Liquid ring vacuum pump cavitation sound
Because the pump cavitation bubbles occur naturally consecutive division, and the accompanying severe water hammer in the high-pressure zone, while noise and vibration.
You can hear the crackling like popcorn-like movement. Assuming that the above division bubbles on the metal surface, the metal surface will suffer continuous severe water hammer, pitting, metal grain loose and falling from a honeycomb, or even perforation.
In addition to the mechanical effects of cavitation damage, but also accompanied by a variety of messy effect electrolysis, chemical corrosion. Actual damage indicated, the pump flow components cavitation damage to the site, just missing part of the bubble. Water ring vacuum pump cavitation corrosion is generated in the bubble and broken parts, non-metallic nominal emerged pitting phenomenon, it will emerge a major cellular destruction. Pump impeller cavitation site has large residual stress, can lead to stress released, cracks.
Because the pump cavitation bubbles in the low pressure area have produced a sudden burst, and along with the strong water hammer generated noise and vibration can be heard crackling like popcorn-like tone. Then the above-mentioned bubble burst in nominally non-metallic, non-metallic nominal will be another strong water hammer emerged pitting, non-metallic grain loose and peeling from a honeycomb and even perforation.
Cavitation destroyed except for the mechanical action, also accompanied by a variety of simple role electrolysis, chemical erosion. Destroy motivation theory indicate that the central pump flow element cavitation destruction of the site, is the bubble disappearance. Vacuum pumps need to use a liquid medium as a job. It is also possible that the threshold value relates to defined characteristic properties of the vibrations which are triggered by cavitation. It can be the case, for example, that vibrations in defined frequencies occur with particular intensity during cavitation.
In addition or as an alternative to the adaptation of the rotational speed, the distance from the cavitation limit can also be increased by virtue of the fact that the pressure in the interior of the pump is increased. For this purpose, the pump can have a duct which extends from outside through the pump housing into the interior of the pump. The duct is provided with a valve which is closed in the normal state. The valve can be opened briefly after the threshold value is exceeded, in order to let gas from the surroundings into the interior of the pump.
As a result, a distance from the cavitation limit is established again. The vibration sensor is preferably connected to the pump housing, with the result that it determines vibrations which occur in the pump housing. The vibration sensor can be arranged where the vibrations which are caused by cavitation are produced, that is to say in the vicinity of the impeller. The vibration sensor can be arranged, for example, on the circumference or on the end side of this region of the housing.
However, no electronic components are normally otherwise arranged in the region of the impeller. If the vibration sensor is arranged there, this has the disadvantage as a result that cables have to be laid additionally. It can therefore be advantageous if the vibration sensor is arranged in a region of the pump housing, in which there are electronic components in any case.
This can be, for example, the region, in which the control unit for the drive is also arranged. This may be suitable, in particular, if the pump is of monobloc configuration. Monobloc configuration means that the pump and the drive are surrounded by a common pump housing.
The vibrations which are produced in the region of the impeller propagate through the pump housing and can also be measured satisfactorily at another location. If the control unit for the drive of the pump is connected to the pump housing, the vibration sensor can be integrated into the control unit.
The pump can be developed by way of further features which are described above with reference to the method. In the following text, the invention will be described by way of example using advantageous embodiments with reference to the appended drawings, in which:. In a liquid ring vacuum pump which is shown in FIG. Liquid in the interior of the pump is driven by the impeller 14 which is in rotation, and forms a liquid ring which extends radially to the inside from the outer wall of the pump housing On account of the eccentric mounting, the vanes of the impeller 14 protrude to different depths into the liquid ring depending on the angular position.
The volume of a chamber which is enclosed between two vanes changes as a result. The liquid ring therefore acts like a piston which moves up and down in the chamber during a revolution of the impeller A duct leads from an inlet opening 16 into the interior of the pump, in which the impeller 14 rotates. The duct 16 opens in the region, in which the vanes of the impeller 14 emerge from the liquid ring, that is to say in which the chamber which is enclosed between two vanes is enlarged.
As a result of 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 into the chamber again during the further rotation of the impeller When the gas is compressed sufficiently by way of the liquid ring which penetrates further, it is output again at atmospheric pressure through an outlet opening A liquid ring vacuum pump of this type serves to evacuate a space which is connected to the inlet opening 16 to a pressure of, for example, 50 millibar.
Moreover, the pump is equipped with a duct which is called a cavitation bore and extends from the outside into the interior of the pump. A solenoid valve is arranged in the duct, by way of which solenoid valve the duct can optionally be opened or closed.
The pump is of monobloc configuration, that is to say the drive and the impeller 14 are accommodated jointly in the pump housing Moreover, a control unit 21 is arranged on the pump housing 20 , via which control unit 21 electrical energy is fed to the drive and the rotational speed of the pump is set.
As the diagrammatic illustration of FIG. Moreover, measured values from an external sensor 27 are fed to the control unit The vibration sensor 22 is connected to the pump housing 20 , in order to determine vibrations of the pump housing The measured values of the vibration sensor 22 are transmitted continuously to the logic module The logic module 23 compares the measured values with a predefined cavitation threshold value 26 see FIG.
If the cavitation threshold value 26 is exceeded, this is evaluated as an indication that cavitation has occurred in the pump. It still cannot be derived, however, solely from the exceeding of the cavitation threshold value whether it is classic cavitation or cavitation on account of an increased liquid content.
Measured values from the external sensor 27 are therefore additionally fed to the logic module, from which measured values the magnitude of the liquid content of the gas to be delivered is derived. The external sensor 27 can be, for example, a sensor which directly measures the liquid content in the feed line to the pump. It is also possible that the external sensor 27 measures values, from which a conclusion can be made indirectly about the liquid content.
These values can concern, for example, the temperature, the pressure or the quantity of supplied steam in the space to be evacuated. In this way, the information is combined in the logic module 23 , using which information a decision can be made as to whether the rotational speed has to be increased or decreased, in order to eliminate the cavitation. If cavitation occurs and the gas to be delivered contains no condensate or only a very small quantity of condensate, the rotational speed is decreased.
If cavitation occurs and the gas to be delivered contains a relatively large quantity of condensate, the rotational speed is increased. A corresponding signal is given to the actuating module 24 by the logic module 23 , with the result that the drive of the pump is set correspondingly. In both cases, the adaptation of the rotational speed leads to the cavitation being stopped again in the pump.
In addition or as an alternative to the rotational speed adaptation, the solenoid valve 28 can be opened briefly via the actuating module 24 , with the result that air from the surroundings can penetrate into the interior of the pump.
The distance from the cavitation limit is also increased by way of the associated pressure increase in the interior of the pump.
In the embodiment according to FIG. Instead, the measured values from the vibration sensor 22 are evaluated in two ways. Firstly, the amplitude of the vibration is compared with the predefined cavitation threshold value. If the amplitude exceeds the threshold value, this indicates cavitation.
Secondly, a Fourier transformation of the measured values is performed and the frequency distribution of the vibrations is taken into consideration. To this end, for example, the third-octave band at 5 kHz and the third-octave band at 10 kHz can be singled out. The classic cavitation is manifested by way of a characteristic distribution in the 5 kHz third-octave band, whereas the cavitation which is caused by way of increased liquid content brings about a characteristic frequency distribution in the 10 kHz third-octave band.
By way of the evaluation of the two third-octave bands in the logic module 23 , it can therefore be determined which type of cavitation it is. In the context of the invention, this evaluation of the frequency bands represents a comparison between a limiting value and measured values which represent the liquid content.
The pump can be used, for example, in such a way that it is operated in a first stage of the method at a rotational speed of, for example, rpm. The minimum rotational speed, above which the liquid ring is stable, lies at approximately rpm. At rpm, the pump is therefore operated considerably below the minimum rotational speed. In this operating state, the pump can be used to transport a quantity of liquid out of the space to be evacuated.
If no more liquid is contained in the space, the pump can change over into vacuum operation in a second stage of the method. The space to be evacuated has a volume of I. The time in seconds is plotted on the horizontal axis. The boiling point is different because the atmospheric pressure pushing down on the water in Denver is less than at sea level, which allows the liquid to convert to vapor at a lower temperature. When liquid is boiling in a cooking pot we call it "dinner".
When the liquid ring in a pump starts to boil, we call it "cavitation". Much like the pot of water example, cavitation in a liquid ring pump is caused when the operating pressure of the liquid ring reaches the vapor pressure of that liquid. This causes some of the liquid to become vapor, forming bubbles that travel around with the liquid ring.
As these bubbles travel inside the pump they collapse, or implode, and can break off pieces of the pump. These pieces travel with the liquid ring and cause further damage through erosion. The answer is more complicated than you might think. Cavitation is a function of both temperature and pressure.
The lower the operating temperature of the liquid ring, the lower the potential for cavitation.