Method for Checking the Properties of a Conveyed Medium of a Centrifugal Pump, in Particular for Detecting a Dry Run

Abstract

A method for checking the properties of a conveyed medium of a centrifugal pump includes detecting dry running, using a speed-controlled centrifugal pump. The method also includes varying a target speed of the speed-controlled centrifugal pump during a period of time in which the target speed is increased. The method also includes determining a motor parameter characterizing the torque of the pump. The method also includes evaluating the motor parameter characterizing the torque of the pump. The determining and evaluating are carried out taking into account a reference value. The method also includes ascertaining a property of the conveyed medium and/or the presence of dry running based on an outcome of the determining and evaluating.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a 371 National Stage Application of PCT/ 2023/085311, filed Dec. 12, 2023, which claims priority from German Patent Application No. 102023100282.1, filed Jan. 9, 2023, the entire disclosure of which is herein expressly incorporated by reference.

BACKGROUND

The disclosure relates to a method for checking the properties of a conveyed medium of a centrifugal pump, in particular for detecting dry running.

Centrifugal pumps are used to convey different conveyed media. For the proper and, in particular, energy-efficient operation of a centrifugal pump, information regarding the conveyed medium can be important. In particular, changes in the characteristic properties of the conveyed medium, such as a change in temperature and a change in density, may be important for efficient pump control or pump regulation, so that there is a legitimate interest in a suitable method for determining these characteristic parameters of the conveyed medium during pump operation.

Another important point for pump operation in wet-running centrifugal pumps is dry running detection, because the medium to be conveyed is regularly used to lubricate the rotating machine parts of the pump unit or to cool the pump, which is why dry running during pump operation can lead to increased wear or even thermal overload of the pump. Dry running of the pump is also an indication of empty pipelines of the hydraulic system, which can lead to damage to the system. For this reason, dry running detection is also important for pumps that are not running wet. In order to avoid any consequential damage, whether to the pump itself or to the hydraulic system, it is therefore desirable to detect such a condition early and reliably, especially regardless of the operating point.

If the pipelines in a hydraulic system are not sufficiently filled with the fluid, i.e. the conveyed medium, the torque acting on the impeller decreases compared to a filled pipeline. This circumstance can be exploited to reliably detect dry running. For this purpose, the torque is recorded in the laboratory with the valve closed but the pipeline filled for different speeds and the measured speed or the torque value pair is stored as a table in the pump. Operation with the valve closed and the resulting zero flow corresponds to the worst-case scenario with the lowest possible torque with filled pipelines. The stored torque values are the lowest that can occur when the pipeline is filled. As soon as the torque during operation is lower than the reference torque stored in the table, it is concluded that dry running is occurring and the pump can issue a corresponding message.

This method works reliably for medium and high speeds, but at low speeds the torque differences between the filled system with a closed valve and the unfilled system are very small, so that a reliable distinction is difficult at such an operating point.

SUMMARY

Against this background, it also makes sense to optimize the process shown in such a way that dry running detection is reliably possible for every operating point.

The problems identified are solved by a method in accordance with the features of this disclosure. Advantageous embodiments of the method are also the subject matter of this disclosure.

According to the disclosure, it is proposed to briefly vary the target speed for the speed-controlled centrifugal pump for the process of checking or detecting dry running. It is particularly preferable to increase the target speed for a short time. When the speed changes, a motor parameter characterizing the torque of the pump is determined and evaluated taking into account at least one reference value. Based on the result of the evaluation, a statement can then be made about a change in the characteristic properties of the conveyed medium and/or the presence of dry running.

A characteristic property which can be determined by means of the method according to the disclosure is, for example, the density and/or temperature of the conveyed medium or a change in the density and/or temperature of the conveyed medium compared to an initial value. Since the density of the conveyed medium and its temperature influence the viscosity of the liquid and thus the required torque of the pump, a recorded torque change at a specific speed can thus be used to make a statement about a deviation or change in the consistency of the conveyed medium. This also applies in the event that there is a pressure drop or dry running on the suction side, so that the corresponding torque to be applied by the pump motor is reduced.

For example, the existing torque of the electric motor can be determined as a motor parameter to be recorded. It is conceivable here to directly record the torque using a torque sensor or, alternatively, to derive the torque from other motor parameters. Torque can also be estimated, especially when using a motor model. It is also conceivable that the electrical power consumption of the motor and/or the consumed motor current are determined or estimated. Since the electrical power consumption or the motor current, in particular the current component forming the motor torque, have a direct influence on the resulting torque or are proportional to it, the method according to the disclosure can also be carried out by evaluating these operating parameters.

As already shown at the beginning, dry running detection works reliably at medium and higher speeds, wherein the variation of the torque is often too small only at lower speeds, so that no reliable indication is possible. Against this background, it is expedient to carry out the proposed method only if the centrifugal pump is currently operating at a speed that is too low. Only in this case is it necessary to artificially vary or increase the target speed in order to be able to carry out a reliable dry running detection or a check of the properties of the conveyed medium. The execution of the method can therefore be subject to the condition that the currently set target speed is less than a definable threshold value.

In the simplest case, the speed variation proposed according to the disclosure can be taken up in a short-term increase of the target speed, wherein the motor parameter for the currently increased speed is determined and compared against a reference value assigned to the increased target speed. For prior determination of the assigned reference value, for example in advance in the laboratory or under real operating conditions of the operating parameters, for example the torque is recorded for different speeds with the valve closed but with the pipeline filled and the measured speed or the torque value pair is stored as a table in the pump. The stored reference values are the lowest that can occur when the pipeline is filled. As soon as the operating parameters, for example the torque, during operation are lower than the reference value stored in the table, it is concluded that dry running is occurring and the pump can issue a corresponding message.

It is particularly preferred if the speed increase sets the target speed to the maximum speed or to a value between 50-90% of the maximum speed, preferably to a value between 65-85% of the maximum speed.

Depending on the use case of the pump, however, the speed increase described above may not be permitted. Alternatively, it is therefore proposed to overlay the current target speed with a time-variable signal, so that there is subsequently a defined temporal change or fluctuation of the target speed over a certain period of time, but preferably only with a limited increase in the magnitude of the speed. The dynamic change of the target speed inevitably causes a dynamic change in the motor parameter to be evaluated. For the further evaluation, a correlating relationship between the time profile of the target speed and the time profile of the motor operating parameter is investigated. On the basis of the correlation, a physical property of the conveyed medium, in particular a change in this property, can be concluded and/or the presence of dry running can be detected. Since the system pressure at the suction nozzle and the properties of the conveyed medium influence the mass inertia of the pump unit, it is possible to draw conclusions about the existing inertia by evaluating the time lapse between the target specification of the speed and the reactive torque or the recorded motor parameter, and based on this, a property of the conveyed medium or possible dry running can be detected.

It is preferred, for example, if the target speed is superimposed with a periodic signal, so that there is a periodic profile of the target speed during the observation period. However, it is conceivable that it could be superimposed with an alternative time-variant signal.

Due to the inertia, there is usually a time delay in the torque profile or the motor parameter compared to the time profile of the target speed. Against this background, the evaluation of the phase offset between the time profile of the target speed and the time profile of the recorded motor parameter is useful. If the viscosity of the conveyed medium increases, the phase offset increases, but if there is a decrease in viscosity, the phase offset inevitably also decreases. In a dry-running scenario, the phase offset is minimal due to the lack of conveyed medium in the area of the impeller.

Alternatively or additionally, the amplitude value of the motor parameter, in particular the torque, can also be determined and evaluated during the fluctuating target speed due to the superposition. With higher inertia, the magnitude of the torque amplitude or of the motor parameter amplitude increases. It makes sense to compare the determined amplitude value with a reference value or to determine the amplitude ratio to a reference amplitude.

For the detection of changes in the physical properties of the conveyed medium or for the detection of dry running, it may therefore be envisaged to maintain corresponding reference values for the phase offset or the amplitude ratio or the amplitude itself, wherein if there is a change in the currently determined phase offset and/or the amplitude ratio compared to the reference value a change in the conveyed medium and, if appropriate, dry running can be inferred.

The one or more reference values for carrying out the method can be obtained, for example, during a training run of the pump, especially during commissioning or at regular times during pump operation. It is also conceivable to determine the reference values by means of preliminary measurements and to update them continuously.

In addition to the method according to the disclosure, the present disclosure also relates to a centrifugal pump, in particular a heating circulation pump, with an integral or external control system/regulation system configured to carry out the method according to the disclosure. Consequently, the centrifugal pump according to the disclosure has the same advantages and properties as have already been explained in detail on the basis of the method according to the disclosure. For this reason, a repetitive description is dispensed with.

Further advantages and properties of the disclosure will be described below on the basis of the exemplary embodiments explained by means of the figures. In the figures:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: shows a diagram of the torque of a pump drive unit against the speed for different pump conditions,

FIG. 2: shows an exemplary representation of the torque profile over time after overlaying with a periodic signal for carrying out the method according to the disclosure, and

FIG. 3: shows an exemplary comparison of the target speed overlaid with a periodic signal and the resulting torque profile against time.

DETAILED DESCRIPTION

FIG. 1 shows the torque of a centrifugal pump unit as a function of the set target speed of the pump. In the diagram representation, a total of two curves are shown. The dotted line represents the profile of the torque P_Q0 over the entire speed range in the case of a suction line completely filled with conveyed medium, but with the valve closed in the suction line, so that the pump operates at a zero flow rate. The solid line shows the profile of the torque P_Dry over the entire speed range when the pump is running dry.

Overall, it can be seen that the deviation between the torque P_Dry when running dry and the torque P_Q0 when the pump is properly filled increases with increasing speed, so the described method works very accurately at medium and high speeds. Especially in the low-speed range, for example at the speed marked with the reference sign 1, the torque differences or differences in power consumption between the filled system with closed valve and the unfilled system are very small or in some cases hardly existent. It is difficult in this speed range to reliably distinguish whether dry running is actually occurring or whether only the valve is closed.

In order to solve the aforementioned problem, it is therefore proposed according to the disclosure to briefly increase the target speed of the speed-controlled centrifugal pump, for example to the speed value marked in FIG. 1 with the reference sign 2. Here, dry running can be detected very reliably. According to the illustration in the figure, the speed for dry running detection is significantly increased, here by a factor of about three. Basically, it can be stated that a short-term increase in the speed to a value in the range between 50% and 100% of the maximum speed enables the most reliable detection. The advantage of the method is that the method is very easy to implement.

Since in certain processes even a temporary speed increase to the extent suggested above may not be permitted, the method can alternatively provide for an excitation of the target speed with a periodic signal, which not only enables dry running detection, but also provides information about the conveyed medium.

It is proposed to apply a periodic signal to the current target speed as part of a parameter identification, in order to then observe the phase shift between the target speed profile and the (measured or estimated) torque profile that occurs. Alternatively, the amplitude of the measured or estimated torque profile can also be considered.

In the case of the speed-controlled centrifugal pump, the current actual speed and the actual torque are available as estimated values from the motor control system. The phase shift or the amplitude of the torque depends on the inertia of the moving machine parts, for example impeller, shaft, etc. (known) and the conveyed medium (to be determined). The method can be divided into two steps.

• • Step 1: Detect the phase shifts or torque amplitude in the normal state. Step 1 takes place in the laboratory. Here, different operating points are approached for a defined reference medium and at a reference temperature (various speed/torque combinations) and the target speed is overlaid with a higher-frequency signal (for example 10 Hz). For each of these operating points, the phase shift between the profile of the target speed and the torque profile or, alternatively, the torque amplitude is recorded. The resulting characteristic map is stored in the pump.

Step 2: Detect Different Parameters in the Field

While the pump is running, the target speed is overlaid at regular intervals or once on request with the same higher-frequency signal as before in the laboratory and the phase shift between the target speed profile and the measured/estimated torque profile or, alternatively, the torque amplitude of the torque profile is recorded. If the phase shift or torque amplitude is significantly lower than the reference value for the phase shift recorded in the laboratory, this can be interpreted as an indication of the presence of dry running. Furthermore, the phase shift can provide information about a density or temperature of the medium that deviates from the reference or an incorrect mixing ratio.

As an example, FIG. 2 shows the simulated torque profile of a centrifugal pump, wherein the static target speed is superimposed with a periodic signal and the periodic profile of the torque marked with reference sign 10 results (corresponds to the laboratory setup). The curve 20 shows the torque profile for an identical test setup, wherein the simulated inertia of the rotating part of the pump has been increased by 50%. It is clearly visible that the amplitude of the curve 20 is higher. The ratio of the amplitudes, here marked with the reference sign 40, is equal to the ratio of the simulated inertias. The pump could detect this 50% deviation from the values recorded in the laboratory and infer a 50% change in inertia. Equivalent to the inertia, the density of the medium could also have changed.

The average value of the torque profiles 30 is identical. This means that the information about the inertia would not be recognizable at a static target speed (without applying a periodic signal).

FIG. 3 now shows the time profiles of the target speed and the recorded torque. The curve 50 corresponds to the target speed, which was overlaid with a periodic signal. The curve 60 shows the resulting torque profile. The reference sign 70 marks the resulting phase offset between the two profiles, which also increases with increasing inertia. As a precautionary measure, it should be noted that in the illustration of FIG. 3, the speed profile 50 and the torque profile 60 have been superimposed on each other for better comprehensibility of the phase offset.

The foregoing disclosure has been set forth merely to illustrate the disclosure and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the disclosure may occur to persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims and equivalents thereof.

Claims

1-11. (canceled)

12. A method for checking the properties of a conveyed medium of a centrifugal pump, the method comprising:

detecting dry running, using a speed-controlled centrifugal pump;

varying a target speed of the speed-controlled centrifugal pump during a period of time in which the target speed is increased;

determining a motor parameter characterizing the torque of the pump;

evaluating the motor parameter characterizing the torque of the pump, wherein the determining and evaluating are carried out taking into account a reference value; and

ascertaining a property of the conveyed medium and/or the presence of dry running based on an outcome of the determining and evaluating.

13. The method as claimed in claim 12, wherein the method is used to determine a change in density and/or a change in temperature of the conveyed medium compared to the reference value.

14. The method as claimed in claim 12, further comprising: estimating a generated torque of the speed-controlled centrifugal pump as the motor parameter characterizing the generated torque of the speed-controlled centrifugal pump, and/or estimating an electrical power consumption of the motor and/or a motor current consumed.

15. The method as claimed in claim 12, further comprising: first checking whether current target speed is below a minimum value and a variation or increase of the target speed is only carried out if the current target speed is below the minimum value.

16. The method as claimed in claim 12, wherein the speed variation provides an increase in the target speed, the method further comprising: determining the motor parameter for the currently increased target speed; and comparing the motor parameter against a reference value assigned to the increased target speed, wherein dry running is detected if the motor parameter is less than the reference value or is less than the reference value by at least a defined value.

17. The method as claimed in claim 12, further comprising: overlaying the target speed for the check with a time-varying signal for a specified period of time with a periodic signal.

18. The method as claimed in claim 17, further comprising: detecting a property of the conveyed medium or a presence of dry running based on a correlation between the resulting target speed profile and the time profile of the motor parameter.

19. The method as claimed in claim 18, further comprising: comparing a phase offset between a time profile of the target speed and the time profile of the motor parameter against a reference value.

20. The method as claimed in claim 17, further comprising: determining an amplitude of the motor parameter during an overlaying of the target speed; and comparing the amplitude against a reference value.

21. The method as claimed in claim 12, further comprising: determining one or more reference values by a training procedure performed under normal operating conditions.

22. A heating circulation centrifugal pump, configured with an integral or external control system for carrying out the method as claimed in claim 12.