EP2791511B1 - Liquid ring vacuum pump with cavitation regulation - Google Patents

Description

The invention relates to a method for operating a liquid ring vacuum pump. In the method, vibration measurement values of the pump are recorded and compared with a predetermined cavitation threshold value. The invention also relates to a liquid ring vacuum pump suitable for carrying out the method.

Liquid-ring vacuum pumps pose the problem that cavitation can occur in different operating states. Operating the pump under cavitation conditions for an extended period of time exposes the components of the pump to high mechanical stress, which can quickly destroy the pump. Previous liquid ring vacuum pumps are therefore designed so that always a sufficient distance is maintained to the operating conditions in which cavitation may occur. Although the pump is thus protected against damage due to cavitation, the distance to the cavitation limit does not exploit part of the potential performance of the pump.

Liquid ring vacuum pumps with control devices for stepless adjustment of the rotational speed as a function of pressure and temperature are known US 4,655,688 A , which is considered to be the closest prior art, and can prevent cavitation.

The invention is based on the object to present a pump and a method for operating a pump, in which the efficiency is increased. Starting from the aforementioned state of Technique, the object is achieved with the features of the independent claims. Advantageous embodiments can be found in the subclaims.

In the method according to the invention, a measured value is recorded which represents the liquid content in the gas to be delivered, and the measured value is compared with a predetermined limit value. The speed of the pump is reduced if the specified cavitation threshold is exceeded and the liquid content is below the specified limit. The speed of the pump is increased if the specified cavitation threshold is exceeded and the liquid content is above the specified limit.

First, some terms are explained. The liquid that forms the liquid ring of the pump is called the working fluid. To distinguish this is a liquid carried by the gas to be conveyed, which is referred to below as condensate. The term condensate is not limited to liquids that have been formed by condensation, but also includes other liquids that are carried by the gas. In particular, it is not necessary that the condensate is a different substance than the operating fluid. If the condensate enters the pump, it can mix with the operating fluid. It is therefore not inevitably challenged the same fluid from the pump, which has occurred as condensate. The term liquid content refers to liquid / condensate entrained by the gas to be delivered.

The cavitation threshold is selected so that it can be concluded from vibration readings above the cavitation threshold that cavitation occurs in the pump, while there is no cavitation in the pump for vibration readings below the cavitation threshold. The concrete value of the cavitation threshold value depends on the design of the pump as well as the type of sensor and the absorption of the Measured values. For each individual pump, the cavitation threshold can be easily determined by experiment.

The limit value for the liquid content is also dependent on the specific shape of the pump. In one pump, even very small amounts of condensate trigger cavitation. With the other pump, a certain amount of condensate can be carried without affecting the operation of the pump. This, too, can easily be determined by tests for each pump. It is also conceivable that the limit value changes depending on the rotational speed of the pump, so that the limit value is a function dependent on the rotational speed. The statement that the measured value is compared with a limit value is to be understood by a wide margin. For example, if the liquid content is deduced from indirect measurements, the comparison with the limit value may be that the indirect measurement identifies features that indicate high or low liquid content.

The invention has recognized that in liquid ring vacuum pumps unlike other types of pumps (cf. DE 35 20 538 A1 ) is not always possible to bring the pump back out of the cavitation by lowering the speed. In fact, decreasing the speed helps only in certain operating conditions, for example, when the cavitation occurs because the pump is operated at high speed and at low suction pressure. This cavitation is called classical cavitation.

Conversely, if cavitation occurs when condensate is added to the pump along with the gas being pumped, it would even be counterproductive to lower the speed of the pump. With the reduced speed, the pump would certainly no longer able to bring out the excess fluid from the pump. In fact, it is possible, the to remove excess fluid from the pump by increasing the speed. The increase in the speed thus causes in this case that the cavitation is eliminated.

With the invention, this finding is used to introduce a method by which the operation of the pump can be automatically adjusted for different types of cavitation. In the method according to the invention, in each case two criteria are combined in order to decide whether the rotational speed is increased or decreased. If the cavitation threshold has been exceeded and the liquid content is low, the speed will be reduced. If the cavitation threshold has been exceeded and the liquid content is high, the speed is increased. The process step to increase the speed of the pump after the occurrence of cavitation is exactly opposite to the usual teaching, according to which it was assumed that in cavitation, the speed must always be lowered.

In the pump, readings from external sensors can be processed to determine the liquid content of the gas being pumped. It can be provided in the space to be evacuated a sensor that measures the liquid content directly. It can also be concluded from other measured values, which concern about the pressure or the temperature in the space to be evacuated, on the liquid content.

Additionally or alternatively, measured values recorded at the pump can be used to determine the liquid content. It is possible, for example, to deduce the liquid content from measured values of a vibration sensor. Although the liquid content can not be measured directly via a vibration sensor. It turns out, however, that the cavitation caused by an excess of condensate causes characteristic oscillations that differ from the oscillations in classical cavitation. By suitable evaluation the measured values of the vibration sensor, these characteristic properties can be determined. For example, a Fourier analysis can be carried out and it can be concluded from the peculiarities of the frequency spectrum whether the cavitation is caused by increased liquid content or not. How the specifics look concretely depends on the design of the pump and the arrangement of the vibration sensor and must be determined in individual cases by tests if necessary.

The measured values to be compared with the cavitation threshold value can be recorded with the same vibration sensor or another vibration sensor. The evaluation of whether there is any cavitation is easier than the evaluation of the different types of cavitation. For example, the cavitation threshold may simply relate to the amplitude of the oscillation. If the amplitude exceeds the cavitation threshold, it can be concluded that cavitation is present.

Another way of determining the liquid content and thus the type of cavitation from measured values recorded on the pump is to evaluate the internal motor data, such as the motor voltage and the motor current.

Occasionally it happens that the cavitation can not be eliminated by adjusting the speed alone. In this case it can be provided to admit additional air via a valve in the working space of the pump. Although this reduces the efficiency of the pump, cavitation is reliably eliminated.

The operation of the pump can be based on a multi-stage sequence. At a first stage of the process, the pump may be operated at a speed below the minimum speed lies. In this case, the minimum speed denotes that speed at which the liquid ring in the pump is just stable. At this stage of the process, the pump is therefore operated without a stable liquid ring. In this operating state, the pump, which is actually designed for conveying gas, can be used to initially convey an amount of liquid out of the space to be evacuated. The blades of the impeller then act as blades, with which the liquid is passed through the pump. A separate condensate pump is thereby superfluous.

If in this way the liquid has been removed from the space to be evacuated, it is possible to proceed to normal vacuum operation, in which the pump is operated at a speed above the minimum speed. The idea of initially operating the pump at a speed below the minimum speed to remove fluid and then continuing the vacuum operation at a speed above the minimum speed has independent inventive content even without taking vibration measurements, determining the fluid content, and speed is adjusted. The following description of further process stages substantiates the independent inventive content.

After the transition to vacuum operation, the liquid ring vacuum pump can be operated at a second stage of the process initially at maximum speed to bring out in the shortest possible time as much gas from the space to be evacuated. In this operating state, there is the risk that with decreasing pressure, classical cavitation will occur in the liquid ring. The classic cavitation can be counteracted by reducing the speed. The pump can be operated in this way close to the cavitation limit, the speed is reduced further and further, the lower the pressure. The term cavitation limit designates an operating state the pump, which shows the first signs of cavitation.

If the pressure in the space to be evacuated has dropped to the desired value, the speed of the pump can be reduced in a third process stage to a value close to the minimum speed. Low-speed operation saves energy. If cavitation occurs at such a low speed, this is usually due to an increased liquid content in the gas to be delivered. So cavitation occurs, this can be counteracted by increasing the speed.

The pump can be used in this way, for example, when disinfecting in hospitals. The object to be disinfected is placed in a chamber and treated with hot steam. Subsequently, the chamber can be evacuated by the method according to the invention. It can first be transported away at low speed, the condensate. By then operating the pump at maximum speed and then lowering the speed along the cavitation limit, time is saved in the actual evacuation. By finally maintaining low pressure through low speed operation, energy is saved.

The invention also relates to a liquid ring vacuum pump which can be operated according to the method of the invention. The pump includes a pump housing, an impeller mounted eccentrically in the pump housing, and a vibration sensor for absorbing vibrations of the pump. According to the invention, a logic module is provided which compares a measured value of the vibration sensor with a predetermined cavitation threshold value and which compares a measured value representing the liquid content of the gas to be conveyed with a first limit value. A control unit of the pump is designed to adjust the speed of the pump. In this case, the control unit is to designed to reduce the speed when the predetermined Kavitationsschwellwert was exceeded and the liquid content is below a predetermined limit. In this case, the control unit is designed to increase the speed when the predetermined Kavitationsschwellwert has been exceeded and the liquid content is above a predetermined limit.

If there is cavitation in the liquid ring of the pump, characteristic oscillations occur which are different from the vibrations during normal operation. With the vibration sensor, first signs of cavitation can be detected before the cavitation is so pronounced that damage to the pump can occur. The predetermined cavitation threshold value is chosen so that it is not exceeded during normal operation of the pump, but only when the pump approaches the cavitation limit.

The predetermined cavitation threshold value is suitably selected for the respective pump. The cavitation threshold may, for example, refer to the amplitude of the oscillations. It is also possible that the threshold value refers to certain characteristic properties of the vibrations that are triggered by cavitation. For example, it may be that with cavitation vibrations in certain frequencies occur with particular intensity.

In addition or as an alternative to adaptation of the rotational speed, the distance to the cavitation boundary can also be increased by increasing the pressure in the interior of the pump. The pump may for this purpose have a channel which extends from the outside through the pump housing into the interior of the pump. The channel is provided with a valve which is normally closed. The valve can be opened briefly after exceeding the threshold value, to release gas from the environment into the interior of the pump. This again establishes a distance to the cavitation boundary.

The vibration sensor is preferably connected to the pump housing so that it detects vibrations occurring in the pump housing. The vibration sensor can be arranged where the vibrations caused by cavitation occur, ie in the vicinity of the impeller. The vibration sensor can be arranged, for example, on the circumference or on the front side of this region of the housing.

However, otherwise usually no electronic components are arranged in the region of the impeller. Consequently, if the vibration sensor is located there, this has the disadvantage that extra cables have to be routed. It can therefore be advantageous if the vibration sensor is arranged in a region of the pump housing in which electronic components are present in any case. This can be, for example, the area in which the control unit for the drive is arranged. This is particularly useful when the pump is designed in block design. Block design means that the pump and the drive are surrounded by a common pump housing. The vibrations generated in the area of the impeller propagate through the pump housing and can also be measured well elsewhere. If the control unit for driving the pump is connected to the pump housing, the vibration sensor may be integrated in the control unit.

The pump can be developed with further features which are described above with reference to the method according to the invention.

Fig. 1:

eine schematische Schnittdarstellung einer erfindungsgemäßen Flüssigkeitsring-Vakuumpumpe;

Fig. 2:

die Pumpe aus Fig. 1 in einer Seitenansicht;

Fig. 3:

eine Steuereinheit einer erfindungsgemäßen Flüssigkeitsring-Vakuumpumpe;

Fig. 4:

die Ansicht aus Fig. 3 bei einer anderen Ausführungsform der Erfindung; und

Fig. 5:

eine schematische Darstellung eines Betriebsablaufs der erfindungsgemäßen Pumpe.

The invention will now be described by way of example with reference to the accompanying drawings, given by way of advantageous embodiments. Show it:

Fig. 1:

a schematic sectional view of a liquid ring vacuum pump according to the invention;

Fig. 2:

the pump off Fig. 1 in a side view;

3:

a control unit of a liquid ring vacuum pump according to the invention;

4:

the view Fig. 3 in another embodiment of the invention; and

Fig. 5:

a schematic representation of an operation of the pump according to the invention.

At an in Fig. 1 shown liquid ring vacuum pump, an impeller 14 is mounted eccentrically in a pump housing 20. Liquid in the interior of the pump is carried by the impeller 14 in rotation and forms a liquid ring extending radially inwardly from the outer wall of the pump housing 20. Due to the eccentric bearing the wings of the impeller 14 protrude different depths depending on the angular position in the liquid ring. The volume of a chamber enclosed between two flights changes as a result. The liquid ring thus acts as a piston which moves up and down in the chamber during one revolution of the impeller 14.

From an inlet opening 16, a channel leads into the interior of the pump, in which the impeller 14 rotates. The channel 16 opens in the area in which the wings of the impeller 14 emerge from the liquid ring, in which thus the enclosed between two wings chamber increases. Through the enlarging chamber gas is sucked through the inlet opening 16 into the chamber. After the chamber has reached its maximum volume, the liquid ring penetrates the further rotation of the impeller 14 back into the chamber. If the gas is sufficiently compressed by the further penetrating liquid ring, it is passed through an outlet opening 17 at atmospheric pressure delivered again. Such a liquid ring vacuum pump serves to evacuate a space connected to the inlet opening 16 to a pressure of, for example, 50 millibars.

The pump is also equipped with a designated as Kavitationsbohrung channel extending from the outside into the interior of the pump. In the channel, a solenoid valve is arranged, with which the channel can be selectively opened or closed.

According to Fig. 2 the impeller 14 is connected via a shaft 18 to a drive motor. The pump is designed in block construction, the drive and the impeller 14 are thus accommodated together in the pump housing 20. On the pump housing 20, a control unit 21 is also arranged, via which the drive supplied electrical energy and the speed of the pump is adjusted.

Like the schematic representation of the Fig. 3 shows, the control unit 21 comprises a vibration sensor 22, a logic module 23 and a control module 24. The control unit 21 are also supplied with measured values from an external sensor 27.

The vibration sensor 22 is connected to the pump housing 20 to detect vibrations of the pump housing 20. The measured values of the vibration sensor 22 are continuously transmitted to the logic module 23. The logic module 23 compares the measured values with a predetermined cavitation threshold value 26 (see Fig. 4 ). If the cavitation threshold 26 is exceeded, this is interpreted as an indication that cavitation has occurred in the pump. However, exceeding the cavitation threshold does not yet determine whether it is classic cavitation or cavitation due to increased liquid content. For this reason, the logic module is additionally supplied with measured values from the external sensor 27, from which it results what the liquid content of the gas to be delivered is. For example, the external sensor 27 may be a sensor that directly measures the fluid content in the supply line to the pump. It is also possible that the external sensor 27 measures values from which the liquid content can be indirectly deduced. These values may, for example, relate to the temperature, the pressure or the amount of the supplied steam in the space to be evacuated.

Thus, in the logic module 23, the information is merged, on the basis of which it can be decided whether the speed must be increased or decreased in order to eliminate the cavitation. If cavitation occurs and the gas to be pumped contains no condensate or only very little condensate, the speed is lowered. If cavitation occurs and the gas to be pumped contains more condensate, the speed is increased. From the logic module 23, a corresponding signal is given to the control module 24, so that the drive of the pump is adjusted accordingly. In both cases, adjusting the speed will cause the pump to exit cavitation again.

In addition or as an alternative to the speed adaptation, the solenoid valve 28 can be opened for a short time via the control module 24, so that air from the environment can penetrate into the interior of the pump. Also by the associated pressure increase in the interior of the pump, the distance to the cavitation limit is increased.

In the embodiment according to Fig. 4 the logic module 23 does not receive information from an external sensor. Instead, the measured values are evaluated by the vibration sensor 22 in two ways. First, the amplitude of the oscillation compared with the predetermined Kavitationsschwellwert. If the amplitude exceeds the threshold, this indicates cavitation. On the other hand, a Fourier transformation of the measured values is made and the frequency distribution of the vibrations is considered. For this purpose, for example, the third-octave band at 5 kHz and the third-octave band at 10 kHz can be singled out. The classical cavitation manifests itself by a characteristic distribution in the 5 kHz third band, while the cavitation caused by increased liquid content causes a characteristic frequency distribution in the 10 kHz third band. By evaluating the two third-octave bands in the logic module 23, it is therefore possible to ascertain the type of cavitation that is involved. This evaluation of the frequency bands in the context of the invention represents a comparison between a limit value and measured values which represent the liquid content.

For example, the pump may be used to operate at a first stage of the process at a speed of, for example, 1000 rpm. The minimum speed from which the liquid ring is stable is about 2000 rpm. At 1000 rpm, the pump is therefore operated well below the minimum speed. In this operating state, the pump can be used to transport an amount of liquid out of the space to be evacuated.

If no liquid is contained in the room, the pump can go on in a second stage of the process in the vacuum operation. In Fig. 5 the second stage of the method is schematically illustrated, where A represents the speed of the pump in Hz, where B shows the measurements taken with the vibration sensor 22 on a relative scale between 0 and 10, and C indicates the pressure in the space to be evacuated in millibars , The room to be evacuated has a volume of 400 l. The horizontal axis shows the time in seconds. To the Time t = 0 is in the room to be evacuated atmospheric pressure of just over 1000 mbar and the vibration sensor measures no vibration of the pump. After the transition to vacuum operation, the pump is accelerated within a short time to the maximum speed of about 5400 rpm. The pressure in the room quickly drops to values of about 500 mbar. At the time t = 20 s, the vibrations measured with the vibration sensor 22 exceed for the first time the in Fig. 5B Dashed shown Kavitationsschwellwert 26. The speed of the pump is then slightly reduced, which causes the vibrations within a short time again falls below the predetermined Kavitationsschwellwert 26. The speed is then increased again slightly until the cavitation limit is reached again. With the method according to the invention, the container, which has a volume of 400 l, evacuated within 80 s to a pressure of 60 mbar. If you operate the same pump with constant speed, the same process takes 113 s.

When the final pressure is reached, a lower speed is sufficient to maintain the pressure. In the third stage of the process, the speed is therefore reduced so that it is just above the minimum speed. If it comes to cavitation in this state, this is usually due to an increased liquid content in the gas to be delivered. In the logic module 23, on the one hand, an excess of the cavitation threshold and, on the other hand, a high liquid content are determined. The logic module 23 will thus convey the instruction to the control unit 24 to increase the speed.

Claims (15)

1. Method for operating a liquid ring vacuum pump having the following steps:

   a. recording of measured vibration values of the pump and comparing of the measured vibration values with a predefined cavitation threshold value (26);

   b. recording of a measured value which represents the liquid content in the gas to be delivered, and comparing of the measured value with a predefined limiting value;

   c. adapting of the rotational speed of the liquid ring vacuum pump,

   i. the rotational speed being reduced if the predefined cavitation threshold value (26) has been exceeded and the liquid content lies below the predefined limiting value;

   ii. the rotational speed being increased if the predefined cavitation threshold value has been exceeded and the liquid content lies above the predefined limiting value.
2. Method according to Claim 1, characterized in that, in step b., measured values from an external sensor (27) are processed.
3. Method according to Claim 1 or 2, characterized in that, in step b., measured values which are recorded at the pump are processed.
4. Method according to Claim 3, characterized in that, in step b., measured vibration values are processed.
5. Method according to Claim 4, characterized in that, in step b., the frequency spectrum of the measured vibration values is taken into consideration.
6. Method according to one of Claims 1 to 5, characterized in that the cavitation threshold value (26) relates to the amplitude of the vibration.
7. Method according to one of Claims 1 to 6, characterized in that air is let into the working space of the pump if the cavitation cannot be eliminated by adaptation of the rotational speed.
8. Method according to one of Claims 1 to 7, characterized in that the pump is operated in a first method stage at a rotational speed which lies below the minimum rotational speed.
9. Method according to Claim 8, characterized in that the pump is operated in a second method stage first of all at a maximum rotational speed, and in that the rotational speed is lowered after the occurrence of cavitation.
10. Method according to Claim 8 or 9, characterized in that the pump is operated in a third method stage at a rotational speed just above the minimum rotational speed.
11. Liquid ring vacuum pump having a pump housing (20), having an impeller (14) which is mounted eccentrically in the pump housing (20), and having a a vibration sensor (22) for recording vibrations of the pump, characterized in that the pump comprises a logic module (23) which compares a measured value of the vibration sensor (22) with a predefined cavitation limiting value (26) and which compares a measured value which represents the liquid content of the gas to be delivered with a first limiting value, a control unit being provided, furthermore, to adapt the rotational speed of the pump:

    i. the control unit being designed to reduce the rotational speed if the predefined cavitation threshold value has been exceeded and the liquid content lies below a predefined limiting value;

    ii. the control unit being designed to increase the rotational speed if the predefined cavitation threshold value has been exceeded and the liquid content lies above a predefined limiting value.
12. Liquid ring vacuum pump according to Claim 11, characterized in that the pump housing (20) has a duct which extends from outside into the interior of the pump, and in that the duct is provided with a valve (28).
13. Liquid ring vacuum pump according to Claim 11 or 12, characterized in that the valve (28) is opened after the predefined cavitation threshold value (26) is exceeded.
14. Liquid ring vacuum pump according to one of Claims 11 to 13, characterized in that the pump is of monobloc configuration.
15. Liquid ring vacuum pump according to one of Claims 11 to 14, characterized in that the vibration sensor (22) is integrated into the control unit (21).

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