An adaptive regulating method and system for preventing cavitation of a boiler feed water pump

Abstract

The present application relates to the technical field of boiler feed water pump cavitation control, and discloses a self-adaptive adjustment method and system for preventing cavitation of a boiler feed water pump, which acquires the inlet pressure, inlet water temperature and feed water flow of the pump in real time, calculates the effective net positive suction head NPSHa and looks up the required net positive suction head NPSHr, calculates the margin coefficient and makes a judgment, and drives the frequency converter and the inlet regulating valve to adjust in coordination according to the judgment result. The system comprises a parameter acquisition unit, a self-adaptive controller, an actuator and a man-machine interaction unit. The present application can effectively prevent cavitation from occurring and reduce the energy consumption of the feed water pump by real-time dynamic calculation and coordinated adjustment of the net positive suction head, and can adapt to the nonlinear pressure variation characteristics of sliding pressure operation, and balance the rapidity of cavitation protection and the economy of operation.

Description

Technical Field

[0001] This invention relates to the field of boiler feedwater pump cavitation control technology, specifically to an adaptive adjustment method and system for preventing boiler feedwater pump cavitation. Background Technology

[0002] In the thermal power generation industry, boiler feedwater pumps are key equipment in the thermal system. Cavitation is one of the main risks threatening the safety of feedwater pumps: when the absolute pressure at the pump inlet is lower than the saturated steam pressure corresponding to the water temperature, water vaporizes to form bubbles. These bubbles collapse in the high-pressure zone, generating shock waves that cause impeller erosion, increased vibration, and decreased efficiency. In severe cases, it can lead to impeller breakage.

[0003] Net positive suction head (NPSH) is a key parameter for measuring cavitation resistance, including effective NPSHa and required NPSHr. Traditional feedwater pump cavitation protection generally adopts a fixed NPSH margin constant pressure operation mode, that is, a fixed margin value is determined according to the design conditions and maintained unchanged throughout the entire operation. However, this method has the following drawbacks: Low-load energy waste is significant. When the unit is running at low load, the deaerator pressure decreases, and the pump's NPSHr decreases accordingly. However, the fixed margin mode still operates according to the design conditions, resulting in NPSHA being much greater than the required value, causing an additional power consumption loss of 8%-15%. There is a risk of cavitation during sudden load increases. During rapid load changes, the deaerator pressure response lags behind changes in feedwater flow. If the margin is not set properly, NPSHA may momentarily drop below NPSHr, triggering cavitation. It cannot adapt to the nonlinear characteristics of sliding pressure operation. Modern large-capacity units generally adopt sliding pressure operation, where the deaerator pressure and load have a nonlinear relationship. Fixed margin strategies cannot track this change, resulting in poor control quality and lag in regulation. Traditional single-variable control has limitations. Simply adjusting the pump speed or valve opening cannot simultaneously achieve both rapid cavitation protection and economical operation.

[0004] Therefore, there is an urgent need for an adaptive adjustment method and system to prevent boiler feedwater pump cavitation in order to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to provide an adaptive adjustment method and system for preventing cavitation in boiler feedwater pumps. By real-time acquisition of three key operating parameters—feedwater pump inlet pressure, inlet water temperature, and feedwater flow rate—the effective net positive suction head (NPSHa) and the required NPSHr are calculated respectively. Based on the margin coefficient judgment result, the frequency converter and the inlet regulating valve are driven to work together to achieve the dual goals of cavitation protection and energy consumption optimization.

[0006] To achieve the above objectives, the present invention provides an adaptive adjustment method for preventing cavitation in boiler feedwater pumps, comprising the following steps: Step S1: Real-time acquisition of three key operating parameters: inlet pressure, inlet water temperature, and feedwater flow rate of the water pump. The inlet pressure is acquired by a pressure transmitter, with a measurement range covering the preset pressure range and an accuracy no lower than the preset accuracy level; the inlet water temperature is acquired by a temperature sensor, with a measurement range covering the preset temperature range and an accuracy within the preset accuracy range; and the feedwater flow rate is acquired by a flow meter, with a measurement range covering the preset flow rate range.

[0007] Step S2: Calculate the effective net positive suction head (NPSHa) based on the collected real-time parameters. This includes: calculating the absolute pressure at the pump inlet based on the pump inlet pressure and installation altitude; calculating the saturated vapor pressure based on the inlet water temperature using a saturated vapor table or the Antoni equation; calculating the hydraulic losses in the inlet pipeline based on the feedwater flow rate and pipeline characteristics; and calculating the effective net positive suction head (NPSHa) based on the pump inlet absolute pressure, saturated vapor pressure, and inlet pipeline hydraulic losses.

[0008] Step S3: Based on the water supply flow rate, dynamically retrieve the required net positive suction head (NPSHr) corresponding to the current flow rate from the preset NPSHr characteristic database.

[0009] Step S4: Calculate the real-time margin coefficient K_ratio = NPSHa / NPSHr, and compare it with the preset margin coefficient range. The preset range has a preset safety lower limit and a preset economic upper limit.

[0010] Step S5: Based on the margin coefficient determination result, drive the frequency converter and the inlet regulating valve to work together. When the margin coefficient is less than the preset safety lower limit, it is determined that there is a risk of cavitation, and the output frequency of the frequency converter and the opening of the inlet regulating valve are increased simultaneously; when the margin coefficient is within the preset range, it is determined that it is in the optimal margin range, and the current operating condition is maintained or finely adjusted to keep the margin coefficient within the preset range; when the margin coefficient is greater than the preset economic upper limit, it is determined that the margin is too large and there is energy-saving space, and the output frequency of the frequency converter is reduced and the opening of the inlet regulating valve is decreased simultaneously.

[0011] In a preferred embodiment of the present invention, a valve position protection and frequency conversion compensation step is also included: when the opening degree of the inlet regulating valve exceeds the preset safe opening degree range, further adjustment of the valve opening degree is stopped, and the frequency converter independently performs compensation adjustment to maintain the margin coefficient within the preset range.

[0012] In a preferred embodiment of the present invention, when the pump is in the optimal margin range and the fine-tuning mode is selected, fine-tuning optimization is performed with the goal of maximizing the efficiency of the water supply pump. This includes gradually reducing the speed and recording changes in pump efficiency. When the efficiency begins to decline, the speed is returned to the previous steady state, and the optimal operating speed is finally determined. During the fine-tuning process, the margin coefficient is monitored in real time to ensure that the speed does not fall below the safety lower limit.

[0013] In a preferred embodiment of the present invention, a parameter self-tuning step is further included: determining the load change direction based on the trend of water flow rate change within a preset time period; increasing the regulation rate parameter when the load shows an upward trend; and decreasing the regulation rate parameter when the load tends to stabilize to avoid oscillation. The threshold for determining load increase / decrease is 3%-8% of the rated flow rate, and the stability threshold is 1%-3% of the rated flow rate; the rate adjustment range is 40%-60% (increase) and 20%-40% (decrease).

[0014] On the other hand, the present invention provides an adaptive adjustment system for preventing cavitation in boiler feedwater pumps, comprising: The system comprises a parameter acquisition unit, an adaptive controller, and actuators. The parameter acquisition unit includes an inlet pressure transmitter, an inlet temperature sensor, an electromagnetic flowmeter, and a feedwater pump outlet pressure transmitter, all outputting standard 4-20mA signals. The adaptive controller includes a signal conditioning module, a saturated vapor pressure calculation module, an NPSHA calculation module, an NPSHAr lookup module, a margin judgment module, and a collaborative control module. The actuators include a feedwater pump frequency converter and an inlet regulating valve. The collaborative control module generates collaborative control commands based on the judgment results of the margin judgment module to synchronously adjust the frequency converter output and the inlet regulating valve opening, ensuring that the real-time margin coefficient approaches and remains within a preset margin coefficient range. The system may also include a human-machine interface unit for displaying operating parameters, alarm information, and parameter settings.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention achieves online dynamic monitoring of NPSH by real-time calculation of the effective NPSH and dynamic retrieval of the required NPSH. Furthermore, it achieves the dual goals of cavitation protection and energy optimization through the coordinated adjustment of the frequency converter and regulating valve. First, the real-time calculation of the effective NPSH comprehensively considers multiple factors such as inlet pressure, water temperature, flow rate, and pipeline losses, accurately reflecting the true NPSH margin state at the pump inlet and eliminating the shortcomings of traditional fixed margin methods that cannot track changes in operating conditions. Second, the dynamic retrieval of the required NPSH is based on a pre-set characteristic curve database, enabling adaptive adjustment of the required NPSH value according to real-time flow, matching the nonlinear characteristics of sliding pressure operation. Third, the coordinated adjustment of the frequency converter and regulating valve utilizes both the influence of speed regulation on NPSHr and the direct influence of valve regulation on NPSHA, achieving a balance between speed and economy. Experimental verification shows that this invention can reduce feedwater pump power consumption by more than 12% under low-load conditions, with a regulation response time of less than 2 seconds.

[0016] This invention also employs interval discrimination by setting a margin coefficient range, enabling precise identification of three different operating conditions: cavitation risk condition, optimal margin condition, and energy-saving space condition. This interval discrimination strategy allows the control system to take differentiated adjustment actions for different conditions, avoiding the coarse, either-or control of traditional methods and improving control accuracy and adaptability. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structural framework of the adaptive adjustment system in this invention; Figure 2 This is a schematic diagram of the framework of the adaptive collaborative adjustment principle based on dynamic discrimination of margin coefficient in this invention; Figure 3 This is a logical framework diagram of the adjustment process from online parameter acquisition to valve position protection in this invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer and more complete, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention in any way. Example 1

[0019] An adaptive control system for preventing cavitation in boiler feedwater pumps includes a parameter acquisition unit, an adaptive controller, an actuator, and a human-machine interface unit. The units interact with each other and transmit commands through a standard signal interface.

[0020] The parameter acquisition unit is used to collect operating parameters at the inlet of the feedwater pump, including inlet pressure, inlet water temperature, and feedwater flow rate. The inlet pressure is acquired by a pressure transmitter installed on the feedwater pump inlet pipeline. The transmitter's range is selected according to actual operating conditions, for example, 0 to 1.6 MPa, with an accuracy class of not less than 0.2, and outputs a standard 4-20mA signal. The inlet water temperature is acquired by a temperature sensor installed on the feedwater pump inlet pipeline. The temperature sensor can be a Pt100 platinum resistance thermometer, with a temperature range of, for example, 0 to 150℃, and an accuracy of ±0.1℃. This temperature temperature is converted into a 4-20mA signal output by the temperature transmitter. The feedwater flow rate is acquired by a flow meter installed on the feedwater pump inlet pipeline. The flow meter can be an electromagnetic flow meter, with its range selected according to the feedwater pump's rated flow rate, and outputs a standard 4-20mA signal. In addition, the parameter acquisition unit also includes a pressure transmitter (range 0 to 20 MPa) installed on the feedwater pump outlet pipeline for real-time acquisition of the outlet pressure, which is used in conjunction with the inlet pressure to calculate the head H.

[0021] The adaptive controller is implemented using a PLC or DCS and includes a signal conditioning module, a saturated vapor pressure calculation module, an NPSHA calculation module, an NPSHR lookup module, a margin discrimination module, and a cooperative control module.

[0022] The NPSHa calculation module calculates the effective net positive suction head (NPSHa) in real time based on inlet pressure, inlet water temperature, and feedwater flow rate. The NPSHr lookup module dynamically determines the required NPSHr for the feedwater pump at a given feedwater flow rate. This module contains pre-set flow-required NPSHr characteristic data for the feedwater pump. This data can be obtained from hydraulic performance test reports provided by the pump manufacturer or through on-site hydraulic performance calibration tests. It includes multiple discrete flow points and their corresponding required NPSHr values, covering a range from minimum flow rate to 120% of rated flow rate. The number of discrete flow points can be adjusted adaptively according to the degree of change in the pump's characteristic curve. For pump types with gentle characteristic curve changes, fewer discrete points can be used; for pump types with drastic characteristic curve changes, more discrete points are recommended.

[0023] The margin discrimination module calculates the real-time margin coefficient, which is the ratio of the effective net positive suction head (NPSHa) to the required NPSHr, and compares it with a preset margin coefficient range. The collaborative control module generates collaborative control commands based on the discrimination results from the margin discrimination module. These commands are used to drive the actuators to perform collaborative adjustment actions. The determination principles for the first, second, third, and fourth rates are as follows: under cavitation risk conditions, the rate is proportional to the degree to which the current margin coefficient deviates from the preset safety lower limit; under energy-saving optimization conditions, the rate is proportional to the degree to which the margin coefficient deviates from the preset economic upper limit.

[0024] Preferably, the adaptive controller may further include a parameter self-tuning module for determining the direction of load change based on the trend of water flow rate change, and adjusting the regulation rate parameter in the coordinated regulation command accordingly.

[0025] The actuators include a feedwater pump frequency converter and an inlet regulating valve. The frequency converter is a voltage-type AC-DC-AC frequency converter; the inlet regulating valve is an electric regulating valve that receives 4-20mA control signals.

[0026] The human-machine interface unit includes a touchscreen and a communication interface, used to display operating parameters, alarm information, and parameter settings. The communication interface can interface with the DCS system using the Modbus RTU protocol to achieve data upload and remote control functions.

[0027] An adaptive adjustment method for preventing cavitation in boiler feedwater pumps includes the following steps: Step S1: Online parameter acquisition.

[0028] Each sensor in the parameter acquisition unit samples data according to a preset control cycle, which can be set according to actual needs, for example, from 50ms to 200ms. In this embodiment, the control cycle is set to 100ms. Within each control cycle, three real-time operating parameters are acquired: inlet pressure P_gauge, inlet water temperature T_in, and feedwater flow rate Q.

[0029] The signal conditioning module filters and performs range conversion on the acquired analog signals. Filtering can use a second-order low-pass filter with a cutoff frequency set to 10Hz, effectively suppressing power frequency interference and random noise. Range conversion converts 4-20mA signals into corresponding engineering unit values: pressure signals to MPa, temperature signals to °C, and flow signals to m³ / h. The processed engineering values ​​are stored in the controller's process image area for subsequent modules to access.

[0030] Step S2: Calculate the effective net positive suction head (NPSHa) in real time based on the inlet pressure, inlet water temperature, and feedwater flow rate.

[0031] The calculation of the effective net positive suction head (NPSHa) is performed by the NPSHA calculation module, which includes the following sub-steps.

[0032] First, determine the absolute pressure at the pump inlet, P_abs. The absolute pressure at the pump inlet is determined by the inlet pressure P_gauge, the local atmospheric pressure, and the installation altitude. The calculation formula is: P_abs = P_gauge + P_atm - H_a × k Where P_atm is standard atmospheric pressure, H_a is the pump installation altitude, and k is the atmospheric pressure correction factor per meter of altitude (valued at 0.012 MPa / m). When the altitude is low and the correction is negligible relative to atmospheric pressure, P_abs = P_gauge + P_atm can be directly taken.

[0033] Secondly, the saturated vapor pressure P_v is determined. Based on the inlet water temperature T_in, the corresponding saturated vapor pressure P_v is determined by consulting a pre-set saturated vapor pressure table. The saturated vapor pressure table can be established based on the IAPWS-IF97 international standard for water and vapor properties, and pre-sets saturated vapor pressure data for multiple discrete temperature points within the range of 0℃ to 150℃. When looking up the table, two adjacent discrete temperature points T_i and T_{i+1} (T_i ≤ T_in < T_{i+1}) are determined based on T_in, and the accurate P_v value is calculated using linear interpolation. When the inlet water temperature exceeds the range of the pre-set table, the calculation can be automatically switched to the Antoni equation for extrapolation.

[0034] Next, determine the hydraulic loss Δh_f in the inlet pipe. Based on the water supply flow rate Q and the characteristics of the inlet pipe, calculate the hydraulic loss Δh_f in the inlet pipe. The calculation formula is: Δh_f = K × Q² / (2gA²) Where K is the pipeline resistance coefficient, determined by on-site calibration tests. The specific method is as follows: under rated flow conditions, measure the pressure difference between the pump inlet and the pressure at a reference point upstream, and calculate the K value using the known flow rate and pipeline parameters. Calibration tests should be completed before the new system is put into operation, and subsequently retested every six months. The value of K ranges from 0.01 to 0.05; g is the acceleration due to gravity; A is the pipeline cross-sectional area, calculated from the inner diameter of the inlet pipe.

[0035] Finally, the effective net positive suction head (NPSHa) is determined. The calculation formula is: NPSHa = (P_abs × - P_v) / (ρg) - Δh_f Where ρ is the density of water and g is the acceleration due to gravity. The calculation results are in meters and represent the actual available cavitation margin at the pump inlet. The NPSHa calculation module stores the calculation results in the process image area for subsequent modules to use.

[0036] Step S3: Dynamically determine the required net positive suction head (NPSHr).

[0037] The NPSHr lookup module dynamically determines the required net positive suction head (NPSHr) of the feedwater pump at the given feedwater flow rate based on the real-time feedwater flow rate.

[0038] This module contains pre-set flow rate-required net positive suction head (NPSH) characteristic data for the feedwater pump. This data is stored in array format and can contain, for example, 50 discrete flow rate points, covering a range from 0% to 120% of the rated flow rate. Each flow rate point corresponds to a required NPSH value obtained through hydraulic performance testing. The characteristic data can be obtained from hydraulic performance test reports provided by the feedwater pump manufacturer or through on-site hydraulic performance calibration tests. The number of discrete flow rate points can be adaptively adjusted according to the degree of variation in the feedwater pump's characteristic curve.

[0039] During the retrieval process, the location of the real-time feedwater flow rate Q in the characteristic data is determined. Specifically, two adjacent discrete flow rate points Q_i and Q_{i+1} are identified, where Q_i ≤ Q < Q_{i+1}. The required net positive suction head (NPSH) values ​​NPSHr_i and NPSHr_{i+1} corresponding to these two flow rate points are obtained. Then, the required NPSHr corresponding to the current flow rate Q is determined using a linear interpolation algorithm. NPSHr = NPSHr_i + (NPSHr_{i+1} - NPSHr_i) × (Q - Q_i) / (Q_{i+1} -Q_i) This interpolation algorithm performs linear fitting between adjacent discrete points, and the interpolation accuracy meets engineering requirements. Besides the aforementioned pre-set database lookup method, the dynamic determination of the required net positive suction head (NPSH) can also be achieved through other methods such as online solving of the pump-hydraulic model or a neural network-based prediction model. The pre-set database lookup method is one preferred implementation.

[0040] Step S4: Calculation and determination of margin coefficient.

[0041] The margin discrimination module calculates the real-time margin coefficient K_ratio = NPSHa / NPSHr and compares and discriminates it with the preset margin coefficient range.

[0042] The preset margin coefficient range is defined by a preset safety lower limit K_L and a preset economic upper limit K_H. The value of K_L should ensure that the feedwater pump does not experience cavitation, and its range is 1.10 to 1.15. The value of K_H should achieve economical operation under safe conditions, and its range is 1.20 to 1.25. In this embodiment, K_L is 1.10 and K_H is 1.20.

[0043] The discrimination logic compares K_ratio with the preset interval [K_L, K_H] and outputs three discrimination results: When K_ratio < K_L, it is determined to be a cavitation risk state; When K_L ≤ K_ratio ≤ K_H, it is determined to be the optimal margin state; When K_ratio > K_H, it is determined to be in an energy-saving optimization state.

[0044] Preferably, to avoid frequent system control state switching caused by fluctuations in the margin coefficient near the threshold, hysteresis control logic is set at the state transition boundary. Specifically, a first hysteresis value Δ1 and a second hysteresis value Δ2 are set; in this embodiment, both Δ1 and Δ2 are set to 0.05. When K_ratio increases from the region less than K_L, K_ratio > K_L + Δ1 is required to determine that the system has entered the optimal margin interval; when K_ratio decreases from the region greater than K_H, K_ratio < K_H - Δ2 is required to determine that the system has entered the optimal margin interval.

[0045] Step S5: Coordinate and adjust the execution of actions.

[0046] Based on the judgment result of the margin judgment module, the collaborative control module generates collaborative control instructions, which are used to drive the actuator to perform the following collaborative adjustment actions: When the judgment result is a cavitation risk state (K_ratio < K_L): The first regulation mode is executed. The collaborative control module generates collaborative control commands to increase the inverter output frequency and increase the opening of the inlet regulating valve. Increasing the inverter output frequency increases the pump speed, reducing the required net positive suction head (NPSHr) of the feedwater pump at this flow rate; increasing the opening of the inlet regulating valve reduces inlet pipeline resistance loss, increases pump inlet pressure, and thus increases the effective NPSHa. The combined effect of these two factors allows the margin coefficient to quickly recover to the safe range.

[0047] At this point, the first rate (the rate at which the inverter output frequency increases) is determined by the following formula: V_1 = 0.3 + (K_L - K_ratio) × 0.4, and V_1 ≤ 1.0 Hz / s. When K_ratio = K_L, V_1 = 0.

[0048] The second rate (the rate of increase in the opening of the inlet regulating valve) is determined by the following formula: V_2 = 0.5% + (K_L - K_ratio) × 0.5%, and V_2 ≤ 2% / s. When K_ratio = K_L, V_2 = 0.

[0049] In the above formula, K_L is set to 1.10 (if the preset safety lower limit is set to other values, the benchmark value in the formula will be adjusted accordingly, but the coefficients will remain unchanged). The rate is directly proportional to the degree to which the margin coefficient deviates from the safety lower limit; the greater the deviation, the greater the rate, thus achieving a smooth transition.

[0050] When the judgment result is an energy-saving optimization state (K_ratio > K_H): The second regulation mode is executed. The collaborative control module generates collaborative control commands to reduce the inverter output frequency and decrease the opening of the inlet regulating valve. Reducing the inverter output frequency directly reduces the pump's power consumption; decreasing the inlet regulating valve opening reduces inlet throttling losses. The two work together to achieve energy-saving operation while ensuring cavitation safety.

[0051] The third rate (inverter frequency reduction rate) is determined by the following formula: V_3 = 0.3 + (K_ratio - K_H) × 0.4, and V_3 ≤ 1.0 Hz / s. When K_ratio = K_H, V_3 = 0.

[0052] The fourth rate (the rate at which the inlet regulating valve opening decreases) is determined by the following formula: V_4 = 0.5% + (K_ratio - K_H) × 0.5%, and V_4 ≤ 2% / s. When K_ratio = K_H, V_4 = 0.

[0053] In the above formula, K_H is set to 1.20 (if the preset economic upper limit is set to other values, the benchmark value in the formula will be adjusted accordingly, but the coefficients will remain unchanged). The rate of change is proportional to the degree to which the margin coefficient deviates from the economic upper limit; the greater the deviation, the greater the rate of change, thus achieving a smooth transition.

[0054] When the judgment result is the optimal margin state (K_L ≤ K_ratio ≤ K_H): The third adjustment mode is executed. The collaborative control module generates control commands to cause the system to perform one of the following two operations: maintain the current inverter output frequency and inlet regulating valve opening unchanged; or make fine adjustments with the goal of maximizing the efficiency of the feedwater pump.

[0055] As the preferred option: When in the optimal margin state and fine-tuning mode is selected, the goal is to optimize the efficiency of the pump to achieve the highest possible efficiency. Specific steps include: The inverter output frequency is gradually reduced by a preset step size, with each reduction step not exceeding 0.2Hz, for example, 0.1Hz; After each frequency reduction, wait for a preset number of control cycles (e.g., 3 cycles) to allow the system to stabilize, and then calculate the real-time operating efficiency η of the feedwater pump. The formula for calculating the efficiency is: η = ρgQH / P Where ρ is the density of water, g is the gravitational acceleration, Q is the water flow rate, H is the head of the water pump, which is calculated by combining the difference between the water pump outlet pressure P_outlet and the inlet pressure P_gauge with the flow velocity head, and P is the motor input power, which is collected in real time by a power transmitter. When a decrease in efficiency η is detected relative to the previous value, it indicates that the point of highest efficiency has been exceeded. At this time, the inverter output frequency is increased back to the frequency value before the efficiency decrease, and this frequency value is locked as the optimal operating frequency and maintained at this frequency.

[0056] This efficiency optimization strategy can adjust the operating point of the feedwater pump to near its highest efficiency while ensuring cavitation safety, thereby further reducing operating energy consumption. It is important to note that before performing efficiency optimization, the current margin coefficient must be confirmed to be ≥ the lower safety limit + 0.05. During the optimization process, the margin coefficient should be monitored in real time. If the margin coefficient drops to within the lower safety limit + 0.02, the optimization should be stopped immediately and the frequency should be reverted to the previous frequency.

[0057] As the preferred option two: The method of this invention may also include valve position protection and frequency conversion compensation functions. Specific steps include: The safe opening range of the inlet regulating valve is preset. This safe opening range has a lower limit opening threshold and an upper limit opening threshold. For example, the lower limit opening threshold is 20% and the upper limit opening threshold is 95%. Real-time monitoring of the actual opening degree of the inlet regulating valve; When the actual opening degree is less than or equal to the lower limit opening degree threshold, the reduction of the inlet regulating valve opening degree is stopped, and the valve opening degree is locked at the current value. After that, the frequency converter independently increases the margin coefficient by increasing the output frequency. When the actual opening degree is greater than or equal to the upper limit opening degree threshold, the opening degree of the inlet regulating valve is stopped from increasing, and the valve opening degree is locked at the current value. After that, the frequency converter independently reduces the margin coefficient by reducing the output frequency.

[0058] This protection mechanism can prevent the valve from deteriorating its throttling characteristics when it is close to the fully closed position, or losing its regulating ability when it is close to the fully open position. At the same time, the frequency converter independently compensates to ensure that the system can still maintain the margin coefficient within the target range under this abnormal operating condition.

[0059] As the preferred option three: The method of the present invention may also include a parameter self-tuning function. Specific steps include: Collect water supply flow values ​​for N consecutive control cycles, where N≥10, for example, N=30; Based on the N water supply flow rates, determine the characteristic value of the water supply flow rate variation trend. This characteristic value can be the slope of the flow rate change obtained by fitting with the least squares method, or it can be other statistical characteristic values ​​that can characterize the variation trend, such as the moving average of the difference between adjacent periods; When the trend characteristic value indicates that the water supply flow rate is increasing and the increase is greater than the first threshold, it is determined that the load is increasing, and the adjustment rate parameter (such as the frequency change rate and the opening change rate) in the coordinated adjustment command is increased by a first percentage, for example, by 50%. When the trend characteristic value indicates that the water flow rate is decreasing and the decrease is greater than the second threshold, it is determined that the load is decreasing, and the adjustment rate parameter is reduced by the second percentage, for example, by 30%. When the absolute value of the trend characteristic value is less than or equal to the third threshold, the load is determined to be stable, and the current adjustment rate parameter is maintained. The first and second thresholds can be set to 5% of the rated flow, and the third threshold can be set to 2% of the rated flow.

[0060] This parameter self-tuning mechanism enables the system to respond faster when the load changes rapidly, and reduces unnecessary adjustment actions when the load is stable, thereby improving the control quality under variable load conditions. Example 2

[0061] This embodiment describes the industrial application of the present invention in a 300MW thermal power unit, and verifies the technical effects of the present invention through actual operating data.

[0062] The unit is equipped with two 100% capacity boiler feedwater pumps, one in operation and one on standby. The feedwater pump has a rated flow rate of 850 m³ / h, a rated head of 1100 m, a rated speed of 2980 r / min, and a matching motor power of 4500 kW. The feedwater pump inlet pipe diameter is DN250, the pipe material is 20G boiler steel, the design pressure is 1.6 MPa, and the design temperature is 150℃. The inlet pressure transmitter has a range of 0 to 1.6 MPa and is installed approximately 2 m from the pump inlet, along with a condensate tank and pressure tapping pipe. The inlet temperature sensor has a measurement range of 0 to 150℃ and is installed on the pipe wall approximately 1 m from the pump inlet, with the temperature sensing sleeve extending approximately 50 mm into the center of the pipe. The electromagnetic flow meter has a range of 0 to 1200 m³ / h and is installed on a straight pipe section approximately 5 m from the pump inlet, with the lengths of the preceding and following straight pipe sections meeting the requirements. The outlet pressure transmitter has a range of 0 to 20 MPa and is installed at the pump outlet flange.

[0063] During the industrial test, the control system operated continuously with a control cycle of 100ms, collecting inlet pressure, temperature, and flow parameters in real time, calculating NPSHa and NPSHr, determining the margin coefficient, and driving the frequency converter and regulating valve for coordinated adjustment. The test period was 30 days, during which operating data were recorded under three typical operating conditions: full load, sliding pressure operation, and low load peak shaving.

[0064] Under full load conditions, the unit load rate is 100%, the main steam pressure is 15.7 MPa, the main steam temperature is 571℃, the feedwater flow rate is approximately 800 m³ / h, and the deaerator pressure is 0.12 MPa. Under the traditional fixed margin operation mode, the fixed NPSH margin value determined according to the design conditions is 40 m, corresponding to a margin coefficient K value of approximately 1.25. Based on real-time calculations, the system of this invention stably controls the margin coefficient at approximately 1.18, with the inverter output frequency at approximately 48.5 Hz (rated frequency 50 Hz) and the regulating valve opening at approximately 75%. Compared to the fixed margin method, the system of this invention reduces the inverter frequency by 1.5 Hz and the regulating valve opening by 5%, achieving a reduction in energy consumption. Tests show a 3.2% reduction in power consumption under full load conditions, equivalent to a reduction in coal consumption of approximately 1.2 g / kWh. Based on 6000 hours of operation per year, this translates to an annual saving of approximately 216 tons of standard coal.

[0065] Under sliding pressure operation, the unit load rate is 75%, the main steam pressure is 11.5 MPa, the main steam temperature is 567℃, the feedwater flow rate is approximately 600 m³ / h, and the deaerator pressure is 0.08 MPa. In traditional fixed margin operation, the margin coefficient K is approximately 1.30. Based on real-time calculations, this invention's system stably controls the margin coefficient at around 1.15, with the inverter output frequency at approximately 45 Hz and the regulating valve opening at approximately 65%. Tests show that power consumption is reduced by 8.7% under sliding pressure operation, equivalent to a reduction in coal consumption of approximately 3.5 g / kWh. This is because the deaerator pressure decreases during sliding pressure operation, increasing the actual margin under fixed margin conditions. This invention's system can dynamically track this change and reduce the margin setpoint, thereby achieving greater energy savings.

[0066] Under low-load peak-shaving conditions, the unit load rate is 50%, the main steam pressure is 7.8 MPa, the main steam temperature is 560℃, the feedwater flow rate is approximately 400 m³ / h, and the deaerator pressure is 0.05 MPa. Under traditional fixed-margin operation, the margin coefficient K is approximately 1.35. Based on real-time calculations, the system of this invention stably controls the margin coefficient at around 1.12, with the inverter output frequency at approximately 42 Hz and the regulating valve opening at approximately 55%. Tests show a 12.4% reduction in power consumption under low-load peak-shaving conditions, equivalent to a reduction in coal consumption of approximately 5.8 g / kWh. The energy-saving effect is most significant under low-load conditions because the required net positive suction head (NPSH) of the feedwater pump decreases as the flow rate decreases. The margin waste caused by the fixed-margin method is more severe under low load, making the dynamic adjustment advantage of the system of this invention more apparent.

[0067] During the entire test, no cavitation alarm events occurred in the system. The pump vibration value remained stable below 0.05 mm (peak-to-peak), a reduction of more than 40% compared to the traditional fixed margin operation mode. No cavitation erosion traces were found during impeller inspection, verifying the effectiveness of the cavitation protection of the system of this invention. Vibration measurements were performed using a portable vibration analyzer, measuring the effective values ​​of vibration velocity in both horizontal and vertical directions at the pump bearing housing, including both the drive end and the non-drive end.

[0068] In the load change rate test, the simulated unit underwent a variable load test at a load change rate of 30% / min. During the test, the feedwater flow rate changed from 500 m³ / h to 800 m³ / h within 30 seconds, with a deaerator pressure response lag of approximately 5 seconds. Throughout the entire load change process, the margin coefficient remained above 1.1, and no cavitation risk alarm occurred. The control system responded quickly and adjusted smoothly, with a response time of approximately 2 seconds from detecting the margin change to completing the adjustment, verifying the good adaptability of the system to sliding pressure operation.

[0069] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments that can be applied to other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An adaptive adjustment method for preventing cavitation in boiler feedwater pumps, characterized in that, Includes the following steps: S1: Real-time acquisition of water pump inlet pressure, inlet water temperature, and water flow rate; S2: Calculate the effective net positive suction head (NPSHa) based on the inlet pressure, inlet water temperature, and feedwater flow rate; S3: Based on the water supply flow rate, dynamically retrieve the required net positive suction head (NPSHr) corresponding to the current flow rate from the preset NPSHr characteristic database; S4: Calculate the margin coefficient K_ratio = NPSHa / NPSHr, and compare the margin coefficient with the preset margin coefficient range to obtain the discrimination result, wherein the preset range has a preset safety lower limit and a preset economic upper limit; S5: Based on the discrimination result, execute the coordinated adjustment action, the coordinated adjustment action including: When the margin coefficient is less than the preset safety lower limit, the output frequency of the water pump inverter and the opening of the inlet regulating valve are increased simultaneously. When the margin coefficient is greater than the preset economic upper limit, the output frequency of the inverter is reduced and the opening of the inlet regulating valve is reduced simultaneously. When the margin coefficient is between the preset safety lower limit and the preset economic upper limit, the third adjustment mode is executed: maintain the current inverter output frequency and inlet regulating valve opening, or make fine adjustments with the goal of maximizing the efficiency of the water pump, so that the margin coefficient is kept within the preset range.

2. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, The calculation of the effective net positive suction head NPSHa in S2 specifically includes: S201: Based on the inlet pressure P_gauge and the pump installation altitude H_a, calculate the inlet absolute pressure P_abs according to the formula: P_abs = P_gauge + P_atm - H_a × k, where P_atm is the standard atmospheric pressure and k is the atmospheric pressure altitude correction factor. S202: Based on the inlet water temperature T_in, the saturated vapor pressure P_v is obtained by querying a preset saturated vapor pressure table and using linear interpolation; S203: Based on the water supply flow rate Q, the pipeline resistance coefficient K, and the pipeline cross-sectional area A, calculate the hydraulic loss Δh_f of the inlet pipeline according to the formula Δh_f = K × Q² / (2gA²), where g is the acceleration due to gravity. S204: According to the formula NPSHa = (P_abs × - P_v) / (ρg) - Δh_f calculates the effective net positive suction head (NPSH), where ρ is the density of water.

3. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, The S3 dynamic retrieval of the required net positive suction head (NPSHr) specifically includes: A pre-set NPSHr characteristic database stores at least 20 discrete flow points and their corresponding required net positive suction head (NPSH) values ​​in array form, the discrete flow points covering the range of 0% to 120% of the rated flow of the feedwater pump; Based on the real-time water flow rate Q, determine the two adjacent discrete flow points Q1 and Q2 in the database, as well as the corresponding required net positive suction head (NPSHr1 and NPSHr2). The NPSHr corresponding to the current flow rate is calculated using the linear interpolation formula NPSHr = NPSHr1 + (NPSHr2 - NPSHr1) × (Q - Q1) / (Q2 - Q1).

4. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, The preset safety lower limit ranges from 1.10 to 1.15, and the preset economic upper limit ranges from 1.20 to 1.

25.

5. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, The coordinated adjustment action in S5 specifically includes: When the margin coefficient K_ratio is less than the preset safety lower limit, it is determined to be a cavitation risk state, and the first adjustment mode is executed: the output frequency of the frequency converter is increased at a first rate, and the opening of the inlet regulating valve is increased at a second rate. When the margin coefficient K_ratio is greater than the preset economic upper limit, it is determined to be in an energy-saving optimization state, and the second adjustment mode is executed: the output frequency of the inverter is reduced at a third rate, and the opening of the inlet regulating valve is reduced at a fourth rate. When the margin coefficient K_ratio is between the preset safety lower limit and the preset economic upper limit, it is determined to be in the optimal margin state, and the third adjustment mode is executed: The third adjustment mode includes a maintenance mode and a fine-tuning mode; In maintenance mode, the current inverter output frequency and inlet regulating valve opening remain unchanged; In fine-tuning mode, optimization is performed with the goal of maximizing the efficiency of the water pump. Specifically, this includes: gradually reducing the output frequency of the frequency converter, with each reduction step not exceeding 0.2Hz; calculating the real-time operating efficiency η of the water pump after each reduction; and when a decrease in the efficiency η is detected relative to the previous value, increasing the output frequency of the frequency converter back to the frequency value before the decrease and locking this frequency value as the optimal operating frequency.

6. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 5, characterized in that, The first rate and the third rate are determined by the following formula: V = 0.3 + ΔK × 0.4, and V ≤ 1.0 Hz / s, where ΔK = K_L - K_ratio when in cavitation risk state, and ΔK = K_ratio - K_H when in energy-saving optimization state; the second rate and the fourth rate are determined by the following formula: V_valve = 0.5% + ΔK × 0.5%, and V_valve ≤ 2% / s.

7. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, It also includes valve position protection and frequency conversion compensation steps: The safe opening range of the inlet regulating valve is preset, and the safe opening range has a lower limit opening threshold and an upper limit opening threshold; Real-time monitoring of the actual opening degree of the inlet regulating valve; When the actual opening degree is less than or equal to the lower limit opening degree threshold, the reduction of the opening degree of the inlet regulating valve is stopped, and the frequency converter independently increases the margin coefficient by adjusting the output frequency. When the actual opening degree is greater than or equal to the upper limit opening degree threshold, the opening degree of the inlet regulating valve is stopped from increasing, and the frequency converter independently reduces the margin coefficient by adjusting the output frequency.

8. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, It also includes a parameter self-tuning step: Collect water supply flow values ​​for N consecutive control cycles, where N≥10; Based on the N water supply flow rates, determine the characteristic value of the water supply flow rate variation trend; When the trend characteristic value indicates that the water flow rate is increasing and the increase is greater than the first threshold, it is determined that the load is increasing, and the rate of change of the inverter output frequency in S5 is increased by the first percentage. When the trend characteristic value indicates that the water flow rate is decreasing and the decrease is greater than the second threshold, it is determined that the load is decreasing, and the rate of change of the inverter output frequency in S5 is reduced by the second percentage. When the absolute value of the trend characteristic value is less than or equal to the third threshold, the load is determined to be stable and the current rate of change is maintained. The first threshold and the second threshold are 3%-8% of the rated flow rate; the third threshold is 1%-3% of the rated flow rate; the first percentage is 40%-60%; and the second percentage is 20%-40%.

9. The adaptive adjustment method for preventing boiler feedwater pump cavitation according to claim 1, characterized in that, S1 to S5 are executed cyclically with a fixed control cycle, which is 50ms to 200ms.

10. An adaptive adjustment system for preventing cavitation in boiler feedwater pumps, characterized in that, It includes a parameter acquisition unit, an adaptive controller, and an actuator; The parameter acquisition unit includes an inlet pressure transmitter, an inlet temperature sensor, and an electromagnetic flow meter, all of which output standard signals to the adaptive controller; the parameter acquisition unit also includes a feedwater pump outlet pressure transmitter, used to acquire the outlet pressure in real time to calculate the head H; The adaptive controller includes a signal conditioning module, a saturated vapor pressure calculation module, an NPSHA calculation module, an NPSHr lookup module, a margin discrimination module, and a cooperative control module; The actuator includes a water pump frequency converter and an inlet regulating valve; The collaborative control module is used to generate collaborative control commands for synchronously adjusting the output frequency of the frequency converter and the opening of the inlet regulating valve based on the discrimination result of the margin discrimination module, so that the real-time margin coefficient approaches and is maintained within the preset margin coefficient range.

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