DE102014008716B4 - Procedure for detecting a dry run - Google Patents

Abstract

Method for detecting dry running within a centrifugal pump unit, characterized in that the rotor of the centrifugal pump unit accelerates from the current operating speed (n_0) for a period of time (T) and the energy (E) required for acceleration is calculated, the calculated energy (E ) is compared with an energy reference value (Eref) and, depending on the result of this comparison, a dry run is concluded, with the energy (E) consumed for acceleration being calculated by measuring the electrical power consumption (P_0) of the centrifugal pump unit at the beginning and in discrete Steps determined during the acceleration process, the differences between the power consumption (P_0) at the beginning of the acceleration process and the respective power consumption (P_akt) summed up during the acceleration process and these differences are then multiplied by the sampling interval.

Description

The present invention relates to a method for detecting dry running within a centrifugal pump unit. Furthermore, the invention relates to pump electronics for a centrifugal pump for carrying out the method and a centrifugal pump with such pump electronics.

With centrifugal pumps, it is particularly important to recognize whether the liquid to be pumped is present or not. Since various rotating parts in the pump unit are lubricated and/or cooled by the liquid to be pumped, dry running inevitably leads to increased wear or increased thermal stress, resulting in shorter service lives and damage to the centrifugal pump unit. The main components affected here are, for example, plain bearings or mechanical seals. Since no liquid is pumped in the event of a dry run, the pump unit consumes energy unnecessarily. Finally, various applications in which the centrifugal pump unit is used require information as to whether the pumped liquid is present or not. Typically, heaters such as boilers or air conditioning use additional hardware (sensors) to protect a system against the threat of running dry.

The prior art knows various ways of detecting dry running. For example, the simplest option is to use a liquid sensor that directly indicates whether liquid is present or not. Another possibility is, for example, measuring the temperature on a critical component, for example a plain bearing, using a temperature sensor. Here, a dry run is indirectly concluded, since the coolant is obviously missing in the case of an increased temperature on the monitored component. Such a solution is, for example, from the German Offenlegungsschrift DE 10 2010 039 531 A1 known. A disadvantage of the options mentioned is that an additional sensor is required, which leads to increased structural complexity and additional costs.

Another alternative is to measure the pressure on the outlet side of the pump unit. The European patent application EP 2 055 965 A1 describes the detection of dry running based on a pressure value in the pump falling below a reference pressure in relation to a specific speed.

If no medium flows through the pump, the pressure in the system drops and the frequency converter feeding the electric motor of the pump unit would increase the speed of the pump unit to compensate for the drop in pressure. It follows that in the event of dry running, the maximum speed would be accelerated, while the power output would decrease, since the power in the case of dry running is proportional to the speed (P~n) during normal operation of the pump unit, i.e. operation with pumped liquid , but the power output is proportional to the 3rd power of the speed (P~n ³ ). The German utility model specification DE 20 2005 007 955 U1 describes a device for pressure control with dry-running protection, in which dry-running is detected in that the output power of the pump falls below half of the maximum power input of the frequency converter at maximum speed.

Other known types of dry-running protection use thermocouples, which measure the frictional heat between the stator and rotor, a probe with an electrode protruding into the liquid flow of the pump and measuring the resistance between the pump housing and the liquid, see published application DE 29 25 830 A1 , or a vibration measurement, because if there is a lack of liquid, increased vibrations will occur on the rotor shaft.

The international patent application WO 2009/006927 A1 describes the determination of the active power or the load current of the electric motor driving the centrifugal pump when the centrifugal pump is started, the determined value or a value derived from it being compared with an active power or load current reference value which represents a sufficient quantity of pumped medium in the centrifugal pump. If the deviation of the value in relation to the active power or load current reference value falls below a defined threshold, this condition is interpreted as dry running. This method works reliably at high speeds.

Based on the attached 1 However, it becomes clear that the use of the active power or the load current is not expedient for low speeds. Mathematically, this can already be seen from the fact that the proportionality ratios in the normal case (P~n ³ ) and in the case of dry running (P~n) differ only slightly at low speeds. This comes in 1 to expression.

1 shows the power curve over the speed of a pump set for a flow rate of zero, see dotted line, and for dry running, see solid line. Another curve is drawn between these curves, which represents the mean value between the other two curves. This mean value is used by the applicant for dry-running detection by comparing the current power with this reference value. If the electrical power consumption is below the reference value for a specific speed, dry running is assumed. On 1 it becomes clear that this decision does not work for speeds below 2000 rpm, since the curves overlap there and no clear statement can be made about the operating status of the pump unit.

From the European patent application EP 2 743 509 A1 a method is known for monitoring the dry running of a feed pump, in which, starting from a known initial state, an applied voltage is removed from the electric drive motor or a defined energy input is introduced into the electric drive motor, whereupon the electric drive motor is allowed to run down to a defined end state, wherein the speed detection sensor detects the number of revolutions of a motor rotor when the electric drive motor runs down to the defined end state and compares it with a previously defined threshold value, and when the defined threshold value is exceeded it is concluded that the feed pump is running dry.

The publication DE 101 01 099 A1 describes a method for monitoring dry running of a dry-installed feed pump, which is driven by an electric drive motor, with the electric current of the drive motor being recorded as an ACTUAL value and compared with a dry-running of the feed pump as a switch-off current as a TARGET limit value and when this TARGET is reached limit value, the electric drive motor is switched off, with the cut-off current (IA) stored in a processor comparing the ACTUAL value with the SET limit value in a range between a dry-running current (IT ) and an operating current (IN) flowing at ZERO delivery of the feed pump.

DE 32 19 818 C2 discloses a torque control device for a hydrostatic transmission of a vehicle coupled to a drive motor.

U.S. 8,774,972 B2 describes a method for controlling the operation of a centrifugal pump with a variable frequency drive (VFD) feeding an AC motor driving the centrifugal pump. The method includes internally automatically monitoring the output current and voltage of the VFD to the AC motor without the need for an external sensor; automatically calculating the output power based on the monitored values of the output current and the output voltage; automatically checking whether the calculated output power is either above a predetermined high power limit or below a predetermined low power limit for a desired setpoint; and automatically initiating a predetermined response action if the calculated output power is either above the predetermined high power limit or below the predetermined low power limit.

Furthermore, from the US 2014/0119966 A1 a protection of displacement pumps known, with a distinction between dangerous operating conditions such as dry running, which can cause damage if left unattended. When using horsepower or torque measurements to detect dry running, it is important to remember that horsepower and torque vary with specific gravity, viscosity, differential pressure and speed changes. RPM is the simplest parameter because it can be measured directly. For systems with different temperatures, the power and torque comparisons must be evaluated at a common specific gravity and viscosity. Therefore, the power and torque readings are corrected to nominal conditions for specific gravity and viscosity changes before evaluation. This can be achieved by entering the specific gravity and viscosity versus temperature curves into the controller. A simple temperature gauge can then be used to correct the power readings.

It is the object of the present invention to provide an alternative method for dry-running detection that does not require the use of additional sensors and can be easily implemented in pump controls without disturbing the conventional pump control.

This object is achieved by a method having the features of claim 1. Advantageous developments of the invention are specified in the dependent claims.

A method for detecting dry running within a centrifugal pump unit is proposed, in which the rotor of the centrifugal pump unit is accelerated from the current operating speed for a specific period of time and the energy required for acceleration is calculated, with the calculated energy being compared with an energy reference value and depending on the Result of this comparison is closed on a dry run.

The core idea of the present invention is based on the finding that the centrifugal pump unit has a significantly different energy consumption in its entire speed range in the event of dry running than in the normal case. The reason for this is that with dry running, only the mass of the rotor and impeller has to be accelerated, i.e. the mechanical inertia of these components has to be overcome, whereas in normal cases the hydraulic inertia of the liquid to be pumped must also be overcome. Experiments have shown that the energy required for an acceleration process in the event of dry running is sufficiently different from that for a corresponding acceleration process with an identical speed jump at the same output speed for each output speed, so that during operation of the centrifugal pump unit each operating speed is clearly identified by a short speed jump whether there is a dry run.

The acceleration process can be started from any operating speed. This has the advantage that the dry-running detection according to the invention can be carried out in any operating state of the pump unit. It is therefore not necessary to start at a specific speed first.

The acceleration process can be terminated according to various criteria. According to a first variant, the acceleration can then be ended when a certain period of time has expired. This means that the period of time for which the acceleration is to take place is specified. This time period can be between 100 ms and 1 s, for example, in particular between 100 ms and 200 ms. The process therefore only needs a tenth of a second to detect a possible dry run.

According to a second variant, which can be used as an alternative or in combination with the first variant, acceleration takes place to a specific target speed. Acceleration stops when the target speed is reached. This target speed can be an absolute speed, for example the maximum speed of the centrifugal pump unit, or a relative speed, for example a speed that is 30% to 50% faster than the current operating speed. In contrast to the first variant, the second variant does not provide for the specification of a period of time but rather for the specification of the target speed, with the period for the acceleration, however, resulting from when the target speed is reached. This depends on the acceleration torque.

According to a third variant, the acceleration is ended as soon as the energy consumed for acceleration differs by a specific amount from the energy required to accelerate the rotor in the case of dry running or in the case of wet running. According to this variant, there is therefore no waiting until a predetermined period of time has elapsed or a specific target speed has been reached. Rather, the acceleration process is already aborted when a clear mathematical decision can already be made about the operating state of the centrifugal pump unit.

The energy theoretically required for acceleration when the centrifugal pump unit is running wet can be used as an energy reference value. A dry run can then be concluded if the calculated energy is less than or less than the energy reference value by a certain amount. Alternatively, the energy theoretically required for acceleration when the centrifugal pump unit is running dry can be used as an energy reference value. In this case, a dry run can be concluded if the calculated energy does not deviate from the energy reference value by more than a specific amount.

Either the energy theoretically required to accelerate the rotor, which is required for dry running, or the energy required for wet running, can be used as an energy reference for the determined actual situation on the centrifugal pump unit. Is the energy consumed for the current acceleration process greater than the energy reference value in the event of a dry run running, there is obviously no dry running. If the energy consumed for the current acceleration process is less than the energy reference value in the case of wet running, then there is obviously dry running.

It should be noted that the third variant described can be used as an alternative or additional termination criterion for the acceleration process compared to the first and/or second variant.

The rotor is preferably accelerated with the maximum torque that the pump assembly can generate. This has the advantage that speed jumps of a certain level can be achieved in a very short time. This in turn has the advantage that a superimposed speed control of the centrifugal pump assembly only has to be deactivated or interrupted for a very short time in order to carry out the method according to the invention for detecting dry running.

According to a variant of the method according to the invention, the energy reference value can be a constant, i.e. a value which is the same over the entire operating range of the pump unit. Such a constant value makes sense above all in the case of the embodiment variant mentioned above, in which a specific jump in speed is carried out for the acceleration process. Based on the starting speed (current operating speed), the jump in speed required to reach the constant energy reference value can be calculated mathematically. The mathematical relationship between the starting speed and the necessary speed jump or necessary target speed in order to reach the specified energy reference value can be expressed in a function that is evaluated before the acceleration process. In this variant, the size of the speed jump that is required to reach the energy reference value is determined.

According to another variant, the energy reference value is not a constant but depends on the speed. Here, a fixed speed jump for the acceleration over the operating range of the pump unit is assumed, for example 100 rpm. Such a fixed speed jump naturally leads to a different energy reference value at low speeds than at high speeds. For example, the energy reference value can be stored as a function dependent on the current operating speed (starting speed) or as a table in which corresponding energy reference values are assigned for a large number of starting speeds. Depending on the starting speed, that energy reference value can then be selected from the table and used for the dry-running detection according to the invention, which is assigned to the current starting speed for a specific speed jump. Alternatively, the energy reference value can be calculated using the stored function for the current starting speed for a specific speed jump and used for the dry-running detection according to the invention.

According to a further embodiment variant, the energy reference value can also be dependent on the speed jump achieved during the acceleration process in addition to being dependent on the current starting speed. In this variant, the speed jump is not fixed. In this case, it is advantageous if the energy reference value is determined by an equation that is constantly calculated during the acceleration process, taking into account the level of the jump in speed that is currently being reached.

For this purpose, it can be provided, for example, that the energy reference value is the energy theoretically required for acceleration when the centrifugal pump assembly is running wet, which energy is calculated during the acceleration process. This means that during the acceleration process, an equation is evaluated that specifies the energy required for acceleration when the centrifugal pump unit is running wet. This energy depends on the current operating speed and the speed jump achieved as a result of the acceleration and can be used as an energy reference value. Since the energy required for acceleration is greater with wet running than with dry running, dry running can be inferred if the calculated energy for accelerating the rotor of the centrifugal pump unit is less than the calculated theoretically required energy for wet running of the centrifugal pump unit.

Alternatively, the energy reference value can be the energy theoretically required for acceleration when the centrifugal pump assembly is running dry, which energy is calculated during the acceleration process. This means that during the acceleration process, an equation is evaluated that specifies the energy required for acceleration when the centrifugal pump unit is running dry. With this variant, there is always a considerable deviation from the energy reference value used during normal operation of the centrifugal pump unit. On the other hand, a dry run can then be concluded when the energy required to accelerate the rotor does not deviate from the energy reference value by a certain amount.

The use of an equation for the energy reference value, which is evaluated at the same time as the calculation of the energy consumed for the acceleration process, can be used to implement the termination criterion in the third variant mentioned above, since the current energy consumption at any point in time corresponds to that for dry running or wet running expected energy value from the equation can be compared.

According to the invention, the energy consumed for the acceleration is calculated by determining the power consumption of the centrifugal pump unit at the beginning and during the acceleration process and summing the differences between the power consumption at the beginning of the acceleration process and the respective power consumption during the acceleration process. The determination of the power consumption during the acceleration process is time-discrete, for example in discrete steps of 125 ms. By integrating a power, one mathematically obtains the energy that is in a physical system at the end of the integration period. In contrast, the integration of the power differences between the power at the start of the acceleration process and the respective points in time during the acceleration process gives the energy put into the centrifugal pump unit for the acceleration process.

The rotor is advantageously accelerated in such a way that the increase in speed caused by the acceleration is higher at a low operating speed than at a high operating speed. This means that the acceleration process at low operating speeds is longer and/or has to reach a higher operating speed than an acceleration process that is started at a higher operating speed. It is true that an acceleration process (duration or jump height) used at a lower operating speed, which leads to a meaningful result, can also be used at a higher operating speed. However, this is not possible the other way around, because at lower speeds a greater increase in speed is required in order to be able to make a clear statement as to whether dry running is present or not.

The method according to the invention can preferably be integrated in pump electronics for a centrifugal pump, which is set up to carry out the method of the type described above. The invention therefore also relates to pump electronics for the centrifugal pump, which is set up to carry out the method described.

In addition, the invention also relates to a centrifugal pump with such pump electronics and a method according to the present invention implemented therein.

• 1: Leistungskurven über der Drehzahl für verschiedene Pumpenbetriebszustände
• 2: Diagramm zur Darstellung des Verfahrensablaufs
• 3: Beschleunigungsvorgang mit Vorgabe einer Zieldrehzahl
• 4: Beschleunigungsvorgang mit Vorgabe eines Beschleunigungszeitraums
• 5: Beschleunigungsvorgang unter Verwendung eines aussagekräftigen Energieverbrauchs als Abbruchkriterium

Further advantages and features of the invention can be found in the following description of exemplary embodiments, which refer to the attached figures. Show it:

• 1 : Power curves versus speed for different pump operating conditions
• 2 : Diagram showing the process flow
• 3 : Acceleration process with specification of a target speed
• 4 : Acceleration process with specification of an acceleration period
• 5 : Acceleration process using a meaningful energy consumption as a termination criterion

As already explained in the introduction to the description 1 three power curves versus speed for a centrifugal pump unit. A first power curve is shown as a dotted line, which shows the course of the power P_Q0 over the speed for a volume flow of zero. Furthermore, the course of the power P_Dry over the speed in the case of dry running is shown as a solid line. It becomes clear that the power curve in the case of dry running is considerably flatter than in the case of normal operation, in which the impeller of the centrifugal pump unit rotates in a liquid to be pumped. This is because in the normal case the power P is proportional to the cube of the speed (P~n ³ ), whereas in the case of dry running there is a simple proportional relationship (P~n).

For dry-running detection, a speed-dependent mean value can be formed between the power consumption in the normal case and the power consumption in the event of a dry run, which is referred to as Ref limit curve can be used to determine whether or not a dry run is present. This speed-dependent mean value is illustrated by the dashed curve P_DryLimit. For dry-running detection, the current power consumption is determined during operation of the centrifugal pump unit and compared with the reference value of the mean value curve at the current speed (starting speed + current speed step height). If the current power consumption is below the reference value, then there is a dry run.

How out 1 further shows that the performance curves are congruent at low speeds. At the in 1 For the pump unit selected as an example, this is the case for all speeds below 2000 rpm, so that no dry-running detection is possible in this speed range. Because a differentiation between the normal case and the case of dry running cannot be made here. This disadvantage is avoided by the method according to the invention.

In the case of dry running, ie without liquid in the impeller, the moment of inertia J for a centrifugal pump unit is only determined by the mass of the rotor and the impeller: J = J _motor . In contrast, the moment of inertia J is normally also formed by a hydraulic component: J = J _motor + J _hydraulics .

_{In general, E rot} =1/2·Jω ² applies to the rotational energy. In relation to the centrifugal pump unit, Erot = 1/2 (J _engine + J _hydraulics )ω ₀ ² _{applies to any operating speed ω 0} = 2π·n ₀ in the normal case. If the pump unit is accelerated starting from this speed ω ₀ , also referred to below as the starting speed, the speed increases by Δω at the end of the acceleration process. If the pump set lacks pumped liquid compared to normal operation, the rotational energy at the end of the acceleration process is E _rot,dry = 1/2. J _motor (ω ₀ + Δω) ² .

The energy E required for an acceleration process then results from the difference between the energy E _end at the end of the acceleration process and the rotational energy E _start that was stored in the centrifugal pump unit at the beginning of the acceleration process. The following then applies to dry running: $$\begin{array}{l}\Delta E_{\mathrm{right}Ot,\mathrm{i.e}\mathrm{right}y}=\Delta E_{\mathrm{right}Ot,\mathrm{i.e}\mathrm{right}y,en\mathrm{i.e}}-\Delta E_{\mathrm{right}Ot,\mathrm{i.e}\mathrm{right}y,sta\mathrm{right}t}\\ =\frac{1}{2}{J}_{MOtO\mathrm{right}}{\left({\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0+\Delta \mathrm{\omega }\right)}^2-\frac{1}{2}{J}_{MOtO\mathrm{<figure-callout\; id="0"\; label="right\; \omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">right</figure-callout>}}{\omega }_{}{}_0{}^2\\ =\frac{1}{2}{J}_{MOtO\mathrm{right}}\left[2{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0\Delta \mathrm{\omega }+\Delta {\mathrm{\omega }}^2\right]\end{array}$$

And for the normal case, the same applies: $$\Delta E_{\mathrm{right}Ot,nO\mathrm{right}mal}=\frac{1}{2}\left(J_{MOtO\mathrm{right}}+J_{Hy\mathrm{i.e}\mathrm{right}a\mathrm{and}lik}\right)\left[2{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0\Delta \mathrm{\omega }+\Delta {\mathrm{\omega }}^2\right]$$

As previously stated, the energy reference value Eref can be a constant, ie a value which is the same over the entire operating range of the pump set. Since the acceleration process can begin at any starting speed ω ₀ , the speed jump Δω that must be reached in order to obtain this constant energy reference value Eref must be calculated. This can be done from the mathematical relationship Δω=f(E _ref ,ω ₀ ) between the starting speed ω ₀ and the required speed jump Δω. We are therefore looking for the level of a speed jump Δω that results in an identical energy difference Eref between normal operation and dry running. This mathematical relationship results from the subtraction of equations 1 and 2 _{with Eref = E normal} - E _{dry = constant:} $$\begin{array}{l}{E}_{\mathrm{right}ef}=\frac{1}{2}\left(J_{MOtO\mathrm{right}}+J_{Hy\mathrm{i.e}\mathrm{right}a\mathrm{and}lik}\right)\left[{\left({\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0+\Delta \mathrm{\omega }\right)}^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2\right]\\ -\frac{1}{2}(J_{MOtO\mathrm{right}}\left[{\left({\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0+\Delta \mathrm{\omega }\right)}^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2\right]-\frac{1}{2}{J}_{Hy\mathrm{i.e}\mathrm{right}a\mathrm{and}lik}\left[{\left({\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0+\Delta \mathrm{\omega }\right)}^2-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2\right]\\ \Rightarrow \Delta \mathrm{\omega }=-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0\pm \sqrt{\frac{2E_{\mathrm{right}ef}}{J_{Hy\mathrm{i.e}\mathrm{right}a\mathrm{and}lik}}+{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}\end{array}$$

ergibt. Aus Gleichung 3 kann nun abhängig von der Startdrehzahl ω₀ sie Sprunghöhe Δω der Drehzahl ermittelt werden, so dass ein konstanter Energieunterschied zwischen Normal- und Trockenlauf über den gesamten Betriebsbereich des Pumpenaggregats angenommen und damit ein konstanter Energiereferenzwert festgelegt werden kann.Only the solution with the positive root makes sense from a physical point of view, so that the speed jump to be determined is $$\Rightarrow \Delta \mathrm{\omega }=-{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0+\sqrt{\frac{2E_{\mathrm{right}ef}}{J_{Hy\mathrm{i.e}\mathrm{right}a\mathrm{and}lik}}+{\mathrm{<figure-callout\; id="0"\; label="\omega "\; filenames="DE102014008716B4\_0005.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_0^2}$$

results. Equation 3 can now be used to determine the jump height Δω of the speed as a function of the starting speed ω ₀ so that a constant energy difference between normal and dry running can be assumed over the entire operating range of the pump unit and a constant energy reference value can thus be defined.

An exemplary process sequence of a variant of the method according to the invention is shown in 2 shown. The method can be carried out again and again at intervals during the operation of a centrifugal pump unit. The distances can be regular or irregular. Alternatively or in combination, the implementation of the method can be specifically triggered, for example, when a monitoring electronic system of the centrifugal pump unit detects an abnormal state. This supposedly abnormal condition can then be verified with the dry-running detection.

The method begins at block 10. First, the electric power P_0 currently consumed by the centrifugal pump unit and the current operating speed n_0 are determined, block 12. It should be noted that n_0=ω ₀ /2π. The determination can in each case be carried out by measurement or calculation from electrical and/or mechanical variables of the centrifugal pump unit. As a rule, however, the speed is already available for the speed control in the pump electronics, so that no separate measurement or calculation of the speed is required. The same applies to centrifugal pump units with extensive functional equipment for the power that is also required for various pump applications, possibly also for the speed control.

The determined current power consumption P_0 and the current operating speed n_0 are then saved, block 14.

The acceleration process then takes place in block 16, in which the rotor of the centrifugal pump unit is accelerated to a higher speed, starting from the current operating speed. This is done with maximum torque.

In block 18, the energy E required for this acceleration process is calculated. This is done by constantly redetermining the current power consumption P_act(i) in discrete time steps during the acceleration process and calculating its difference from the power consumption P_0 at the beginning of the acceleration process. These differences are summed up and then, multiplied by the sampling interval T_abt, which can be 125 µs, for example, result in the energy E required for the acceleration.

In the next step 20, there is a check as to whether a termination criterion for the acceleration process has been met. If the abort criterion is not met, the process is further accelerated so that the method continues at step 16. If, on the other hand, the termination criterion is met, the acceleration process is terminated and a return is made to the conventional speed control of the centrifugal pump unit, which was present before the dry-running detection method according to the invention was carried out.

The end of a specified acceleration period, e.g. T=100ms, the achievement of a specified target speed n_x or the positive comparison of the calculated energy consumption E with Equation 2 (for wet running) that was also evaluated during the acceleration process can be used as a termination criterion.

In order to return to the conventional speed control, the target speed n_x is reset to the speed n_0 that was present at the start of the acceleration process, step 22.

In the next step 24, it is then checked whether the energy E required for the acceleration process is less than an energy reference value E_ref. For example, the energy theoretically required for acceleration when the centrifugal pump unit is running wet is used as the energy reference value E_ref, which must be significantly higher than when running dry, as the comparison of equations 1 and 2 shows. If the comparison in step 24 shows that the energy E required for acceleration is less than or preferably a certain amount less than the energy reference value E_ref (equation 2) based on the normal case, dry running is assumed. This State can then be displayed by the centrifugal pump unit, step 28, for example optically, acoustically, via a telecommunications line or directly on the centrifugal pump unit.

However, if the energy E required for the acceleration process is not smaller, in particular not smaller by the specific amount, than the energy reference value Eref, the normal case is present, i.e. an impeller rotating in the pumped liquid, so that the method according to the invention can be used without detecting a fault condition in the form of dry running can be finished, step 26.

3 shows a first speed profile for the acceleration process in step 16. In this variant, a specific target speed n_x is sought for the acceleration process, which is to be achieved starting from the starting speed n_0. The period of time T elapses before the target speed n_x is reached. After the target speed n_x has been reached, the original starting speed n_0 is sought again. The power differences are then integrated over the entire acceleration period T, which can be between 0.1 s and 1 s depending on the target speed n_x.

4 shows an alternative variant of the acceleration process, in which no target speed n_x but the time period T is specified, for which the rotor of the centrifugal pump unit is to be accelerated. The rotational speed reached at the end of the acceleration process is irrelevant, since the acceleration process is terminated as soon as the time period T has expired. The period can be between 100ms and 200ms.

5 shows a third variant of the acceleration process, in which a target speed n_x of the speed control of the centrifugal pump unit is specified in order to trigger the acceleration process, but this target speed n_x is irrelevant for the acceleration process itself. It only serves to get the frequency converter to supply the electric motor with maximum current so that a maximum acceleration torque acts on the rotor.

The acceleration process is aborted as soon as the calculated energy required for the acceleration is meaningful in order to determine whether a dry run is present or not. This determination can be made by comparing the currently calculated energy consumption and Equation 2, which was evaluated at the same time. A meaningful result may already be available after an acceleration period T that is less than 100 ms. Accordingly, the acceleration variant according to 5 the one in which the method according to the invention takes the shortest time. This has the advantage that the overriding speed control of the centrifugal pump unit has to be interrupted for a minimally short period of time in order to be able to carry out the method for dry-running detection according to the invention.

The new process for detecting dry running is therefore based on the energy required for a short acceleration process. The difference in energy required to accelerate the rotor due to its inertia is significantly different from the energy required to accelerate the rotor and the liquid in the impeller. The acceleration procedure takes about 100ms or less and another 100ms to return to the original speed. Fast jumps in speed, as required by the method according to the invention, can be achieved with a conventional speed control, for example with field-oriented control (FOR). Due to the rapid dynamics, the steady state of the hydraulic system is not affected and the new method for detecting dry running is not noticed by a user of the centrifugal pump unit. In this respect, the method according to the invention is particularly user-friendly and in no way impairs the conventional control of the centrifugal pump unit.

The method according to the invention works safely and reliably in the entire speed range of the centrifugal pump unit. If the energy reference value is also calculated during the acceleration process, it is no longer necessary to carry out a large number of measurements to create a reference curve such as the dashed power curve in 1 represents is required.

The method according to the invention provides failsafe protection and reliable dry-running detection, which works reliably even at low speeds. Due to the small jumps in speed and/or the very short acceleration times, the overriding speed control of the centrifugal pump unit is not impaired.

Claims (12)

Method for detecting dry running within a centrifugal pump unit, characterized in that the rotor of the centrifugal pump unit accelerates from the current operating speed (n_0) for a period of time (T) and the energy (E) required for acceleration is calculated, the calculated energy (E ) is compared with an energy _{reference value (E ref} ) and, depending on the result of this comparison, a dry run is concluded, with the energy (E) consumed for acceleration being calculated by measuring the electrical power consumption (P_0) of the centrifugal pump unit at the beginning and in determined in discrete steps during the acceleration process, the differences between the power consumption (P_0) at the beginning of the acceleration process and the respective power consumption (P_akt) during the acceleration process are summed and these differences are then multiplied by the sampling interval. procedure after claim 1 , characterized in that the acceleration is terminated when the time period (T) has expired. procedure after claim 1 or 2 , characterized in that the period is between 100ms and 200ms. Method according to one of the preceding claims, characterized in that the acceleration is ended when a specific target speed (n_x) has been reached. procedure after claim 4 , characterized in that the target speed (n_x) is the maximum speed of the pump unit. Method according to one of the preceding claims, characterized in that the energy _{reference value (E ref} ) is the energy theoretically required for acceleration when the centrifugal pump unit is running wet, a dry run being concluded if the calculated energy (E) is less than or around a certain value amount (e _ref) is less than the energy reference value, or that the energy reference value (e _ref) the power theoretically required for the acceleration in a dry running of the centrifugal pump assembly is being closed to a dry run if the calculated energy (e) no more than one certain amount from the energy reference value (Eref). procedure after claim 6 , characterized in that the energy _{reference value (E ref} ) is calculated during the acceleration process. Method according to one of the preceding claims, characterized in that the acceleration is terminated as soon as the energy (E) consumed for acceleration differs _{from the energy reference value (E ref ) by a certain amount.} Method according to one of the preceding claims, characterized in that the acceleration takes place with maximum torque. Method according to one of the preceding claims, characterized in that the acceleration takes place in such a way that the acceleration-related speed increase (Δω) is greater at a low current operating speed (n_0) than at a high current operating speed (n_0). Pump electronics for a centrifugal pump, characterized in that they are used to carry out the method according to one of Claims 1 until 10 is set up. Centrifugal pump, in particular glandless pump, characterized by pump electronics claim 11 .

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