FEATURE • PUMPING GASEOUS LIQUIDS

WORLD PUMPS October 2002 0262 1762/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved2

Many applications in processing plantsrequire pumps that are – at leasttemporarily – able to handle liquidswith gas or vapour entrainments,without risk of flow interruption.

Liquids pumped near their boilingpoint, such as condensates, liquifiedgases, hydrocarbons, aerosols orrefrigerants, demand a lot of a pump.The operator should attempt toeliminate all risks. Radial centrifugalpumps allow only limited gasentrainments in the pumped liquid;with slight gas contents of ca. 5% theirhead and flow decline dramatically.Higher gas contents cause totalinterruption of the pump flow.

The SERO Multifunction Pump SRZS, a side-channel pump of specialconstruction (Figure 1), has no prob-

lems with gas inclusions, even underextreme conditions. For example,during venting of the suction line, theSRZS has the outstanding ability toreliably deliver the gas with the helpof only a small amount of liquid left in the pump.

Design features andoperating principles

The multifunction pump combinesthe advantages of two pumpingsystems:• the side-channel pump hydraulics

for gas transport allow the pump to attain pump heads that are up to four times greater than thatgenerated by radial flow centrifugalpumps;

• the radial flow centrifugal pump hydraulics of the inlet stage helps

achieve extremely low NPSHvalues.

Priming processDuring the process of priming,operation of the side-channel pumphydraulics is similar to that of a gashandling vacuum pump or acompressor. The side-channel pumphas the ability to work temporarily asa liquid ring vacuum pump andtherefore may also be considered as arotary displacement pump. Similar tothe operating principle of a liquidring vacuum pump, a circulatingliquid ring has a displacement effect,the auxiliary liquid entering (piston-like) the impeller and the sidechannel and exiting during eachcirculation. The centrifugal effectcauses collection of the liquid in theouter region of the side channel andimpeller cells and thus forms theliquid ring, which is necessary for thepriming process. The gas builds up inthe inner region.

The special arrangement of the sidechannel gives the liquid ring aneccentric course, bringing about thedisplacement effect of the liquid ringvacuum pump. The produced vacuumallows an independent venting ordegassing of the suction line and apriming of the liquid (Figure 2a).The operation of the multifunctionpump as a mere vacuum pump is onlypossible over a limited period. Non-supply of further auxiliary liquid atnormal temperature leads to a gradualheating and finally to a vaporization

Multifunction pump for handlinggaseous liquids in processengineeringPumping liquids with a high gas content can be problematic, causing sharp losses in headand flow or even total cessation of pump flow. German pump manufacturer SERO believesits Multifunction SRZS Pumps provide the ideal solution for such challenging situations,offering a combination of the advantages of side-channel and radial flow centrifugal pumphydraulics. Together these achieve high pump heads, even at relatively high gasentrainments, combined with low NPSH values.

Figure 1. Cutawaydiagram of SERO’sSEMA-S side-channel,multifunction pumpwith magnetic drive.

FEATURE • PUMPING GASEOUS LIQUIDS

www.worldpumps.com WORLD PUMPS October 2002 3

of this liquid.

The multifunction pump has gainedwidespread use because of its uniqueadvantage of insusceptibility tovapour or gas inclusions in thepumped liquid and at the same timeits ability to achieve substantial heads.Gas bubbles are entrained through thepump along with the liquid, with amaximum allowable gas content of50%. Liquid–gas mixtures can behandled problem-free with no need ofany auxiliary devices. Gas contents of10%, for example, have virtually noimpact on the steeper characteristiccurve compared with radial flowcentrifugal pumps (Figure 3).

Commonly a side-channel pump iscalled ‘self-priming’, whereas ‘self-venting’ would be a much better des-cription. The self-venting effect is agreat advantage in the process of partial vaporization. Even at the verybeginning of cavitation, the perfor-mance (Q-H) curve of a normalcentrifugal pump drops steeply,whereas the flow of the multifunctionpump does not break down.

The ability to handle such a high gascontent is not common for all pumps ofthis type. Minor constructional modifi-cations may have a great impact on thegas-handling ability. For example, theUS type of side-channel pump –known as the ‘peripheral’, ‘regenera-tive’ or ‘turbine’ pump – was modifiedto be more efficient, yet can onlyhandle small gas contents. In contrast,the multifunction pump is capable of

handling relatively high gas volumes.

Liquid transferAfter initial priming, the flow processchanges automatically from displace-ment over mixture transport (withliquid being pumped as a foam-likemixture into the pressure area), topure liquid transfer. When completelyfilled with liquid, the multifunctionpump can be considered as a normalcentrifugal pump.However, the specific way thepumped liquid is directed through theside-channel pump hydraulics gives ita substantially higher energy thanthat attained with conventionalimpellers of radial design. This higherpressure figure must be considered as asignificant advantage over ‘normal’centrifugal pumps.

On its way from the inlet to theoutlet of the side channel, thepumped liquid circulates many timesin between the individual vanespaces. Pressure is created as the sumof individual impulses during pumpcirculation. This process is an ‘innermultistage effect’ (Figure 2b). With acentrifugal pump, impeller diameterand revolutions per minute deter-mine the volume speed that istransferred into pressure (pressurefigure = 1). The total pressure of themultifunction pump is created as anaccumulation of impulses, with apressure figure of about 4.

The specific flow through the side-channel stages with intended varia-tions causes a little decrease in pump

Figure 3. Impact of air contents on the characteristic curves of amultifunction pump.

Figure 2. The principles of the operation of the side-channel pump hydraulics: (a) gasdelivery; (b) liquid delivery.

FEATURE • PUMPING GASEOUS LIQUIDS

WORLD PUMPS October 2002 www.worldpumps.com4

efficiency, as do losses caused bypartial flows at the clearance surfacesof the impeller hub, which arenecessary for hydraulic impellerbalancing and for lubrication.Optimal casting techniques andhighly precise manufacturing tech-nologies allow achieved efficiencyrates of up to 40%.

Multifunction pumps are used forapplications where other pump types or technologies can not attainbetter efficiencies or where otherpump designs require higherexpenditure on plant and/or pumpdesign. As a rule, this is the case atlow flows and middle to high heads,i.e. at low specific revolutions perminute, nq, of 8 to 10. At theseworking points the efficiency rate isabove that of centrifugal pumps thatare running at low flows in a partialload range with reduced efficiency(Figure 4).

Power consumption of the multi-function pump is approximately onethird below that of standard waterpumps (DIN EN 733) and standardchemical pumps (DIN EN 735).

Extremely low NPSHvalue

The multifunction pump combinesthe advantages of side-channelpumps, to handle high gas entrain-ments and to be self-priming, withthe principle of a centrifugal stage, asthe first step towards achieving lowerNPSH values. In addition, its suctioncasing has a flow-friendly axial inletthat is provided with a speciallydesigned large cross-section, allowinglow flow speeds. These slow flowspeeds help the multifunction pumpto bring about low NPSH values atthe inlet into the suction impeller.The radial suction impeller is alsodesigned according to the latest

insights into cavitation to achievespecially low NPSH values.

The head produced by the suction im-peller is sufficient to reliably developthe necessary inlet pressure for thenext side-channel stage, even underthe most unfavourable workingconditions.

The low motor speed of n = 1450rpm (at 60 Hz, n = 1750 rpm), whichis possible due to the high pressurefigure, allows the multifunctionpump to have lower NPSH valuescompared to a faster running pump.Another advantage of this pump’sslow speed is its extended lifetime, asa consequence of a combinedmastering of mechanical loads,partial load behaviour and noise.

The NPSH values are between 20 cm(0.6 ft) and 1 m at n = 1450 rpm,depending on the required flow. Thisallows inlet heads of less than 0.5 m(1.5 ft) for boiling liquids (Figure 5),resulting in substantial savings onsystem costs.

The lower its specific speed, the betterthe priming ability of the pump andthe lower the risk of cavitation.

The multifunction pump has nodefinite fixed cavitation limit line butan area in which the pump can beoperated without cavitation. Atvariable vapour pressure, themultifunction pump is considerablyless susceptible to cavitation than aradial flow centrifugal pump. Withthis increased operational safety, themultifunction pump guaranteestrouble-free production processes.

In practice

As with conventional radial flowcentrifugal pumps, the flow of amultifunction pump is proportionalto speed, whereas the head varieswith the square. However, theoperating principle is completelydifferent.

The multifunction pump has a differ-ent power curve from a centrifugalpump. Its power consumption reduceswith increasing flow, so that the old

Figure 5. Net positivesuction head curve forthe multifunctionpump.

Figure 4. Comparisonof the efficiencies ofmultifunction andcentrifugal pumps intheir typical nqapplication ranges.

FEATURE • PUMPING GASEOUS LIQUIDS

www.worldpumps.com WORLD PUMPS October 2002 5

centrifugal pump ‘law’ – to start thepump only against a closed valve – isnot valid. Multifunction pumps mustnever be operated against a closeddischarge line.

Centrifugal and multifunction pumpsalso differ considerably in theirperformance (Q-H) curves: the side-channel multifunction pump has asteep, almost straight performancecurve (Figure 6a). This characteristicallows a very exact throttle regu-lation, because changes of the on-sitecounter pressure entail only smallchanges of the pump flow rate.

With reference to the overalloperating process, slight pressuredeviations only have a small influenceon the multifunction pump’s flow.

It is not possible to vary the pressureof a multifunction pump by remanu-facturing the impellers. However,due to the very distinct graduation of the various types, this pump adaptsexactly to the required workingconditions, with the advantage ofrelatively good efficiencies over thewhole performance range.

Pumps are required to adapt tovariable working conditions, that is,they need to be controllable duringoperation. The almost linear pumpcurve of the multifunction pumpfavours exact pressure-dependentcontrol in processing services, whilethe NPSH curve must of coursealways be observed. Possibleregulation methods are speed controlby means of a frequency converter

and throttling in the discharge line,or bypass. In the case of bypassregulation, the bypass flow shouldnot be circulated directly into thesuction line but into the suctiontank, for technical reasons.

Before the first start-up, the multi-function pump must be filled – atleast partially – with the pump liquid,to ensure that the liquid ringnecessary for proper operation can beformed. For all subsequent startsenough liquid is kept within thepump.

Trouble-freeoperation in processcirculationsThe proven technology of the multi-function pump (Figure 7) has beenapplied in thousands of applicationsand is the optimal solution for hand-ling two phase flows (i.e. liquid–gasmixtures).

Applications for this pump type coverthe transport of low boiling liquids,such as liquified gas, as well as thedelivery of various refrigerants, such asammonia (NH3) or liquid carbondioxide (CO2), down to –60 °C.

Here, the required pump operationfeatures are:

• insusceptibility to gas inclusions• generation of relatively high

differential pressures• reliable seals• large, axially arranged suction

casing openings. Figure 6. Comparison of (a) performance (Q-H), and (b) power consumption (p) curves

of a radial centrifugalpump and a

multifunction pump.

a)

b)

Figure 7. Cross-sectionaldiagram of the

multifunction pump.

FEATURE • PUMPING GASEOUS LIQUIDS

WORLD PUMPS October 2002 www.worldpumps.com6

For transportation of hydrocarbons,solvents and fuels in storage tankinstallations, use is made of the highself-priming ability to fully empty the

containers or road tankers. Thepump’s compact design is also a greatadvantage.

Another very important applicationof the multifunction pump is as aboiler feed or condensate pump foroperating temperatures up to 180 °C;in short, wherever vapour is neededto keep a process running, e.g. incondensate recovery systems, hotwater circulation or in distillatecolumns. The multifunction pump’slow NPSH values and insuscept-ibility to cavitation make them avery suitable choice for theseapplications.

Within effective environmentalprotection, for example in flue gasdesulphurization, these pumps are alsoan excellent choice for use inauxiliary installations, for example inneutralization of chemicals. The multifunction pump is availablefor flows up to Q = 35 m3/h (154 gpm)and heads up to H = 300 m (Figure 8).

The most usual construction is theprocessing design with a standardmotor on a common baseplate. Inaddition, vertical inline and closecoupled versions are available asoptions, on a limited performancebasis.

Decisive factors for the selection ofmaterials and sealing systems are theliquid being pumped and theoperating conditions. Availablematerials range from cast ironthrough ductile iron to high gradestainless steel alloys. Shaft sealscover single and double actingmechanical seals.

If the demand is for sealless pumps,SERO offers magnetically driven orcanned motor pumps.

User benefits

Total life cycle costs should be anessential factor to consider in pumpselection. Instead of making highinvestments for expensive monitoringor diagnosis systems, the pump usershould first make a correct pumpselection.

In many cases, pumps are operatedoutside their designed performancerange and therefore do not workoptimally. Most pump failures occurdue to NPSH problems, gas inclusionsin the pumped liquid, dry run oroperation outside their designedrange, e.g. at too low a counterpressure.

The characteristics of the multifunc-tion pump offer important advan-tages over the major causes of failureof ‘normal’ centrifugal pumps. Theirgood self-priming ability togetherwith their insusceptibility tocavitation make them an idealchoice for handling liquids near theirvapour pressure.

Due to their considerable pressurerating and the compact dimensions ofa modular design, SERO believes itsmultifunction pumps are the optimalsolution, technically and also eco-nomically, for low flow – high headapplications involving low-viscousand pure liquids. ■

CONTACTAlbert L. ZientekSERO Pumpenfabrik GmbH & Co. KGIndustriestraße 31, 74909 Meckesheim,GermanyTel: +49-6226-9201-0Fax: +49-6226-9201-40E-mail: info@seroweb.dewww.seroweb.de

Figure 8. Characteristic head/flow curves for SERO’s multifunction pumps.

RES No. – USE THE ENQUIRY SERVICE @ www.worldpumps.com