EP2791511A2

EP2791511A2 - Liquid ring vacuum pump with cavitation regulation

Info

Publication number: EP2791511A2
Application number: EP12799568.6A
Authority: EP
Inventors: Heiner KÖSTERS; Matthias Tamm; Daniel SCHÜTZE
Original assignee: Sterling Industry Consult GmbH
Current assignee: Sterling Industry Consult GmbH
Priority date: 2011-12-12
Filing date: 2012-12-12
Publication date: 2014-10-22
Anticipated expiration: 2032-12-12
Also published as: MX2014007011A; CN104066994A; BR112014014150A2; JP2015502486A; EP2791511B1; US9169838B2; MX348628B; CN104066994B; JP5657846B1; WO2013087708A3; US20140377084A1; BR112014014150B1; WO2013087708A2

Abstract

The invention relates to a method for operating a liquid ring vacuum pump in which vibration measurements of the pump are taken and compared with a prescribed cavitation threshold (26). In addition, a measurement representing the liquid content in the gas to be conveyed is taken. This measurement is compared with a prescribed threshold. The rotational speed of the liquid ring vacuum pump is reduced if the prescribed cavitation threshold (26) has been exceeded and the liquid content is less than the prescribed threshold. The rotational speed is increased if the prescribed cavitation threshold has been exceeded and the liquid content is greater than the prescribed threshold. The invention also relates to a liquid ring vacuum pump designed for implementing the method. Due to the regulation depending on the oscillations of the pump in accordance with the invention, the pump can be operated near the cavitation boundary without any risk of damage.

Description

Liquid ring vacuum pump with cavitation control

The invention relates to a method for operating a liquid ring Va kuumpumpe. 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. If the pump is operated for a long period under cavitation conditions, this is a high mechanical ^¬ cal strain on the components of the pump through which the pump can be quickly destroyed. Previous liquid ring vacuum pumps are therefore designed so that a sufficient distance is always maintained to the operating conditions in which cavitation can occur. Thus, the pump is indeed protected from damage by cavitation, by the distance to Kavi ^¬ tion limit but a part of the potential performance of the pump is not used.

The invention is based on the object to present a pump and a method for operating a pump, in which the efficiency is increased. Based on the above-mentioned prior art, 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 forming the liquid ring of the pump is referred to as operating liquid ^¬ ness. Must be distinguished from an entrained by the gas to för ^¬-promoting fluid, which is hereinafter referred to as condensate. The term condensate is not limited to liquids formed by condensation ha ^¬ ben, but also includes other liquids that are entrained by the gas. In particular, it is not necessary that the condensate is a different substance than the operating fluid. Occurs, the condensate in the pump, it can mix with the Be ^¬ operating liquid. 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 value is selected such that from vibration ^{measurement values} above the cavitation ^threshold value it can be concluded that cavitation occurs in the pump, whereas there is no cavitation in the pump for vibration measurement values below the cavitation threshold value. 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. Is closed, for example, from indirect measurements on the liquid content, the comparison with the limit value may be that are identified in the indirect measurement Merkma ^¬ le, indicating a high or low liquid content.

The invention has recognized that, unlike other types of pumps (cf., for example, DE 35 20 538 A1), in liquid-ring vacuum pumps it is not always possible to ^recirculate the pump out of the cavitation by lowering the rotational 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.

Creates the cavitation on the other hand in that the pump together ^¬ men supplied with the gas condensate to be delivered, it would be counterproductive to lower the speed of the pump. With the reduced speed, the pump would certainly no longer able to herauszubefordern the excess fluid from the pump. In fact, it is possible, the Squeeze out excess fluid by increasing the speed of the pump. 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 is reduced ver ^¬ . If the cavitation threshold has been exceeded and the liquid content is high, the speed is increased. The method step to increase the speed of the pump after the occurrence of cavitation, the common teaching exactly entgegenge ^¬ sets, according to the one assumed that the rotational speed must always be lowered in cavitation.

In the pump measured values can be processed by external sensors to the liquid content of the gas to be conveyed to convey it ^¬. It can be provided in the space to be evacuated a sensor that measures the liquid content directly. It may also consist of other metrics that affect about the pressure or the temperature in the room to evakuierendem be closed to the flues ^¬ sigkeitsgehalt.

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 If the measured values of the vibration sensor are obtained, 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. Exceeds the amplitude of the Kavita ^¬ tionsschwellwert, 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.

It occasionally happens that the cavitation can not be eliminated solely by ei ^¬ ne adjustment of the speed. In this case it can be provided to admit additional air via a valve in the working space of the pump. The efficiency of the pump while decreases cavitation but reliably be seitigt ^¬.

The operation of the pump can be based on a multi-stage sequence. In a first process step the pump with egg ^¬ ner speed can be operated 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 mode can be used to the pump, which is actually designed for conveying gas, initially herauszubefordern a quantity of liquid from the evaku to ^¬ ierenden space. The blades of the impeller then act as blades, with which the liquid is passed through the Pum ^¬ pe. A separate condensate pump is thereby superfluous.

In this way, the liquid removed from the space to be evacuated, can be passed to the normal vacuum operation in which the pump is operated at a temperature above the minimum rotational speed ^¬ number. The idea to operate the pump initially at a speed below the minimum rotational speed in order to transport fluid, and then continues ^¬ release the vacuum operation at a speed above the minimum speed has independent inventive content, be included without vibration measurement values of the liquid ^¬ content determined and the speed is adjusted. The following description of additional process steps to ^¬ specifies 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 challenge in the shortest possible time as much gas from the space to be evacuated. In this operating state, there is a risk that it comes with decreasing pressure to classical cavitation in the liquid ^¬ ring. The classic cavitation can be counteracted by a speed ^¬ rundown. 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 boundary refers to a operating state of 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. By operating at low speed energy is saved ^¬ . If, at such a low speed to Ka ^¬ vitation, this is regularly increased liquid ^¬ content in the gas to be conveyed. 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 subject matter to be disinfected ^¬ Ge is introduced into 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 the pump is then operated at maximum speed and the rotational speed ^¬ is then lowered along the cavitation, is saved in the actual evacuation time. By finally maintaining low pressure through low speed operation, energy is saved.

The invention also relates to a liquid ring vacuum pump ^¬ which operated according to the method of the invention who can ^¬. The pump comprises a pump housing, an eccentrically mounted in the pump housing impeller and a vibration ^¬ sensor for receiving vibrations of the pump. According to the invention a logic module is provided which is similar to ver ^¬ a measured value of the oscillations ^¬ supply sensors with a predetermined-Kavitati onsschwellwert and a liquid content of the gas to be conveyed representing the measured value equals to a first limit ^¬ ver. 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 ^¬ signs of cavitation can be detected ^¬ to before the cavitation is such a serious nature that can cause damage to the pump. The predetermined Kavitations ^¬ threshold value is chosen so that it is not exceeded during normal operation of the pump, but only when the Pum PE approaches the Kavitationsgrenze.

The predetermined Kavitationsschwellwert is selected suitable for the respective pump, the Kavitationsschwellwert can relate, for example, to the amplitude of the vibrations. 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 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 comprise a channel for this purpose, he extends ^¬ from the outside through the pump housing into the interior of the pump. The channel is provided with a valve which is closed in the normal state. The valve can be opened briefly after the threshold value has been exceeded in order to remove gas from the converter. 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 finds occurring in the pump housing oscillations ^¬ conditions. 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 may be advantageous because of the ^¬ when the vibration sensor is disposed in a region of the pump housing are already present in the electronic compo nents ^¬. 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. When 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.

The invention is described below with reference to the accompanying drawings on the basis ^¬ appended advantageous embodiments in ^¬ way of example. Show it: Fig. 1: a schematic sectional view of a liquid ring vacuum pump according to the invention;

Fig. 2: the pump of Figure 1 in a side view.

Fig. 3: a control unit of a liquid ing-vacuum pump according to the invention;

Fig. 4: the view of Figure 3 in another embodiment of the invention. and

5 shows a schematic representation of an operating sequence of the pump according to the invention.

In a liquid ring vacuum pump shown in FIG. 1, 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. By getting ver ^¬ größernde chamber gas is sucked through the inlet port 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 forced through an outlet opening 17 in the atmosphere. rendruck again delivered. 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 ^¬ sets.

As the schematic representation of FIG. 3 shows, the control unit 21 comprises a vibration sensor 22, a logic module 23 and an adjustment module 24. The control unit 21 is 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 value 26 is exceeded, this is interpreted as an indication that cavitation has occurred in the pump, but the excess of the cavitation threshold value does not yet result 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 brought together, can be decided by which if the speed must he ^¬ höht be lowered or to the cavitation beseiti ^¬ gen. If cavitation and the pumped gas contains condensate or very little Condensate, the speed is lowered. Occurs cavitation and the pumped gas contains ei ^¬ ne larger amount of 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 set 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 adjusting 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 any information from an external sensor. Instead ^¬ the readings from the vibration sensor 22 evaluated in two ways. On the one hand, the amplitude of the vibration compared with the predetermined cavitation threshold value, the amplitude exceeds the threshold value, 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 From ^¬ evaluation of the two third octave bands in the logic module 23 can be determined, then, to make it look han ^¬ delt what kind of cavitation. This evaluation of the frequency bands situated in the context of the invention represents a comparison between a threshold value and values ^¬ measurement representing 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 mode, the pump can be used to herauszutransportieren a quantity of liquid from the evakuie to ^¬ Governing space.

If there is no liquid left in the room, the pump can go into vacuum operation at a second stage of the process. In Fig. 4, the second stage of the process is shown schematically, where A represents the speed of the pump in Hz, where B shows the measured values taken with the vibration sensor 22 on a relative scale between 0 and 10 and where C is the pressure in the to be evacuated Room in millibar indicates. The room to be evacuated has a volume of 400 liters. The horizontal axis shows the time in seconds. At time t = 0, there is atmosphere in the room to be evacuated. pressure of just over 1000 mbar and the vibration sensor does not measure any 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 predetermined cavitation threshold 26 shown in dashed lines in FIG. 4B. The speed of the pump is then slightly reduced, which causes the vibrations to fall below the level within a short time predetermined cavitation threshold 26 decreases. 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 1, evacuated within 80 s to a pressure of 60 mbar, If one operates the same pump at a 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, that is, a rewriting processing of Kavitationsschwellwerts ^¬ and secondly a high liquid content on the one hand determined. The logic module 23 will thus convey the instruction to the control unit 24 to increase the speed.

Claims

claims

Method for operating a liquid ring vacuum pump, comprising the following steps:

a. Recording vibration measurements of the pump and comparing the vibration measurements with a predetermined cavitation threshold (26);

b. Receiving a measurement value that represents the liquid content in the gas to be conveyed, and Verglei ^¬ chen of the measured value with a predetermined threshold value; c. Adjusting the speed of the liquid ring vacuum pump, wherein

i. the speed is reduced when the predetermined cavitation threshold (26) has been exceeded and the fluid content is below the specified limit;

ii. the speed is increased when the predetermined cavitation threshold has been exceeded and the fluid content is above the specified limit.

A method according to claim 1, characterized in that in step b. Measuring values of an external sensor (27) verar ^¬ beitet.

A method according to claim 1 or 2, characterized in that in step b. Measured values recorded on the pump are processed.

A method according to claim 3, characterized in that in step b. Vibration measurement values are processed.

5. The method according to claim 4, characterized in that in

Step b. the frequency spectrum of the vibration measurements is considered.

6. The method according to any one of claims 1 to 5, characterized in that the Kavitationsschwellwert (26) refers to the amplitude of the oscillation.

7. The method according to any one of claims 1 to 6, characterized marked ^¬ characterized in that air is admitted into the working space of the pump, if the cavitation can not be eliminated by adjusting the speed.

8. The method according to any one of claims 1 to 7, characterized marked ^¬ characterized in that the pump is operated at a first process stage with a speed below the minimum speed.

9. The method according to claim 8, characterized in that the pump is first operated at a second process stage with maxima ^¬ ler speed and that the speed is lowered after the occurrence of cavitation.

10. The method according to claim 8 or 9, characterized in that the pump is operated on a third process stage with a speed just above the minimum speed.

11. A liquid ring vacuum pump having a pump housing (20) with an eccentrically mounted in the pump housing (20) impeller (14) and with a vibration sensor (22) for receiving vibrations of the pump, characterized in that the pump is a logic module ( 23) which compares a measured value of the vibration sensor (22) with a predetermined Kavitationsschwellwert (26) and the one representing the liquid content of the gas to be conveyed Comparing the measured value with a first limit value, further comprising a control unit for adjusting the rotational speed of the pump, wherein:

i. the control unit is designed to reduce the speed when the predetermined Kavitationsschwellwert has been exceeded and the

Liquid content is below a predetermined limit;

ii. the control unit is adapted to increase the rotational ^¬ figure, when the predetermined has been exceeded and the flues Kavitationsschwellwert ^¬ sigkeitsgehalt is above a predetermined limit value.

12. Liquid-ring vacuum pump according to claim 11, characterized ge ^¬ indicates that the pump housing (20} has a channel which extends from the outside into the interior of the pump, and that the channel is provided with a valve (28).

13. Liquid ring vacuum pump according to claim 11 or 12, characterized in that after exceeding the predetermined Kavitationsschwellwerts (26) the valve (28) is geöff ^¬ net.

14. Liquid ring vacuum pump according to one of claims 11 to

13, characterized in that the pump is designed in block construction.

15. Liquid ring vacuum pump according to one of claims 11 to

14, characterized in that the vibration sensor (22) in the control unit (21) is integrated.