EP3569865A1 - Monitoring of a rotary pump powered by an electric motor - Google Patents

Abstract

The invention relates to a method for monitoring a centrifugal pump (1) driven by an electric motor (3). During a teach-in operation, the centrifugal pump (1) is operated with a specific for the centrifugal pump (1) minimum flow (Q _min ) and the operation of the centrifugal pump (1) with the minimum flow (Q _min ) is an active power (P _min ) of the electric motor (3) determined. From the determined at the minimum flow (Q _min) active power (P _min), a lower real power threshold value is determined. In a normal operation of the centrifugal pump (1) is a current active power (P) of the electric motor (3) and compared with the lower active power threshold continuously. The centrifugal pump (1) is switched off in normal operation when the instantaneous active power (P) of the electric motor (3) falls below the lower active power threshold.

Description

The invention relates to a method and a monitoring device for monitoring a driven by an electric motor centrifugal pump.

A centrifugal pump is a turbomachine, wherein by means of a rotating impeller, a conveying medium, usually a liquid, suspension or gas-containing liquid, is moved. The through a supply line, which is usually a pipe, incoming conveying medium is thereby promoted by the rotating impeller, that is accelerated. The accelerated fluid then exits via a drain line from the centrifugal pump. The drive of the centrifugal pump is usually carried out by an electric motor. In centrifugal pumps usually mechanical seals are used to seal a rotating shaft of the impeller against the centrifugal pump housing. But also pumps with a magnetic coupling or canned motor pumps are known, which do not require a mechanical seal. The liquid pumped medium takes over in centrifugal pumps in particular also the function of cooling the mechanical seals and / or bearings. Therefore, a sufficient amount of liquid delivery medium is always required for the operation of centrifugal pumps. As a result, overheating of the mechanical seals and / or the bearings and possibly other centrifugal pump components and thus damage to the centrifugal pump can be avoided.

If the centrifugal pump is insufficiently supplied with pumped liquid, it is called a dry run. This can be done, for example, that inadvertently suction-side valves are closed or a centrifugal pump with the pumped liquid supplying reservoir is empty. Dry running quickly leads to overheating of pump components. In particular, in an environment where there is a risk of explosion (so-called EX or ATEX area), ignition source formation due to equipment used in this area should be avoided. If, for example, a centrifugal pump is used within such an environment, it must be avoided that the centrifugal pump causes a hazard as an ignition source. For example, in a dry run, a centrifugal pump may act as an ignition source if it is filled with an ignitable delivery medium or if an ignitable mixture is present in an outer space of the delivery pump which is flammable by a hot surface of the centrifugal pump. In particular in explosive environments, suitable precautions must therefore be taken to avoid dry running of a centrifugal pump.

In order to avoid dry running of a centrifugal pump, for example, so-called liquid level limit switches or flow sensors are used to monitor the sufficient presence of liquid in the centrifugal pump.

WO 2009/006927 A1 discloses a method for monitoring a driven by an electric motor centrifugal pump to prevent dry running, in which an active power or a load current of the electric motor is detected and evaluated for detecting a dry running hazard.

The invention has for its object to provide a method and a monitoring device for monitoring a driven by an electric motor centrifugal pump, which are improved in particular with regard to the avoidance of dry running of the centrifugal pump.

The object is achieved by a method having the features of claim 1 and a monitoring device having the features of claim 11.

Advantageous embodiments of the invention are the subject of the dependent claims.

In the method according to the invention for monitoring a centrifugal pump driven by an electric motor, the centrifugal pump is operated during a teach-in operation with a minimum flow which is specific for the centrifugal pump, and an active power of the electric motor is determined during operation of the centrifugal pump with the minimum flow. From the active power determined at the minimum flow, a lower active power threshold is determined. In a normal operation of the centrifugal pump, a momentary active power of the electric motor is continuously determined and compared with the lower active power threshold value. The centrifugal pump is switched off in normal operation when the instantaneous active power of the electric motor falls below the lower active power threshold.

The invention therefore provides to monitor an active power of the electric motor driving the centrifugal pump and to switch off the centrifugal pump when the active power falls below a lower active power threshold value. The lower active power threshold value is determined with a teach-in operation from an active power resulting at a minimum flow rate of the centrifugal pump. The minimum flow is, for example, a flow indicated by the manufacturer of the centrifugal pump, under which the centrifugal pump should not be operated. Alternatively, the minimum flow can be determined, for example, based on a setpoint characteristic curve provided by the manufacturer of the centrifugal pump for the active power as a function of the flow rate of the centrifugal pump or based on empirical values.

By switching off the centrifugal pump falls below the lower active power threshold, in particular a dry run of the centrifugal pump can be avoided. For the dry running monitoring, therefore, no sensors such as liquid level limit switches or flow sensors are required, which are located in the pumped from the centrifugal pump fluid and therefore need to be maintained regularly. This eliminates the need to use such sensors The effort involved in the planning, procurement, installation, commissioning, maintenance and servicing of these sensors and the costs for dry running monitoring are reduced overall. Further advantages result from the elimination of disadvantages of using the sensors. Such disadvantages are that these sensors can be damaged or impaired by corrosion or by ingredients of the conveyed medium such as fibers or lint, that the sensors do not allow adjustment of a triggering sensitivity that the sensors can not detect whether the pump is filled with the pumped medium that a sensor arranged on the suction side can not detect zero delivery when the pressure-side fitting is closed and can not detect a mistakenly closed suction-side fitting arranged behind the sensor, and that a change in the position of a sensor is usually associated with extensive remodeling work if the installation location of the sensor is known as not optimal for dry running protection.

The determination of the lower active power threshold by the teach-in operation allows a reliable determination of this threshold under the actual operating conditions. Furthermore, the threshold value can be checked by a repetition of the teach-in operation and corrected if necessary, for example in order to adapt it to a change in the pumped medium, a change of system components and / or a change of process conditions.

An embodiment of the invention provides that the lower active power threshold value is at least as great as the active power of the electric motor determined at the minimum flow rate. For example, the lower active power threshold is determined as the product of the active power of the electric motor determined at the minimum flow and a first safety factor that is greater than one. This protects the centrifugal pump not only against dry running, but also before operation with a flow below the minimum flow. The determination of the lower The active power threshold, as the product of the active motor electric power determined at the minimum flow rate and a first safety factor greater than one, allows to account for measurement inaccuracies that may occur in setting the minimum flow and in determining the effective power of the electric motor.

A further embodiment of the invention provides that during the teach-in operation, the centrifugal pump is also operated with a reference flow, which is greater than the minimum flow, and in the operation of the centrifugal pump with the reference flow, an active power of the electric motor is determined, and the normal operation of the centrifugal pump is received only when the determined at the reference flow active power of the electric motor is greater than the determined at the minimum flow effective power of the electric motor. For example, an active power quotient of the determined at the minimum flow active power of the electric motor is formed as a dividend and the determined at the reference flow active power of the electric motor as a divisor, it is a reference quotient, which is less than one, defined and the normal operation of the centrifugal pump is only recorded, if the active power quotient is smaller than the reference quotient.

By determining the active power of the electric motor at the reference flow and the comparison of this active power with the determined at the minimum flow active power can be checked whether the centrifugal pump behaves in an expected manner, for example, corresponds to a desired characteristic of the centrifugal pump. If this is not the case, this indicates that the centrifugal pump system is configured incorrectly, for example, has incorrect components or is mounted incorrectly. In this case, the normal operation of the centrifugal pump is therefore not included. The formation of an active power quotient from the active power determined at the minimum flow rate and the active power of the electric motor determined at the reference flow rate and the comparison of the active power quotient with a reference quotient allows a quantitative assessment of whether the centrifugal pump behaves as expected. A reference quotient of less than one makes it possible, in particular, to test whether the active power increases with increasing flow in a measure corresponding to the expectation.

A further embodiment of the invention provides that during the teach-in operation, the centrifugal pump is also operated with a specific for the centrifugal pump maximum flow from the determined at the maximum flow active power an upper active power threshold is determined in the normal operation of the centrifugal pump continuously the current Active power of the electric motor is compared with the upper active power threshold and in normal operation, the centrifugal pump is switched off or the active power of the electric motor is reduced when the instantaneous active power of the electric motor exceeds the upper active power threshold. Preferably, the upper active power threshold is at most as large as the effective power of the electric motor determined at the maximum flow. For example, the upper active power threshold is determined as the product of the effective power of the electric motor determined at the maximum flow and a second safety factor less than or equal to one.

The aforementioned embodiment of the invention makes it possible to detect an overload of the centrifugal pump in that the active power of the electric motor exceeds an upper active power threshold. The upper active power threshold is determined analogously to the lower active power threshold with the teach-in operation. A determination of the upper active power threshold as the product of the effective power of the electric motor determined at the maximum flow and a second safety factor which is less than or equal to one, in turn, takes into account possible inaccuracies in the measurement of the maximum flow and can occur in the determination of the active power of the electric motor.

As an alternative to the aforementioned embodiment of the invention, it can be provided that an upper active power threshold value is determined from the active power of the electric motor determined at the reference flow rate and the active power of the electric motor determined at the minimum flow rate. In the normal operation of the centrifugal pump, the instantaneous active power of the electric motor is continuously related to the upper one Active power threshold is compared and in normal operation, the centrifugal pump is switched off or the active power of the electric motor is reduced when the instantaneous active power of the electric motor exceeds the upper active power threshold. For example, the upper active power threshold is expressed as the quotient of the square of the active power of the electric motor determined at the reference flow as a dividend and the active power of the electric motor determined as the divisor at the minimum flow or as the product of this quotient with a second safety factor which is less than or equal to one. educated.

In the aforementioned alternative embodiment of the invention, the upper active power threshold value is determined by extrapolation from the active power determined at the reference flow rate and the active power of the electric motor determined at the minimum flow rate. This refinement of the invention can be used, for example, when the maximum flow rate can not be approached during the teach-in operation, for example due to excessive pressure losses.

A monitoring device according to the invention for carrying out the method according to the invention comprises an active power determination device for determining the active power of the electric motor and a control unit, which is designed to determine the lower active power threshold, in the normal operation of the centrifugal pump continuously the current Compare active power of the electric motor with the lower active power threshold and turn off the centrifugal pump in normal operation, when the current active power of the electric motor falls below the lower active power threshold. According to the aforementioned embodiments of the method, the control unit can also be designed to determine the upper active power threshold value, in the normal operation of the centrifugal pump continuously to compare the instantaneous active power of the electric motor with the upper active power threshold and turn off the centrifugal pump in normal operation or the active power of the electric motor reduce when the instantaneous active power of the electric motor exceeds the upper active power threshold. The control unit and / or the active power determination device can be designed, for example, as an engine management system such as the applicant's SIMOCODE pro system, as a soft starter or as a frequency converter, or integrated into an engine management system, a soft starter or a frequency converter.

Since a monitoring device according to the invention serves to carry out the method according to the invention, the advantages of the monitoring device correspond to the advantages of the method according to the invention set out above and are therefore not listed separately here again.

The outlined task is also solved by a computer program product according to the invention. The computer program product is designed to be executable in a monitoring device, in particular a control unit of the monitoring device. The computer program product can be stored as software or firmware in a memory storable and executable by an arithmetic unit. Alternatively or additionally, the computer program product can also be at least partially embodied as a hard-wired circuit, for example as an ASIC. The computer program product is designed to receive measured values, to evaluate and to generate commands to an electric motor. According to the invention the computer program product is designed to implement and perform at least one embodiment of the outlined method for monitoring a centrifugal pump driven by an electric motor. In this case, the computer program product can combine all partial functions of the method, ie be monolithic. Alternatively, the computer program product can also be segmented and each distribute sub-functions to segments that are executed on separate hardware. For example, the determination of the active power threshold value in one segment can be carried out and the comparison of an instantaneous active power with a lower threshold value in another segment. A part of the monitoring method may also be carried out in one monitoring device, in particular a control unit of the monitoring device, and another part of the monitoring method in a higher-level control unit, such as a PLC, a manual parameterizing device or a computer cloud.

FIG 1

ein Blockdiagramm eines Kreiselpumpensystems mit einer von einem Elektromotor angetriebenen Kreiselpumpe und einer Überwachungsvorrichtung zum Überwachen der Kreiselpumpe,

FIG 2

eine Kennlinie einer Kreiselpumpe,

FIG 3

ein Ablaufdiagramm eines Verfahrens zum Überwachen einer von einem Elektromotor angetriebenen Kreiselpumpe.

The above-described characteristics, features, and advantages of this invention, as well as the manner in which they will be achieved, will become clearer and more clearly understood in connection with the following description of exemplary embodiments which will be described in detail in conjunction with the drawings. Showing:

FIG. 1

a block diagram of a centrifugal pump system with a driven by an electric motor centrifugal pump and a monitoring device for monitoring the centrifugal pump,

FIG. 2

a characteristic of a centrifugal pump,

FIG. 3

a flowchart of a method for monitoring a driven by an electric motor centrifugal pump.

FIG. 1 shows a block diagram of a centrifugal pump system. The centrifugal pump system comprises a centrifugal pump 1, an electric motor 3, a reservoir 5 and a monitoring device 7 for monitoring the centrifugal pump 1.

The centrifugal pump 1 is filled with a delivery medium which is stored in the reservoir 5, the centrifugal pump 1 is supplied via a valve 9 and a supply line 11 and is conveyed by means of the centrifugal pump 1 in a discharge line 13. The centrifugal pump 1 is driven by the electric motor 3.

The monitoring device 7 comprises a flow rate determination device 15, an active power determination device 17 and a control unit 19.

The flow determination device 15 is designed to determine the flow Q of the centrifugal pump 1. For example, the flow rate determination device 15 has a stationary or mobile flow meter, with which the flow rate Q is measured directly as a volume flow in the discharge line 13 or at another suitable location. Alternatively, the flow rate detecting device 15 has a pressure measuring device for detecting a pressure difference between a suction side pressure and a pressure side pressure of the centrifugal pump 1, that is, a pressure difference between a pressure in the supply line 11 and a pressure in the discharge line 13. In the latter case, the flow rate detecting device 15 or the control unit 19 is adapted to determine the flow Q by means of the pressure difference, for example based on a corresponding characteristic of the centrifugal pump 1 and possibly using other parameters such as a delivery of the centrifugal pump 1. Alternatively, a based on a different method of flow determination flow determination device 15 are used.

The active power determination device 17 is designed to determine an active power P of the electric motor 3.

The control unit 19 is designed in particular for carrying out the method steps S2, S3, S4, S6 and S7 of the method described with reference to FIG. 3 for monitoring the centrifugal pump 1.

FIG. 2 shows an example of a characteristic of the centrifugal pump 1 for a power, here for the active power P of the electric motor 3 in response to the flow Q of the centrifugal pump 1. The curve is approximately linear in the example shown here with a positive slope. Q _min denotes a specific for the centrifugal pump 1 minimum flow, which should not be exceeded, Q _ref denotes a reference flow and Q _max denotes a specific for the centrifugal pump 1 maximum flow, which should not be exceeded. P _min , P _ref and P _max denote the corresponding active powers. Q _min and Q _max are, for example, limit values of the flow Q predetermined by the manufacturer of the centrifugal pump 1 for the operation of the centrifugal pump 1. Q _ref is greater than Q _min . For example, Q _{ref is} an optimum flow rate specified by the manufacturer of the centrifugal pump 1 for operating the centrifugal pump 1 with optimum efficiency. Alternatively, Q _min, Q _ref and / or Q _max, for example, based on one of the manufacturer of the centrifugal pump 1 target characteristic or made available on the basis of empirical values determined.

FIG. 3 shows a flowchart of an embodiment of the method according to the invention for monitoring the centrifugal pump 1 by means of the monitoring device 7 with method steps S1 to S7.

In a first method step S1, during a teach-in operation, the centrifugal pump 1 is operated with the minimum flow rate Q _min specific to the centrifugal pump 1, and during the operation of the centrifugal pump 1 with the minimum flow rate Q _min the active power P _{min of} the electric motor 3 is determined. Furthermore, in the first method step S1 during the teach-in operation, the centrifugal pump 1 is operated with the reference _flow rate Q _ref , and in the operation of the centrifugal pump 1 with the reference _flow rate Q _ref , the active power P _{ref of} the electric motor 3 is determined. For this purpose, the minimum flow rate Q _min or the reference flow rate Q _{ref of} the centrifugal pump 1 is set with the aid of the control unit 19 by controlling the electric motor 3 using the flow rate determination device 15 and then the active power P _min or P _{ref of} the electric motor 3 is determined by the active power determination device 17.

Alternatively, the minimum flow Q _min and the reference flow Q _{ref of} the centrifugal pump 1 can be set manually, and then the respective active power P _min or P _{ref of} the electric motor 3 can be determined by means of the active power determination device 17.

In a second method step S2, the active power P _{ref of} the electric motor 3 determined at the reference _flow rate Q _{ref is} compared with the active power P _{min of} the electric motor 3 determined at the minimum flow rate Q _min with the aid of the control unit 19. For this purpose, for example, a reference quotient which is smaller than one is defined, from the active power P _{min of} the electric motor 3 determined at the minimum flow Q _min and the active power P _{ref of} the electric motor 3 determined at the reference flow Q _ref , an active power quotient P _min / P _ref is formed, and it is checked whether the active power quotient is smaller than the reference quotient. For example, the reference quotient has the value 0.8. If the test result is negative, that is, if the active power quotient is greater than the reference quotient or coincides with the reference quotient, the method is continued with a third method step S3. Otherwise, the method is continued with a fourth method step S4. The comparison of the active power quotient with the reference quotient is a plausibility test, in which it is checked whether P _min and P _{ref are} in a relationship limited by the reference quotient. A negative test result indicates that the centrifugal pump system is configured incorrectly, for example, has incorrect components or is mounted incorrectly.

In the third method step S3, the method is ended by the control unit 19. Further, a warning may be issued on a possible misconfiguration of the centrifugal pumping system and / or a request to verify the configuration of the centrifugal pumping system.

In the fourth method step S4, a lower active power threshold is determined by the control unit 19 from the active power determined at the minimum flow Q _min , which is at least as large as the active power P _{min of} the electric motor determined at the minimum flow Q _min . For example, the lower active power threshold value is determined as the product of the active power P _{min of} the electric motor 3 determined at the minimum flow rate Q _min and a first safety factor that is greater than one. For example, the first safety factor is 1.1.

In a fifth method step S5, a momentary active power P of the electric motor 3 is determined by the active power determination device 17 in a normal operation of the centrifugal pump 1.

In a sixth method step S6, the instantaneous active power P of the electric motor 3 determined in the fifth method step S5 is compared by the control unit 19 with the lower active power threshold value determined in the fourth method step S4. If the instantaneous active power P of the electric motor 3 falls below the lower active power threshold value, the method is continued with a seventh method step S7. Otherwise, the method is continued with the fifth method step S5.

In the seventh method step S7, the centrifugal pump 1 is switched off by the control unit 19. As a result, in particular dry running of the centrifugal pump 1 is prevented.

In the following, alternative embodiments of the method according to the invention will be described, the further developments of the above with reference to FIG. 3 described embodiment and provide extensions of several method steps S1 to S7.

A first refinement provides that in the first method step S1 during the teach-in operation, the centrifugal pump 1 is additionally operated with the maximum flow rate Q _max specific to the centrifugal pump 1 and the active power P during the operation of the centrifugal pump 1 with the maximum flow rate Q _{max max of} the electric motor 3 is determined. In the second step S2 is then also from the determined at the maximum flow Q _max active power P _max of the control unit 19, an upper real power threshold value is determined which is at most as large as the true power at the maximum flow Q _max P _max of the electric motor 3. For example, the upper active power threshold value is determined as the product of the active power P _{max of} the electric motor 3 determined at the maximum flow rate Q _max and a second safety factor that is less than or equal to one. For example, the second safety factor is 0.9.

An alternative refinement provides that the upper active power threshold value in the second method step S2 is determined from the active power P _{ref of} the electric motor 3 determined at the reference _flow rate Q _ref and the active power P _{min of} the electric motor 3 determined at the minimum flow rate Q _min . For example, the upper active power threshold is expressed as the quotient (P _ref ) ² / P _{min of} the square of P _ref as a dividend and P _min as a divisor or as the product of this quotient with a second confidence factor less than or equal to one and, for example, the value 0.9 has formed. This refinement can be used, for example, if the maximum flow Q _max can not be approached during the teach-in operation, for example because of excessive pressure losses.

In both aforementioned refinements of the above with reference to FIG. 3 In the sixth method step S6, the instantaneous active power P of the electric motor 3 determined in the fifth method step S5 is additionally compared by the control unit 19 with the upper active power threshold value. If the instantaneous active power P of the electric motor 3 exceeds the upper active power threshold value, the method is continued with the seventh method step S7. In the seventh method step S7, if the upper active power threshold value has been exceeded, instead of switching off the centrifugal pump 1, it may alternatively be provided to reduce the active power P of the electric motor 3 and to continue the method with the fifth method step S5.

While the invention has been further illustrated and described in detail by way of preferred embodiments, the invention is not limited by the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims (13)

Method for monitoring a centrifugal pump (1) driven by an electric motor (3), wherein - During a teach-in operation, the centrifugal pump (1) with one for the centrifugal pump (1) specific minimum flow (Q _min ) is operated and in the operation of the centrifugal pump (1) with the minimum flow (Q _min ) an active power (P _min ) of the electric motor (3) is determined, - from the at the minimum flow (Q _min) calculated active power (P _min), a lower real power threshold value is determined, - In a normal operation of the centrifugal pump (1) continuously a momentary active power (P) of the electric motor (3) is determined and compared with the lower active power threshold - And the centrifugal pump (1) is switched off in the normal mode, when the instantaneous active power (P) of the electric motor (3) falls below the lower active power threshold. The method of claim 1, wherein the lower active power threshold is at least as large as the minimum power (Q _min ) determined active power (P _min ) of the electric motor (3). Method according to claim 1 or 2, wherein the lower active power threshold is determined as the product of the active power (P _min ) of the electric motor (3) determined at the minimum flow rate (Q _min ) and a first safety factor greater than one. Method according to one of claims 1 to 3, wherein during the teach-in operation, the centrifugal pump (1) with a reference flow (Q _ref ), which is greater than the minimum flow rate (Q _min ) is operated, and in the operation of the centrifugal pump ( 1) with the reference flow (Q _ref ) an active power (P _ref ) of the electric motor (3) is determined, and wherein the normal operation of the centrifugal pump (1) is only recorded if the active power (P _ref ) of the electric motor (3) determined at the reference flow rate (Q _ref ) is greater than the active power (P _min ) of the electric motor determined at the minimum flow rate (Q _min ) (3) is. Method according to Claim 4, wherein an active power quotient comprises the active power (P _min ) of the electric motor (3) determined as a dividend at the minimum flow rate (Q _min ) and the active power (P _ref ) of the electric motor (3) determined at the reference flow rate (Q _ref ). is formed as a divisor, a reference quotient, which is less than one, is defined and the normal operation of the centrifugal pump (1) is only recorded when the active power quotient is smaller than the reference quotient. Method according to one of claims 1 to 5, wherein during the teach-in operation, the centrifugal pump (1) is operated with a maximum flow (Q _max ) specific to the centrifugal pump (1), - is determined from the at the maximum flow (Q _max) detected active power (P _max), an upper real power threshold value, - In the normal operation of the centrifugal pump (1) continuously the instantaneous active power (P) of the electric motor (3) is compared with the upper active power threshold - And in normal operation, the centrifugal pump (1) is switched off or the active power (P) of the electric motor (3) is reduced when the instantaneous active power (P) of the electric motor (3) exceeds the upper active power threshold. The method of claim 6, wherein the upper active power threshold is at most as large as the maximum power (Q _max ) determined active power (P _max ) of the electric motor (3). The method of claim 6 or 7, wherein the upper active power threshold value than the product of the maximum flow rate (Q _max ) determined active power (P _max ) of the electric motor (3) and a second safety factor which is less than or equal to one, is determined. The method of claim 4 or 5, wherein from the at the reference flow (Q _ref ) determined active power (P _ref ) of the electric motor (3) and at the minimum flow (Q _min ) determined active power (P _min ) of the electric motor (3) an upper Active power threshold is determined - In the normal operation of the centrifugal pump (1) continuously the instantaneous active power (P) of the electric motor (3) is compared with the upper active power threshold - And in normal operation, the centrifugal pump (1) is switched off or the active power (P) of the electric motor (3) is reduced when the instantaneous active power (P) of the electric motor (3) exceeds the upper active power threshold. The method of claim 9, wherein the upper real power threshold value as the quotient of the square of at the reference flow rate (Q _ref) detected active power (P _ref) of the electric motor (3) as the dividend and at the minimum flow (Q _min) calculated active power (P _min) of the electric motor (3) is formed as a divisor or as the product of this quotient with a second safety factor which is less than or equal to one. Monitoring device (7) for carrying out the method according to one of the preceding claims, comprising the monitoring device (7) - An active power detecting device (17) for determining the active power (P) of the electric motor (3) and - A control unit (19) which is adapted to determine the lower active power threshold, in the normal operation of the centrifugal pump (1) continuously comparing the instantaneous active power (P) of the electric motor (3) with the lower active power threshold and the centrifugal pump (1) in normal operation, when the instantaneous active power (P) of the electric motor (3) falls below the lower active power threshold. A monitoring device (7) according to claim 11, wherein when dependent on one of claims 6 to 10, wherein the control unit (19) is adapted to determine the upper active power threshold, in the normal operation of the centrifugal pump (1) continuously the instantaneous active power (P) of the Electric motor (3) to compare with the upper active power threshold and in normal operation, the centrifugal pump (1) or to reduce the active power of the electric motor (3) when the instantaneous active power (P) of the electric motor (3) exceeds the upper active power threshold. Computer program product executable in a monitoring device (7), characterized in that the computer program product is designed to carry out a method according to one of claims 1 to 10.

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