrifugal pumps.

[0028] 2. To prevent repeated switching between series and parallel states caused by fluctuations in parameters such as back pressure and inlet pressure, continuous multiple data acquisitions and the introduction of an efficiency span Δη are used to comprehensively determine the series and parallel states, effectively improving the stability of pump unit operation.

[0029] 3. The pump set automatically determines and operates within the entire stable operating range (OABEDO area) according to system needs, and has a wide range of automatic operating conditions.

[0030] 4. In the transition zone (OCEO region), the pump set selects the series or parallel connection state of the pump set according to the efficiency, resulting in higher operating efficiency. Attached Figure Description

[0031] Figure 1 It is the structural composition of the equipment;

[0032] Figure 2 This is a diagram illustrating the series and parallel connection states and the direction of flow;

[0033] Figure 3 These are the flow-head curves at different speeds in series configuration;

[0034] Figure 4 These are the flow-head curves at different speeds in parallel operation;

[0035] Figure 5 This refers to the range of stable operating conditions in series configuration;

[0036] Figure 6 This refers to the range of stable operating conditions in parallel operation.

[0037] Figure 7 This refers to the range of stable operating conditions in both series and parallel configurations.

[0038] Figure 8 It is the system flow-pipeline resistance characteristic curve. Detailed Implementation

[0039] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0040] An automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to the present invention includes:

[0041] 1. Through experiments, test the hydraulic performance (flow rate, head, efficiency) of centrifugal pumps at different speeds in series and parallel configurations;

[0042] 2. Determine the stable operating range of the pump in series and parallel states respectively. It can be set according to 30% to 120% of the rated flow rate at each speed.

[0043] 3. Measure the pipeline resistance characteristics and establish the flow-pipeline resistance relationship curve. Generally, it can be approximated that the pipeline resistance is proportional to the quadratic method of the flow rate.

[0044] 4. The frequency converter collects the inlet pressure P1 and receives the flow rate Q1 and back pressure P from local input or sent by the upstream system. b ;

[0045] 5. Calculate the pipeline resistance h based on the flow rate Q1 and the resistance characteristic curve;

[0046] 6. Based on the pipeline resistance h, inlet pressure P1, and back pressure P b Calculate the required head H;

[0047] 7. Based on the pump set performance curves and efficiency, determine the series or parallel connection status of the pump set, as well as the speed;

[0048] 8. Switch between series and parallel operation to adjust the pump unit speed.

[0049] Preferably, the stable operating range of the series-parallel centrifugal pumps is divided into three regions: the series region (OABECO region), the parallel region (OEDO region), and the transition region (OCEO region). Based on back pressure, inlet pressure, flow rate, pipeline resistance, etc., the series-parallel state of the pump set and the operating speed are determined. A frequency converter and an electric actuator are used to realize automatic switching and speed regulation between the series and parallel states. During operation, the pumps can be adjusted in real time according to changes in parameters such as back pressure and required flow rate, thus realizing automatic operation control of the variable back pressure series-parallel centrifugal pumps.

[0050] Preferably, to prevent fluctuations in parameters such as back pressure and inlet pressure from causing repeated switching between series and parallel states, continuous multiple data acquisitions are used to determine the series and parallel states, effectively improving the stability of the pump unit's operation.

[0051] Preferably, to prevent repeated switching between series and parallel states caused by fluctuations in parameters such as back pressure and inlet pressure, an efficiency span Δη is introduced to determine the series and parallel states in the transition zone (OCEO region), which effectively improves the stability of pump unit operation.

[0052] Preferably, the pump set automatically determines and operates within the entire stable operating range (OABEDO region) according to system needs, with a wide range of automatic operating conditions.

[0053] Preferably, in the transition zone (OCEO region), the series or parallel connection state of the pump set is selected according to the pump set operating efficiency, resulting in higher operating efficiency.

[0054] Example:

[0055] I. Equipment Introduction

[0056] The pump unit designed in this invention mainly consists of a motor 1, an outlet pressure transmitter 2, an inlet pressure transmitter 3, a series-parallel centrifugal pump 4, an electric actuator 5, and a frequency converter 6. Figure 1 As shown in the diagram. Motor 1 drives the pump rotor. Outlet pressure transmitter 2 and inlet pressure transmitter 3 measure the pump inlet and outlet pressures. Electric actuator 5 drives the switching valve 7 to rotate, achieving the switching between series and parallel operation. The series and parallel operation states and their internal flow directions are shown in the diagram. Figure 2 As shown. The frequency converter 6 is used for the start, stop, and speed control of the series-parallel pumps. It can receive back pressure and flow requirements from the upstream system, and can collect inlet and outlet pressures through the inlet pressure transmitter 3 and outlet pressure transmitter 2. It can control the series and parallel connection switching of the electric actuator 5. The rated speed of the pump set is n0, the rated flow rate in series mode is Q0, and the rated head in series mode is H0.

[0057] II. Preliminary Preparations

[0058] 1) Through testing, the hydraulic performance (flow rate, head, efficiency) of the centrifugal pump at different speeds when in series is tested. The test speeds should be evenly distributed within the possible operating speed range, such as: 100%n0, 80%n0, 60%n0, 40%n0, 20%n0.

[0059] 2) Through testing, the hydraulic performance (flow rate, head, efficiency) of the centrifugal pump at different speeds in parallel operation is tested. The test speeds should be evenly distributed within the possible operating speed range, such as: 100%n0, 80%n0, 60%n0, 40%n0, 20%n0.

[0060] 3) Determine the stable operating range of the pump in series and parallel states respectively. It can be set according to 30% to 120% of the rated flow rate at each speed. Figure 5 The OABO region represents the range of stable operation conditions in series configuration. Figure 6 The OCDO region represents the stable operating range under parallel conditions. The stable operating ranges under series and parallel conditions are combined into a single graph, forming... Figure 7 . Figure 7 The OAECO region represents the stable operating condition range in series connection (series zone), while the OEDO region represents the stable operating condition range in parallel connection (parallel zone). Both series and parallel pump sets can operate stably within the OCEO region (transition zone).

[0061] 4) Measure the pipeline resistance characteristics and establish the flow-pipeline resistance relationship curve. Generally, it can be approximated that the pipeline resistance is proportional to the quadratic method of the flow rate.

[0062] III. Overall Approach

[0063] 1) The frequency converter collects the inlet pressure P1 (absolute pressure), and receives the flow rate Q1 and back pressure P from local input or sent by the upstream system. b (Absolute pressure)

[0064] 2) Based on the flow rate Q1 and Figure 8 Calculate the pipeline resistance h.

[0065] 3) Based on the pipeline resistance h, inlet pressure P1, and back pressure P b Calculate the required head H.

[0066] 4) Based on the pump set performance curve ( Figure 7 Based on efficiency, determine the series or parallel operation of the pump set, as well as the rotational speed n. If (Q1, H) is located in the OABECO region (including the boundary line), the pump set operates in series. If (Q1, H) is located in the OABECO region (including the boundary line, excluding points O and E), the pump set operates in parallel. If (Q1, H) is located in the OEDO region (excluding the boundary line), the series or parallel operation of the pump set is determined based on efficiency.

[0067] 5) To prevent repeated switching between series and parallel states caused by parameter fluctuations, multiple continuous data acquisitions and the efficiency range Δη are used to comprehensively determine the series and parallel states.

[0068] IV. Control Strategies

[0069] (I) Startup process:

[0070] 1) Power on the frequency converter and switch to automatic operation mode;

[0071] 2) Start the pump unit;

[0072] 3) The frequency converter receives the flow rate Q1 and back pressure P from local input or from the upstream system. b The signal is collected by the inlet pressure P1. If P1 / ρg is less than the required net positive suction head, an alarm signal is issued.

[0073] 4) Based on the flow rate Q1 and Figure 8 Calculate the pipeline resistance h;

[0074] 5) Based on the pipeline resistance h, inlet pressure P1, and back pressure P b Calculate the required head H:

[0075] H = h + Pb / ρg - P1 / ρg

[0076] ρ is the density of water, and g is the acceleration due to gravity.

[0077] 6) If (Q1, H) is located in Figure 7 Central OABECO region (including boundary line):

[0078] a) If the switching valve is already in series, the electric actuator will not operate; otherwise, the frequency converter will control the electric actuator to operate, causing the switching valve to rotate into the series state (see...). Figure 2 );

[0079] b) Using the series state performance curve closest to point (Q1, H), the hydraulic performance similarity conversion algorithm is used to determine the rotational speed n corresponding to the (Q1, H) working condition;

[0080] c) Start the pump unit, increase the speed, target n;

[0081] 7) If (Q1, H) is located in Figure 7 Central OABECO region (including the boundary line, excluding points O and E):

[0082] a) If the switching valve is already in parallel operation, the electric actuator will not operate; otherwise, the frequency converter controls the electric actuator to rotate the switching valve to the parallel operation state (see...). Figure 2 );

[0083] b) Using the parallel state performance curve closest to point (Q1, H), the hydraulic performance similarity conversion algorithm is used to determine the rotational speed n corresponding to the (Q1, H) working condition;

[0084] c) Start the pump unit, increase the speed, target n;

[0085] 8) If (Q1, H) is located in Figure 7 Central OEDO region (excluding boundary lines):

[0086] a) Using the series performance curve closest to point (Q1, H), the hydraulic performance similarity conversion algorithm is used to determine the efficiency η1 and rotational speed n1 corresponding to the (Q1, H) working condition;

[0087] b) Using the parallel state performance curve closest to point (Q1, H), the hydraulic performance similarity conversion algorithm is used to determine the efficiency η2 and rotational speed n2 corresponding to the (Q1, H) working condition;

[0088] c) If η1 ≥ η2, and the switching valve is already in series, the electric actuator will not operate; otherwise, the frequency converter controls the electric actuator to rotate the switching valve into series configuration (see...). Figure 2 Start the pump unit, increase the speed, target n1;

[0089] d) If η1 < η2, and if the switching valve is already in parallel operation, the electric actuator will not operate; otherwise, the frequency converter will control the electric actuator to rotate the switching valve to the parallel operation state (see...). Figure 2 Start the pump unit, increase the speed, target n2;

[0090] 9) If (Q1, H) is not in Figure 7If the system is in a stable operating range, an alarm will be issued, and the process will return to step 3.

[0091] (II) Operation Process

[0092] 10) If the frequency converter receives a shutdown command from the local or upstream system, the pump unit will stop;

[0093] 11) Execute steps 3), 4), and 5) continuously m times (Note: Collect data continuously m times to avoid fluctuations in system pressure, pressure transmitter data, and other parameters that could cause repeated switching between series and parallel states. m can be changed according to the debugging situation).

[0094] 12) If (Q1, H) are all located in m consecutive times Figure 7 Central OABECO region (including boundary line):

[0095] ① If the switching valve is already in series, the electric actuator will not operate; otherwise, the frequency converter will control the electric actuator to operate, causing the switching valve to rotate into the series state (see...). Figure 2 );

[0096] ② Using the average of the last m (Q1, H) values, find the nearest series performance curve and use the hydraulic performance similarity conversion method to determine the rotational speed n corresponding to the average (Q1, H) value;

[0097] ③ Adjust the rotation speed, target n;

[0098] 13) If (Q1, H) are all located in m consecutive times Figure 7 Central OABECO region (including the boundary line, excluding points O and E):

[0099] ① If the switching valve is already in parallel operation, the electric actuator will not operate; otherwise, the frequency converter controls the electric actuator to rotate the switching valve to the parallel operation state (see...). Figure 2 );

[0100] ② Using the average of the last m (Q1, H) values, find the performance curve of the nearest parallel state, and use the hydraulic performance similarity conversion method to determine the rotational speed n corresponding to the (Q1, H) average working condition;

[0101] ③ Adjust the rotation speed, target n;

[0102] 14) If (Q1, H) is located m times consecutively in Figure 7 Central OEDO region (excluding boundary lines):

[0103] ① Using the average of the last m (Q1, H) values, find the nearest series performance curve, and use the hydraulic performance similarity conversion method to determine the efficiency η1 and rotational speed n1 corresponding to the average (Q1, H) values;

[0104] ② Using the average of the last m (Q1, H) values, find the performance curve of the nearest parallel state, and use the hydraulic performance similarity conversion method to determine the efficiency η2 and rotational speed n2 corresponding to the average (Q1, H) values;

[0105] ③ If η1-η2≥△η, and if the switching valve is already in series, the electric actuator will not operate; otherwise, the frequency converter controls the electric actuator to rotate the switching valve towards the series state (see...). Figure 2 Adjust the rotation speed to target n1; (Note: The efficiency span Δη is used to avoid fluctuations in system pressure, pressure transmitter acquisition and other parameters, which may cause repeated switching between series and parallel states. Δη can be changed according to the debugging situation).

[0106] ④ If η2-η1≥△η, and if the switching valve is already in parallel operation, the electric actuator will not operate; otherwise, the frequency converter will control the electric actuator to rotate the switching valve to the parallel operation state (see...). Figure 2 Start the pump unit, increase the speed, target n2;

[0107] 15) If point (Q1, H) is not in the m-times of the data collection and calculation... Figure 7 If the operating condition is within the stable range, an alarm will be issued.

[0108] 16) Return to step 10).

[0109] This invention solves the problems of existing technologies, such as only single-line or parallel connection, lack of variable back pressure adaptation, and poor stability, by implementing working condition zoning, variable back pressure adaptation, intelligent series and parallel connection determination, and dynamic adjustment. It realizes efficient, stable, and fully automatic operation of variable back pressure series and parallel centrifugal pump sets, which can be widely used in municipal water supply, industrial fluid transportation and other fields, and has significant economic and technical value.

Claims

1. An automatic operation control method for a series-parallel centrifugal pump set with variable back pressure, characterized in that, Includes the following steps: S1: Test the hydraulic performance of centrifugal pumps in series and parallel configurations at different speeds. The hydraulic performance includes flow rate, head, and efficiency. S2: Determine the stable operating range of the pump in both series and parallel states, and divide the stable operating range of the series and parallel states into three regions: series region, parallel region, and transition region. The transition region is the region where the pump can operate stably in both series and parallel states. S3: Measure the pipeline resistance characteristics and establish a flow-pipeline resistance relationship curve. S4: The frequency converter collects the centrifugal pump inlet pressure P1 and receives the target flow rate Q1 and back pressure Pb from local input or the upstream system. S5: Calculate the pipeline resistance h based on the target flow rate Q1 and the flow-pipeline resistance relationship curve. S6: Calculate the pipeline resistance h based on the pipeline resistance h, inlet pressure P1, and back pressure Pb. b Calculate the required head H of the pump set; S7: Determine the series or parallel connection status and operating speed n of the pump set based on the efficiency of the pump set performance curve; S8: Switch the series or parallel connection status of the pump set through the electric actuator, and adjust the pump set speed to the operating speed n through the frequency converter; S9: During operation, based on back pressure P b In accordance with the changes in the target flow rate Q1, repeat steps S4-S8 to adjust the series and parallel status and operating speed of the pump set in real time.

2. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In step S2, the stable operating range of the pump in series and parallel states is determined as follows: the stable operating range of the pump in series and parallel states is set according to 30% to 120% of the rated flow rate of the pump at each speed.

3. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In step S2, the series region is the region that can operate stably only in the series state, and the parallel region is the region that can operate stably only in the parallel state.

4. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In steps S7 and S9, to prevent back pressure P b Fluctuations in the inlet pressure P1 parameter cause repeated switching between series and parallel operation. By continuously collecting data multiple times, the series and parallel operation status can be determined, effectively improving the stability of the pump unit operation.

5. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In steps S7 and S9, to prevent back pressure P b Fluctuations in the inlet pressure P1 parameter cause repeated switching between series and parallel states. By introducing the efficiency span Δη, the series and parallel states in the transition region can be determined, effectively improving the stability of pump unit operation.

6. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 5, characterized in that, Calculate the efficiency η1 for the series connection and the efficiency η2 for the parallel connection respectively. If η1-η2≥△η, then choose the series connection; if η2-η1≥△η, then choose the parallel connection.

7. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In step S6, the required head H of the pump set is calculated using the formula: H = h + P b / ρg-P1 / ρg, where ρ is the density of the transported medium and g is the gravitational acceleration.

8. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In step S7, based on the pump set performance curve and efficiency, the series / parallel connection status and operating speed n of the pump set are determined. Specifically: if the target flow rate Q1 and the head H are in the series region (including the boundary line), the pump set is selected to operate in series mode; if the target flow rate Q1 and the head H are in the parallel region (including the boundary line but excluding points O and E), the pump set is selected to operate in parallel mode; if the target flow rate Q1 and the head H are in the transition region (excluding the boundary line), the pump set is selected to operate in series or parallel mode based on efficiency.

9. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, In step S8, the series and parallel states of the pump set are switched by the electric actuator. Specifically, the frequency converter outputs a control signal to the electric actuator, and the electric actuator drives the switching valve to rotate to the corresponding position. If the switching valve is already in the target series or parallel state, the electric actuator does not operate. After the switching is completed, the frequency converter adjusts the pump set speed to achieve coordinated control of switching and speed regulation.

10. The automatic operation control method for a series-parallel centrifugal pump set with variable back pressure according to claim 1, characterized in that, Step S4 also includes alarm judgment: if P1 / ρg is less than the required net positive suction head of the pump, the frequency converter will issue an alarm signal and suspend the pump group startup; if the target flow rate Q1 and head H are not in the stable operating range after multiple consecutive collections and calculations in step S9, the frequency converter will issue an alarm signal.

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