WO2009006927A1 - Method for preventing dry running in a centrifugal pump, pump monitoring module and arrangement - Google Patents

Abstract

The invention relates to a method for preventing dry running in a centrifugal pump (P), to a pump monitoring module (PÜB) and to an arrangement. To detect and prevent dry running in a centrifugal pump (P) which is driven by an electric motor (M), a technical solution is proposed without it being necessary to use an additional liquid sensor, a) in which solution the effective power or the load current of the electric motor (M) during the start-up of the centrifugal pump (P) is determined, b) the determined value from a) or a value derived therefrom is supplied to a pump monitoring module (PÜB), c) the pump monitoring module (PÜB) compares the value from b) with an effective power or load current reference value which represents a sufficient quantity of liquid delivery medium in the centrifugal pump (P), d) in the event that the deviation of the value from b) in relation to the effective power or load current reference value undershoots a defined threshold, the pump monitoring module (PÜB) interprets this state as dry running.

Description

description

Method for avoiding dry running in a centrifugal pump, pump monitoring module and arrangement

The invention relates to a method for monitoring a driven by an electric motor centrifugal pump to prevent dry running and a Pumpenuberwachungsbaustein and an arrangement.

A centrifugal pump is a flow machine, wherein a Fordermedium, usually a liquid, is moved by means of a rotating impeller. Through a access line (pipe) entering Fordermedium is entrained by the rotating impeller, i. radially accelerated. The accelerated delivery medium 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, mechanical seals are used to seal the rotating shaft against the centrifugal pump housing. The liquid Fordermedium takes over in centrifugal pumps in particular the function of cooling the mechanical seals. Therefore, a sufficient amount of liquid Fordermedium is always required for the operation of centrifugal pumps to avoid overheating of the mechanical seals and possibly other centrifugal pump components and thus damage to the centrifugal pump.

If the centrifugal pump is insufficiently supplied with liquid Fordermedium, one speaks of so-called dry run. This can be done, for example, by accidentally closing suction-side fittings or by running the liquid storage tank empty. Dry running leads quickly to overheat the pump components, in particular the

Mechanical seals, and must therefore be avoided.

From the internal state of the art it is known by means of temperature sensors to avoid overheating of the centrifugal pump by dry running. However, this solution is to wear, because when exceeding permissible temperatures often damage has already occurred.

From the internal state of the art is also known an additional sensor, a so-called Flussigkeits- limit switch, also known as Liquiphant to install at a defined location in the centrifugal pump. Properly located in the centrifugal pump, this liquid limit switch indicates the sufficient presence of liquid in the centrifugal pump. The evaluation of this information is subsequently used to avoid dry running. The additional sensor, however, means an increased cost-side effort and additional expenses with regard to the configuration in the process control system and the monitoring of the centrifugal pump.

It is therefore the object of the present invention to develop a technical solution for detecting and avoiding dry running in a centrifugal pump without having to use an additional liquid sensor.

The object is achieved by a method for monitoring a centrifugal pump driven by an electric motor for avoidance of dry running,

a) in which the active power or the load current of the electric motor is determined when starting the centrifugal pump, b) the value determined from a) or derived from it

Value is supplied to a pump monitoring module,

c) the pump monitoring module compares the value from b) with a real power or load current reference value, which represents a sufficient amount of liquid Fordermedium in the centrifugal pump,

d) in the event that the deviation of the value from b) in relation to the active power or load current reference value falls below a specified threshold, the pump monitoring module interprets this condition as dry running.

The object is further achieved by a pump monitoring module which is designed to be connected to an electric motor driving a centrifugal pump via an engine control unit for monitoring the centrifugal pump driven by the electric motor to avoid dry running, in which case there is a connection

a) the pump monitoring module can evaluate the active power measured by the motor control unit or the measured load current of the electric motor when starting up the centrifugal pump,

b) the pump monitoring module can compare the evaluated value from a) with a real power or load current reference, which represents a sufficient amount of liquid Fordermedium in the centrifugal pump,

(c) in the event that the deviation of the value of (a) from the actual or load current reference value falls below a specified threshold, the pump overflow This status is interpreted as dry run.

The object is further characterized by an arrangement comprising a Pumpenuberwachungsbaustein, which with a a

Centrifugal pump driving electric motor via a Motorsteue- tion device for monitoring the driven by the electric motor centrifugal pump to prevent dry running is solved solved, wherein

a) the pump monitoring module evaluates the active power measured by the engine control unit or the measured load current of the electric motor when starting up the centrifugal pump,

b) the pump monitoring module compares the evaluated value from a) with a real power or load current reference, which represents a sufficient amount of liquid Fordermedium in the centrifugal pump,

c) in the event that the deviation of the value from a) in relation to the active power or load current reference value falls below a specified threshold value, the pump monitoring module interprets this condition as dry running.

Through the inventive solution is shortly after switching on the centrifugal pump or the centrifugal pump driving at ^¬ electric motor, that is recognized when starting the centrifugal pump, the dry run, and it can be taken appropriate measures to prevent damage to the centrifugal pump by overheating , Typically, the detection of dry running takes place after a few hundred milliseconds. A development of the invention results from the dependent claim.

Advantageously, the method is further developed in that the pump monitoring module throttles or shuts off the electric motor. As a result, after the detection of the dry running, the overheating of the centrifugal pump, in particular its mechanical seals, can already be avoided when the centrifugal pump starts up.

Further advantages of the invention will become apparent from the following description, which explains the invention with reference to two exemplary embodiments in conjunction with the accompanying drawings.

It shows in a schematic representation of the

1 shows a Pumpenuberwachungsbaustein within a

Motor module of a process control system with the detection and avoidance of dry running in one

Centrifugal pump is performed, and the

FIG. 2 shows a typical measuring diagram which shows the active power over time for the case of dry running and in the case of a sufficient supply of the

Centrifugal pump with liquid Fordermedium represents.

FIG. 1 shows a pump monitoring structure in a PUB, which has an evaluation unit AE, a memory SP connected to the evaluation unit AE, and one with the evaluation unit

AE connected input / output unit has. The pump monitoring module PUB is connected to a motor module MBS. The pump monitoring module PUB and the motor module MBS are part of a process control system, for example a Simatic PCS7. An engine control unit MSTG is compatible with the engine MBS, the evaluation unit AE of Pumpenüberwachungsbau- stone PUB and connected to the electric motor M. The electric motor M drives the centrifugal pump P. The centrifugal pump P is supplied with liquid Fordermedium via the supply line ZL and pumps the Fordermedium via the drain line AL out. At the discharge line AL, a flow sensor S for measuring the volume flow is attached, which passes measured data to the evaluation unit AE of the pump monitoring module PUB. Bidirectional traffic between these components is illustrated by double arrows and unidirectional traffic between the named components by simple arrows in the direction of the arrow.

When the electric motor M is switched on by means of the input / output unit EA of the pump monitoring module PUB or the start-up of the centrifugal pump P, the power data of the electric motor M are continuously detected by the motor control unit MSTG and forwarded to the evaluation unit AE of the pump monitoring module PUB. These performance data include in particular the active power and / or the load current of the electric motor M. Since the motor voltage is largely constant, the course of the active power and the load current in the operation of centrifugal pumps is substantially uniform. It is therefore sufficient to supply one of the two parameters to the evaluation unit AE. Also, the value of the active power or the

Load current already in the engine control unit MSTG signal technically transformed, for example, be digitized. Then, this value, also referred to as the value derived from the determined active power or the determined load current, is fed to the evaluation unit AE in the pump monitoring module PUB. In the following, the invention is described for a supplied active power value, but it can also be carried out for a supplied load current value. In the evaluation unit AE of the pump monitoring module PUB, the supplied active power value described above is compared with an active power reference value which represents a sufficient amount of liquid Fordermedium in the centrifugal pump P. This active power reference value is determined in each case for a specific centrifugal pump P experimentally supplying the centrifugal pump P with sufficient amount of liquid Fordermedium or theoretically from known technical data of the electric motor M, such as torque of the electric motor M, moment of inertia of the rotor of the electric motor M and the coupling unit between the electric motor M and centrifugal pump P and calculated from known technical data of the centrifugal pump P, such as moment of inertia of the impeller of the centrifugal pump P and the mass of the liquid Fordermedium.

For example, US Pat. No. 6,260,004 B1 shows a method for calculating power curves from technical performance data of an electric motor driving a centrifugal pump, the centrifugal pump and the liquid medium used, as well as a method for converting and evaluating measured electrical variables and their comparison with the calculated reference values ,

The active power reference values used in the evaluation unit AE for comparison with the determined effective power of the electric motor M are stored in the memory SP.

If the evaluation unit AE determines that the determined active power value deviates unacceptably in relation to the active power reference, ie, falls below defined threshold values, this deviation is interpreted as the presence of dry running and the evaluation unit AE will instruct the engine control unit MSTG in this case. to throttle the electric motor M or even shut it off completely in order to Heat of the mechanical seals of the centrifugal pump P to avoid.

In addition, the signal curves or the interpreted state of the dry run are displayed on the input / output unit EA of the pump monitoring module PUB and possibly an alarm message is output.

The physical-technical reason for a detected unacceptable deviation between a determined active power value and a real power reference value is due to the fact that in the case of dry running or insufficient supply of the centrifugal pump P with liquid Fordermedium that liquid Fordermedium or its mass must be mitbeschleunigt. The electric motor M has to apply an additional power for this, which is reflected in an increased compared to the state of dry running active power value. Conversely, if there is no liquid Fordermedium in the centrifugal pump P, the electric motor M requires less power to start the centrifugal pump P, which is expressed by a low determined active power value.

FIG. 2 shows a typical measuring diagram which represents the active power over time for the case of dry running (dotted line) and for the case of sufficient supply of the liquid medium to the centrifugal pump (solid line). The Y-axis shows the measured active power or the measured load current. The time is shown on the X-axis. The impeller of the centrifugal pump P is thereby accelerated to a speed which is well below the rated speed, ie that speed which is required for continuous operation under full load of the centrifugal pump P. At these lower speeds, damage to slip gaskets will occur even in dry conditions. run avoided. If a permissible operating behavior is determined, the speed can then be switched to continuous operation.

The electric motor M is turned on at the time Tl. After switching on the electric motor M, ie between the times Tl and T2, the electric motor M requires a high active power, since he must mitbeschleunigen the liquid medium in the centrifugal pump P (rising edge). Between the times T2 and T3, the electric motor has reached its maximum effective power (maxima) and after the impeller in the centrifugal pump P has accelerated sufficiently, the active power is again smaller (period between T3 and T4). After the time T4, a state of continuous operation with partial load (low speed) of the centrifugal pump P is achieved in the centrifugal pump P with sufficient existing liquid Fordermedium.

The course of the measured active power in dry running over time is similar to and between the respective times Tl to T4 the course with sufficient existing liquid Fordermedium in the centrifugal pump P. However, at the respective times T2 to T4 and between the times Tl to T4 and after T4 measured much lower active power values.

Depositing one or more of the measured active power values with sufficiently present liquid Fordermedium (solid line) as active power reference values eg at T2 and T3 (these are also referred to as maxima of the active power reference values) and defines thresholds such that deviations of up to 30% of these maxima If the active power reference values are still considered to be harmless, it can be seen that for T2 and T3 the measured maximums of the active power values for dry running (dotted line) are below thresholds in an undue manner. This undershooting of the permissible threshold values is interpreted in the evaluation unit AE as the presence of the state of dry running and the electric motor M is throttled or switched off.

Although, as described above, after detection of the rising edge between T1 and T2 by the evaluation unit AE as a rule, the maximums of the active power reference values (solid line) and the measured maxima (dotted line) are interpreted with respect to the presence of the state of dry running and optionally be used to shutdown or throttling of the electric motor M, the evaluation by the evaluation unit AE could also be accelerated. Typically, measured values of active power are recorded every 100 ms. Already in the time interval of 500 ms between Tl and T2, i. Within the rising flank, you can thus identify significant deviations and use them for evaluation. This measurement and evaluation method thus reacts extremely quickly to impermissible operating states, in particular when starting up the centrifugal pump P.

It was also possible to theoretically calculate the active power curve during dry running and then to define a minimum threshold which must be exceeded at least if an operating state during startup of the centrifugal pump P is not to be interpreted as inadmissible or as dry running.

The individual parameters must each be determined or calculated in advance depending on the system. Nevertheless, the method offers the possibility centrifugal pumps P already within the first second after switching off again to prevent the destruction of the centrifugal pump P. The described method is implemented as a sort of pretest. If no dry run is detected during the pre-test, the centrifugal pump P is then accelerated up to nominal speed (not shown here).

For the described method for avoidance of dry running or for carrying out the preliminary test, the electric motor M can be designed with devices known from the prior art, such as a star-delta circuit, a soft-start device or a frequency converter with ramp function.

The described comparison between determined active power values and active power reference values also makes it possible to detect heavy-duty starting of the centrifugal pump P after a long standstill and in continuous operation of the centrifugal pump P.

In particular, it should be mentioned as essential to the invention that centrifugal pumps P and their process control system PLS anyway have different components, such as engine component MBS, engine control unit MSTG, liquid sensor S in the outflow line AL of the centrifugal pump P and record active power data anyway. Thus, with intelligent use of already existing components in a process control system PLS controlling a centrifugal pump P, it is possible to detect and avoid dry running without having to install an additional sensor or liquid level switch in the centrifugal pump P.

The monitoring method can still be extended if, in addition, the flow in the outflow line AL of the centrifugal pump P during startup of the centrifugal pump P is determined, then the determined flow value or a value derived therefrom is supplied to the evaluation unit AE of the pump monitoring module PUB and then the evaluation unit unit AE of the pump monitoring module PUB the above compares designated flow value with a flow reference value representing a sufficient amount of liquid Fordermedium in the centrifugal pump P and finally in the case that the deviation of the flow value with respect to the flow reference value falls below a predetermined threshold, the pump monitoring module PUB interpreted this state as dry running. This measure represents, so to speak, a double protection, in addition to the described based on the comparison of performance data of the electric motor M method to perform a rapid detection and avoidance of dry running when starting the centrifugal pump P. Again, no additional Flussigkeitsgrenz- switch is needed because the flow sensor S is already present in the drain line AL.

The invention is not limited to the specific embodiments, but includes other modifications not explicitly disclosed, as long as use is made of the core of the invention.

In particular, it is known to the person skilled in the art that the pump monitoring module PUB can also be embodied within the engine module MBS. The motor module MBS is not absolutely necessary for the invention, but is usually present in process control systems PLS. It is also not essential to the invention in which component the evaluation of the active power values or active power reference values takes place. In particular, the evaluation unit AE and / or the memory SP could also be arranged outside the pump monitoring module PUB, in which case evaluation unit AE or memory SP must of course be connected to the pump monitoring module PUB. Also, there may be two separate, structurally separated evaluation units, namely a first evaluation unit for the evaluation of the active power values or active power reference values and a second evaluation unit Evaluation unit for the evaluation of the flow values or

Flow reference values. Circuit design of the individual components of the system as well as software engineering implementations are known in the art.

The switching on or off of the centrifugal pump P can also take place at an operating unit of the engine component MBS, at another location in the process control system PLS or at another component. Likewise, tasks for controlling the electric motor M can also be implemented in the pump monitoring module PUB.

A deviation of the active power value or load current reference value in relation to the active power or load current reference value is also to be encompassed by the invention. After predetermined inversion of the electrical signals to be evaluated, the defined threshold is exceeded and the pump monitoring module PUB interprets this state as dry running.

Claims

claims A method of monitoring a centrifugal pump (P) driven by an electric motor (M) to prevent dry running a) in which the active power or the load current of the electric motor (M) when starting the centrifugal pump (P) is determined, b) the determined value from a) or a value derived therefrom is supplied to a pump monitoring module (PUB), c) the pump monitoring module (PUB) compares the value from b) with a real power or load current reference value which represents a sufficient amount of liquid Fordermedium in the centrifugal pump (P), d) in the event that the deviation of the value from b) with respect to the active power or load current reference value falls below a defined threshold value, the pump monitoring module (PUB) interprets this state as dry running. 2. Method according to claim 1, characterized in that the pump monitoring module (PUB) the electric motor (M) throttles or shuts off. 3. Pump monitoring module (PUB) which is designed to be connected to a centrifugal pump (P) driving electric motor (M) via a Motorsteuerungsgerat (MSTG) for monitoring of the electric motor (M) driven centrifugal pump (P) to prevent dry running whereby, in the case of a connection, a) the pump monitoring module (PUB) can evaluate the active power measured by the motor control unit (MSTG) or the measured load current of the electric motor (M) when starting up the centrifugal pump (P), b) the pump monitoring module (PUB) can compare the evaluated value from a) with a real power or load current reference value, which represents a sufficient amount of liquid Fordermedium in the centrifugal pump (P), c) in the event that the deviation of the value from a) in relation to the active power or load current reference value falls below a defined threshold value, the pump monitoring module (PUB) interprets this state as dry running. 4. Arrangement comprising a pump monitoring module (PUB), which is connected to a centrifugal pump (P) driving electric motor (M) via a Motorsteuerungsgerat (MSTG) for monitoring of the electric motor (M) driven centrifugal pump (P) to prevent dry running, in which, a) the pump monitoring module (PUB) evaluates the active power measured by the motor control unit (MSTG) or the measured load current of the electric motor (M) when the centrifugal pump (P) starts up, b) the Pumpüberwachungsbaustein (PUB) the evaluated Value from a) compares with a real power or load current reference value representing a sufficient amount of liquid Fordermedium in the centrifugal pump (P), In the event that the deviation of the value from a) in relation to the active power or load current reference value falls below a defined threshold value, the pump monitoring module (PUB) interprets this status as dry running.