Method for vibration avoidance in pumps

Abstract

The invention relates to a method for avoiding 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 of a pump operating parameter and checks the signal oscillations in order to identify occurring mechanical vibrations of the pump and, in order to reduce the detected vibrations, changes the pump speed with the aid of the frequency converter.

Description

Technical Field

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

[0002] Mechanical vibrations in centrifugal pumps lead to increased wear and unwanted noise generation during operation. The causes of vibrations can be various. For example, externally excited vibrations due to the rotation of the pump impeller or free vibrations due to the natural frequency of the built-in pump can be the cause.

[0003] Free vibrations occur in particular in solids pumps. Solids pumps are centrifugal pumps used to convey pumped media with highly abrasive solid fractions, such as suspensions of slag, coal or ore in the mining industry. Occasionally, the pumped medium may also contain stones or other rigid elements, which during operation of the pump may cause impacts when they hit the pump structure, which lead to the excitation of free vibrations of the pump. This effect is also increasingly occurring in pumps used in the wastewater sector.

[0004] Particularly unfavorable situations exist when the rotational frequency of the impeller, i.e. the set pump speed, falls on the natural frequency of the built-in pump or corresponds to an integer multiple of this natural frequency. In this case, resonance occurs, i.e. two vibration causes amplify each other. Similarly problematic is when the set rotational frequency of the impeller coincides with a pipe resonance of the conveying system.

[0005] exist Figure 1 Such a resonance situation is shown by way of example in . The figure shows the frequency response of a centrifugal pump that is built in a ready-to-operate manner. The natural frequencies of the free vibration of the system have frequency values ​​f1, f2, f3. The frequency response, i.e. the position of the natural frequencies f1, f2, f3, depends on the specific pump structure, the selected installation position, the materials used and the installed bearings. If the rotational frequency of the pump wheel set by means of a frequency converter is the same as one of the natural frequencies f1, f2, f3 shown or is alternatively an integer multiple of one of the natural frequencies f1, f2, f3 shown, the system is excited by the externally excited rotation of the impeller and an amplified resonance of the pump occurs. If the rotational frequency of the impeller is alternatively within the range of one of the anti-resonances af1, af2 drawn here, the effect is minimal and no vibration or only very small vibration occurs. Summary of the invention

[0006] The concept of the present application is based on the above-mentioned knowledge and proposes a method which reduces the risk of possible vibrations, in particular resonances, occurring to a minimum by means of targeted measures during the operation of the pump.

[0007] This object is achieved by the method according to the invention for avoiding or reducing mechanical vibrations of a pump during operation of the pump.

[0008] For the implementation of the method, the use of a frequency converter for changing the speed of the pump is essential. However, it is irrelevant whether such a frequency converter is integrated into the pump, mounted on the pump housing or installed separately from the pump. The same applies to a pump controller for the implementation of the method, which can be an integral component of the pump, but can also be installed as a separate unit relative to the pump, optionally in combination with a separate frequency converter.

[0009] The solution according to the invention of the present application is that in the case of a pump with a frequency converter, the speed is varied by the pump controller during operation of the pump in such a way that the mechanical vibrations of the pump are reduced as optimally as possible. Another core aspect of the invention is that during operation the pump independently identifies its existing natural frequency by suitable signal evaluation in order to be able to optimally adapt the set pump speed based on this knowledge.

[0010] The pump therefore does not require information about its frequency response that has already been generated in advance and stored in the pump, but can instead determine this information independently during operation. To this end, the pump records a signal during operation of the pump, which characterizes pump operating parameters that are influenced by the mechanical vibrations that occur. The recorded signal is then checked by the pump for possible vibrations, in particular the presence of resonances. Such vibrations are then reduced by a suitable change in the rotational speed.

[0011] In the recorded signal, signal fluctuations caused by mechanical vibrations of the pump can be detected in particular. The amplitude of the detected oscillation frequency of the signal is reduced by a suitable change in the rotational speed. Therefore, according to an advantageous embodiment of the method, the frequency spectrum of the recorded signal is taken into account. Advantageously, the signal is first transformed into its frequency spectrum by means of a transformation, in particular by means of a fast Fourier transformation, in order to thus detect the corresponding frequency values ​​and the associated amplitudes of the occurring signal oscillations.

[0012] One or more motor currents of the pump drive have proven to be suitable operating signals for detecting possible vibrations. The current values ​​are always present in the frequency converter used, so that no further sensors are required. Since the mechanical vibrations of the pump system are also reflected in the motor windings of the pump drive and accordingly in the current of the motor by magnetic induction, the motor acts as an effective and readily available sensor. By means of a corresponding current analysis, mechanical vibrations of the pump system can then be detected with sufficient accuracy. This possibility exists regardless of the type of motor used for the electric pump drive.

[0013] As an alternative or additional operating parameter for determining the frequency response of the pump, for example, the pump pressure, in particular the final pressure of the pump, is suitable. Here, too, mechanical vibrations are reflected in the signal course. The final pressure of the pump can be determined, for example, by means of an existing pressure sensor and can be transformed into its frequency spectrum by signal transformation, in particular a fast Fourier transformation.

[0014] However, suitable sensors do not necessarily have to be available for signal detection. Alternatively, the current pump pressure can be determined computationally, for example, by means of an operating point estimate. A possible method for this is disclosed in DE 10 2018 200 651, the content of which is hereby fully incorporated.

[0015] According to one possible embodiment, the method can be carried out iteratively with a changing pump speed in order, for example, to identify the pump speed at which the amplitude of the detected vibration is as small as possible. Therefore, after a change in the pump speed, the frequency spectrum of the repeatedly recorded signal is analyzed again and a check is performed to see whether the change in speed has resulted in a corresponding reduction in the amplitude.

[0016] Iterative implementation of the method steps can provide arbitrary or random or controlled speed changes. If the amplitude increases, for example, the speed change carried out between two iterations is reversed, otherwise it is maintained. It is also conceivable to completely run through a certain speed range and then set a speed with the smallest amplitude for pump operation.

[0017] Instead, suitable methods and algorithms are used to identify local or global amplitude minima at the associated rotational speeds. Interval halving methods and / or optimization methods, such as the active set method and / or Newton's method, are conceivable in order to determine the suitable rotational speed that leads to the amplitude minimum as quickly as possible. Genetic algorithms are also conceivable, which, although relatively slow, nevertheless enable the identification of the global minimum of the frequency response.

[0018] The setting of the rotational speed or its change during the method iteration also depends on which operating conditions are predefined, for example, by the pump operator. It is conceivable, for example, that the pump operator predefined a constant pump rotational speed or only a small tolerance range for the rotational speed change. During the method iteration, the rotational speed is then changed only within the previously defined tolerance range. In this case, it is usually sufficient to iterate the method execution in which all permissible rotational speeds or at least a part of the permissible rotational speeds are run in order to determine the corresponding amplitude minimum for this range.

[0019] If, however, the operator does not predefine a permissible speed range, ie, the entire technically possible speed range of the pump can instead be utilized, it is expedient for the method to use one of the above-mentioned methods for determining a suitable speed.

[0020] However, according to another advantageous embodiment of the invention, the method can be used not only to reduce the vibrations that occur, but the determination of the frequency response according to the invention is also suitable for pump monitoring, in order to, for example, be able to detect wear or possible damage to the pump mechanism at an early stage. As already explained in detail above, a core aspect of the invention is to determine the frequency response of the pump. This frequency response depends primarily on the pump construction, its installation position, the materials used and the installed bearing components. Changes in one of these factors, such as due to wear or material damage, lead to changes in the frequency response of the pump. Therefore, the pump preferably stores the determined frequency response and monitors the frequency response by continuously repeating the measurement of the frequency shift of the identified decisive frequency. If such a frequency deviation is recognized, this is an indication of a wear phenomenon or a pump damage. The pump can then generate a corresponding warning message or take appropriate measures.

[0021] By further examining the frequency change, it is also possible to distinguish between wear and damage. Wear usually leads to a slow change in the frequency response, while pump damage, such as bearing damage or impeller breakage, leads to a sudden change in the frequency response. The pump therefore takes into account the time component of the detected change in its evaluation in order to distinguish between wear and damage. The degree of the change can also be included.

[0022] In addition to the method according to the invention, the invention further relates to a pump, preferably a centrifugal pump, particularly preferably a wastewater pump or a solids pump or a supply pump, with an internal or external frequency converter and an internal or external pump controller for carrying out the method according to the invention. Accordingly, such a pump is characterized by the same advantages and properties as have been explained in detail above with reference to the method according to the invention. For this reason, a repeated description is omitted.

[0023] Furthermore, the present application proposes the use according to the invention of a pump, in particular a centrifugal pump, as a wastewater pump, a solids pump or a supply pump. The minimization of the mechanical vibrations that occur according to the invention is particularly important in the case of wastewater pumps or solids pumps, so that the use of the method according to the invention in this type of pump brings far-reaching advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further advantages and properties of the invention will be explained in more detail below with reference to the exemplary embodiment shown in the drawings.

[0025] Figure 1 : shows the possible frequency response of a centrifugal pump installed and ready for operation,

[0026] Figure 2 : shows a time diagram of a periodic signal, and

[0027] Figure 3 : Shows from Figure 2 The calculated spectrum of the time signal. DETAILED DESCRIPTION

[0028] The invention according to the present application describes a method for the targeted avoidance of undesired vibration amplification in the event of resonances by means of a frequency converter during the operation of a pump, in particular a solids pump, a wastewater pump or other supply pump. The basis for the targeted avoidance of these resonances is that such resonances must first be recognized by the pump controller, but as far as possible it is not necessary to equip the pump with special sensors, such as acceleration sensors. However, there is nothing to prevent the pump from being equipped with additional sensors, such as acceleration sensors, which can increase the accuracy of the method if necessary.

[0029] Since mechanical vibrations are the result of the interaction of the construction structure with the motor forces, these mechanical vibrations can also be recognized as a superposition of the drive current of the pump current of the pump drive. Since the intensity of the respective superimposed vibrations is of interest here, the evaluation of the motor current is performed by analyzing the frequency spectrum of the recorded motor signal, which the pump controller obtains by performing a fast Fourier transformation (FFT).

[0030] This approach can be briefly described in terms of Figure 2 , 3 diagram to illustrate. Figure 2 The time diagram of the recorded signal is shown, which for simplicity is generated here by superposition of three sinusoidal signals with different frequencies. By applying FFT, the time signal can now be decomposed into its harmonic components and the harmonic components are obtained. Figure 3 The frequency-magnitude spectrum shown in , as expected, allows the individual frequencies of the sinusoidal signal to be read from the frequency-magnitude spectrum.

[0031] Due to the FFT of the motor current, the pump can therefore identify the mechanical vibrations reflected in the recorded motor current. In a subsequent step, the pump or the pump controller then attempts to set the pump speed so that the resulting rotational frequency of the impeller does not fall on the natural frequency of the pump or a multiple of this natural frequency. To this end, the speed is first changed and in a further step, the spectral analysis of the currently recorded motor current is performed again with a change in the speed. If the amplitude of the current oscillations that occur has become smaller, this is an indication that the mechanical vibrations can be successfully reduced by the change in speed. The method is now carried out iteratively in order to achieve the smallest possible amplitude values ​​for the fluctuations that occur in the current signal. Finding the ideal speed can in principle be carried out according to two scenarios:

[0032] Scenario 1: The required rotation frequency is subject to fixed requirements.

[0033] According to scenario 1, the rotational frequency can have only certain values. This may require a certain (fixed) rotational speed for energy-related reasons or for the intended use. In this case, the pump operator defines a tolerance value in the pump controller by which the rotational frequency can deviate from the nominal value, for example ±3 Hz. The pump controller then changes the rotational speed within the permissible tolerance range and iteratively finds the rotational speed at which the vibration amplitude is minimal. Often, even small changes here are sufficient to deviate from the natural frequency of the system and thus minimize the mechanical vibrations that occur.

[0034] Scenario 2: No special requirements for rotation frequency .

[0035] If there are no technological requirements for the rotational frequency, the pump controller can change the pump speed as desired. This enables a targeted search for anti-resonance and setting the final operating speed of the pump to this anti-resonance. The simplest approach (and thus the approach with the lowest memory and technological requirements) for determining the appropriate speed (anti-resonance) from the available speed range is based on a dichotomy. Mathematical optimization methods, such as the "active set method" or the "Newton method", are faster and more effective. The global optimum can also be reliably determined with the aid of a genetic algorithm.

[0036] Alternatively or in addition to the motor current, the signal of the final pressure of the pump can also be checked, wherein here, similar to the motor current, the spectrum is also analyzed by means of a fast Fourier transform and evaluated according to the corresponding resonance frequency. The final pressure can be calculated, for example, using a pressure sensor of the pump or by means of an operating point estimation.

[0037] In order to improve the signal quality, the two signals (final pressure and motor current) can also be combined by means of sensor data fusion. If this is not possible, the current and pressure signals can also be evaluated separately. For sensor fusion, the individual signal values ​​can be evaluated, for example, as shown above, and then combined by means of weighting. It is also conceivable to define frequency ranges in which the individual results of the individually evaluated signals are weighted differently. For example, the result of the evaluation of the motor current in the frequency range between 10 Hz and 200 Hz is used, while the result of the higher-frequency final pressure evaluation is taken into account.

[0038] The particular advantage of the method presented here is that the pump itself can find its natural frequency and therefore does not require a mathematical process model that would have to be developed in a complex manner. The main application of the method presented here is to avoid or reduce vibrations in order to reduce wear and noise during pump operation. In addition, the method can also contribute to wear and damage monitoring and warn the user in the event of damage.

[0039] Wear monitoring

[0040] In the method presented, the frequency response of the built-in pump is permanently monitored. However, as mentioned above, this frequency response depends on the construction, installation position, materials and bearings of the pump. Therefore, a change in the frequency response is in any case an indication that one or more of these variables have changed, for example due to wear. This information can then be used for wear monitoring, for example also in combination with the solution from DE 102018200651, to which reference is explicitly made here. The combination of these two approaches enables a more precise assessment of the wear condition.

[0041] Warning of damage

[0042] Unlike wear, which results in a very slow change in the frequency response, pump damage will suddenly and significantly change the frequency response. Damage may be a broken bearing or impeller, among many other damages. Due to the rapid change in the frequency response, the pump controller can reliably separate wear from damage and, in the case of damage, can output a warning to the operator.

Claims

1. A method for avoiding or reducing mechanical vibrations of a pump during pump operation, wherein a frequency converter and a pump controller are provided, and the pump controller detects at least one signal of a pump operating parameter and checks the signal oscillations in order to identify occurring mechanical vibrations of the pump, and in order to reduce the detected vibrations, the pump speed is changed by means of the frequency converter, and the method is iteratively carried out with a changing pump speed in order to identify a pump speed at which the amplitude of the mechanical vibrations detected in the frequency-amplitude spectrum is minimum.

2. The method according to claim 1, characterized in that The pump is a centrifugal pump.

3. The method according to claim 1, characterized in that The frequency spectrum of the detected signal is calculated by Fast Fourier Transformation.

4. The method according to any one of the preceding claims, characterized in that At least one of the checked signals corresponds to the motor current of the pump drive.

5. The method according to any one of claims 1 to 3, characterized in that At least one of the checked signals corresponds to a hydraulic final pressure of the pump. 6 . The method according to claim 5 , wherein the final pressure is determined sensor-wise by means of a pressure sensor and / or by estimating the operating point of the pump.

7. The method according to claim 1, characterized in that The pump speed is varied within a definable tolerance range.

8. The method according to any one of claims 1 to 3, characterized in that The method is carried out iteratively at any variable rotational speed in order to detect at least one anti-resonance of the pump and to operate the pump at the anti-resonance.

9. The method according to claim 7, characterized in that: The rotational speed is varied by means of an interval halving method and / or an optimization method and / or with the aid of a genetic algorithm.

10. The method according to claim 9, characterized in that The rotational speed is varied by means of the effective set method and / or Newton's method.

11. The method according to claim 8, characterized in that The rotational speed is varied by means of an interval halving method and / or an optimization method and / or with the aid of a genetic algorithm.

12. The method according to claim 11, characterized in that The rotational speed is varied by means of the effective set method and / or Newton's method.

13. The method according to any one of claims 1 to 3, characterized in that The frequencies of the identified resonances are stored and the method is repeatedly performed in order to detect changes in the frequency of the identified resonances.

14. The method according to claim 13, characterized in that The pump is able to determine material wear of the pump and / or structural damage to the pump using the detected frequency change. 15 . A pump device having a frequency converter and a pump controller, the pump controller being configured to carry out the method according to claim 1 .

16. The pump device according to claim 15, characterized in that The pump device is in the form of a centrifugal pump structure.

17. The pump device according to claim 15, characterized in that The pump device is a wastewater pump or a solids pump or a supply pump.

18. Use of the pump device according to claim 15 as a wastewater pump or a solids pump or a supply pump.

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