Liquid Ring Vacuum Pumps & Compressors_Tech Details - Sterling Fluid Systems Group

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Liquid Ring Vacuum Pumps & CompressorsTechnical Details & Fields of Application

STERLING FLUID SYSTEMS GROUPwww.sterlingfluidsystems.com

www.sterlingfluidsystems.com

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21

Sterling Fluid Systems (Austria)Wien

Telephone: +43 (0)1 680 00 50Fax: +43 (0)1 680 0521

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Telelphone: +49 (0) 4821 9000-0Fax: +49 (0) 4821 9000-501E-Mail: [email protected]

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Sterling Fluid Systems (USA)Grand Island

Telephone: (1) 716 773 6450Fax: (1) 716 773 2330E-Mail:[email protected]

Sterling PCU (USA)Dayton

Telephone: (1) 937 299 5594Fax: (1) 937 299 3843


Sterling Fluid Systems (Canada)Guelph

Telephone: (1) 519 824 4600Fax: (1) 519 824 7250E-Mail: [email protected]

Sterling Fluid Systems (Asia)Singapore

Telephone: (65) 6863 0828Fax: (65) 6863 0868

E-Mail:[email protected]

Sterling Fluid Systems (Malaysia)Selangor Darul Ehsan

Telephone: (60) 3 8070 0198Fax: (60) 3 8070 0204


SIHI (Australia)Bayswater

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Sterling Fluid Systems (China)Shanghai

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Sterling Fluid Systems (Taiwan) Taipei

Telephone: (886) 2 8631 2138Fax: (886) 2 8631 2184


Liquid Ring Vacuum Pumpsand Liquid Ring Compressors

Technical Details and Fields of Application

Manufacturing Programme

Liquid Centrifugal Pumps

• Volute Casing Pumps• Chemical Pumps• Side Channel Pumps• Multistage Ring Section • Pitot Tube Pumps• Sealless Pumps• Heat Transfer Pumps• Self Priming Pumps

Vacuum Technology

• Liquid Ring Vacuum Pumps• Liquid Ring Compressors• Dry Running Vacuum Pumps• Gas Ejectors

Engineered Systems

• Engineered Vacuum Systems• Engineered Compressor

Systems• Vacuum Based Membrane

Systems• Filling Systems

Service Support

• Field Service• In-house Report/Service• Total Pump Management• Energy Consultancy

Centrifugal PumpsVacuum TechnologyEngineered Systems

Service Support

ContentsVacuum pumps and compressors 4

Liquid ring vacuum pumps and liquid ring compressors 6

Characteristics 6

Operating principle and types of design 7

Gas and liquid flow passages 13

Working ranges 15

Operating characteristics 17

Operating modes 36

Drive 41

Controlling the inlet volumetric flow rate 43

Types of construction 47

Emissions 53

Combining liquid ring vacuum pumpswith gas ejector vacuum pumps 56

Accessories 60

Acceptance specifications 63

Applications 65

Literature 69

Appendix 70

Vapour pressure of water 70

Materials and material combinations commonly usedin the manufacture of liquid ring gas pumps 70

Vacuum pumps andcompressorsVacuum pumps and compressors aremachines designed for the compression ofgases and vapours. They are used for thoseduties in the field of process technologywhich would otherwise be uneconomical,unsafe or impossible to carry out.Vacuum pumps and compressors compressthe gases or gas vapour mixtures generated invarious processes from the “suction pressure”to the “discharge pressure”. With vacuumpumps, the suction pressure is lower thanatmospheric, whereas the discharge pressurewith compressors is higher than atmospheric.According to DIN 28400, vacuum is dividedinto the following pressure ranges (valuesindicated in mbar):• rough vacuum: 1x103 to 1• medium vacuum: 1 to 1x10-3

• high vacuum: 1x10-3 to 1x10-7

• ultra-high vacuum: <1x10-7

Most of the basic operations in processtechnology are performed in the roughvacuum range. At low suction pressures, it isoften necessary to combine several vacuum

4

Fig. 1: The SterlingSIHI range of liquidring vacuum pumpsis available in singleand two stagedesigns, with suctioncapacities up to12000 m3/h andsuction pressures in the range 33 to1013 mbar.

Vacuum Ranges

Compressors

pumps of different design. For example, high-vacuum pumps in general are connected torough-vacuum pumps.There are several different types of vacuumpumps and compressors. Figure 2 shows asummary.The following criteria should be consideredwhen selecting the required design:

• working range i.e. suction and dischargepressure,

• inlet volumetric rate of flow at the requiredpressures,

• requirements resulting from the type andproperties of the gas or vapour to becompressed,

• requirements in respect of theenvironmental and operating conditions,

• requirements with regard to safety ofoperation,

• economic efficiency.

Vacuum pumps and compressor 5

Combination ofseveral pumps

Fig. 2: Various typesof vacuum pumpsand compressors

Mechanical compressors

Displacement compressors

Reciprocating pistoncompressors

Diaphragmcompressor

Pistoncompressor

Screw typecompressor

Rotary slide valve compressor

Rotary piston lobecompressor

Liquid ringcompressor

Radial flowcompressor

Axial flowcompressor

Gas ringcompressor

Rotating pistoncompressors

Turbo compressors

Ejector type compressor

Liquidejector

Gasejector

Vapourejector

Liquid ring vacuumpumps and liquidring compressorsCharacteristicsLiquid ring vacuum pumps and liquid ringcompressors are compressors in which aliquid ring formed from the service liquidserves to transmit the energy required for thecompression of gases and vapours.

The medium being pumped comes intocontact with the service liquid, thus allowingthese media to affect each other. Liquid ringvacuum pumps and compressors thereforeoccupy a special position among the varioustypes of compressors and are characterisedby a great number and variety of propertiesunequalled by any other design:

• liquid ring vacuum pumps andcompressors are capable of compressingalmost any gas or vapour,

• no frictional contact between individualparts; the materials of construction can beselected to suit the operating conditions,

• the compression process is more or lessisothermal,

• the pumps offer the highest degree ofsafety in compressing explosivesubstances and toxic or carcinogenicmedia,

• if the medium pumped has condensibleconstituents, the inlet volumetric rate offlow can increase,

• single or double mechanical seals andmagnetic drives ensure low leakage rates,

6

Liquid ringserving as energytransmitter

Many distinctivefeatures

• operation is characterised by low noiseemission and a low vibration level,

• a high degree of operational safety withminimum maintenance,

• liquid can be pumped along with the gas flow.

Operating principle and types of designWithin the scope of this book, the term “liquidring gas pump” is used whenever theinformation given refers to both liquid ringcompressors.

When the medium being pumped comes intocontact with the service liquid, this causesthermodynamic effects and makes it possiblefor the gas to react with the liquid.

When compressing dry gases or vapourswhich neither condense in the inlet chambernor during the compression process, theliquid ring gas pump operates as adisplacement compressor. With an increasein volume, the medium is drawn in, and witha reduction of volume, it is compressed.

If the medium pumped is a gas/vapourmixture whose vapour portion condensesafter entering the inlet chamber or duringcompression, the liquid ring gas pumpoperates as a displacement compressor witha condensing effect.

When pumping media which are absorbed byor react with the service liquid, the liquid ringgas pump can be regarded as a displacementcompressor-cum-absorption machine.

In general, large and medium-sized liquid ringgas pumps are of single-acting design.


Liquid ring gaspump

Single-actingdesign

Figure 3 shows an exploded view of theprinciple of operation.

A multi-bladed impeller (2) is mountedeccentrically in a circular casing (1). When thecasing is partially filled with liquid and theimpeller is set into rotary motion, this causesthe liquid ring (3) to be formed as a result ofcentrifugal force. Due to the eccentricity, theinside contour of the liquid ring contacts theimpeller at points (20) and (21), and the liquidcreates a piston action within each set ofimpeller blades, causing flow into and out of theimpeller blade cells. This flow occurs with eachrevolution of the impeller and results in avolumetric expansion in the section of theoutflowing liquid ring, thus causing the mediumto be drawn in via the inlet port (5) in the guide

8 Liquid ring vacuum pumps and liquid ring compressors

Fig. 3: Operatingprinciple of a liquidring gas pump

20

5

4 3 1 21 2 7

6

plate (4) connected to the suction nozzle. In thearea of the inflowing liquid ring, the volume isreduced, thus causing the medium to becompressed. On completion of compression,the medium is discharged via the outlet port (7)in the guide plate (6) which is connected to thedischarge nozzle.

The double-acting design is used for specialapplications, in particular on compressorsemployed for high differential pressure. Here,the casing is designed so that the liquid ringflows into and out of the impeller blade cellstwice during each revolution of the impeller.

Figure 4 shows a section of a single-acting,figure 5 of a double-acting liquid ring gas pump.

Differing immersion depths of the liquid ring intothe impeller, which is a requirement for theproper functioning of a liquid ring gas pump, canalso be achieved by means of an impellerrotating concentrically in the casing and channel-like chambers of varying depths arranged nextto the impeller.


Double-actingdesign

Fig. 4:Longitudinal andtransverse sectionsthrough a single-stage, single actingliquid ring gas pump1 Casing2 Impeller3 Liquid Ring4 Guide plate5 Inlet port6 Guide plate7 Outlet port8 Shaft

4 3 2 81 6 1

5 3

7

2

Pumps of this type are suitable for pumpingsmall inlet volumetric rates of flow and canalso be used for the pumping of liquids. Alongitudinal section through a so-called sidechannel pump is shown in figure 6.

In figures 3 to 6, the inlet and outlet ports arearranged in flat guide plates located at each

side of the impeller. The gas enters andescapes by way of the impeller faces.In another design variation, the inlet andoutlet ports are located in guide platesextending into the impeller hub.


6 1 2 4 8 3

7

7

5

5

2 2

3 1 4 3 1 54

Fig. 5:Longitudinal andtransverse sectionsthrough a single-stage, double actingliquid ringcompressor1 Casing2 Impeller3 Liquid Ring4 Guide plate5 Inlet port6 Guide plate7 Outlet port8 Shaft

Fig. 6:Longitudinal sectionthrough a two-stage,liquid ring gas pumpof the side channeltype1 Casing2 Impeller3 Guide plate4 Guide plate5 Shaft

With a relatively small impeller width, the inletand outlet ports need only be arranged on oneside to adequately fill the impeller blade cellswith gas and empty them. In this case, theports are either arranged in the same guideplate, or the inlet port is arranged in one guideplate and the outlet port in the guide plateopposite, as shown in figures 3 and 5.

With comparatively wide impellers, intake anddischarge of the gas is effected via portsarranged on either side. Figure 4 shows anexample of this arrangement. In contrast, theimpeller blade cells of pump stages designedfor low suction pressures, and thus for low-density gases, can be adequately fed with gas through a single inlet port, even whererelatively wide impellers are used.

The inlet ports extend across almost the entireangle at which the liquid ring flows out of theimpeller blade cells.

The outlet ports are located in the sectionwhere the liquid ring enters the impeller bladecells. The angular extension and geometricalconfiguration of the ports are a function of theratio of the discharge to the suction pressurefor which the pump stage is designed, takinginto consideration that the stage has to workeconomically at the specified pressure ratioand also at lower pressure ratios. The startingpoint of the outlet port represents a “built-inpressure ratio” on each stage. This pointshould, therefore, theoretically shift for eachpressure ratio. In practice, however, this typeof pump can be operated across a relativelywide range of pressure ratios.


Location of inletand outlet ports

Number of inletand outlet ports

Liquid ring vacuum pumps must be capable ofoperating over a very wide pressure range. If,for example, during a pull-down on a vacuumvessel, the suction pressure, after initiallyequalling the atmospheric pressure, islowered to 33 mbar, the pressure ratio risesfrom 1 to approximately 30.

When the ratio of the pressures produced bythe stages of liquid ring vacuum pumpsexceeds a value of approximately 7, as in thecase of vacuum pumps designed for a suctionpressure of less than 150 mbar, there are twopossibilities:

Either the outlet ports are fitted with some sortof valve, allowing automatic adjustment of thestarting point of the outlet port to the pressureratio, or the liquid ring vacuum pump isdesigned as a two-stage pump, so that thetotal pressure ratio is divided between twostages. Figure 7 shows a longitudinal sectionof a two-stage liquid ring vacuum pump.

The valves can be ball or plate type, the lattersubdivided into rigid or flexible plates. Thesealing elements of the valve are moved as aresult of the different pressures in front of and


Wide pressurerange

Fig. 7:Longitudinal sectionthrough a two-stage,single acting liquidring vacuum pump

behind the guide plates. Figure 8 shows how theball port valves work: when the pressure in theimpeller blade cells exceeds the pressure in thedischarge chamber, the ball lifts off the valveseat, allowing the gas to escape from theimpeller blade cells into the discharge chamber.As soon as the pressure in the dischargechamber rises above that in the impeller bladecells, the ball is pressed onto the valve seat andstops both gas and liquid from flowing back intothe impeller.

Gas and liquid flow passagesPart of the liquid forming the liquid ring is ejectedfrom the outlet ports together with the gas. Aliquid ring gas pump must therefore be suppliedwith an uninterrupted flow of service liquid.

The service liquid is supplied through the serviceliquid connection at a pressure usually equal to,or slightly lower than that at the pump discharge nozzle.

Gas and liqiud flow passages 13

Supply ofservice liquid

Outlet port

Valve bore

Fig. 8:Operating principle ofa ball port valve

After the gas/liquid mixture has beendischarged from the pump, the liquid can beseparated from the gas in a separator andthen re-used as service liquid after havingbeen cooled sufficiently, or after having beenmixed with an adequate amount of cool liquid.

Cooling is necessary because most of the heatgenerated during compression and thevarious condensation processes is absorbedby the liquid. Only in cases where it is justifiedfrom an economic viewpoint is the liquid notcirculated for re-use. If there is no economicnecessity to separate the gas from the liquid,the installation of a separator may bedispensed with.

By means of a suitably designed separator,the portion of liquid contained by the gasleaving the separator is kept small.


Fig. 9:Liquid ring vacuumpump with separatorand heat exchanger.

Re-use ofthe liquid

Figure 10 shows the direction of gas andliquid flow in a liquid ring gas pump andseparator. The mode of operation depicted isreferred to as “combined liquid operation”.With this mode of operation, part of the liquidseparated from the gas in the separator is re-used as service liquid.

The separator shown in figure 10 may also beof a different design. It can be mounted on topof the discharge nozzle, or serve as abaseplate for the vacuum pump or thecompressor and the motor.

Working rangesLiquid ring gas pumps are built for inletvolumetric rates of flow ranging fromapproximately 1m3/h to more than 20000m3/h. The physical properties of the serviceliquid are particularly critical with regard to

Working ranges 15

Fig. 10:Direction of gasand liquid flow

Designs of liquid separators

Outlet for liquid

Separator

Circulating liquidFresh liquidService liquid

Liquid ring gas pump

Gas

Gas

Gas/liquid mixture

the suction pressures vacuum pumps canachieve. If the service liquid is water at 15˚c,suction pressures of up to 33 mbar atrelatively good suction volumetric flow ratesare feasible.

It should be noted in this context that thedischarge pressure is equal to or higher than atmospheric.

Lower suction pressures can be achieved bycombining the liquid ring gas pump with a gasejector pump, a vapour ejector pump or alobular pump. (Roots blower type)

At atmospheric suction pressure, liquid ringcompressors work up to an absolutedischarge pressure of approximately 9 bar,dependent on the design.

The working ranges of liquid ring gas pumpsare shown in figure 11.


Liq

uid

rin

g va

cuum

p

ump

s co

mb

ined

with

ga

s ej

ecto

r va

cuum

pum

ps

Liq

uid

rin

g va

cuum

pum

ps

Liq

uid

rin

g co

mp

ress

ors

1 10 100 1000 mbar

100000

10000

1000

100

10

1

m3/h

Suction pressure 1 10 barDischarge pressure

Inle

t vo

lum

etri

c ra

te o

f flo

w

Effect on thesuction pressure

Fig.11:Working ranges ofliquid ring gas pumps

Operating characteristicsThe determining factors for the characteristicsand the operating behaviour of liquid ring gaspumps are the method of energy transferfrom the liquid ring to the medium beingpumped, and the question of whether themedium comes into contact with the serviceliquid.

Inlet volumetric rate of flowThe inlet volumetric rate of flow Vtheortheoretically possible for a pump working as apositive displacement compressor is equal tothe product of the impeller blade cell volumeVcells available to the gas during eachrevolution of the impeller and the speed n:

with Vtheor expressed in m3/h, Vcells in m3, andn in r.p.m.

In practice, however, the inlet volumetric rateof flow is smaller because the impeller cellsare never entirely emptied of gas, and flowlosses in the suction and discharge ports aswell as clearance losses are to be considered.

If the gas drawn into the impeller blade cells isdry, it will try to saturate itself with vapourfrom the service liquid as soon as it hasentered the pump. The saturation processcauses the temperature of the gas and theliquid to drop, because heat is required for thevapour to form. As a result, the spaceavailable for the gas in the impeller blade cellsis reduced by the portion taken up by thevapour.

The ratio of the portions of space taken up bythe gas and the vapour is expressed by


Theoretical inletvolumetric rateof flow

(1)V theor =.

60 x Vcells x n

.

.

.

Dalton’s law of partial pressures, which statesthat in a mixture of gaseous substances, theratio of the individual volumes equals that ofthe ratio of their partial pressures:

where V stands for volume, and p forpressure, with the indices G for gas and D forvapour; p1 is the suction pressure.

The inlet volumetric rate of flow VL indicated inlists and catalogues is usually based on dry airof 20ºC as the pumped medium, and water of15ºC (20ºC in the case of compressors) as theservice liquid. if the aspirated dry air has atemperature other than 20ºC, the ratio of theabsolute temperatures of air and service wateras well as the vapour portion of the air/watervapour mixture in the impeller blade cellschanges accordingly. The result is a differentinlet volumetric rate of flow. The same applieswhen the service water temperature deviatesfrom the indicated values.

Assuming that the temperature t1 of thepumped air has changed to temperature t3 ofthe service water by the time it enters theimpeller blade cells, and that Dalton’s law isstill applicable to air/water vapour mixturesand to this case in particular, the inletvolumetric rate of flow V1 for an airtemperature other than 20ºC and a servicewater temperature other than 15ºC would intheory be calculated according to equation (3)as follows:


Dalton’s law V

= =G

VD

PG

PD

P1

PD

PD

–

V1

t1

t3

20–

273+1+V

L=p

1p

D

p1

–

– 17.04

. .

(2)

(3)

.

.

Here, the figure 17.04 stands for the vapourpressure (in mbar) of water at a temperatureof 15ºC.In practice, however, the change in the inletvolumetric rate of flow does not fully complywith these rules. Because they are only validfor ideal gases, there is only a limited amountof time and space available for the vapour toform, and are counteracted by certainconditions specific to the pump itself.To allow for the dependency of the inletvolumetric rate of flow on the actual vapourpressure of the service liquid to be taken intoaccount, empirical equations were developedfrom the results of measurements. The inletvolumetric rate of flow is therefore calculatedfor service liquid temperatures other than15ºC as follows:

For single-stage vacuum pumps and forcompressors

and for two-stage vacuum pumps

p1 = suction pressure in mbarpD = vapour pressure of the service liquid in

mbarV1 = inlet volumetric rate of flow in m3/hVL = inlet volumetric rate of flow in m3/h

when pumping dry air (20ºC) withwater (15ºC) as the service liquid.


Effect of thevapourpressure on theinlet volumetricrate of flow

V =.1

V.L.

Iλ

p1p1 1(0.27 In p – 0.0783) – 1.05 17.04

=Iλ

one ..

.1(0.27 In p – 0.0783)–1.05 pD

.

p p1 Dp1 1(0.35 In p – 0.1) – 17.04

=Iλ

two .1(0.35 In p – 0.1)–.

(4)

(5)

(6)

.

.

. ..

.

. .

.

.

.

The results of equations (5) and (6) are meanvalues. These apply to water as the serviceliquid, an air temperature t1 of 20ºC, a valuefor pD of between 17 and 123 mbar, and a p1of between 33 and 1013 mbar.

An excessive amount of vapour in thepumped liquid can lead to cavitation in thepump and to damage of pump components,in particular the impellers and guide plates.From practical experience it is known that acertain volumetric flow rate of air understandard conditions or some other non-condensible gas has to be present in the liquidpumped to avoid cavitation. This flow isindependent of the service watertemperature, yet dependent on the size andthe rotational speed of the pump.

The values obtained from equations (5) and(6) are depicted in graphical format in figure12. The line designated as the cavitation limitis to be considered a standard value forvacuum pumps capable of reaching a suction


Cavitation

30 40 50 60 80 100 150 200 300 400 500 600 800 1000 mbar

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

Single stage pump

Two stage pump

Suction Presure

15ºC Service water temperature

50ºC

40ºC

30ºC

25ºC

20ºC

λ Ι

Cavitation Limit

17ºC

Fig. 12:Effect of the servicewater temperatureon the inletvolumetric rate offlow

pressure of 33 mbar free from cavitation whenhandling dry air.

To avoid damage due to cavitation, liquid ringvacuum pumps are equipped with cavitationprotection in the form of a flow of a non-condensable medium which is eitherreturned or supplied to the impeller cells via a connection provided for this purpose. This connection is normally connected to thegas chamber of the separator.

If the pumped medium is a dry gas or avapour which is not condensed in the suctionchamber, the inlet volumetric flow rate is onlyaffected by the temperature of the mediumpumped, if we leave its solubility in theservice liquid out of consideration. Throughmeasurements, the contributory effect hasbeen determined, allowing us to calculate theinlet volume flow rate on handling dry air witha temperature t1 from the inlet volume flowrate of dry air at 20ºC.

The inlet volumetric rate of flow of dry air witha temperature t1 and water with a temperaturet3 is thus calculated using the followingequation:

where

Figure 13 shows a graphic illustration of factorλIII as a function of the air temperature forseveral service water temperatures.


Protectionagainstcavitation

High gastemperatureincreases theinlet volumetricrate of flow

V =.1

V.L.

Iλ .

IIIλ

IIIλ 0.66

= 1(t – 20)1+

3t + 273

(7)

(8)

. .. .

The temperature of the medium pumpedshould not exceed certain maximum values.Depending on the pump size, the maximumpermissible temperature for volumemanufactured liquid ring gas pumps isbetween 60 and 200ºC. The permissiblemaximum is also influenced by the mediumbeing pumped.

If the medium pumped contains vapourswhich condense in the pump, the inletvolumetric rate of flow is higher than whenhandling dry gases. In this case, condensationtakes place in the suction chamber or in theimpeller cells which are connected to thesuction chamber.


1.5

1.4

1.3

1.2

1.1

1.0

0.90 50 100 150 200

Temperature of the air entering theliquid ring gas pump

λ ΙΙΙ

Servic

e wate

r tem

peratu

re 15

ºC 30ºC

45ºC

ºC

Fig.13: Inletvolumetric flow rateas a funtion of thetemperature of themedium pumpedwhen pumping dry gas

Condensingvapoursincrease theinlet volumetricrate of flow

The increase is dependent on the inletpressure, the temperature of the gas/vapourmixture, the temperature of the service liquidand on the size of the pump.

On the basis of measured values, an empiricalequation has been developed to calculate theincrease of the inlet volumetric rate of flow ofwater vapour, saturated air and watercompared to the inlet volume rate of flow fordry air with a temperature of 20ºC.

where

d = impeller diameter in mh = impeller width impinged with gas/vapour

through an inlet port expressed in mp1 = suction pressure in mbarpD = vapour pressure of the service liquid

in mbarpS = vapour pressure of water at the

temperature of the air/water vapourmixture in mbar

V1 = inlet volume rate of flow in m3/hVL = inlet volumetric rate of flow when

handing dry air (20ºC) with water (15ºC)as the service liquid in m3/h.

Equation (9) is valid for air/water vapourmixtures and values for pD and pS of between17 and 123 mbar, and for a p1 of between 33and 1013 mbar.


Empiricalequation on thebasis ofmeasured values

IIλ =

11[0.75 p (In p – 0.2877)]E.

11[0.75 p (In p – 0.2877)]E–0.75 pS. .

E 0.082 0.793+=pD

17.04. h

d

0.0369

V =.1

V.L.

Iλ .

IIλ (9)

(10)

(11)

.

.

. .. .

.

. .

.

Figure 14 shows factor II as a function of theabove-mentioned contributory factors for adesign size h/d of 0.43.

As far as the cavitation limit is concerned, therule also applies here that a certain volumetric

λ


Air/water vapourmixture temperature

Service watertemperature 15ºC

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.030 40 60 80 100 200 400 600 800 1000

Suction pressure mbar

50ºC40ºC

30ºC

20ºC

λ II

2.2

2.0

1.8

1.6

1.4

1.2

1.030 40 60 80 100 200 400 600 800 1000




50ºC40ºC

30ºC

20ºC

λ II



50ºC

40ºC

30ºC

20ºC

30 40 60 80 100 200 400 600 800 1000Suction pressure mbar

1.8

1.6

1.4

1.2

1.0

λ II

rate of flow of air, or of some other non-condensable gas under standard conditions,has to be present in the product being pumped.

In most applications the service liquid iswater. However, in the field of processtechnology, it is often necessary to use liquidswhose chemical and physical properties arevery different from those of water. The serviceliquid varies according to the processrequirements. It is important to note that thevapours contained by the medium pumpedcondense for the most part inside the pumpand are discharged through the dischargenozzle together with the gas/liquid mixture inthe form of condensate. The obvious choiceof a service liquid in this case is a liquid whichis already available as condensate.

Where the physical properties of the serviceliquid vary from those of water, this has aneffect on the inlet volumetric rate of flow, thepump input power, the service liquid flow andon the temperature of the mixture of gas andliquid on the discharge side.

Considering the fact that the vapour pressureis determined by the type of liquid, and thatit increases with rising temperature, the inletvolumetric rate of flow is dependent on thetype of liquid, particularly in the case of lowsuction pressures. The inlet volumetric rateof flow theoretically tends to zero as soon asthe inlet pressure becomes equal to thevapour pressure of the service liquid which,therefore, represents the lower physical limitof the attainable suction pressure.


Different serviceliquids

Fig.14: (opposite): Effect of the medium pumped on thevolumetric rate of flow when pumping water-vapour-saturated air

The power Pis required for the isothermalcompression of a gas from an inlet pressurep1 to a discharge pressure p2 is proportionalto the suction pressure, the inlet volumetricrate of flow V1 and the natural logarithm of thecompression ratio:

wherep1 = suction pressure in mbarp2 = discharge pressure in mbarPis = isothermal compression rating in kWV1 = inlet volumetric rate of flow in m3/h

As the compression power is derived from theenergy generated by the rotary movement ofthe liquid ring, the amount of energy itcontains should be at least equivalent to theisothermal compression rating.

In order to evaluate the amount of energycontained by the liquid ring, it can beassumed that it varies in proportion to thevolume of the rotating liquid, its density andto the square of the rotational speed of theliquid ring. At a given impeller speed, thedensity of the service liquid is thereforecritical for the energy contained in the liquidring, and consequently for the possiblecompression power.

In addition to this, the inner limit of the liquidring is influenced both at the suction anddischarge end. As a result, the inlet volumetricrate of flow and the power requirement aredependent on the density of the service liquid.On the basis of the similarity law, it is possible


Isothermalcompressionpower Pis =

V.. .

.

p1

3.6 104

p2p1

1 In (12)

.

.

. ..

.

to calculate the inlet volumetric flow rate andthe power required for a change in density ofthe service liquid from given operating data.

If the service liquid consists of immis-cible components and the densities of itscomponents vary, they will form a layeredliquid ring. The inlet volumetric rate of flowthen depends on whether the portion withthe lower boiling point has the lesser orgreater density.

In general, the viscosity of the service liquidhas only a minor effect on the inlet volumetricrate of flow. The improved sealing effect inthe clearances between the impeller and theguide plate which comes with a higherviscosity is lost, because the rate of flow ofthe service liquid entering the flow is smallerthan usual.

At 20ºC, water has a specific heat of 4.183 kJ/(kg K). The liquids used in process technologyoften have a lower heat capacity. As a result,the temperatures of the liquid ring and thegas/liquid mixture exiting the discharge nozzleare higher.

The evaporation heat is the physical quantitywhich is critical for the heat flow generatedby the condensation of the vapour particles inthe pumped medium, the greater portion ofwhich is absorbed by the liquid ring, and forthe heat required for saturation, which ismainly taken from the liquid ring when drygases are handled.

The data indicated in lists and cataloguesapply to the compression of air. Liquid ringgas pumps, however, are suitable for handlingmost media in the gas or vapour state as wellas mixtures of the two.


Similarity lawenablesconversion

Influence of theviscosity

Specific heatcapacity

Evaporationheat

The following properties, conditions andeffects of the medium pumped have a directinfluence on the inlet volumetric rate of flow:

• temperature• saturation state• condensation in the suction chamber• density (usually significant only in the case

of hydrogen and helium, and gas mixturesincluding these gases)

• solubility in the service liquid• entrainment of liquid with the gas flow

(other than the service liquid)• reaction with the service liquid

The following have an indirect effect:• condensation during compression• specific heat capacity• evaporation heat

Power requirementThe power input required by liquid ring gaspumps depends only to a negligible extent onthe vapour pressure of the service liquid. Theviscosity of the service liquid affects thefriction losses and thus also the power input. Ifpower input PL is known for water as theservice liquid, input power P can be calculatedby solving equation (13) for a meancircumferential speed of the impeller and akinematic viscosity v of the service liquid of upto 6mm2/s:

Impeller speedThe speed of the impeller determines the inletvolumetric rate of flow and the power input ofthe pump. Figures 15 and 16 show the inlet


Effects of themedium pumpedon the inletvolumetric rate offlow

Characteristiccurves

P P= υ.L0.05 (13).


n normal x 1.4

n normal x 1.2

Speed n normal

n normal x 0.8

n normal x 1.4

n normal x 1.2

Speed n normal

n normal x 0.8

Po

wer

inp

ut

kW

Inl

et v

olu

met

ric

rate

of f

low

m

3 /h

30 40 60 100 200 400 600 1000


Po

wer

inp

ut

kW

Inl

et v

olu

met

ric

rate

of f

low

m

3 /h

1000 1500 2000 2500

Absolute discharge pressure mbar

n normal x 1.35

n normal x 1.2

Speed n normal

nnormal x 0.8

n normal x 1.35

n normal x 1.2

Speed n normal

n normal x 0.8

Fig. 15:Inlet volumetric rateof flow and powerinput of a liquid ringvacuum pump atdifferent speedsMedium pumped: air,20ºC; service liquid:water, 15ºC;discharge pressure1013 mbar

Fig. 16:Inlet volumetric rateof flow and powerinput of a liquid ringcompressor atdifferent speedsMedium: air, 20ºC;service liquid: water,20ºC; suctionpressure 1013 mbar

volumetric rate of flow and the input power asa function of speed n for a vacuum pump anda compressor, respectively.

Discharge pressure and suctionpressureThe discharge pressure of liquid ring vacuumpumps and the inlet pressure of liquid ringcompressors can deviate considerably fromthe atmospheric pressure prevailing undernormal circumstances, and can thereforeresult in a change in inlet volumetric rate offlow and pump power input. If the differencein pressure levels results in a high mechanicalloading of components, its acceptabilityshould be examined.

Figures 17 and 18 show the inlet volumetricrate of flow and the pump power input atconstant speed using water as the serviceliquid for a single-stage vacuum pump atseveral discharge pressures, and for a single-stage compressor at several inlet pressures,respectively.

Outlet temperatureWith vacuum pumps and compressors, all thepower needed to compress gaseoussubstances is converted to heat.

If the medium contains vapours whichcondense in the suction chamber or duringcompression, condensation heat is generatedwhich has to be eliminated.

The heat produced inside the pump causedby the medium reacting with the service liquidalso has to be eliminated.

If liquid is pumped with the gas, itstemperature change releases a heat flow. It is


Variations fromatmosphericpressure

Generation ofheat

Operating characteristics 31P

ow

er in

put

k

W

I

nlet

vo

lum

etri

c ra

te o

f flo

w

m3 /

h

30 40 60 80 100 200 400 600 1000Suction pressure mbar

Po

wer

inp

ut

kW

Inl

et v

olu

met

ric

rate

of f

low

m

3 /h

Discharge pressure 1013 mbar1500 mbar

2000 mbar

2500 mbar

Cavitation limit

2500 mbar

2000 mbar

1500 mbar

1013 mbar

Cavitation limit

Inlet pressure 1800 mbar

1400m

bar

1000 mbar

800m

bar

Inlet pressure 800 mbar 1000 mbar 1400 mbar

1800 mbar

1000 2000 3000 4000Absolute discharge pressure mbar

Fig. 17:Inlet volumetric rateof flow and powerinput of liquid ringvacuum pump atdifferent dischargepressuresMedium pumped:air, 20ºC; serviceliquid:water, 15ºC

Fig. 18:Inlet volumetric rateof flow and powerinput of a liquid ringcompressor atdifferent suctionpressuresMedium pumped:air, 20ºC; serviceliquid:water, 20ºC

only in the case of extreme differences intemperature that any heat worth consideringdissipates via the surface of the pump.A large portion of the heat generated in liquidring gas pumps is transferred to the liquid ringand discharged together with the gas/liquidmixture. The following heat flow rates are aresult of this process:

Compression rating and friction losses:

P= pump power input in kWQV= heat flow rate in kJ/h

Heat exchange between the medium pumpedand the service liquid:

whereQG = heat flow rate kJ/hcp = specific heat capacity of the product

pumped in kJ/(kg K)m

G = mass flow of the gas in kg/ht1 = temperature at the suction nozzle in °C

t2 = temperature at the discharge nozzle in °C

Condensation heat:

wheremD1 = mass flow of condensing vapour in

kg/hmD2 =mass flow of the vapour escaping

through the discharge nozzle in kg/hr = evaporation heat in kJ/kgQK = heat flow rate in kJ/h

If both the cooling of the vapour to the


Heattransferred tothe liquid ring

QG =. . .

Gm

pc 1(t 2t– )

QV =. .P 3600 (14)

(15)

(16)

.

.

.

.

.

.D1

m.D2

mQK =. .r ( – ). . .

. .

.

.

.

.

condensation temperature, and the cooling ofthe condensate to the temperature of the liquidring plus any reaction heat available are nottaken into consideration, the heat flow rate Qabsorbed by the liquid ring is calculatedusing equation (17) as follows:

If the mass and heat flows, or their equivalents,flowing in and out of the pump are inequilibrium, the following applies:

whereQ = heat flow rate in kJ/hB = service liquid flow rate in m3/hρ3 = density of the service liquid in kg/m3

c = specific heat of the service liquid inkJ/(kg K)

t2 = temperature at the discharge nozzle in °Ct3 = temperature at the service liquid

connection in ºC

This results in a temperature increase in theliquid ring gas pump of:

Heat flow rates QG and QK can only becalculated if temperature t2 in the dischargenozzle of the pump and the vapour pressure ofthe substances present in the vapour state atthis temperature are known. Temperature t2can, therefore, not be directly derived fromequation (19). In practice, it is determined bymeans of an iterative calculation which can beperformed after establishing the coherence ofthe temperature and the vapour pressure in theform of an equation.


Heat flow rate

Temperatureincrease

(17)=Q

.Q.

V+ Q.

G+ Q.

K

Q =. . ..B c3 2

(t3

t– )ρ(18)

Q.

= = . .B c32t 3t– ρ∆ t

(19)

.

.

.

. . . .

.. . .

.

. .

As an example, figure 19 shows the differ-ential temperature ∆t = t2 – t3, as a function ofthe suction pressure for a liquid ring vacuumpump handling dry and water-vapour-satur-ated air at t1=20ºC.

Similarity lawThe similarity law states that withgeometrically similar vacuum pumps orcompressors, if the pressure ratio p2/p1 andthe k-value are equal, the compressionprocesses are similar, and the impellerutilisation and the isothermal efficiency arealso equal. In this context the term impellerutilisation λR is defined as the volumetricefficiency of an impeller.

d = impeller diameter in mb = impeller width in mn = impeller speed in r.p.m.


30 40 60 80 100 200 400 600 800 1000 Inlet pressure mbar

12

10

8

6

4

2

0

Pumped medium: Air/water vapour mixture

Dry air

Fig. 19:Differentialtemperature∆t = t2 – t3 for aliquid ring vacuumpump handling drywater-vapoursaturated air

Impellerutilisation

λ V.

.R =

601

..4

1π

b .nd2(20)

.

.. . .

The isothermal efficiency ηis the ratio of the isothermal compression power tothe pump power input. Equation (21) gives the following:

where

p1 = suction pressure in mbarp2 = discharge pressure in mbarV1 = inlet volumetric rate of flow in m3/hP = pump power input in kW

The k-value is the ratio of the isothermalcompression power to the power of therotating liquid ring, and hence a relativemeasure indicating to what extent the energyof the liquid ring is consumed in generatingthe compression power:

where

p1 = suction pressure in mbarρ3 = density of the service liquid in kg/m3

u = circumferential speed of the impellerin m/s

In equation (22), the inlet volumetric rate offlow cancels out the volumetric flow rate ofthe liquid ring, and the natural logarithmfrom the pressure ration p2/p1 is taken to beequal to 1.


Isothermalefficiency

k-value= ..

p1

23

ρk 102

u2

η =is

V..

.

.

.

p1

3.6 104

p2p1

1 In

P

(21)

(22)

.

.. .

. .

.

.

Operating modesThe following methods detail how the serviceliquid is handled outside the pump.

Figures 20 to 22 show some examples ofsingle pumps and separators of standarddesign. Generally, the illustrations also applyto separators of different design and toarrangements of several pumps connected toa common separator. A distinction is madebetween three modes of operation which arebriefly described below:

• combined flow operation• recirculation flow operation• make-up liquid operation

Combined flow operationCombined flow operation is the modecommonly used under normal circumstances.The flow of make-up liquid is limited to thecapacity required to remove the heat.

The arrangement is shown in figure 20:Service liquid B consists of a mixture of make-up liquid F and circulating liquid U. The make-up liquid is tapped from the mains supply,whereas the circulating liquid is obtained fromthe separator. Before entering the serviceliquid connection, the two flows U and F aremixed to form flow B. The required make-upliquid flow F is calculated using the followingequation:

where

F = make-up liquid flow rate in m3/hB = service liquid flow rate in m3/h


Small supply ofmake-up liquidrequired

Make-up liquidflow rate

BF = 2t

3t–.

2t

4t–

(23).

t2=temperature at the discharge nozzle in ºCt3=temperature of the service liquid in ºCt4=temperature of the make-up liquid in ºC

The discharged liquid flow A is the sum of thefollowing: make-up liquid flow F, vapourportions of the pumped product condensingin the pump, and any liquid portions of theproduct flow available.

If the discharge pressure is higher thanatmospheric, the drainage liquid either has tobe removed via a liquid trap, or the separatormust be fitted with a regulating device toensure that the liquid is maintained at therequired level.

Operating modes 37

uA

F

Liquid level

mU

XBp

MII

M I

uC uMII tMI

uMI

ul

PG

uml

ue

uBuseuelB

mU

uU

XBp

uA

A

uMIIuB

PG

uMI

uB

tMI

tB

UF

lF hF i F

Fig. 20:Combined flowoperationA Drainage liquidB Service liquidF Make-up liquidM1 Medium

pumped-suctionside

M11 Mediumpumped-suctionside

PG Liquid ringvacuum pumpor compressor

XBp SeparatorU Circulating

liquidhF Shut-off valveiF Regulating valvelB Service liquid

linelF Make-up liquid

linemU Liquid level tubet ThermometertM1 Non-return

valveuA Liquid outletuB Service liquid

connectionuc Cavitation

protectionue Drainu1 Connection for

vent clockuM1 Suction line

connectionuM11 Discharge line

connectionuml Connection for

drain valveuse Dirt drainuU Circulating flow

connection

Recirculation flow operationThis mode of operation is used for processesinvolving corrosive, waste-water-polluting orhazardous fluids, as well as in condensaterecovery systems.

Figure 21 illustrates the principle of operation.In this mode of operation, the liquid separ-ated from the product flow is re-used asservice liquid. The liquid contained in thesystem is therefore continuously recirculated(closed circuit).

During continuous operation, the liquidheated up during compression has to be


Fig. 21:Circulating flowoperationA Drainage liquidB Service liquidK Cooling liquidF Make-up liquidM1 Medium

pumped-suctionside

M11 Mediumpumped-discharge side

PF Liquid pumpPG Liquid ring

vacuum pumpor compressor

XBp SeparatorbK Heat exchangerhK Shut-off valveiK Regulating valveiU Regulating valvelB Service liquid

linelK Cooling liquid

linemB Manometer

vacuum guagemU Liquid level tubet ThermometertM1 Non-return







drain valveuse Dirt drainuU Circulating flow

connection

uA

Liquid level

mU

XBp

MII

MI

uC uMII tMI

uMI

ul

PG

uml

ue

uBuseuelB

mUuU

XBp uA

A

uMIIuB

PG

uMI

uB

tMI

t B

U

lB PF i U

uUbK

K

mB

bK

i K

hK

lK

cooled by means of a heat exchanger installedin the circulating flow line. The pressure dropof the heat exchanger has to be small if thereis no liquid pump installed in the circulatingflow line.

The heat flow rate to be discharged throughthe heat exchanger is calculated according toequation (17).

A liquid pump must be installed in thecirculating flow line to boost the pressurewhenever the viscosity of the service liquid ishigh (>2mm2/s), or if the liquid ring gas pumpis operated with a small differential betweenthe discharge and suction pressures.

The portions of the flow condensing in theliquid ring gas pump as well as the liquidconstituents of the product being pumped willdrain out of the separator. If the dischargepressure is higher than atmospheric, eitherthe drainage liquid is removed via the liquidtrap, or the separator is fitted with some sortof control device in order to maintain therequired liquid level.

If the gas outlet from the separator (productfrom the discharge line MII) has a highervapour mass flow rate than the productentering the pump (MI), the difference has tobe made up to prevent the liquid level in thepump from dropping below a given minimumlevel. Gas must be prevented from enteringthe recirculation line.

Make-up liquid operationThis mode of operation is used when re-useof the service liquid as such is not required. Inthis case, the entire volume of service liquidneeded to operate the system is taken fromthe mains water supply.

Operating modes 39

Booster pump

Monitoring theliquid level

The principle of operation is shown in figure22. Drainage flow A is the sum total of thefollowing: Make-up liquid flow F, the vapourportions condensing in the pump, and anyliquid constituents contained in the product. Ifthe discharge pressure is higher thanatmospheric, either the drainage flow has tobe removed via a liquid trap, or the separatormust be fitted with some sort of controldevice for the purpose of maintaining therequired liquid level. There is no need for aseparator as long as the medium beingpumped and the liquid do not have to bedrained separately.


Fig. 22:Make-up liquidoperationA Drainage liquidB Service liquidF Make-up liquidM1 Medium

pumped-suctionside

M11 Mediumpumped-discharge side

PG Liquid ringvacuum pumpor compressor

XBp SeparatorhF Shut-off valveiF Regulating valvelB Service liquid

linelF Make-up liquid

linemB Manometer

vacuum gaugemU Liquid level tubet ThermometertM1 Non-return







drain valveuse Dirt drain

uA

F

Liquid level

mU

XBp

MII

MI

uC uMII tMI

uMI

ul

PG

uml

ue

uBuseuelB

mU

XBp

uA

A

uMIIuB

PG

uMI

uB

tMI

tB

F

lF hF i F mB

Drive.MotorsLiquid ring gas pumps are normally driven bythree-phase electric motors suitably protectedin accordance with the operating conditions.For higher drive ratings, high-voltage motorsare used. Other prime movers such as internalcombustion engines or steam turbines canalso be used.

Torque transmissionAs liquid ring gas pumps have a relativelyuniform torque, its transfer to the pump doesnot call for any sophisticated technical solutions.

The small to average pump sizes are usuallydirect driven via a flexible coupling. Largerpumps running at a relatively low speed arenormally driven by a four-pole motor and agear to reduce the pump speed.

The types of reduction drives used are gearboxes, V-belt or flat-belt drives.

Belt drives generate a radial load acting on theshaft and bearings which is dependent on thepower to be transmitted, the speed and thediameter of the pulley, and on the initialtension of the belt. For these reasons, and inorder not to reduce the operating life of thebelt, the minimum diameter of the pulley hasto be specified.

As the single-acting stage of a liquid ringvacuum pump also exerts a radial force on thepump shaft, the direction of pull of the belt isimportant. The overhung belt load can resultin a higher bearing load and thereforeincrease the deflection of the shaft, or canlower the load on the bearings and thereforereduce the deflection of the shaft.

Drive 41

Direct drive bymeans of anelectric motor

Belt drive ispossible

Moment of inertia and load torqueVacuum pumps and compressors haverelatively low moments of inertia and loadtorques. The moment inertia is specific to themachine. Besides being dependent on thepump size, the torque during start-up alsodepends on the liquid level inside the pump,the type of liquid, and on the pressuresprevailing in the suction and dischargenozzles during the starting phase.

Start-upGenerally speaking, direct-on-line startingis the preferred starting method. Motors withstar-delta-starting can be used, if therequirement is for an extremely low start-ing current. However, it must be checkedwhether the torque of the star-connectedmotor is sufficient.


Fig. 23:Package Unit with alobular vacuumpump and a liquidring vacuum pump

Preference givento d.o.l. starting

By using a soft starter, the current surgeaccompanying the start-up of an electricmotor is avoided, and the drive can be startedwith an almost constant torque. Soft startersor acceleration rate controllers are also usedas a starting aid for the drives of large liquidring vacuum pumps with magnetic couplings.Soft starting prevents decoupling of themagnetic coupling.

Start/stop frequencyThe permissible start/stop frequency dependsmore on the drive than on the pump. As aresult of the high starting currents needed forthe start-up, electric motors become very hot.Acceleration of the rotating element, and inmany cases the gears, leads to high loadsacting on the couplings.

Under normal circumstances, 15 starts perhour are considered acceptable. Underexceptional conditions, consultation with thecoupling and motor manufacturer isrecommended. When starting larger pumps,the loads on the mains power should also beconsidered.

Controlling the inlet volumetricflow rate Against the background of the rising cost ofelectricity, the end-users of vacuum pumpsand compressors increasingly analyse thecommercial efficiency of their machines witha view to minimising running costs.

If the inlet volumetric flow rate is notcontrolled, the pump automatically runs up tothe suction pressure at which the processvolumetric flow rate is equal to the inletvolumetric flow rate. This can entail an


High frequencyof start/stopcycles

unnecessarily high power and cooling waterconsumption.

Figure 24 shows how the suction pressure ofa liquid ring vacuum pump responds to achange in the process volumetric flow rate.The different types of control generally used

with liquid ring gas pumps to adjust the inletvolumetric rate of flow to specified operatingconditions are explained below. Measures forsaving energy are highlighted.

Speed ControlBy controlling the speed, it is possible toadjust the inlet volumetric flow rate to suit therequirements of the installation and saveenergy at the same time. However, certainlimiting parameters must be observed whenselecting the speed.


Fig. 24:Effect of a change insuction pressure onthe volumetric rate of flow

Energy Saving

Pressure decrease

Process volumetric flow rate

Pressure increase

Design point

Inlet volumetric flow rate of a

liquid ring vacuum pump


Volu

met

ric

flow

rat

e

m3/htoo high

as designed

too low

1000

The minimum speed is determined by thecircumferential speed of the impeller requiredto produce the necessary compression power.In this context, it is to be noted that themaximum compression power of a vacuumpump is given at a suction pressure ofapproximately 400 mbar, if the dischargepressure is 1013 mbar (compare to figure 15).

The maximum speed is determined by theload-carrying capacity of the rotatingcomponents in the pump, in particular theshaft and the impeller.

As well as a change in the compressionpower, a change in the speed will also resultin a change in the power loss. On average,liquid ring vacuum pumps are characterisedby an exponential dependency of the pumppower input on the circumferential speed ofthe impeller and, therefore, on the speedof rotation. In order to save energy, it istherefore important to select as low a speedas possible.

The inlet volumetric rate of flow can becontrolled by speed in the range from, say, 50to 100% of the maximum inlet volumetric flowrate. In practice, however, the control range isusually much smaller, because the liquid ringgas pumps available are not ideally suited tothe individual pumping requirement of theapplication.

Controlling the service liquidtemperatureThe costs of make-up and cooling liquidsrepresent a major portion of the total runningcosts. By selecting a suitable method ofoperation and ensuring that there is a largedifference between the temperatures of theservice liquid and the make-up liquid or the


Minimum speed

Maximum speed

Control range

cooling liquid, and if the control is effected bymeans of the make-up or cooling liquid flowrates, it is possible to reduce the running coststo a minimum.

Furthermore, it must be remembered that the inlet volumetric flow rate of a liquid ringvacuum pump also depends on the vapourpressure and therefore on the temperature ofthe service liquid. By making the make-upliquid flow rate constant, the efficiency of theunit cannot be maximised. This flow must beadjusted automatically. In practice,thermostat-type control dependent on suctionpressure is the better solution, although it ismore complicated.

Attempting to save liquid by reducing theservice liquid flow rate would result in anuncontrolled reduction of the inlet volumetricrate of flow, which could lead to unacceptablevibration of the pumps

Connecting liquid ring gas pumps in parallelBy sharing the process volumetric flow rateamong several pumps, pump operation canbe adapted more efficiently to therequirements of the installation. In addition tothe regulating aspect. there is the advantageof operating at a reduced flow rate should onepump fail.

Bypass controlThis is the type of control most frequentlyused, because it allows the processvolumetric flow rate to be set from zero to amaximum. This method of control is notoptimal if energy saving is the objective.

Figure 25 shows a typical example of bypasscontrol: the excess gas discharged by the


Automaticadjustment ofthe make-upliquid flow rate

Increasedavailability

liquid ring gas pump is returned to the suctionpipe via a pipe equipped with a control valve,this being fitted between the gas outlet line ofthe separator and the pump suction pipe.

Under no circumstances should the gasbypass line be connected between thedischarge pipe containing the gas-liquidmixture and the suction pipe of the pump,because a mixture of gas and liquid ofundefined composition would enter thesuction nozzle. If the gas being handled by theliquid ring vacuum pump is allowed to bemixed with air, the discharge of this gas canbe reduced by feeding air to the suctionnozzle of the pump.

Types of constructionLiquid ring gas pumps are produced invarious configurations. The difference is in the


Liquid ringgas pump

Inletvolumetricflow rate

Processvolumetricflow rate Bypass

volumetricflow rate

Control element

Dischargevolumetricflow rate

Separator

Fig 25: Bypass control

number of stages, type of bearings, method ofsealing the shaft, the materials of the pumpcomponents and the static test pressures.

Number of stagesThe compression power to be generated aswell as the operating behaviour and economicefficiency of the stages are all critical whenselecting the number of stages required bythe pump.

Liquid ring vacuum pumps are designed withone or two stages. Liquid ring compressorsfor absolute discharge pressures of up toapproximately 3.5 bar usually have one or twostages, but two or more stages where higherdischarge pressures are required.

Shaft bearingsAs a rule, the shaft has two radial bearingsThese can be arranged either one at each endof the shaft or two at one end. Examples of ashaft arrangement with one bearing at eachend are illustrated in figures 4 to 7. Figures 26and 27 show designs with two bearingsmounted at one end of the shaft.


Single ormultistage

Fig 26: Longitudinal section of a liquid ring vacuum pumpof motor pedestal design

Shaft sealsThe type of seal is selected according to thedegree of sealing integrity required.Compared with other types of compressor,the advantage of liquid ring gas pumps is thatliquid is available at the shaft seals, and can beused for sealing or flushing purposes, forlubricating mating sliding components, or forremoval of friction heat. The types of sealusually fitted on liquid ring gas pumps aresingle or double stuffing boxes.

Glandless pumpsWhere liquid ring gas pumps are used forpumping toxic, carcinogenic or malodorousgases and vapours, or for compressingradioactive media, the requirements to be metin respect of sealing are so stringent that theycan only be fulfilled by glandless pumps. Thistype of pump is fitted with either a magneticcoupling or a canned motor.

The common characteristics of both designsare a shrouded can and a shaft running in plainbearings. Magnetic couplings basically consist


Conformingto therequirements ofoperation

Emphasis onsealingcapability

Fig 27: Longitudinal section of a liquid ring vacuum pumpof motor pedestal design

of two magnet holders, one rigidly connectedto the pump shaft, the other to the motor shaft.The can is positioned between the two magnetholders. This thin-walled tube is securedtightly to the pump casing at one end, andclosed at the other (for this reason it is alsocalled an isolation shroud). The designprinciple is illustrated in figure 28.

The magnets are made from rare earth metalcobalt alloys and rare earth metal iron alloyswhich are renowned for their superior densityof magnetic energy and their magnetic

stability. Metal cans give rise to eddy currentlosses which generate heat. The magnitude ofthe heat loss depends on the magnetic massand the speed of the coupling. To remove theheat, the inner magnet (rotor) is surroundedby service liquid.

Figure 29 shows the longitudinal section of atwo-stage liquid ring vacuum pump with amagnetic drive. The shaft is supported by tworadial bearings and a thrust bearing. The


Magnets

Rotor

Pump shaft

Cylindricalisolating body(can isolatingshroud)

Magnet bonnet

Motor shaft

Fig 28: Designprinciple of amagnetic coupling

Fig 30 (opposite): Liquid ring vacuum pump system

bearings are flushed with service liquid forlubrication and heat dissipation purposes.

Under more difficult starting conditions,where, for example, there is a high densityservice liquid, an automatic soft starter is usedto stop the magnets becoming desynchron-ised when the pump is started. It is recom-mended that the drive of a magnetic couplingbe switched off immediately after loss ofsynchronisation to prevent the componentsfrom being damaged.


Fig 29:Longitudinalsection of a twostage liquid ringvacuum pumpwith a magneticcoupling

Figure 31 shows liquid ring compressor with acanned motor capable of meeting the moststringent sealing requirements.

Materials of constructionLiquid ring gas pumps are manufactured from a combination of different materials bestsuited to meet the operational requirements(see appendix) The materials of theaccessories such as the separator, pipes andfittings are selected to suit the materials of thebasic pump design.

Static test pressuresExplosion-protected pumps are used forhandling inflammable substances (explosive


Fig 31: Longitudinal section of a liquid ring compressorwith a canned motor which meets the most stringentsealing requirements

gases or vapours). They are subjected to highstatic test pressure.

Emissions

SoundA pump in operation produces vibrations,some of which are emitted in the form ofsound. All moving solid, liquid and gaseousparts and substances are sources of sound,which travels as an air or structure-bornenoise from its source to the casing and anyother surfaces of the pump, from where theyare emitted to the surroundings in the form ofair and structure-borne noise.

The speed of the liquids and gases flowingthrough the pump is a function of the speed ofthe impeller and the suction and dischargepressures of the pump. These values,therefore, have a direct effect on the soundpower emitted. It is possible to establish arelationship between the sound power andthe sound pressure by means of the size ofthe sound emitting area.

National and international standards (e.g. DINEN 23742 and ISO 1996) specify themeasuring techniques and instruments to beused in sound measurement.

The sound pressure levels measured in twodifferent sized liquid ring vacuum pumps areshown in figure 32. The peaks of the curvesare found at a frequency which is the productof the speed and the number of impellerblades. The letters NR stand for “noise rating”.

Emissions 53

Sources ofsound

Soundpressure andsound power

Low soundpressure level

The operating data of these pumps and theA-rated sound pressure and sound powerlevels are listed in table 1.

VibrationsThe magnitude and acceptability of vibrationsare evaluated according to VDI guideline2056, which gives details about the measuringdevices, mounting arrangement of the test


60 125 250 500 1000 2000 4000 8000Midfrequency of octave band Hz

100

90

80

70

60

50

dB

NR 80

NR 75

NR 70

So

und

pre

ssur

e le

vel

ΙΙ

Ι

Fig 32:Measuring thesound pressurelevel of liquid ringvacuum pumps

Tab. 1:Operating data andA-rated soundpressure and soundpower levels

Pump size I IISuction pressure mbar 80 80Discharge pressure mbar 1013 1013Pump power input kW 9.5 64Speed r.p.m 1450 735Sound Pressure level dB(A) 68 79Sound pressure level dB(A) 81 94

piece and evaluation of the vibration intensity.The vibrations produced by liquid ring gaspumps result from the imbalance of theirrotating parts and the sudden changes in thepressure levels taking place in the impellerblade cells and discharge chambers each timean impeller blade cell passes over thedischarge port. Those places where thevibration energy is transmitted to other parts ,i.e. the bearings, feet and pump flanges,usually serve as measuring points.

VDI guideline 2056 divides the test pieces intodifferent groups distinguished by the fact thatthe acceptable vibration level increases withincreasing machine size. Figure 33 shows acomparison of the measured values with thelimiting values of an average-sized liquid ringvacuum pump. The term “effective velocity” isa measure of the vibration intensity.

Emissions 55

Fig 33:Measuring thevibration velocity ofa liquid ringvacuum pump

Little vibrationexperienced

16 31.5 63 125 250 500 1000

63.0

31.5

16.0

8.0

4.0

2.0

1.0

0.5

0.25

µm

Machine group M

not tolerable

effective velocity =

71 mm

/s2.8 mm

/s1.1 m

m/s

still tolerableacceptablegood

Mean perception threshold

of a human being 0.11 m

m/s

Vibration frequency f

Dis

pla

cem

ent

s

Hz

LeakageWhen handling harmful, explosive ormalodorous media, every precaution must betaken to prevent leakage to the atmosphere. Inmany applications, leakage from theatmosphere into the system is equallyundesirable, whether it be for safety ortechnical purposes. Even in those caseswhere both the medium being pumped andthe service liquid added are harmless, shouldleakage be avoided, if only because it isunsightly.

It is obvious that any requirement in respect ofsealing implies that testing must be carriedout. Hydrostatic pressure tests includingvisual examination of the test piece are notconsidered sealing tests in this sense. Thesealing between static components greatlyinfluences the overall degree of sealing.

Leakage rates of less than 1.10-3 mbar 1/s canbe achieved with clean, carefully machinedstandards parts. The leakage rate of shaftseals is given in the information andwarranties provided by the seal manufacturer.Leakage tests can only be performed onmachines which are not in operation.

Extremely low leakage rates can only beattained on glandless designs such as onpumps driven via a magnetic coupling.


Avoidingleakage

Low leakageflow rates

Combining liquid ring vacuumpumps with gas ejector vacuum pumpsThe vapour pressure of the service liquidlimits the operating range of liquid ringvacuum pumps on account of the suctionpressure. The operating range can be exten-ded to approximately 4 mbar byconnecting the pump to an ejector whichutilises the pressure gradient between theatmospheric pressure and the suctionpressure of the liquid ring vacuum pump.

The arrangement of a gas ejector vacuumpump is shown in figure 34. In the ejectornozzle, the motive gas, i.e. atmospheric air orgas from the separator compressed toatmospheric pressure, is propelled at avelocity governed by the suction pressure ofthe ejector.

Combining pumps 57

Extendingthe range ofoperation

Medium pumped

Casing Motive gas

Ejector nozzle

Mixing nozzle

Diffuser

Liquid ringvacuum pump

Fig 34:Liquid ring vacuumpump combinedwith a gas ejectorvacuum pump

This causes the gas being pumped to bedrawn into the mixing nozzle via the suctionchamber of the ejector. Inside the diffuser, thevelocity energy of the mixture is converted topressure energy.

Ejectors are designed for use with liquid ringvacuum pumps by selecting their dimensionsaccording to the suction pressures and inletvolumetric flow rates required. The rightcombination of pumps is therefore crucial.There are pump combinations for differentpressure ranges and for water at differenttemperatures than the service liquid. The inletvolumetric flow rates of a liquid ring vacuumpump with and without ejectors are illustratedin figure 35.

By reducing the inlet volumetric flow rate ofthe liquid ring vacuum pump, the dischargepressure of the ejector increases. However,this pressure must not exceed a given limit. Ifthis is exceeded, the inlet volumetric flow rate


The rightcombinationis crucial.


Liquid ringvacuum pumpcombined witha gas ejectorvacuum pump

Service water temperature 15ºC

4 6 8 10 20 40 60 80 100 200 400 600 1000


Inle

t vo

lum

etri

c flo

w r

ate

m3 /

h

0

Fig 35: Inlet volumetric rate of flow of a liquid ring vacuumpump with and without a gas ejector vacuum pump

of the ejector drops. An increase in the inletvolumetric flow rate of the liquid ring vacuumpump will reduce the discharge pressure ofthe ejector. However, over a wide range thisdoes not affect its inlet volumetric rate of flow.The operating behaviour of an ejector can beseen from the example of a characteristiccurve shown in figure 36.

The connection of an ejector in front of theliquid ring vacuum pump does not affect theoperating behaviour of the latter. When thesuction side of the ejector is closed, thesuction pressure of the liquid ring vacuumpump is outside the cavitation range,provided that the correct pump combinationis selected. The ejector is either mounteddirectly on the suction nozzle of the liquid ringvacuum pump, or fitted inside the suctionpipe. If required, a non-return valve can befitted between the two pumps. Care must betaken to ensure that the valve produces nomore than a small pressure drop (1 to 2 mbar).If, for process reasons, it is not possible to use

Combining pumps 59

Suction pressure 30mbar

20mbar

10mbar

0 20 40 60 80 100

Dicharge pressure mbar

Inle

t vo

lum

etri

c flo

w r

ate

m3 /

h

No change inthe operatingbehaviour

Fig 36:Characteristiccurves of a gasejector vacuum pump

air as the motive gas, gas from the gas outletline of the separator is used for this purpose.This arrangement is shown in figure 37.In the high pressure range, the inletvolumetric flow rate of a pump combination is

smaller than that of the liquid ring vacuumpump alone (see figure 35). In thoseapplications where the requirement is for ashort evacuation period, the ejector isbypassed in this pressure range, or the motivegas connection has to be at least shut.

It must be noted that with atmospheric air asthe motive gas, the altitude of the installationhas an effect on the motive gas pressure.

The materials of construction of the ejectorcan be selected to suit the conditions ofoperation.

AccessoriesSeparatorsThe main purpose of a separator is toseparate the mixture of gas and liquiddischarged from the discharge nozzle of the


Separatinggas and liquid

Separator


Motive gas line

Gas ejectorvacuum pump

Non-return valve

Fig 37:Tapping the gasfrom the separator

liquid ring gas pump into a gas and a liquidflow. However, separators are also used forother purposes, such as:

• service liquid ring storage tanks• gas storage tanks• settling tanks for the separation of dirt,

or to separate liquids of different specificdensity

• heat exchangers for cooling liquid.

Separators are available in different designs.The basic designs are:

• upright standing version – stands besidethe pump

• top-mounted version – mounted on top ofthe pump

• base-mounted version – arrangedunderneath the pump

• integrated version – integrated in the pump

When selecting the appropriate design, the duties of the separator, the requiredquality of separation, and the conditionsprevailing on site all have to considered. Theseparator design is determined by the inletvolumetric flow rate of the liquid ring gaspump, it’s dimensions and the flow rate of theservice liquid.

Separators intended for use in recirculating orclosed-circuit operation must have a sufficientsupply of liquid and should be equipped withmeans of monitoring the level of the liquid.

Separators intended for use on liquid ringcompressors must be provided with a safetyvalve connection. The regulations forconstruction and operation of pressurevessels have to be observed.

Accessories 61

Types ofdesigns

Additionaluses

Figure 38 shows an example of an uprightstanding separator; the top-mounted versionof a separator is shown in figure 39, each typewith the necessary connections.

Liquid outletThe liquid outlet of separators used inconjunction with liquid ring compressorsmust not be open to the atmosphere, as this


Fig 38:Upright standingversion of aseparator

Fig 39:Top mountedversion of aseparator

Liquid outlet

Pump dischargepipe connection

Vent lineconnection forliquid outletfor compressortype operation

Safety valveconnection forcompressoroperation

Gas outlet line

Liquidstand pipeconnections

Circulatingflow lineconnectionDrain

Gas outlet

Circulating flowline connection

Connection to thepump discharge nozzle

Liquid outlet

Discharge ofliquid atworkingpressure

could result in a mixture of gas and liquidescaping. This is prevented by means of valvetype traps.

Baseplates and baseframesSmall and medium-sized liquid ring gaspumps are usually mounted together with themotor on a common baseplate. For the largerpump sets, baseframes made from rolledsteel sections are used.

Flexible couplingsCouplings are selected according to thetorque to be transmitted, the startingfrequency and the period of operation. Largepump sets are preferably fitted with couplingswhich have a spacer unit, because this allowsthe pump and driver to be disconnectedwithout impairing their axial alignment.

Coupling guardsAccording to industrial safety regulations,couplings, belt drives and exposed shaft endsmust be guarded so that they cannot betouched accidentally. Standards (PD 5304;DIN 31001) specify the details to be observedfor the design of the guard. In installationssubject to explosion hazards, the safety guardmust be spark proof.

Acceptance specificationsThe acceptance specifications include adescription of the test equipment and the testprocedures applied during acceptancetesting. In some cases, the equations given inthe various acceptance specifications forcorrecting the inlet volumetric rate of flow andthe pump power input vary from thoseapplied by individual manufacturers.

Acceptance specifications 63

Observeindustrialsafetyregulations

DIN 28431Acceptance ruled for liquid ring vacuumpumps.Although this standard has been writtenespecially for liquid ring vacuum pumps, itcan also be applied without restrictions toliquid ring compressors.The usual tolerances of the inlet volumetricflow rate and the required power input arelisted as maximum deviations in a tableforming part of this standard. An example of atest arrangement is shown in figure 40.

PNEUROP 6612Acceptance specification and performancetests for liquid ring vacuum pumps.

ISO 1217 Acceptance tests; Ref. No. ISO 1217Displacement compressors.

HEIPerformance standard for liquid ring vacuumpumps.HEAT EXCHANGE INSTITUTE (HEI),Cleveland, Ohio 44115, USA.


M

M

P t

t1

t3P3

V3

P1 n;P

P2

Atmosphere

Outflowing liquid

Service liquid

Air

t2

·

V·

Flow measurement

Pressure measurement

Temperature measurement

Liquid ring gas pump

Electric motor

Separator

Throttle valve

Fig 40:Test arrangementson the basis ofDIN 28431

ApplicationsIn many cases, the typical properties of liquidring gas pumps render this pump type themost favourable or even the only suitablepump capable of meeting the specifiedrequirements. Some of its outstandingfeatures to be highlighted in this context are:

• it is capable of handling almost all gasesand vapours used in the chemical andpharmaceutical industries. For eachrequirement, the service liquid is selectedto suit the product being pumped.

• compression accompanied by a very smalltemperature increase.

• liquid ring vacuum pumps are alsocapable of compression to overpressures,and liquid ring compressors can also beoperated at suction pressures far belowatmospheric.

Without claiming to be complete, thefollowing summary lists the numerouspossibilities for the application of liquid ringgas pumps:

Chemical process technologyLiquid ring gas pumps are used for distilling,drying, condensing, absorbing, evaporatingand filtering purposes.

Venting of steam turbine condensersLiquid ring vacuum pumps are used for thestart-up and condensing operations of steamturbine condensers.

Filter installationsLiquid ring vacuum pumps are suitable for usewith any type of vacuum filter.

Applications 65

Advantagesdue tospecificproperties

Fields ofapplication

Steam and gas sterilisersBefore and after sterilisation, the steriliserchambers are evacuated with the help ofliquid ring vacuum pumps.

Plastics extruders Liquid ring vacuum pumps are used to de-gasand dry the plastic compound.

Vacuum calibration of plastic profiles Liquid ring vacuum pumps are employed togenerate a vacuum which draws the plasticprofile against the walls of the mould.

Paper machinesDe-watering of the wet section of papermachines is performed with liquid ring vacuumpumps.

Sugar millsIn sugar mills, liquid ring compressors are usedfor compressing carbon dioxide; liquid ringvacuum pumps are employed in the vacuum-pan house and for drying purposes.

Vacuum driers For the purpose of vacuum drying, liquid ringvacuum pumps are used in connection withdifferent types of vacuum drier.

Solvent recovery To avoid health hazards and environmentalpollution, exhaust flows containing solventsare cleaned. The processes applied to recoverthe solvents could not be carried out withoutthe use of liquid ring vacuum pumps or liquidring compressors.

Lowering of the ground water tableLiquid ring vacuum pumps are used toevacuate a tank. Air and ground water areextracted from filter pipes over the tank.



Drying and impregnation of woodWood can be dried quickly and withoutcracking if it is put under vacuum before beingdipped into an impregnation bath.

Sludge extraction vehiclesA liquid ring vacuum pump is used to producea vacuum in a collecting tank into which thesludge is then pumped automatically.

Water worksLiquid ring compressors are used in waterworks for the oxidation process, to aerate thewater, and to effect a reverse flow through thefilters.

Vacuum toiletsSewage is pumped pneumatically into acollecting tank under vacuum.

Mineral water production plantsThe gases contained in natural mineral waterare removed by means of evacuation.

Applications 67

Fig 41:Liquid ringcompressors installed in a drinking watertreatment plant

Automobile manufacture The cooling, braking and air conditioningsystems are evacuated before being filledwith liquid.

Milk collecting tank trucksMilk is drawn out of the tanks in dairy farmsand into the tanker by means of a liquid ringvacuum pump.

Brick manufactureTo improve the quality of the clay used in brickmanufacture, it is de-gassed before pressing.

Automatic bottling plantBottles and cans are evacuated before filling.This process creates a low-oxygenatmosphere which prevents the beveragefrom oxidizing.

Bleaching of chemical pulp Liquid ring compressors are used in oxygen -ozone - bleaching section for chemical pulp


Fig 42:Liquid ring vacuumpump on achemical plant

Vacuum mixing equipment Vacuum mixing equipment is used inproduction of liquid and paste - like products;by continuous deaeration and degassing, avery high degree of homogeneity of productsis achieved.

Mine gas extraction The gas which forms during coal mining isdrawn off continuously to eliminate all risk ofexplosion.

Applications 69


AppendixVapour pressure of water

Materials and materialcombinations commonly used inthe manufacture of liquid ringgas pumps

The numbers indicated are material numbersas per DIN 17007.

70 Appendix

Temp. Press. Temp. Press. Temp. Press. Temp. Press.˚C mbar ˚C mbar ˚C mbar ˚C mbar10 12.7 18 20.62 26 33.60 45 95.811 13.12 19 21.96 28 37.78 50 123.412 14.01 20 23.37 30 42.41 55 157.413 14.97 21 24.85 32 47.53 60 199.2014 15.97 22 26.42 34 53.18 65 250.115 17.04 23 28.08 36 59.40 70 311.6016 18.17 24 29.82 38 66.24 75 385.517 19.36 25 31.66 40 73.75 80 473.60

Material combinationComponent 0A 0B 3B 4A 4BSuction/discharge cover 0.6025 0.6025 2.1050 1.4308 1.4408Guide plate element 0.6025 0.6025 2.1050 1.4308 1.4408(port plate)Casing 0.6025 0.6025 2.1050 1.4038 1.4408

1.0038 1.0038 1.4541Impeller 2.0196 0.7040 2.1052 1.4460Cu 1.4460Cu

2.0975 0.7043 1.4541 1.45711.40271.0570

Shaft, in contact 1.4021 1.4021 1.4301 1.4301 1.4571with the liquidShaft not in contact 1.0060 1.0060 1.0060 1.0060 1.0060with the liquidShaft protection sleeve 1.4027 1.4027 1.4410 1.4410 1.4410

1.4581Bearing housing, 0.6025 0.6025 0.6025 0.6025 0.6025bearing cover

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Liquid Ring Vacuum Pumps & Compressors_Tech Details - Sterling Fluid Systems Group