Installing Liquid Ring Vacuum Pumps

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The first lesson for operating liquid-ring vacuum pumps is installing them properly. The liquid-ring vacuum pump is a specific form of rotary positive-displacement pump utilizing liquid as the principal element in gas compression. The compression is performed by a ring of liquid formed as a result of the relative eccentricity between the pump's casing and a rotating multibladed impeller.

The eccentricity results in near-complete filling, and then partial emptying, liquid ring vacuum pump pid each rotor chamber during every revolution. The filling and emptying actions create a piston action within each set of rotor or impeller blades. The pump's components are positioned in such a manner as to admit gas liquid ring vacuum pump pid the rotor chamber is emptying the liquid, and then allowing the gas to discharge once compression is completed.

Sealing areas between the inlet and discharge ports are provided, liquid ring vacuum pump pid close the rotor areas, and to separate the inlet and discharge flows. The proper installation of a liquid-ring vacuum pump is critical to its subsequent operation and maintenance.

The following installation guidelines are general recommendations that apply to nearly all types of liquid-ring vacuum pumps, but users should refer to the specific recommendations of each manufacturer to ensure the best performance. As with any pump, care should be taken in unpacking the pump so as not to damage or misalign the assembly. For pump and motor units mounted on a baseplate, the unit should liquid ring vacuum pump pid lifted by the base only.

Slings or hooks should not be attached to the pump or motor, since this can cause misalignment. Also, the pump should not be run until it is properly installed, nor should it be run without a sealing liquid.

Normally, a pump's components are protected with a water-soluble preservative, which should be flushed from the unit if any fluid other than water is utilized in a closed-loop system.

Pumps made of stainless steel or other non-ferrous materials may be shipped without preservative, that is, "dry. Liquid-ring vacuum pumps are basically slow-speed, smooth-operating rotating devices. Nonetheless, it is important to ensure that the pump's frame or baseplate is mounted level and firmly anchored.

Pumps that are about 50 hp and above are best placed on a concrete pad. Smaller units may be mounted on existing floors and skids. All joints in piping, whether flanged or screwed, should be free of strain and checked for leaks prior to start-up. Normally, pumps that are supplied direct coupled to motors are aligned and test-run in the factory prior to shipment.

However, because of unforeseen forces and moments imposed on the pump during shipment and installation, it is necessary to check the coupling's alignment prior to startup.

To do this correctly, the guidelines of the coupling manufacturer should be followed as a minimum, and exceeded where possible. For pumps utilizing V-belt drives, it is necessary to ensure that the sheaves are properly installed and aligned before attempting to tension the drive. The V-belts should liquid ring vacuum pump pid placed over the sheaves and in the grooves liquid ring vacuum pump pid forcing them over the sides of the grooves.

When all belts are in their grooves, the centers are adjusted to take up all slack and leave liquid ring vacuum pump pid belts fairly taut. When the pump is operating, the slack side should have a slight bow. After several days of operation, re-tension the belts if necessary.

Slipping squealing at startup are indications of insufficient tension. Excessive tension can shorten bearing life. If the unit is idle for an extended period of time, the tension on the belts should be removed. The belts should never be mixed or switched from one groove to another on the sheaves, and should be replaced only with a matched set.

Belt dressing should never be applied, and the sheaves should remain free of oil and grease. The working principle of the liquid ring vacuum pump pid pump liquid ring vacuum pump pid dependent upon a continuous supply of clean seal liquid normally water, but other suitable liquids can also be used.

This liquid enters the pump through a connection on the casing and is discharged from the pump, along with the gas. Three basic piping arrangements for the seal liquid can be used for vacuum pump applications: All these arrangements have four elements: In this design, seal liquid is taken directly from a main and supplied to the pump Figure 1.

The liquid discharge is separated from the gas and wasted to a drain. No recirculation or recovery takes liquid ring vacuum pump pid. This is a common arrangement where conservation or contamination of the seal liquid is not a concern. An automatic solenoid liquid ring vacuum pump pid ensures the flow of the seal liquid in conjunction with the pumpmotor's operation i.

With a manual seal-liquid shut-off valve, care should be taken to flash cautions to open the valve immediately before turning the motor on, and shutting the valve immediately before the motor is stopped. This arrangement provides for total recirculation of the seal liquid. A liquid ring vacuum pump pid exchanger is added to the system to remove the heat of compression and condensation from the seal liquid before it is reintroduced into the pump Figure 3.

For prolonged operation at high suction pressures, and when the system heat exchanger, piping, valves, and so on has excessive pressure drop losses, a circulating pump may be necessary. With partial or total recovery arrangements, the seal-liquid level in the separator-recirculation tank should be at, or slightly below, the centerline of the pump shaft.

Provisions may also be made for high-level overflow and low-level makeup on total recovery systems. These level controls help prevent the starting of the pump with the casing full of water, since this could overload the motor and damage the pump. In fact, liquid-ring vacuum pumps in any piping arrangement should not be started with a full casing of seal liquid. Thus, provisions are normally made to liquid ring vacuum pump pid the pumps in the event they become flooded.

These provisions may vary somewhat from one manufacturer to another. In general, it is not necessary to drain the pump if the incoming seal liquid is shut off simultaneously. An automatic valve can be used to control this procedure. Many liquid-ring vacuum pumps that incorporate a standard packing or gland arrangement for shaft sealing are also fitted with lantern rings and a gland connection provided for cooling liquid.

A suitable source of cooling liquid must be provided, at around 5 psig above the operating pressure. A common supply for both liquid ring vacuum pump pid seal liquid and the gland cooling is normally used. If mechanical seals liquid ring vacuum pump pid employed, a supply of cooling and flush liquid is liquid ring vacuum pump pid required. It is recommended that a separate and clean source of seal liquid for mechanical seals is used. Double mechanical seals require a monitoring device to detect a leak on the inboard seal.

To begin, the suction and discharge flanges on pumps are normally marked by arrows on the casing. The suction and discharge lines should be the same size as the pump connections. Ideally, the discharge line from the pump to the separator should be at as low an elevation as possible. However, if it is necessary, a discharge leg can be used with minimum elevation above the pump's discharge flange. Liquid ring vacuum pump pid high an elevation in the discharge line can cause a back pressure on the pump, overloading the motor and affecting the pump's capacity.

The seal-liquid supply piping should be the same size as the connection on the pump. For fully recirculated seal systems that do not use a recirculation pump, a larger pipe size is often used to reduce the pressure drop.

Remove the protective coverings from the pump openings just before connecting the pipe work. Check that all foreign matter, such as welding slag, nuts, bolts, rags and dirt, have been thoroughly cleared out of pipe work before connecting to the pump.

When connecting the pipe work, check that the flanges fit easily without strain, and that the flange holes are in perfect alignment. The flange gaskets must not protrude into the interior bore of the pipe or pump flange. All pipe work must be supported independently on each side of the pump, and must fit easily without transmitting strain to the pump casing.

It is recommended that during the first three weeks of operation, a protective mesh be fitted at the pump's suction inlet. Forces and moments caused by piping connections to the liquid ring vacuum pump pid should be held to a minimum. Ideally, there should be no forces or moments exerted on the pump casing, which can be achieved by completely supporting the piping.

Finally, the line sizes should be at least the same size as the pump connections, to eliminate any unnecessary pressure drops. Standard induction motors are suitable for driving liquid-ring pumps.

Starting liquid ring vacuum pump pid are low, and so across-the-line operation is normally employed. It is recommended that a motor controller with over-current protection of the heater or fuse be used. The full-load current rating, stamped on the motor nameplate, should be used in making the selection for protection rating.

A disconnect switch should also be installed between the motor controller and the power supply. After the electrical work is completed, the pump should be turned by hand. It may be necessary to slacken the gland rings in order for the shaft to turn freely.

The direction of rotation is marked by an arrow on the pump. Prime the system, turn on the seal liquid, bump the motor turn it on and off to check the pump's rotation, and turn off the seal liquid. If the direction is wrong, reverse any two of the three motor leads and recheck. A V, single-phase supply should be used for control circuits. Solenoid valves, vacuum and pressure switches, level controllers, and alarms should be supplied with only V, to comply with electrical-safety-code requirements.

Liquid-ring pumps come with many accessories, supplied by the manufacturer or by other companies in the field. An application's particular requirements, mode of operation, and type of control scheme dictate the necessity of various items. The following covers some of the more commonly used items.

Isolation valves separate the pump from the system whenever it is shut down for extended periods of time or for maintenance procedures. Gate valves or full-port ball valves are recommended for minimizing pressure drops liquid ring vacuum pump pid lines that are 3 inches or above in size. Butterfly valves are a more economical choice Inlet check valves prevent the gas and seal liquid from flowing back to the process when the pump is stopped. Swing-check, double flexible-seal, or equal-type valves must be installed horizontally.

Inlet vacuum relief valves protect the pumpfrom cavitation. When the pump's suction pressure is below the setting of a vacuum-relief valve, the valve will open and bleed in atmospheric air or process gas if connected back to the pump'sdischarge side. Most inexpensive vacuum-relief valves are basedon atmospheric pressure and need to be calibrated periodically. Flexible connectors are used to correct for slight misalignmentsbetween a pump and a process, or if a minimal amount of expansion is anticipated.

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The invention relates to a method for operating a liquid ring vacuum pump. In the method, measured vibration values of the pump are recorded and are compared with a predefined cavitation threshold value. Moreover, the invention relates to a liquid ring vacuum pump which is suitable for carrying out the method. In liquid ring vacuum pumps, there is the problem that cavitation can occur in different operating states.

If the pump is operated over a relatively long time period under cavitation conditions, this represents a high mechanical load for the components of the pump, by way of which high mechanical loads the pump can quickly be destroyed.

Previous liquid ring vacuum pumps are therefore designed in such a way that a sufficient distance is always maintained from the operating states, in which cavitation can occur. Although the pump is therefore protected against damage as a result of cavitation, part of the possible performance capability of the pump is not utilized as a result of the distance from the cavitation limit.

A pump and a method for operating a pump is proposed wherein the efficiency is increased. In the method, a measured value is recorded which represents the liquid content in the gas to be delivered, and the measured value is compared with a predefined limiting value. The rotational speed of the pump is reduced if the predefined cavitation threshold value is exceeded and the liquid content lies below the predefined limiting value. The rotational speed of the pump is increased if the predefined cavitation threshold value is exceeded and the liquid content lies above the predefined limiting value.

First of all, some terms will be explained. The liquid which forms the liquid ring of the pump is called operating liquid. A distinction is to be made between this and a liquid which is driven by the gas to be delivered and is called condensate in the following text. The term condensate is not restricted to liquids which have formed as a result of condensation, but rather also comprises other liquids which are driven by the gas.

In particular, it is not necessary that the condensate is a different material to the operating liquid. If the condensate enters into the pump, it can mix with the operating liquid. The same liquid which has entered as condensate is therefore not necessarily delivered out of the pump. The cavitation threshold value is selected in such a way that a conclusion can be made from measured vibration values above the cavitation threshold value that cavitation is occurring in the pump, whereas there is no cavitation in the pump in the case of measured vibration values below the cavitation threshold value.

The specific value of the cavitation threshold value depends both on the design of the pump and on the type of sensor and the recording of the measured values. The cavitation threshold value can be determined readily for every individual pump by way of experiments. The limiting value for the liquid content is likewise dependent on the specific design of the pump.

In one pump, very small quantities of condensate already trigger the cavitation. In another pump, a certain quantity of condensate can be driven, without the operation of the pump being impaired. This can also be determined readily for every pump by way of experiments. It is also conceivable that the limiting value changes depending on the rotational speed of the pump, that is to say that the limiting value is a function which is dependent on the rotational speed.

The specification that the measured value is compared with a limiting value is to be understood broadly. If, for example, a conclusion is made about the liquid content from indirect measurements, the comparison with the limiting value can be that features are identified in the indirect measurement which indicate a high or low liquid content.

It has been recognized that it is not possible in every case in liquid ring vacuum pumps, as opposed to other types of pumps cf. A reduction of the rotational speed actually helps only in certain operating states, for example, if the cavitation is produced by the fact that the pump is operated at a high rotational speed and with a low intake pressure.

This cavitation is called classic cavitation. If, in contrast, the cavitation is produced by the fact that condensate is fed to the pump together with the gas to be delivered, it would even be counter-productive to lower the rotational speed of the pump. At the reduced rotational speed, the pump would namely even more no longer be in the position to convey the excess liquid out of the pump.

However, it is actually possible to convey the excess liquid out of the pump by way of an increase in the rotational speed. The increase in the rotational speed therefore brings it about in this case that the cavitation is eliminated.

This finding is utilized to propose a method, by way of which the operation of the pump can be adapted automatically in the case of different types of cavitation. In the method, in each case two criteria are combined, in order to decide whether the rotational speed is increased or decreased.

If the cavitation threshold value has been exceeded and the liquid content is low, the rotational speed is reduced. If the cavitation threshold value has been exceeded and the liquid content is high, the rotational speed is increased.

The method step of increasing the rotational speed of the pump after the occurrence of cavitation is precisely contrary to the established teaching, according to which it has been assumed that the rotational speed always has to be lowered in the case of cavitation. Measured values from external sensors can be processed in the pump, in order to determine the liquid content of the gas to be delivered. To this end, a sensor which directly measures the liquid content can be provided in the space to be evacuated.

A conclusion can also be made about the liquid content from other measured values which concern, for instance, the pressure or the temperature in the space to be evacuated. In addition or as an alternative, measured values which are recorded at the pump can be used to determine the liquid content. It is possible, for example, to make a conclusion about the liquid content from measured values of a vibration sensor. Although the liquid content cannot be measured directly via a vibration sensor, it is shown that the cavitation which is caused by an excess of condensate causes characteristic vibrations which differ from the vibrations in the case of the classic cavitation.

These characteristic properties can be determined by way of a suitable evaluation of the measured values of the vibration sensor. For example, a Fourier analysis can be performed and a conclusion can be made from the features of the frequency spectrum as to whether the cavitation is caused by increased liquid content or not. The specific appearance of the features depends on the design of the pump and the arrangement of the vibration sensor and possibly has to be determined in the individual case by way of experiments.

The measured values which are to be compared with the cavitation threshold value can be recorded by way of the same vibration sensor or another vibration sensor. The evaluation as to whether cavitation is present at all is simpler than the evaluation with regard to the different types of cavitation. For example, the cavitation threshold value can relate simply to the amplitude of the vibration.

If the amplitude exceeds the cavitation threshold value, a conclusion can be made therefrom that there is cavitation. Another possibility for making conclusions about the liquid content and therefore the type of cavitation from measured values which are recorded at the pump involves evaluating the internal motor data, such as the motor voltage and the motor current.

It occurs occasionally that the cavitation cannot be eliminated solely by way of an adaptation of the rotational speed. In this case, it can be provided to let additional air into the working space of the pump via a valve. Although the degree of efficiency of the pump drops as a result, the cavitation is eliminated reliably. The operation of the pump can be based on a multiple-stage sequence. In a first method stage, the pump can be operated at a rotational speed which lies below the minimum rotational speed.

Here, the minimum rotational speed denotes that rotational speed, at which the liquid ring in the pump is just stable. In this method stage, the pump is therefore operated without a stable liquid ring. In this operating state, the pump which is actually designed to deliver gas can be utilized to first of all convey a quantity of liquid out of the space to be evacuated.

The vanes of the impeller then act like blades, by way of which the liquid is guided through the pump. A separate condensate pump becomes superfluous as a result. If the liquid has been removed from the space to be evacuated in this way, a transition can be made to normal vacuum operation, in which the pump is operated at a rotational speed which lies above the minimum rotational speed.

The concept of first of all operating the pump at a rotational speed below the minimum rotational speed, in order to transport away liquid, and then of continuing the vacuum operation at a rotational speed above the minimum rotational speed has independent inventive content, even without measured vibration values being recorded, the liquid content being determined and the rotational speed being adapted.

The following description of further method stages substantiates the independent inventive content. After the transition to vacuum operation, the liquid ring vacuum pump can first of all be operated at a maximum rotational speed in a second method stage, in order to convey as large a quantity of gas as possible out of the space to be evacuated in as short a time as possible.

In this operating state, there is the risk of classic cavitation occurring in the liquid ring with decreasing pressure. The classic cavitation can be counteracted by way of a reduction in the rotational speed. The pump can be operated close to the cavitation limit in this way, the rotational speed being reduced further and further as the pressure becomes lower. Here, the term cavitation limit denotes an operating state of the pump, in which first signs of cavitation are exhibited.

If the pressure in the space to be evacuated has dropped to the desired value, the rotational speed of the pump can be reduced to a value close to the minimum rotational speed in a third method stage. Energy is saved as a result of the operation at a low rotational speed. If cavitation occurs at a low rotational speed of this type, this is as a rule a result of an increased liquid content in the gas to be delivered. If cavitation therefore occurs, it can be counteracted by way of an increase in the rotational speed.

In this way, the pump can be used, for example, during disinfection in hospitals. The object to be disinfected is introduced into a chamber and is treated with hot steam.

Subsequently, a chamber can be evacuated by way of the method according to the invention. The condensate can first of all be transported away at a low rotational speed.

By the pump subsequently being operated at a maximum rotational speed and the rotational speed then being lowered along the cavitation limit, time is saved during the actual evacuation. Energy is saved by the low pressure finally being maintained by way of operation at a low rotational speed. Moreover, the invention relates to a liquid ring vacuum pump which can be operated in accordance with the method.

The pump comprises a pump housing, an impeller which is mounted eccentrically in the pump housing, and a vibration sensor for recording vibrations of the pump.

A logic module is provided which compares a measured value of the vibration sensor with a predefined cavitation threshold value 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 of the pump is designed to adapt the rotational speed of the pump. Here, the control unit is 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. Here, the control unit is 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.

If cavitation occurs in the liquid ring of the pump, characteristic vibrations occur which differ from the vibrations during normal operation. First signs of cavitation can be determined by way of the vibration sensor, before the cavitation is pronounced to such an extent that damage to the pump can occur. The predefined cavitation threshold value is selected in such a way that it is not exceeded during normal operation of the pump, but rather only when the pump approaches the cavitation limit.

The predefined cavitation threshold value is selected in a suitable manner for the respective pump. The cavitation threshold value can relate, for example, to the amplitude of the vibrations. 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.