DE102023100282A1 - Method for checking the properties of a pumped medium of a centrifugal pump, in particular for dry running detection - Google Patents

Abstract

This invention relates to a method for checking the properties of a pumped medium of a centrifugal pump, in particular for dry running detection, using a speed-controlled centrifugal pump, wherein for the check the target speed of the centrifugal pump is briefly varied, in particular increased, and a motor parameter characterizing the torque of the pump is determined and evaluated taking into account a reference value in order to determine a property of the pumped medium or dry running.

Description

The invention relates to a method for checking the properties of a pumped medium of a centrifugal pump, in particular for dry running detection.

Centrifugal pumps are used to pump different types of fluid. Information about the fluid can be important for the proper and, in particular, energy-efficient operation of a centrifugal pump. In particular, changes in the characteristic properties of the fluid, such as a change in temperature or a change in density, can be important for efficient pump control or pump regulation, so there is a legitimate interest in a suitable method for determining these characteristic parameters of the fluid during pump operation.

Another important point for pump operation with wet-running centrifugal pumps is dry-run detection, because the medium to be pumped 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 that the pipes in the hydraulic system have run dry, which can lead to damage to the system. For this reason, dry-run detection is also important for pumps that do not run wet. To avoid any subsequent damage, whether to the pump itself or the hydraulic system, it is therefore desirable to detect such a condition early and reliably, especially regardless of the operating point.

If the lines in a hydraulic system are not sufficiently filled with the liquid, i.e. the pumped medium, the torque acting on the impeller decreases compared to a filled pipeline. This circumstance can be exploited to reliably detect dry running. To do this, the torque is recorded in the laboratory with the valve closed but the line filled for different speeds and the measured speed or the pair of torque values is stored in a table in the pump. Operation with the valve closed and the resulting zero flow rate corresponds to the "worst-case" scenario with the lowest possible torque with filled pipelines. The stored torque values are the lowest that can occur with a filled pipeline. As soon as the torque during operation is lower than the reference torque stored in the table, dry running is concluded 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 closed valve and the unfilled system are very small, making a reliable distinction difficult at such an operating point.

Against this background, it also makes sense to optimize the procedure shown so that dry-run detection is reliably possible for every operating point.

The problems identified are solved by a method according to the features of claim 1. Advantageous embodiments of the method are the subject of the dependent claims.

According to the invention, it is proposed to briefly vary the target speed for the speed-controlled centrifugal pump for the process of checking or detecting dry running. The target speed is particularly preferably increased for a short period of time. When the speed changes, a motor variable characterizing the torque of the pump is then 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 pumped medium and/or the presence of dry running can be detected.

A characteristic property that can be determined using the method according to the invention is, for example, the density and/or temperature of the conveying medium or a change in the density and/or temperature of the conveying medium compared to an initial value. Since the density of the conveying medium or its temperature influences the viscosity of the liquid and thus the required torque of the pump, a detected torque change at a specific speed can be used to make a statement about a deviation or change in the consistency of the conveying medium. This also applies if 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.

The existing torque of the electric motor can be determined as an engine parameter to be recorded. It is conceivable that the torque can be recorded directly using a torque sensor or alternatively that the torque can be derived from other engine parameters. Torque can also be estimated, in particular using a motor model. It is also conceivable that the electrical power consumption of the motor and/or the motor current consumed 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 invention 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, only at lower speeds the variation in torque is often too small, so that no reliable statement is possible. Against this background, it is expedient to only carry out the proposed method if the centrifugal pump is currently operating at too low a speed. Only in this case is it necessary to artificially vary or increase the target speed in order to be able to carry out reliable dry-running detection or to check the properties of the pumped medium. The execution of the method can therefore be subject to the condition that the currently set target speed is lower than a definable threshold value.

In the simplest case, the speed variation proposed according to the invention can be limited to a short-term increase in the target speed, whereby the motor parameter for the currently increased speed is determined and compared with a reference value assigned to the increased target speed. To determine the assigned reference value in advance, the operating parameter, e.g. the torque with a closed valve but a filled line, is recorded for different speeds in advance in the laboratory or under real operating conditions, and the measured speed or the pair of torque values 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 parameter, e.g. the torque during operation, is lower than the reference value stored in the table, dry running is assumed 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 application of the pump, however, the speed increase described above may not be permitted. As an alternative, it is therefore proposed to superimpose a time-variable signal on the current target speed, resulting in a defined temporal change or fluctuation in the target speed over a certain period of time, but preferably only with a limited increase in the speed. The dynamic change in the target speed inevitably causes a dynamic change in the motor parameter to be evaluated. For further evaluation, a correlating relationship between the time course of the target speed and the time course of the motor operating parameter is examined in particular. On the basis of the correlation, a conclusion can be drawn about a physical property of the pumped medium, in particular a change in this property, and/or the presence of dry running can be detected. Since the system pressure at the suction port and the properties of the pumped medium influence the mass inertia of the pump unit, the existing mass inertia can be determined by evaluating the time curves between the target speed and the reactively adjusted torque or the recorded motor parameter and, based on this, a property of the pumped medium or any dry running can be detected.

It is preferred, for example, if the target speed is superimposed with a periodic signal so that the target speed has a periodic course during the observation period. However, superimposition with an alternative time-variant signal is conceivable.

Due to the mass inertia, there is usually a time delay in the torque curve or the motor parameter compared to the time curve of the target speed. Against this background, it is useful to evaluate the phase shift between the time curve of the target speed and the time curve of the recorded motor parameter. If the viscosity of the pumped medium increases, the phase shift increases; if, however, the viscosity decreases, the phase shift inevitably also decreases. In a dry-running scenario, the phase shift is minimal due to the lack of pumped medium in the area of the impeller.

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

In order to record changes in the physical properties of the conveying medium or to detect dry running, it can therefore be provided to provide corresponding reference values for the phase offset or the amplitude ratio or the amplitude itself, whereby a change in the currently determined phase offset and/or the amplitude ratio compared to the reference value can be used to conclude that there has been a change in the conveying medium and, if necessary, that the medium is running dry.

The one or more reference values for the process execution can be obtained, for example, during a training run of the pump, in particular during commissioning or at regular times during pump operation. It is also conceivable to determine the reference values by means of preceding measurements and to update them continuously.

In addition to the method according to the invention, the present invention also relates to a centrifugal pump, in particular a heating circulation pump, with an integral or external control/regulation that is configured to carry out the method according to the invention. As a result, the centrifugal pump according to the invention has the same advantages and properties as have already been explained in detail with reference to the method according to the invention. For this reason, a repeated description is omitted.

• 1: eine Diagrammdarstellung des Drehmomentes eines Pumpenantriebsaggregates gegenüber der Drehzahl für unterschiedliche Pumpenzustände,
• 2: eine exemplarische Darstellung des zeitlichen Drehmomentverlaufs nach Überlagerung mit einem periodischen Signal für die Ausführung des erfindungsgemäßen Verfahrens, und
• 3: eine exemplarische Gegenüberstellung der mit einem periodischen Signal überlagerten Soll-Drehzahl sowie des resultierenden Drehmomentverlaufs über der Zeit.

Further advantages and properties of the invention will be described below with reference to the embodiments explained with the aid of the figures. They show:

• 1 : a diagram showing the torque of a pump drive unit versus the speed for different pump conditions,
• 2 : an exemplary representation of the temporal torque curve after superposition with a periodic signal for the execution of the method according to the invention, and
• 3 : an exemplary comparison of the target speed superimposed with a periodic signal and the resulting torque curve over time.

1 shows the torque of a centrifugal pump unit as a function of the set target speed of the pump. The diagram shows a total of two curves. The dotted line shows the torque P _Q0 over the entire speed range for the case of a suction line completely filled with pumped medium, but with a closed valve in the suction line so that the pump operates with zero flow. The solid line shows the torque P _Dry over the entire speed range when the pump is running dry.

Overall, it is clear that as the speed increases, the deviation between the torque P _Dry when running dry and the torque P _Q0 when the pump is properly filled increases. The method described therefore works very precisely at medium and high speeds. Particularly in the low speed range, for example at the speed marked with reference number 1, the torque differences or differences in power consumption between the filled system with a closed valve and the unfilled system are very small or in some cases almost non-existent. It is difficult to reliably distinguish whether dry running is actually occurring or whether the valve is just closed in this speed range.

To solve the above-mentioned problem, the invention therefore proposes to increase the target speed of the speed-controlled centrifugal pump for a short time, for example to the 1 with the reference number 2. Dry running can be detected very reliably here. According to the figure, the speed is increased significantly for dry running detection, here by a factor of about three. In principle, it can be said 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 it is very easy to implement.

Since a temporary increase in speed to the extent suggested above may not be permitted in certain processes, 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 allows information on the pumped medium to be obtained.

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 curve and the resulting (measured or estimated) torque curve. Alternatively, the ampli tude of the measured or estimated torque curve.

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. The phase shift or the amplitude of the torque depend on the inertia of the moving machine parts, e.g. impeller, shaft, etc. (known) and the pumped medium (to be determined). The process can be divided into two steps.

Step 1: Recording the phase shifts or torque amplitude in the normal state. Step 1 takes place in the laboratory. Different operating points are approached with a defined reference medium and at a reference temperature (various speed/torque combinations) and the target speed is superimposed by a higher frequency signal (e.g. 10 Hz). For each of these operating points, the phase shift between the target speed curve and the torque curve or alternatively the torque amplitude is recorded. The resulting characteristic map is stored in the pump.

Step 2: Identifying different parameters in the field

While the pump is running, the target speed is superimposed at regular intervals or once on request with the same higher frequency signal as previously in the laboratory and the phase shift between the target speed curve and the measured/estimated torque curve or alternatively the torque amplitude of the torque curve is recorded. If the phase shift or the 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 dry running. The phase shift can also provide information about a density or temperature of the medium that deviates from the reference or an incorrect mixing ratio.

An example is 2 the simulated torque curve of a centrifugal pump, where the static target speed is superimposed with a periodic signal and the periodic torque curve marked with the reference number 10 results (corresponds to the laboratory setup). Curve 20 shows the torque curve with an identical test setup, where the simulated mass inertia of the rotating part of the pump was increased by 50%. It is clearly visible that the amplitude of curve 20 is higher. The ratio of the amplitudes, marked here with the reference number 40, is equal to the ratio of the simulated mass inertias. The pump could recognize this deviation of 50% compared to the values recorded in the laboratory and conclude from this that the mass inertia has changed by 50%. The density of the medium could also have changed in the same way as the mass inertia.

The mean value of the torque curves 30 is identical. This means that the information on the mass inertia would not be recognizable at a static target speed (without the application of a periodic signal).

In 3 The time curves of the target speed and the recorded torque are now shown. Curve 50 corresponds to the target speed, which was superimposed with a periodic signal. Curve 60 shows the resulting torque curve. Reference number 70 marks the resulting phase offset between the two curves, which also increases with increasing mass inertia. As a precaution, it is pointed out that in the illustration of the 3 the speed curve 50 and the torque curve 60 were shifted over each other to make the phase shift easier to understand

Claims (11)

Method for checking the properties of a pumped medium of a centrifugal pump, in particular for dry running detection, using a speed-controlled centrifugal pump, wherein for the check the target speed of the centrifugal pump is briefly varied, in particular increased, and a motor parameter characterizing the torque of the pump is determined and evaluated taking into account a reference value in order to determine a property of the pumped medium and/or to detect the presence of dry running. Procedure according to Claim 1 , characterized in that by means of the method a change in the density and/or a change in temperature of the conveying medium compared to a reference value is determined. Method according to one of the preceding claims, characterized in that the torque generated by the pump is determined, in particular estimated, as a motor parameter characterizing the torque of the pump, and/or the electrical power consumption of the motor and/or the motor current consumed is determined or estimated. Method according to one of the preceding claims, characterized in that it is first checked whether the current target speed is below a minimum value and a variation or increase in the target speed only occurs if the current target speed is below the minimum value. Method according to one of the preceding claims, characterized in that the speed variation provides a short-term increase in the target speed, the engine parameter for the currently increased target speed is determined and compared with a reference value assigned to the increased target speed, wherein dry running is detected if the engine parameter is smaller than the reference value or is smaller than the reference value by at least one defined value. Method according to one of the preceding Claims 1 until 4 , characterized in that the target speed for the check is superimposed for a certain period of time with a time-variable signal, in particular with a periodic signal. Procedure according to Claim 6 , characterized in that a property of the pumped medium or the presence of dry running is detected on the basis of a correlation between the resulting target speed curve and the time curve of the motor parameter. Procedure according to Claim 7 , characterized in that a phase offset between the time course of the target speed and the time course of the engine parameter is evaluated and preferably compared against a reference value. Method according to one of the Claims 6 until 8th , characterized in that the amplitude of the engine parameter is determined during the superposition of the target speed and is preferably compared against a reference value. Method according to one of the preceding claims, characterized in that the one or more reference values are determined by a training procedure carried out under normal conditions. Centrifugal pump, in particular heating circulation pump, with an integral or external control configured to carry out the method according to one of the preceding claims.