Patent Application
20220186749
The invention relates to a method for preventing or reducing mechanical vibrations during the operation of a pump, in particular a centrifugal pump.
Mechanical vibrations in centrifugal pumps lead to increased wear and tear and unwanted noise during operation. The causes of vibrations can be manifold. Causes can be externally excited vibrations, for example due to the rotation of the pump impeller, or free vibrations due to the natural frequencies of the built-in pump.
Free vibrations occur especially in solid pumps. Solid pumps are centrifugal pumps for the transport of pumped media with strongly abrasive solid parts, for example, suspensions of slag, coal or ore in mining. Occasionally, the pumped medium may also contain stones or other rigid elements which, when hitting the pump structure, may produce shocks during pump operation which cause the free vibrations of the pump to be excited. This effect also occurs increasingly in pumps for the waste water sector.
A particularly unfavorable case occurs if the rotational frequency of the impeller, i.e. the set pump revolution rate, equals the natural frequency of the built-in pump or corresponds to an integer multiple of the natural frequency. In this case, resonance vibrations occur, i.e. the two causes of vibration mutually amplify each other. It is similarly problematic when the set rotational frequency of the impeller coincides with the pipeline resonance of the conveying system.
Such a resonance case is exemplified in
The idea of the present application builds on the above knowledge and proposes a method, which by targeted measures during the operation of the pump reduces the risk of the occurrence of possible vibrations, especially resonances, to a minimum.
This object is achieved by a method according to the features of claim 1. Advantageous embodiments are the subject of the dependent claims.
For the implementation of the method, the use of a frequency converter for changing the revolution rate of the pump is decisive. However, it does not matter whether such a frequency converter is integrated into the pump, attached to the pump housing or installed separately from the pump. The same applies to the pump controller for the implementation of the method, which may be an integral part of the pump, but also may be installed as a separate unit to the pump, optionally in conjunction with a separate frequency converter.
The solution according to the invention of the present application consists in varying the revolution rate by a pump controller for a pump with a frequency converter during the operation of the pump in such a way that mechanical vibrations of the pump are reduced as optimally as possible. Another core aspect of the invention also consists of the pump independently identifying its existing natural frequencies during operation by means of suitable signal evaluation in order to be able to optimally adapt the set pump revolution rate based on this knowledge.
The pump therefore does not need information about its frequency response which has already been generated in advance and stored in the pump but can instead determine this independently during operation. For this purpose, the pump records a signal during pump operation which characterizes a pump operating parameter, which is influenced by occurring mechanical vibrations. The recorded signal is subsequently investigated by the pump for the presence of any vibrations, in particular resonance vibrations. Such a vibration is subsequently reduced by a suitable revolution rate change.
In the recorded signal, in particular signal fluctuations can be identified which are caused by mechanical vibrations of the pump. The amplitude of the identified oscillation frequency(ies) of the signal is reduced by a matching change of revolution rate. According to the advantageous embodiment of the method, therefore, the frequency spectrum of the recorded signal is considered. It is advantageous if the signal is first transformed into its frequency spectrum by means of transformation, in particular by means of a Fast Fourier Transformation, so as to identify the corresponding frequency values and associated amplitudes of occurring signal vibrations.
The motor current or currents of the pump drive proves to be a suitable operating signal for the identification of any vibrations. The current values are available to the frequency converter used anyway, so that no further sensors are required. Since mechanical vibrations of the pump system are also reflected by magnetic induction in the motor windings of the pump drive and accordingly in the current of the motor, the motor therefore acts as an effective sensor that can be available at any time. By appropriate current analysis, mechanical vibrations of the pump system can be identified with sufficient accuracy. This possibility exists independently of the motor type of the electric pump drive used.
As an alternative or additional operating parameter for the determination of the frequency-response of the pump, the pump pressure is suitable, for example, in particular the final pressure of the pump. Here, too, mechanical vibrations are reflected in the signal profile. The final pressure of the pump can be determined, for example, by means of existing pressure sensors and can be transformed into its frequency spectrum by signal transformations, in particular a Fast Fourier Transformation.
For the signal acquisition, however, a suitable sensor does not necessarily have to be kept available. Alternatively, for example. the current pump pressure can be determined mathematically by means of operating point estimation. A possible method for this is disclosed in DE102018200651, the content of which is fully included at this point.
According to a possible embodiment, the method can be carried out iteratively with varying pump revolution rate, for example, to identify that pump revolution rate at which the amplitude of an identified vibration is as minimal as possible. The pump thus analyzes the frequency spectrum of the repeatedly recorded signal again after the change of revolution rate and checks whether the variation of the revolution rate has led to a decrease in the corresponding amplitude.
The iterative implementation of the steps of the method can provide an arbitrary or random or else controlled change of revolution rate. If the amplitude increases, for example, then the change of revolution rate carried out between two iterations is reversed, otherwise it is retained. It is also conceivable to drive continuously through a certain full range of revolution rates and subsequently set the revolution rate with the lowest amplitude for pump operation.
An alternative is the use of suitable methods and algorithms for identifying a local or global amplitude minimum with the associated revolution rate. An interval halving method and/or an optimization method are conceivable, such as an active-set method and/or a Newton method, to determine as quickly as possible the appropriate revolution rate which leads to an amplitude minimum. A genetic algorithm is also conceivable which, although comparatively slow, enables the identification of a global minimum frequency response.
The setting of the revolution rate or the variation thereof during the iterations of the method also depends on which operating conditions are predetermined, for example by the pump operator. It is conceivable, for example, that the pump operator specifies a constant pump revolution rate or specifies only a small tolerance range for revolution rate changes. During the iterations of the method, a revolution rate variation is then carried out only within the previously defined tolerance range. In such a case, an iterative implementation of the method is usually sufficient, in which all or at least some of the permitted revolution rates are operated at to determine the corresponding amplitude minimum for this range.
If, on the other hand, no specification has been made by the operator for a permissible revolution rate range, i.e. it can instead be the full, technically possible revolution rate range of the pump, it is expedient if the method uses one of the aforementioned methods for identifying the appropriate revolution rate.
According to a further advantageous embodiment of the invention, however, the method can not only serve to reduce occurring vibrations, but the determination according to the invention of the frequency response is also suitable for pump monitoring, for example, to detect wear or any damage to the pump mechanism at an early stage. As has already been explained in detail above, a core 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 one of these factors, for example due to wear or material damage, leads to a change of the frequency response of the pump. The pump therefore preferably stores the determined frequency response and monitors this by running repeating measurements for frequency shifts of the identified relevant frequencies. If such a frequency deviation is detected, this is an indication of wear and tear or of pump damage. The pump can then produce a corresponding warning message or carry out an appropriate measure.
Further investigation of the frequency change can also distinguish between wear and damage. Usually wear leads to a creeping change in the frequency response, while pump damage, for example bearing damage or impeller breakage, results in a sudden change in frequency response. The pump therefore takes into account in its evaluation the time-related component of the detected change to differentiate between wear and damage. The degree of change can also be included.
In addition to the method according to the invention, the present invention also relates to a pump, preferably a centrifugal pump, particularly preferably a waste water 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 already been explained in detail above based on the method according to the invention. A repeated description is omitted for this reason.
In addition, the use according to the invention of a pump, in particular a centrifugal pump, as a waste water pump, a solids pump or a supply pump is proposed by the application. The minimization according to the invention of occurring mechanical vibrations is especially important for waste water pumps or solids pumps, so that the application of the method according to the invention in such pump types brings far-reaching advantages.
Further advantages and properties of the invention are to be explained in more detail below on the basis of an exemplary embodiment shown in the figures.
The invention according to the present application describes a method for the targeted prevention of undesirable vibration amplifications in the resonant case during the operation of a pump, in particular a solids pump, a waste water pump or another supply pump, by means of a frequency converter. The foundation for the targeted prevention of these resonant vibrations is that such resonance cases must initially be detected by the pump controller, but preferably without having to retrofit the pump with a special sensor system such as accelerometers. However, there is nothing to prevent fitting the pump with additional sensors, for example accelerometers, which may increase the accuracy of the method if necessary.
Since the mechanical vibrations are a consequence of the interaction between the structure and the force of the motor, these mechanical vibrations can also be seen as a superposition in the drive currents of the pump current of the pump drive. Since the intensity of the individual superimposed vibrations is of interest here, the evaluation of the motor currents is carried out by analyzing the frequency spectrum of the recorded motor signal, which the pump controller obtains by executing the Fast Fourier Transformation (FFT).
This procedure can be briefly illustrated based on the representations of the
Due to the FFT of the motor currents, the pump can therefore detect mechanical vibrations which are reflected in the recorded motor current. In the following step, the pump or the pump controller then seeks to set the pump revolution rate so that the resulting rotational frequency of the impeller does not fall on a natural frequency of the pump or a multiple of such a natural frequency. For this purpose, the revolution rate is initially varied and in a further step a spectrum analysis of the currently recorded motor current is again performed at a changed revolution rate. If the amplitude of the occurring current oscillation has become smaller, this is an indication that the mechanical vibration could be successfully reduced by the revolution rate variation. The method is now carried out iteratively to achieve as small an amplitude value of the occurring fluctuations in the current signal as possible. Finding the ideal revolution rate can in principle be carried out according to two scenarios:
According to scenario 1, the rotational frequency may only have a certain value. This may have energy-related reasons or the intended purpose requires a certain (fixed) revolution rate. In this case, the pump operator defines a tolerance value in the pump controller by which the circulating frequency may deviate maximally from the setpoint, for example ±3 Hz. The pump controller then varies the revolution rate within the allowable tolerance range and iteratively finds out the revolution rate at which the vibration amplitude is minimal. Often even very small variations are sufficient to depart from the natural frequency of the system and thus to minimize the occurring mechanical vibrations.
If there are no process-side requirements for the rotational frequency, the pump controller can change the pump revolution rate at will. This allows a targeted search for an anti-resonance and setting the final operating revolution rate of the pump to this anti-resonance. The easiest way (and thus the one with the lowest memory and process requirements) to determine the appropriate revolution rate (antiresonance) from the available revolution rate range is based on bisection. Mathematical optimization methods are faster and more effective, such as the “active-set method” or the “Newton method”. A global optimum can also be reliably determined by means of a genetic algorithm.
Alternatively or in addition to the motor currents, the signal of the final pressure of the pump can also be examined, in that similarly to the motor current here too the frequency spectrum is analyzed and evaluated for corresponding resonance frequencies by means of Fast Fourier Transformation. The final pressure can be calculated, for example, with a pressure sensor of the pump or else by means of operating point estimation.
To increase the signal quality, both signals (final pressure and motor current) can also be merged by means of sensor data fusion. If this is not possible, current and pressure signals can also be evaluated individually. For the sensor fusion, for example the individual signal values can be evaluated as shown above and then merged by means of weighting. It is also conceivable to define ranges of interest in which the individual results of the separately evaluated signals can be weighted differently. For example, the result of the evaluation of the motor currents for frequency ranges between 10 and 200 Hz is used, while the result of the final pressure evaluation for higher frequencies is taken into account.
A particular advantage of the method presented here is that the pump itself can find its natural frequencies and therefore no mathematical process model, which would be complex to develop, is required. The main application of the method presented here is the prevention or reduction of vibrations to reduce wear and noise during pump operation. In addition, the process can also provide a contribution to wear and damage monitoring and can warn the user in case of damage.
With the presented method, the frequency response of the built-in pump is permanently monitored. However, as mentioned above, this depends on the construction of the pump, the installation position, the materials and the bearings. 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 and tear. This information can then be used for wear monitoring, for example in combination with the solution from DE 10 2018 200 651, to which express reference is made at this point. A combination of these two approaches makes it possible to evaluate the wear condition more precisely.
In contrast to wear, which leads to a very slow change in frequency response, pump damage would change the frequency response abruptly and significantly. Damage can be, among many other things, a bearing or impeller break. Due to the rapid change of the frequency response, the pump controller can reliably separate wear and tear and damage and in the event of damage can issue a warning to the operator.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 002 826.0 | Apr 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/060432 | 4/14/2020 | WO | 00 |
