METHOD FOR VIBRATION PREVENTION IN PUMPS

Description

[0001] The invention relates to a method for avoiding or reducing mechanical vibrations during the operation of a pump, in particular a centrifugal pump.

[0002] Mechanical vibrations in centrifugal pumps lead to increased wear and unwanted noise during operation. The causes of these vibrations can be varied. They can be caused by externally excited vibrations, for example, due to the rotation of the pump impeller, or by free vibrations due to the natural frequencies of the pump itself.

[0003] US patents 2006 / 266913 A1 and 2007 / 017672 A1 disclose a generic method for vibration avoidance in pumps. The method described in US patent 2004 / 199480 A1 involves monitoring pumps for cavitation, clogging, and seal failure by evaluating the motor current using a neural network. DE 103 34 817 A1 discloses a method for fault detection in pumps based on frequency analysis of a pressure signal. JP 2000 303872 A and JP 2013 027105 A are two examples of the application of an antiresonance approach in remote technical fields. Further reference is also made to DE 102018200651.

[0004] Free vibrations are particularly noticeable in solids pumps. Solids pumps are centrifugal pumps used to transport fluids containing highly abrasive solid particles, such as slag, coal, or ore slurries in mining. Occasionally, the pumped fluid may also contain stones or other rigid elements that, upon impact with the pump structure during operation, can generate shocks leading to the excitation of free vibrations within the pump. This effect is also increasingly observed in pumps used in wastewater applications.

[0005] A particularly unfavorable situation arises when the impeller's rotational frequency, i.e., the set pump speed, coincides with the natural frequency of the installed pump or corresponds to an integer multiple of its natural frequency. In this case, resonant vibrations occur, meaning the two sources of vibration reinforce each other. A similar problem arises when the set impeller rotational frequency coincides with the piping resonance of the conveying system.

[0006] One such case of resonance is exemplified in Figure 1This figure shows the frequency response of a centrifugal pump installed in operation. The natural frequencies at which the system oscillates freely have the frequency values ​​f₁, f₂, and f₃. The frequency response, i.e., the position of the natural frequencies f₁, f₂, and f₃, depends on the specific pump design, the chosen installation position, the materials used, and the bearings installed. If the rotational frequency of the impeller, set by the frequency converter, is identical to or instead an integer multiple of one of the natural frequencies f₁, f₂, or f₃ shown, the system is excited by the externally excited rotation of the impeller, resulting in amplified resonant oscillation of the pump. If, instead, the rotational frequency of the impeller lies in the range of one of the anti-resonances af₁ or af₂ shown here, this effect is minimal, and no oscillation or only a very small oscillation occurs.

[0007] The idea of ​​the present application builds on the above finding and proposes a method which, through targeted measures during pump operation, reduces the risk of possible vibrations, in particular resonances, to a minimum.

[0008] This problem is solved by a method according to the features of claim 1, a pump arrangement according to the features of claim 8, and a use of such a pump arrangement according to the features of claim 9. Advantageous embodiments are the subject of the dependent claims.

[0009] The use of a frequency converter to change the pump's speed is crucial for the execution of the process. However, it is irrelevant whether such a frequency converter is integrated into the pump, mounted on the pump housing, or installed separately. The same applies to the pump control system for process execution, which can be an integral part of the pump or installed as a separate unit, possibly in conjunction with a separate frequency converter.

[0010] The solution according to the invention of the present application consists of varying the speed of a pump with a frequency converter during pump operation by means of a pump controller in such a way as to reduce mechanical vibrations of the pump as optimally as possible. Another key aspect of the invention is that the pump independently identifies its existing natural frequencies during operation by means of suitable signal evaluation in order to optimally adjust the set pump speed based on this knowledge.

[0011] The pump therefore does not require pre-generated and stored information about its frequency response, but can instead determine this independently during operation. To do this, the pump records a signal during operation that characterizes a pump operating parameter influenced by mechanical vibrations. The recorded signal is then analyzed by the pump for the presence of any vibrations, particularly resonant vibrations. Subsequently, such vibrations are reduced by appropriately adjusting the pump speed.

[0012] In the recorded signal, signal fluctuations caused by mechanical vibrations of the pump can be identified. By appropriately changing the rotational speed, the amplitude of the identified vibration frequency(ies) of the signal is reduced. According to an advantageous embodiment of the method, the frequency spectrum of the recorded signal is therefore considered. It is advantageous if the signal is first transformed into its frequency spectrum by means of a transformation, in particular by means of a Fast Fourier Transform, in order to identify the corresponding frequency values ​​and associated amplitudes of the occurring signal oscillations.

[0013] The motor current(s) of the pump drive prove to be a suitable operating signal for identifying any vibrations. The current values ​​are already available to the frequency converter, so no additional sensors are required. Since mechanical vibrations of the pump system are also reflected in the motor windings of the pump drive and consequently in the motor current via magnetic induction, the motor acts as an effective and readily available sensor. Through appropriate current analysis, mechanical vibrations of the pump system can then be identified with sufficient accuracy. This method is available regardless of the type of motor used in the electric pump drive.

[0014] As an alternative or additional operating parameter for determining the pump's frequency response, the pump pressure, especially the pump's ultimate pressure, is suitable. Here, too, mechanical vibrations are reflected in the signal. The pump's ultimate pressure can be determined, for example, using an existing pressure sensor and transformed into its frequency spectrum via signal transformations, particularly the Fast Fourier Transform.

[0015] However, a suitable sensor is not strictly necessary for signal acquisition. Alternatively, the pump pressure can be calculated, for example, using operating point estimation. A possible method for this is disclosed in DE102018200651.

[0016] According to the invention, the method is performed iteratively with varying pump speeds in order to identify the pump speed at which the amplitude of an identified oscillation is minimized. Thus, after the speed has been changed, the pump re-analyzes the frequency spectrum of the repeatedly recorded signal and checks whether the variation in speed has led to a decrease in the corresponding amplitude.

[0017] The iterative execution of the process steps can involve arbitrary or random changes in rotational speed, or a controlled change. For example, if the amplitude increases, the change in rotational speed between two iterations is reversed; otherwise, it is maintained. It is also conceivable to run through a specific rotational speed range completely and then set the speed with the lowest amplitude for pump operation.

[0018] Alternatively, suitable methods and algorithms can be used to identify a local or global amplitude minimum and its corresponding rotational speed. Possible approaches include interval bisection and / or optimization methods, such as an active set method and / or Newton's method, to determine the appropriate rotational speed leading to an amplitude minimum as quickly as possible. A genetic algorithm is also conceivable; while comparatively slow, it enables the identification of a global minimum in the frequency response.

[0019] Setting the rotational speed or varying it during process iterations also depends on the operating conditions specified, for example, by the pump operator. It is conceivable, for instance, that the pump operator specifies a constant pump speed or only a small tolerance range for speed changes. During the process iterations, speed variations then only occur within the previously defined tolerance range. In such a case, an iterative process execution is usually sufficient, in which all or at least some of the permissible speeds are run to determine the corresponding amplitude minimum for this range. However, such an approach is not part of the claimed invention.

[0020] If, however, the operator has not specified a permissible speed range, i.e., the full technically possible speed range of the pump can be used, it is advisable for the procedure to use one of the aforementioned methods to identify the appropriate speed.

[0021] According to a further advantageous embodiment of the invention, the method can not only be used to reduce vibrations, but the determination of the frequency response according to the invention is also suitable for pump monitoring, for example, to detect wear or any damage to the pump mechanics at an early stage. As already explained in detail above, a key aspect of the invention is to determine the frequency response of the pump. This depends essentially on the pump design, its installation position, the materials used, and the installed bearing components. A change in any of these factors, e.g., due to wear or material damage, leads to a change in the pump's frequency response. The pump therefore preferably stores the determined frequency response and monitors it by continuously repeating measurements for frequency shifts of the identified key frequencies.If such a frequency deviation is detected, it indicates wear or pump damage. The pump can then generate a corresponding warning message or take appropriate action.

[0022] Further analysis of the frequency change allows for differentiation between wear and damage. Wear typically leads to a gradual change in the frequency response, while pump damage, such as bearing failure or impeller breakage, results in a sudden change in the frequency response. Therefore, the pump takes the temporal component of the detected change into account during its evaluation to differentiate between wear and damage. The degree of change can also be considered.

[0023] In addition to the method according to the invention, the present invention also relates to a pump, preferably a centrifugal pump, and particularly preferably a wastewater, solids, or supply pump, with an internal or external frequency converter and an internal or external pump control for carrying out the method according to the invention. Accordingly, such a pump is characterized by the same advantages and properties as have already been described in detail above with reference to the method according to the invention. Therefore, a repetitive description is omitted.

[0024] Furthermore, the application proposes the use of a pump, in particular a centrifugal pump, as a wastewater pump, solids pump, or supply pump according to the invention. The minimization of mechanical vibrations according to the invention is of particular importance for wastewater or solids pumps, so that the application of the method according to the invention offers significant advantages for such pump types.

[0025] Further advantages and features of the invention will be explained in more detail below with reference to an exemplary embodiment shown in the figures. The figures show: Fig. 1 : a possible frequency response of an installed and operational centrifugal pump, Fig. 2 : a time diagram of a periodic signal and Fig. 3 : the calculated frequency spectrum of the time signal from Figure 2 .

[0026] The invention according to the present application describes a method for selectively preventing undesirable vibration amplifications during resonance in the operation of a pump, in particular a solids, wastewater, or other supply pump, by means of a frequency converter. The foundation for the targeted prevention of these resonance vibrations is that such resonance events must first be detected by the pump control system, ideally without having to retrofit the pump with special sensors such as accelerometers. However, there is nothing to prevent equipping the pump with additional sensors, e.g., accelerometers, which may increase the accuracy of the method.

[0027] Since the mechanical vibrations result from the interaction between the design and the motor force, these vibrations can also be detected as a superposition in the drive currents of the pump drive. Because the intensity of the individual superimposed vibrations is of interest here, the motor currents are evaluated by analyzing the frequency spectrum of the recorded motor signal, which the pump control receives by performing a Fast Fourier Transform (FFT).

[0028] This approach can be briefly illustrated using the representations of the Figures 2 , 3 illustrate. Figure 2 This shows a time diagram of a recorded signal, which, for simplicity, was generated here by superimposing three sine waves with different frequencies. By applying the FFT, the time signal can now be decomposed into its harmonic components, resulting in the following: Figure 3The frequency amplitude spectrum shown, from which the individual frequencies of the sine signals can be read as expected, is shown.

[0029] By performing FFT analysis of the motor currents, the pump can detect mechanical vibrations that are reflected in the recorded motor current. In the next step, the pump or pump controller attempts to adjust the pump speed so that the resulting impeller rotational frequency does not fall at the pump's natural frequency or a multiple thereof. To achieve this, the speed is first varied, and then a spectrum analysis of the currently recorded motor current is performed again at the changed speed. If the amplitude of the resulting current oscillation has decreased, this indicates that the mechanical vibration has been successfully reduced by varying the speed. The process is then executed iteratively to achieve the smallest possible amplitude value for the fluctuations in the current signal. Finding the ideal speed can, in principle, be carried out according to two scenarios: Scenario 1: The required rotation frequency is subject to fixed requirements.

[0030] According to scenario 1, the rotational frequency may only have a specific value. This may be due to energy efficiency reasons, or the intended use may require a specific (fixed) speed. In this case, the pump operator defines a tolerance value in the pump control system by which the rotational frequency may deviate from the setpoint, e.g., ± 3 Hz. The pump control system then varies the speed within the permissible tolerance range and iteratively finds the speed at which the vibration amplitude is minimal. Often, very small variations are sufficient to deviate from the system's natural frequency and thus minimize the resulting mechanical vibrations. However, this scenario is not part of the claimed invention. Scenario 2: There are no special requirements regarding the rotation frequency.

[0031] If there are no process-related requirements regarding the circulation frequency, the pump controller can change the pump speed at will. This allows for the targeted search for an antiresonance and the adjustment of the pump's final operating speed to this antiresonance. The simplest way (and therefore the one with the lowest memory and process requirements) to determine the appropriate speed (antiresonance) from the available speed range is based on bisection. Mathematical optimization methods, such as the "active set method" or the "Newton method," are faster and more effective. A global optimum can also be reliably determined using a genetic algorithm. According to the invention, the method is actually performed iteratively with arbitrarily varying pump speeds in order to identify at least one antiresonance of the pump and to operate the pump in this antiresonance.

[0032] Alternatively or additionally to the motor currents, the pump's ultimate pressure signal can also be analyzed. Analogous to the motor current, the frequency spectrum is analyzed using a Fast Fourier transform and evaluated for corresponding resonance frequencies. The ultimate pressure can be calculated, for example, using a pressure sensor on the pump or by estimating the operating point.

[0033] To improve signal quality, both signals (final pressure and motor current) can be combined using sensor data fusion. If this is not possible, current and pressure signals can also be evaluated individually. For sensor fusion, the individual signal values ​​can be evaluated as described above and then combined using weighting. It is also conceivable to define frequency ranges in which the individual results of the separately evaluated signals are weighted differently. For example, the result of the motor current evaluation is used for frequency ranges between 10 and 200 Hz, while the result of the final pressure evaluation is considered for higher frequencies.

[0034] A particular advantage of the method presented here is that the pump itself can find its natural frequencies, thus eliminating the need for a complex and costly mathematical process model. The primary application of this method is the prevention or reduction of vibrations to minimize wear and noise during pump operation. Furthermore, the method can also contribute to wear and damage monitoring and warn the user of any damage. Wear monitoring

[0035] In the presented method, the frequency response of the installed pump is continuously monitored. However, as mentioned above, this response depends on the pump's design, installation position, materials, and bearings. A change in the frequency response therefore always indicates that one or more of these parameters have changed, for example, due to wear. This information can then be used for wear monitoring, for example, in combination with the solution from DE 10 2018 200 651, to which explicit reference is made here. Combining these two approaches allows for a more precise assessment of the wear condition. Warning of damage

[0036] Unlike wear, which leads to a very slow change in the frequency response, pump failure would cause a sudden and significant change in the frequency response. This failure could be, among many other things, a bearing or impeller breakage. Due to the rapid change in the frequency response, the pump control system can reliably distinguish between wear and failure and issue a warning to the operator in the event of a failure.

Claims

1. Method for preventing or reducing mechanical vibrations of a pump, in particular a centrifugal pump, during pump operation, wherein a frequency converter and a pump controller are provided, and the pump controller detects at least one signal and investigates it for signal oscillations to detect occurring mechanical vibrations of the pump and changes the pump revolution rate by means of the frequency converter to reduce a detected vibration, characterized in that the signal is a signal of a pump operating parameter and in that the method is carried out iteratively with an arbitrarily varying pump revolution rate to identify at least one antiresonance of the pump and to operate the pump in this antiresonance.

2. Method according to Claim 1, characterized in that the frequency spectrum of the detected signal is calculated by Fast Fourier Transformation.

3. Method according to one of the preceding claims, characterized in that at least one investigated signal corresponds to the motor current of the pump drive.

4. Method according to one of the preceding claims, characterized in that at least one investigated signal corresponds to the hydraulic final pressure of the pump, wherein the final pressure is preferably determined by sensor by means of a pressure sensor and / or by estimating the operating point of the pump.

5. Method according to one of Claims 1 to 4, characterized in that the revolution rate is varied by the interval halving method and / or the optimization method, in particular by the active set method and / or the Newton method, and / or by means of the genetic algorithm.

6. Method according to one of the preceding claims, characterized in that in that frequencies of identified resonance vibrations are stored and the method is carried out repeatedly to detect frequency changes of identified resonance vibrations.

7. Method according to Claim 6, characterized in that the pump can detect material wear of the pump and / or any damage to the pump structure on the basis of detected frequency changes.

8. Pump arrangement, preferably of a centrifugal pump design, particularly preferably a waste water or solids or supply pump, with a frequency converter and a pump controller configured to carry out the method according to one of the preceding claims.

9. Use of a pump arrangement according to Claim 8 as a waste water pump or a solids pump or a supply pump.