Method for detecting cavitation and / or a bubbling-in or inflow of air within a hydraulic system

Abstract

This invention relates to a method for detecting cavitation and / or a bubbling-in or inflow of air within a hydraulic system comprising at least one electromotively driven pump for conveying a liquid medium to be conveyed, comprising the steps of: - detecting at least one operating parameter of the drive motor that influences the torque of the pump drive, - determining the variance of the detected operating parameter in a defined observation period, and - detecting cavitation and / or a bubbling-in or inflow of air in the hydraulic system if the determined variance value exceeds a defined threshold.

Description

[0001] Description

[0002] Method for detecting cavitation and / or air bubbling or

[0003] Air inflow within a hydraulic system

[0004] The invention relates to a method for detecting cavitation and / or air bubbling or air inflow within a hydraulic system comprising at least one electric motor-driven pump for conveying a liquid conveying medium within the hydraulic system.

[0005] Modern pumps are increasingly being equipped with comprehensive functions for monitoring the current pump operation, as well as more advanced functions for monitoring the entire hydraulic system. Potential anomalies in the pump or within the hydraulic system should be detected efficiently and quickly, enabling alarms to be generated or countermeasures to be taken if necessary.

[0006] One possible anomaly, for example, is the occurrence of cavitation, which can occur in the area of ​​the impeller of centrifugal pumps. Cavitation is the formation and dissipation of vapor-filled cavities in the pumped medium, which can lead to high-frequency shocks on the pump impeller. The persistent presence of cavitation can lead to mechanical damage to the pump, particularly the impeller, so it is desirable to detect such a phenomenon early on in order to be able to remedy the situation. Air bubbling or air flow into the hydraulic system can also lead to shocks on the impeller and mechanical damage. The occurrence of cavitation in hydraulic systems, especially in closed hydraulic systems, is primarily promoted by a drop in pressure at the pump's suction port.Against this background, the idea is to use detected cavitation to also infer a possible negative pressure situation in the suction area of ​​the pump, which ultimately also enables the detection of a leak within the closed system.

[0007] The object of the present invention is to be able to reliably and effectively detect the anomalies described above using a novel method.

[0008] This object is achieved by a method according to the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims.

[0009] According to the invention, a suitable operating parameter of the electric motor-driven pump is continuously or cyclically recorded during pump operation within the hydraulic system. The operating parameter to be recorded is an operating parameter that either characterizes the torque of the pump drive or at least has a significant dependency on or influence on the resulting torque of the pump drive.

[0010] For the recorded operating parameter, a possible variance of the operating parameter is then determined over a defined observation period. In other words, the degree of any potential signal fluctuation or noise in the recorded operating parameter is determined by determining the variance. The determined variance value serves as an indicator for the occurrence of cavitation in the pump and / or air bubbles / air inflows into the hydraulic system. Since cavitation caused by vacuum or high-frequency shocks from bubbling air leads to variances in the resulting torque and thus to an increased and detectable signal fluctuation, an increase in the variance value above a definable threshold can be interpreted as the presence of cavitation or air bubbles / air inflows.

[0011] The duration of the observation period for determining the variance value can be in the range between 1 - 10 minutes, preferably between 1 - 5 minutes and particularly preferably between 1 - 2 minutes.

[0012] A possible operating parameter whose variance value is determined could be, for example, the torque itself. It is also conceivable to measure or calculate an electrical quantity of the drive motor that is characteristic of the resulting motor torque. For example, operating parameters such as the electrical motor power consumption and / or the motor current, in particular a current component that forms the motor torque, such as the quadrature component iq of the stator current, are conceivable.

[0013] The operating parameter is recorded over a definable period of time, and its variance is determined using statistical methods. By comparing the measured values ​​with a definable threshold or reference value, it can be estimated whether the calculated variance lies within the usual range of values ​​or deviates from it, i.e., whether the variance of the operating parameter has increased. An increase in variance indicates an anomaly in pump operation, which can be caused in particular by cavitation and / or air bubbling / air inflow in the closed circuit. The respective operating parameter can either be measured directly, derived from other variables, or estimated using an engine model based on measured variables.

[0014] As an alternative to the aforementioned operating parameters, the actual pump speed could also be recorded and evaluated. The actual speed can be measured using a speed sensor or, alternatively, estimated using a motor model. In a pump control system with speed control, cavitation or air bubbles / air inflow can trigger speed fluctuations, so cavitation detection is possible by recording the motor speed and, in particular, determining the speed variance.

[0015] The occurrence and degree of cavitation can be influenced by external conditions. Therefore, the use of a dynamic threshold value adapted to the external conditions to compensate for the variance is advisable. Therefore, the use of an adaptive threshold value dependent on the current operating mode and / or the current pump operating point is generally recommended. In particular, the adaptive threshold value can depend on the current system pressure in the hydraulic system and / or the current pump speed and / or the current actual temperature of the pumped medium. The medium temperature in particular has a significant influence on the formation of cavitation, because as the temperature of the pumped medium increases, the probability of cavitation also increases.

[0016] It is particularly advantageous if, in parallel with the variance determination, the mean value of the operating parameter or the mean value of a variable derived from the operating parameter is also determined. A moving mean calculation is preferred here, which consequently determines the mean value for a continuously moving time window. For example, the electrical power consumption of the motor is derived from the operating parameter, in particular the motor current, and the actual motor speed. By observing the mean value, in particular the power consumption, any drop in pump performance can be detected, which could be characteristic of a possible pressure loss in the system, e.g., due to a leak within the system.However, since a power drop could also be triggered by a regular load change, combined monitoring of the mean and variance values ​​can reliably distinguish between a power drop triggered solely by a load change and a power drop caused by leaks. A power drop is detected, for example, when the mean changes, particularly abruptly, by a certain amount.

[0017] If the system detects a drop in performance and, at the same time, an increased variance of the recorded operating parameter occurs, it can be concluded that there is a pressure loss accompanied by cavitation, which is a reliable indication of the occurrence of possible leaks in the hydraulic system.

[0018] To avoid any miscalculations and to verify a leak, it may be useful to briefly specify an adjusted target speed for the pump's motor control in order to temporarily operate the motor at the upper current limit, provided the motor has not yet been operated at the current limit during regular operation. This can further optimize the accuracy and reliability of the variance value recording. The operating parameter is recorded again at a temporarily adjusted speed, and the variance is determined. If the operating parameter's dispersion increases further and the threshold is exceeded during this operating state, a normal load change can be ruled out and a leak in the system can be verified instead.

[0019] For example, the target speed of the speed controller of the motor control is adjusted or increased for operation at the current limit for a defined time interval, in particular for a period of 1 - 5 minutes, in particular for a period of 1 - 4 minutes, preferably for a period of approximately 3 minutes.

[0020] As already explained above, the hydraulic system is preferably a closed system.

[0021] In addition to the method according to the invention, the present invention also relates to a pump, preferably a centrifugal pump, particularly preferably a heating circulation pump, for example for solar thermal energy, with a pump control or

[0022] Pump control configured to implement the method according to the invention. The pump control or pump regulation can be a frequency converter. The same advantages and properties apply to the pump as already explained above with reference to the method according to the invention. For this reason, a repetitive description is omitted. The pump control or pump monitoring can be an integral component of the pump, but alternatively, it can also be designed as an external component.

[0023] Finally, the invention relates to a hydraulic system comprising at least one pump circulating a pumped medium in a closed circuit, as well as a pump control or pump monitoring system configured to implement the method according to the invention. The pump control or pump monitoring system can be an integral component of the pump; alternatively, however, it can also be configured as an external component that merely communicates with an integral pump control system and retrieves the necessary operating parameters for evaluation from the pump control system.

[0024] Further advantages and features of the process will be explained in more detail below using the figures shown. They show:

[0025] Figure 1 : a simplified representation of the hydraulic system according to the invention,

[0026] Figure 2: a flow chart of the method according to the invention for leakage

[0027] Detection and

[0028] Figure 3: an example time diagram representation of the recorded

[0029] Current component i q the stator current of the pump drive.

[0030] The invention is based on the finding that cavitation and / or air bubbles / air inflows lead to high-frequency and violent shocks on the impeller of a pump, which in turn can cause strong signal fluctuations in the current measurement and in the downstream speed control of the pump. These signal fluctuations can be detected in the motor control system using a novel logic system according to the invention, allowing cavitation to be reliably detected.

[0031] Furthermore, in closed hydraulic systems, a leak or a drop in the system pressure of the hydraulic system can be reliably detected in this way, especially at high medium temperatures.

[0032] Figure 1 schematically shows the possible structure of such a hydraulic system. It depicts a closed hydraulic circuit 1, for example, a standard heating circuit for a residential building to be heated. Possible consumers within the heating circuit are designated simply by the reference numeral 2, and the installed circulation pump is marked by the reference numeral 10. The pump 10, designed as a centrifugal pump, is driven by an electric motor 11, in particular a synchronous motor, particularly preferably a permanent magnet motor, which is controlled in an energy-efficient manner by means of the frequency converter 12 and the speed control implemented in the frequency converter's controller.

[0033] A flowchart illustrating the process steps for leak detection in closed circuits is shown in Figure 2. The entire process is implemented in the pump control or the integral frequency converter 12 of pump 2; however, in principle, the process could also be executed externally by the pump and the required operating parameter(s) could instead be communicated from pump 2 to an external unit for process execution.

[0034] In block 15 of the diagram in Figure 2, the operating parameter is first determined, whereby the motor current consumed by the motor 11 is determined in the frequency converter of the motor control by a three-phase current measurement. Using the motor model 20 stored in the microprocessor, the quadrature component i can be calculated based on the measurement result. q (t) of the stator current, which is a causal factor for the motor torque, can be calculated. The current component iq (t) is sent to block 30 for variance calculation together with the setpoint i q _set(t) of the integral current controller. In block 30, the scatter around the setpoint iq_set(t) can then be calculated using statistical methods. The observation period for the current component i q (t) is typically set to a time interval of 1 to 2 minutes. The calculated variance is then compared in block 40 against an adaptively adjustable threshold. If the threshold is exceeded, cavitation or air bubbles / air inflow is assumed to be occurring, and the process continues in block 70 or 80. If the threshold is not exceeded, no anomaly is assumed, and pump operation continues normally with continuous current measurement in block 15.

[0035] If, in addition to the cavitation effects, a vacuum scenario or a leak in the closed circuit is to be detected using the method, the necessary detection 50 of a power drop, which is necessary in parallel with the variance determination, is useful. For this purpose, the current electrical power consumption of the pump motor 11 is determined and the average value is calculated over a limited time window (block 51). For this purpose, block 51 receives from the motor model 20, in addition to the current motor current i qalso the estimated speed n of the motor to calculate the electrical power consumption. The time window is sliding, so that the power value is constantly updated. In block 52, it is then examined whether there has been a significant change in the mean value, in particular a decrease in power consumption by a certain minimum value. If such a decrease in power has occurred and, at the same time, the variance calculated in block 30 is above the threshold value, it is assumed that there is a loss of pre-pressure and a resulting occurrence of cavitation or air bubbling / air inflow in the suction area of ​​pump 10. For verification, a suspected pre-pressure loss test is carried out in block 80.

[0036] Otherwise, if a power decrease is detected but the calculated variance value does not exceed the threshold, it is assumed that the power decrease was caused solely by a regular load change and not by a leakage-related pressure loss. The process then returns to the current measurement in block 15.

[0037] The suspected test in block 80 is used to verify cavitation or a possible leak. For the suspected test, the target speed in the speed controller of pump 10 is temporarily set for a short period of time, e.g., approximately 3-5 minutes, so that the motor operates at the current limit. This measure further amplifies the signal fluctuations caused by air bubbling / air inflow or cavitation, allowing them to be detected more accurately and reliably. The higher the current strength, the greater the current variances that occur, and the detection can thus be verified.

[0038] If the variance threshold is exceeded again during this pre-pressure loss test (80), the suspicion of cavitation or a leak in the system can be confirmed, and a corresponding alarm message (100) can be generated. It can generally be assumed that the variance will increase again during operation at the current limit.

[0039] Figure 3 shows the time course of the calculated torque forming current component i q (t). In the period 0 - 100 seconds, the current intensity of component i q(t) at 5 A, with the signal curve showing slight measurement fluctuations. At time 100 seconds there is a massive drop in power and the average current drops suddenly to around 3.75 A, which is caused by a drop in pressure in the system from 4 bar to 2 bar. This also results in noticeably stronger signal fluctuations in the current component and the calculated current variance may be above a predetermined threshold, so that in this case a leak in the system can be concluded, which not only forces the drop in power but also simultaneously leads to cavitation and the corresponding current signal fluctuations. At time 200 seconds there is a further drop in power due to a drop in system pressure from 2 bar to 0.5 bar. As a result, the current variances increase again significantly.Using the described method, a potential leak can be reliably detected in closed hydraulic systems, which can be identified by a drop in pump motor performance or by the occurrence of excessive current signal fluctuations. Since the operating parameters used for the monitoring logic are already recorded in most control modules for pump drives, the method can be retrofitted without additional hardware and without extensive reconfiguration of the pump control system, thus cost-effectively retrofitting the protection of existing pump systems.

Claims

1. A method for detecting cavitation and / or air bubbling or air inflow within a hydraulic system (1) comprising at least one electric motor-driven pump (10) for conveying a liquid conveying medium, comprising the steps: - detecting at least one operating parameter of the drive motor (11) influencing the torque of the pump drive (11), - Determining the variance of the recorded operating parameter in a defined observation period, - Detection of cavitation and / or air bubbling or air inflow in the hydraulic system (1 ) when the determined variance value exceeds a defined threshold value.

2. Method according to claim 1, characterized in that the operating parameter is the motor torque and / or the absorbed electrical power of the motor (11) and / or the motor current, in particular the current component i forming the motor torque. q , is about.

3. Method according to claim 1, characterized in that the operating parameter is the actual speed of the pump (10) or the motor (11), in particular the estimated actual speed.

4. Method according to one of the preceding claims, characterized in that the defined threshold value for the comparison with the determined variance is selected depending on the operating mode and / or operating point, in particular depending on from the current system pressure and / or the speed of the pump (10) and / or the temperature of the pumped medium.

5. Method according to one of the preceding claims, characterized in that the mean value of the operating parameter or the mean value of a variable derived from the operating parameter is determined in parallel to the variance determination.

6. Method according to claim 5, characterized in that the mean value is formed over a defined period of time or is determined for a sliding time window.

7. Method according to one of the preceding claims 5 or 6, characterized in that a drop in the pump's performance is detected on the basis of the mean value, in particular when the mean value changes by a certain difference amount.

8. Method according to one of the preceding claims, characterized in that a leak in the system (1) is detected when a drop in performance as well as cavitation and / or air bubbling or air inflow has been detected, in particular when the variance value exceeds the threshold value and the mean value shows a sudden change by a difference amount.

9. Method according to one of the preceding claims, characterized in that when the defined threshold value is exceeded and / or when a drop in power is detected, the pump motor (11) is controlled to operate at the current limit, in particular by adjusting or increasing the target speed of a speed controller of the motor control (12), wherein the operation at the current limit is preferably carried out over a time interval of 1 to 5 minutes, in particular of approximately 3 minutes.

10. Method according to claim 9, characterized in that during operation of the pump (10) at the current limit, the variance value is determined again in order to to verify a loss of pre-pressure of the pump (10) or negative pressure in the suction area of ​​the pump (10).

11. Method according to claim 10, characterized in that during operation at the current limit, the determined variance value is compared with the previously determined variance value and in particular a pre-pressure loss is determined if the variance value determined during operation at the current limit is greater.

12. Method according to one of the preceding claims, characterized in that the predetermined time period for the variance value determination is in the range of 1 to 10 minutes, preferably in the range of 1 to 5 minutes, particularly preferably in the range of 1 to 2 minutes.

13. Method according to one of the preceding claims, characterized in that the hydraulic system (1) is a closed system.

14. Pump (10, 11), preferably a centrifugal pump, particularly preferably a heating circulation pump, with a pump control (12) configured to carry out the method according to one of the preceding claims.

15. Hydraulic system (1) comprising at least one pump (10, 11) circulating a pumping medium in a closed circuit and a pump control / monitoring device (12) configured to carry out the method according to one of claims 1 to 14.

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