EP2500579A1 - Detection of cavitation and gas entrainment in an electric rotary pump - Google Patents

Abstract

The power or load current of the electromotor (M) is determined, and the derived values are fed to a pump control module (PUB). A first indicator signal is determined from the values formed in a first predetermined frequency range, by the pump control module, and is compared with a first predetermined threshold value. A signal for displaying the information about cavitation formed in the liquid medium used in the centrifugal pump (P) is output, when the first indicator size exceeds the first predetermined threshold value. Independent claims are included for the following: (1) computer program comprising computer-executable program code instructions for implementing monitoring method of electromotor propelled centrifugal pump; (2) computer program product for monitoring electromotor propelled centrifugal pump; and (3) electromotor propelled centrifugal pump assembly.

Description

The invention relates to a method for monitoring a centrifugal pump driven by an electric motor, in which the power consumption or the load current of the electric motor is determined and values derived therefrom are fed to a pump monitoring module according to the preamble of claim 1. Furthermore, the invention relates to an arrangement with a pump monitoring module according to FIG the preamble of claim 9 for carrying out such a method.

A centrifugal pump is a turbomachine, wherein by means of a rotating impeller, a fluid, usually a liquid, is moved. Incoming medium entering through an access line is entrained by the rotating impeller, d. H. 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.

From the WO 2009/006927 A1 a method for preventing dry running in a centrifugal pump is known. This occurs when the centrifugal pump is insufficiently supplied with liquid pumped medium. Dry running of a centrifugal pump can occur, for example, if valves on the intake side are inadvertently connected or if a liquid reservoir in front of the access line has run empty. Since dry running can quickly lead to overheating of the pump components, in particular of mechanical seals, a method for detecting and avoiding dry running in a centrifugal pump is described there.

In the operation of centrifugal pumps, however, may still other disorders occur, which, if they are longer, to a Damage to the pump can lead or optimal driving style of a process in which the centrifugal pump is used, are detrimental. Examples include an entrainment of gas or cavitation in a funded liquid medium. Cavitation can even cause a material removal on the impeller of the centrifugal pump after a longer exposure time. When a gas Mitförderung the flow rate of the pump decreases.

The invention is therefore based on the object to find a technical solution that allows monitoring of a driven by an electric motor centrifugal pump on co-promotion of gas or cavitation in a subsidized liquid medium.

To solve this problem, the new method of the type mentioned in the features specified in claim 1. Advantageous developments of the invention are described in the dependent claims, in claim 7 a computer program suitable for carrying out the method, in claim 8 a corresponding computer program product, and in claim 9 an arrangement suitable for carrying out the method.

The invention is based on the finding that the gas bubbles imploding during cavitation as well as the gas bubbles compressed in the co-transport of gas cause impact torques on the blades of the impeller, which are detectable in the drive as torque pulses. Since the power consumption or the load current of the electric motor is decisive for the formation of torque, their values can advantageously be determined and evaluated for the purpose of detecting such pulses. Since values of the power consumption or of the load current usually exist in motor control systems in any case, this has the advantage that no additional sensors are required for monitoring the centrifugal pump. When considering the power consumption or the load current in the frequency domain, such torque pulses, which have a comparatively broadband effect, are, in particular, elevated amplitude values of the spectral lines in the vicinity of the rotational frequency in three-phase electric motors, recognizable. Calculating so in the frequency domain spectral amplitude values of power consumption or load current, we obtain values of an indicator size, which differ from each other in normal operation and interference mode of the centrifugal pump and thus are significant for the differentiation of the different modes. The distinction can therefore be made by simply comparing the signal components in a predetermined frequency range with a predetermined threshold.

In an advantageous embodiment of the invention, the reliability of the diagnostic statement is further improved by the threshold value used for comparison being adapted to the specific model. During normal operation of the respective centrifugal pump, values of the first indicator variable are determined, and the first threshold value is set so that the values determined in normal operation lie at a certain distance below the first threshold value. Thus, since the first threshold only in an accident, d. H. If an entrainment of gas or cavitation in the pumped liquid medium is exceeded, thus a reliable diagnosis is possible.

In a particularly advantageous embodiment of the invention, a more accurate classification of the cause of the fault is also made, ie, it is distinguished between the cause of the fault entrainment of gas and the cause of the fault cavitation. For this purpose, spectral amplitude values in a second predetermined frequency band, which is separated from the first frequency band of the first indicator variable, are calculated as a second indicator variable in the frequency domain. If the second indicator size deviates greatly from the first indicator size, this indicates a presence of gas in the pumped liquid medium, otherwise cavitation is likely to cause the disturbance. Due to the hydraulic conditions on the impeller of the centrifugal pump namely expressing entrainment of gas and cavitation different in frequency range. Cavitation caused by torque pulses are relatively broadband in the frequency range, while by entrainment of gas signal components are rather narrow band around the rotational frequency of the motor current around increased.

This finding can be used in a further advantageous embodiment to increase the reliability of the diagnosis statement by means of a statistical evaluation of the signal components in a plurality of comparatively narrow frequency bands. If the standard deviation of the signal components exceeds a third predetermined threshold value, entrainment of gas in the liquid medium is indicated, otherwise the presence of cavitation. Another possibility for evaluating the signal components is an envelope method in which an amplitude curve described by a polynomial is applied via the amplitudes of the individual spectral lines in the frequency domain, the polynomial parameters of which are used as indicator quantities.

A control system of a process plant, in which the centrifugal pump is used, can also be supplemented by a way to assess the condition of the centrifugal pump, which is important for improving plant operation, for example in the sense of preventive maintenance to avoid unnecessary plant downtime. For this purpose, the pump monitoring module uses the first indicator size to grading a determined cavitation into different levels of cavitation intensity. With a larger indicator size, a stronger cavitation level is assigned. Operating times of the centrifugal pump are weighted with the respectively determined for these stages of cavitation and estimated the remaining life of the centrifugal pump based on the occurred during their previous operation total load.

A widely used type of drive for centrifugal pumps is the asynchronous motor with squirrel cage rotor. Since the stator current is decisive for the torque generation of the motor, it is possible to detect disturbances, in particular due to increased amplitude values of the spectral lines, which are close to the rotational frequency of the stator current. In the case of the frequent case of an asynchronous motor, such determination of the first frequency range is therefore particularly well suited for being adjacent to the rotational frequency of the load current, which is referred to here as the stator current, but does not include the rotational frequency itself. This frequency range can additionally be mirrored around the rotational frequency, so that it consists of two bands, which are arranged in mirror image to the rotational frequency exists.

The method is preferably implemented in software or in a combination of software / hardware, so that the invention also relates to a computer program with computer-executable program code instructions for implementing the method. In this context, the invention also relates to a computer program product, in particular a data carrier or a storage medium, with a computer program executable by a computer. Such a computer program is preferably part of the process control system or is kept in a memory of the process control system or can be loaded into this memory, so that during operation of the process control system it automatically performs the pump monitoring according to the method.

With reference to the drawings, in which an embodiment of the invention is shown, the invention and refinements and advantages are explained in more detail below.

Figur 1

ein Blockschaltbild eines Teils einer technischen Anlage mit einer Kreiselpumpe und

Figur 2

ein Ablaufdiagramm eines Verfahrens zur Überwachung einer Kreiselpumpe.

Show it:

FIG. 1

a block diagram of a part of a technical system with a centrifugal pump and

FIG. 2

a flowchart of a method for monitoring a centrifugal pump.

According to FIG. 1 a pump monitoring module PÜB has an evaluation unit AE, a memory SP connected to the evaluation unit AE and an input / output unit EA likewise connected to the evaluation unit AE. The pump monitoring module PÜB is connected to a motor module MBS. The pump monitoring module PÜB and the motor module MBS are part of a process control system PLS, for example a SIMATIC PCS7. An engine control unit MSTG is connected to the engine block MBS, the evaluation unit AE of the pump monitoring module PÜB and to an electric motor M. The electric motor M drives a centrifugal pump P. The centrifugal pump P is supplied with liquid conveying medium via a supply line ZL and pumps out the pumped medium via a drain line AL. Data traffic between the mentioned components is indicated by arrows.

During operation of the centrifugal pump P, the power consumption of the electric motor M from the engine control unit MSTG continuously with high sampling rate, z. B. 400 kHz, and passed in the form of a temporal sequence of digital values to the evaluation unit AE of the pump monitoring module PÜB. In the evaluation unit AE of the pump monitoring module PÜB, the digital values of the current consumption, which represent the power consumption or the load current of the electric motor M, are subjected to a fast Fourier transformation (FFT) in order to determine signal components of the digital values in different frequency ranges. For the detection of a disturbance and its classification on the basis of the signal components, predetermined threshold values are still required, which are stored in the memory SP. The respective diagnosis result is indicated by a signal by means of the input / output unit EA.

Based FIG. 2 the sequence of the method for monitoring a centrifugal pump driven by an electric motor is explained in more detail below.

mit

• K1 - erste Indikatorgröße,
• A_fi - Amplitude der Spektrallinie bei der Frequenz fi mit einer Frequenzauflösung von in diesem Beispiel 0,2 Hz.

After the method has been started with a step S1, a first indicator variable K1 is calculated according to the formula in a step S2 $$K1=\sqrt{\frac{1}{40}{\displaystyle \underset{\mathit{fi}=40.2\mathit{Hz}}{\overset{48\mathit{Hz}}{\Sigma }}}{A}_{\mathit{fi}}^2}+\sqrt{\frac{1}{40}{\displaystyle \underset{\mathit{fi}=52\mathit{Hz}}{\overset{59.\mathrm{8th}\mathit{Hz}}{\Sigma }}}{A}_{\mathit{fi}}^2}.$$

With

• K1 - first indicator size,
• A _fi - amplitude of the spectral line at the frequency fi with a frequency resolution of 0.2 Hz in this example.

In this consideration, it is assumed that an asynchronous drive motor with squirrel cage rotor for the pump, ie a drive with almost constant speed and a rotation frequency of the stator current of 50 Hz. Due to the cavitating gas bubbles in the cavitation and compressed in Gasmitförderung gas bubbles shock moments on the blades of the Impeller of the pump. Since the stator current is decisive for the torque formation of the motor, in the event of such disturbances in the spectral lines, increased signal components, in particular in the vicinity of the rotational frequency of 50 Hz, can be detected. If the rms value is formed from the signal components in a frequency range which is additionally mirrored by the rotational frequency 50 Hz, the first indicator variable K1 is obtained, which is significant for distinguishing between normal operation and interference-prone operation by gas promotion or cavitation. In the example shown, the first indicator quantity K1 is calculated on the basis of the signal components in a frequency range composed of the subregions between 40.2 and 48 Hz and between 52 and 59.8 Hz. The distinction between normal operation and faulty operation is made in a subsequent step S3, in which the first indicator variable K1 is compared with a first threshold SW1 associated therewith. The first threshold SW1 is on-hand previously predetermined calculations of the first indicator size K1 in normal operation, for example, by multiplying the mean value of the indicator variable K1 calculated from several measurements by a factor of 1.3.

In an alternative exemplary embodiment, the indicator variable K1 is predetermined in such a way that it lies a small distance above the highest value of the first indicator variable K1 determined in normal operation. Since the first threshold value is determined on the same centrifugal pump on which the diagnosis is carried out later, it is thus adapted to the specific model and manufacturing variations advantageously have no influence on the reliability of the diagnostic statement.

Is in accordance with step S3 FIG. 2 If it is determined that the first indicator size is greater than the first threshold value SW1, then a next step S4 is entered. This transition is in FIG. 2 marked by a "J" at the transitional arrow. If, on the other hand, the first indicator size K1 is not greater than the first threshold value SW1, then, according to an arrow marked "N", a step S5 is entered in which a signal indicates that the pump is in normal operation and there is no disturbance that would be caused by Gasmitförderung or cavitation.

If a fault is detected, a second indicator quantity K2 is calculated in step S4, which results as the sum of the signal components in the frequency ranges 32.2 to 40 Hz and 60 to 67.8 Hz. The formula used for this differs from the above calculation formula for K1 only in terms of the summation limits. In a subsequent step S6, it is checked whether the difference between the first indicator size K1 and the second indicator size K2 is smaller than a second predetermined threshold value SW2. If this is the case, a transitional arrow labeled "J" is used to proceed to a step S7 in which, depending on the size of the first indicator variable K1, a cavitation in FIG one of three different strength levels is displayed. If, on the other hand, the difference between the first indicator quantity K1 and the second indicator size K2 is greater than the second threshold value SW2, the transition corresponding to an arrow marked "N" takes place to a step S8, in which a signal causes a disturbance due to entrainment of gas is displayed in the conveyed liquid medium. Due to the hydraulic conditions on the impeller of the centrifugal pump, namely, a entrainment of gas and a cavitation in the spectral components in the frequency range expressed differently. In cavitation, the spectral signal components decrease with increasing distance to the rotational frequency of the stator current of an asynchronous motor weaker than mitführördertem gas. Therefore, the above calculated difference of the two indicator sizes K1 and K2 in the case of entrainment of gas is greater than in the case of cavitation. In the manner described, it is thus easy to make a distinction between the two causes of the fault.

As a further distinguishing feature of Gasmitförderung and cavitation, the scattering of the amplitudes of the spectral signal components in the Gasmitförderung is significantly higher than in the cavitation case, a distinction between the two causes of failure is possible as an alternative to the procedure described above, in which the standard deviation of the amplitudes of different spectral signal components over a larger frequency range than a third indicator size K3 is calculated. In practical experiments, it has been shown that the standard deviation for a gas co-promotion is about 1.5 times higher than that for cavitation. Therefore, this distinguishing method also allows a comparatively reliable diagnosis if the third indicator quantity K3 is compared with a third threshold, which is suitably predetermined. As another alternative to that FIG. 2 described embodiment is still called a procedure in which an envelope is placed over the spectral signal components in a predetermined frequency range and by a polynomial is approximated. In order to distinguish between the different causes of the disturbance, the polynomial parameters can then be used as indicator quantities.

After a cavitation has been detected during the monitoring of the centrifugal pump, it is divided into the three classes "weak cavitation", "medium strong cavitation" and "strong cavitation" with the help of further limit values. Taking into account the operating time of the centrifugal pump, the pump running times are calculated in each case at low cavitation, moderate cavitation and strong cavitation, which are taken into account in the preventive maintenance control system to avoid unnecessary downtime due to an unexpected failure of the centrifugal pump.

It is known to the person skilled in the art that a pump monitoring module, unlike the embodiment shown, can be embodied within the engine module or within the engine control unit. It is also not essential to the invention in which component the evaluation of the power consumption or the load current or the determination of the various threshold values takes place. In particular, the evaluation unit or the memory may also be arranged outside the pump monitoring module, in which case the evaluation unit or memory must of course be connected to the pump monitoring module. Engine control units, in which often the functions of a motor starter, a soft starter and / or an engine management system are realized, have their own arithmetic unit and can realize the evaluation described above in addition to the actual control task and pass the calculated indicator sizes to a control system. Furthermore, the engine block for the invention is not mandatory, but usually available in process control systems.

Claims (9)

Method for monitoring a centrifugal pump (P) driven by an electric motor (M), a) in which the power consumption or the load current of the electric motor is determined, b) in which determined values from a) or values derived therefrom are fed to a pump monitoring module (PÜB),
characterized, c) that the pump monitoring module determines, as a first indicator variable, the signal components of the values from a) in a first predetermined frequency range and compares it with a first predetermined threshold value, and d) that the pump monitoring module outputs a signal for indicating a co-promotion of gas or cavitation in a funded by the centrifugal pump liquid medium when the first indicator size exceeds the first predetermined threshold. Method according to claim 1, characterized in that e) that for the determination of the first threshold values of the first indicator size in normal operation of the centrifugal pump (P) are determined, and f) that the first threshold value is predetermined as a function of the values from e). Method according to claim 1 or 2, characterized g) that the pump monitoring module (PÜB) determines, as a second indicator variable, the signal components of the values from a) in a second predetermined frequency range, which is separated from the first frequency range, and h) that the pump monitoring module outputs a signal for indicating a entrainment of gas in the liquid medium, when the deviation of the second indicator size from the first indicator size exceeds a second threshold, and that the pump monitoring module otherwise, a signal indicating cavitation is output. Method according to claim 1 or 2, characterized i) the pump monitoring module (PÜB) determines the signal components of the values from a) in a multiplicity of further predetermined frequency ranges, which are separated from one another and from the first frequency range, j) that the pump monitoring module calculates the standard deviation of the signal components from i) as a third indicator variable and compares it with a third predetermined threshold value and k) that the pump monitoring module outputs a signal indicating an entrainment of gas in the liquid medium when the standard deviation exceeds the third threshold, and that the pump monitoring module otherwise outputs a signal indicative of cavitation. Method according to one of the preceding claims, characterized 1) that the pump monitoring module (PÜB) on the basis of the first indicator size gradation of a detected cavitation makes such that at larger first indicator size a stronger level of cavitation is determined, and m) that the pump monitoring module estimates the remaining service life of the centrifugal pump as a function of its operating times weighted with the respective cavitation levels. Method according to one of the preceding claims, characterized in that the electric motor (M) is an asynchronous motor with squirrel cage rotor and that the first predetermined frequency range is adjacent to the rotational frequency of the load current. Computer program with computer executable program code instructions for implementing the method according to one of claims 1 to 6, when the computer program is executed on a computer. Computer program product, in particular data carrier or storage medium, having a computer-executable computer program according to claim 7. Arrangement comprising a pump monitoring module (PÜB) which is connected to an electric motor (M) driving a centrifugal pump (P) via an engine control unit (MSTG) for monitoring the centrifugal pump (P) driven by the electric motor (M), n) wherein the pump monitoring module is designed to evaluate values of the power consumption or the load current of the electric motor or values derived therefrom, characterized o) that the pump monitoring module is further adapted to determine the signal components of the values from n) in a first predetermined frequency range as a first indicator variable, to compare with a first predetermined threshold value and a signal for indicating a co-promotion of gas or cavitation in one output by the centrifugal pump conveyed liquid medium when the first indicator size exceeds the first predetermined threshold.

Images