0262 1762/04 © 2004 Elsevier Ltd. All rights reserved WORLD PUMPS September 200430

Centrifugal pumps can be tracedback to the late 1600s, whenDenis Papin, a French-borninventor, experimented withstraight vane impellers. Britishinventor John G. Appold intro-duced a curved vane impeller in1851. Centrifugal pumps havecontinued to evolve since then.Next to the electric motor theyare the most popular machines inthe world.

Pumps will provide satisfactoryservice to the user, assuming theyare in a system offering a flooded suction or ‘suction head’(fluid is located above thecentreline of the pump). Thedrawback of standard endsuction centrifugal pumps is thatthey do not fare well when theliquid is below the pumpcentreline. Liquid must bedelivered to the pump so theprocess can begin. The pumpcannot lift liquid vertically tobegin the process. The reason is

air in the suction side of thepump cannot be evacuated fromthe casing. The centrifugal pumpwill become ‘air bound’ andincapable of pumping anyadditional liquid. The onlyremedy for this situation is toraise the liquid level to the pumpinlet and have the operator ventthe suction line before starting.

To eliminate this problem, self-priming centrifugal pumps weredeveloped. The term self-primingis actually an industry term thatdescribes the ability of a pump tocreate a partial vacuum bypurging air from the suction line.The self-priming pump unit usesan initial quantity of liquid(usually water) to create thevacuum at the impeller eye andcontinuously ‘digest’ or removeair from the suction line. Thisbenefit is not without cost. Self-priming pumps are, in general,slightly less efficient than an endsuction pump.

Marlow Pumps manufactured thefirst self-priming centrifugalpump in 1932. This design ofpump would go to the source andget liquid to be pumped.

How it works

Atmospheric air exerts a pressureof 101.3 kPa (14.7 psi) all aroundus. The pump creates a partialvacuum as it removes air from thesuction line. The vacuum causesatmospheric pressure to pushwater to the pump through thesuction line. In a laboratory – withperfect vacuum – the atmosphericair would push liquid 10.3 m (33.9ft) up a column. The practicalapplication limit for self primingpumps is about 7.62 m (25 ft) ofliquid.

The self-priming process occursautomatically once the pump is started with the initial quantity of liquid. Withoutoperator involvement, the pump can prime itself with thepumpage and begin pumping. Ifvacuum is broken, the pump is able to reprime and continuepumping. The savings in time,effort and cost are substan-tial: especially in dewateringapplications such as in miningwhere pumps often run dry forbrief periods.

The advantages of using a self-priming pump are now clear. Itshould be pointed out thatnumerous varieties of self-priming methods are in existencetoday. Most are related to the twomethods that will be discussednow: volute priming and diffuserpriming.

Self-priming centrifugalpumps: a primerSince they were first introduced in the 1930s, self-priming centrifugal pumps have becomewell established in many applications where the source liquid is located below the centrelineof the pump. In this article John Kanute, an application engineer with Goulds Pumps,provides a comprehensive overview of the topic of self-priming, from the basic principles ofoperation to a detailed look at the methods of volute and diffuser priming and the specialconsiderations for the use of this type of pump.

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Figure 1. From left toright: closed, open andsemi-open impellers.

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Volute priming

Just as in a standard centrifugalpump, everything starts with theimpeller.

There are three commonimpeller designs (Figure 1).

• The closed impeller hasshrouds on both sides of thevanes. It is a desirable designfor higher pressures and clearliquids.

• The semi-open impeller hasa shroud on one side of thevanes. It can handle amoderate amount of solids inthe liquid.

• The open impeller has theshroud cut back completelyexcept where the vanes arelocated. It can pass a highconcentration of solids.

The impeller is placed inside aprogressively expanding spiral-shaped casing called a volute. Atone point on the inside of thevolute, there is a close clearancebetween it and the outer edgeof the impeller. This point ofclose clearance is usually calledthe ‘cutwater’ or ‘peeler’. Thepurpose of the volute is tocollect liquid being flung off thewhirling impeller vanes anddirect it to the discharge of thepump. The volute takes highvelocity liquid from the impellerand increases the flow areagradually; liquid entering it at a high velocity is gradually

slowed. As the liquid slowsdown, the pressure is increased,since these two are inverselyrelated. It is this action thatproduces the pressure that thepump develops. The volute caneither be cast into the pumpcasing or bolted in, dependingon the design.

The priming process begins withfilling the casing with liquid.The liquid is usually addedthrough a tapped plug in thecasing. This initial liquid is usedto create the vacuum and sealagainst leakage. The casing isdivided into two sides: thesuction and discharge sides. Thedivision is made by a solidpartition inside the casing withthe only opening between thetwo sides at the impeller ‘eye’ orcentre. Once the casing is filledfrom an external source it isready to begin the process.

As the pump is started, theimpeller takes liquid from the‘eye’ and throws it to thecircumference (Figure 2). Thisaction will create a vacuum atthe eye that is the source for theself-priming pump. The liquidfilling the casing is pulledthrough the impeller from thesuction side. Once the initialliquid is pulled from the suctionside, the impeller will continueto create the vacuum and beginpulling air from the suction line.

Air is drawn towards thevacuum in the impeller eye and

mixes with the liquid at theimpeller tips. The air/liquidmixture is peeled away by thecutwater. The air/water mixturethen percolates upwardsthrough the casing because it isnow less dense than liquidaround it. The air separates as itrises and is vented toatmosphere in the discharge linewhile the liquid falls back to thecasing to be used again (Figure3a). It is this action that digeststhe air from the suction line andcreates the partial vacuum. Theprocess continues until all theair has been evacuated and fullprime is achieved. It is importantto vent the air on the dischargeside. The pump will notcompress the air and will notprime unless the air is allowed toescape.

Figure 2. Theimpeller rotates and

flings out liquid.

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Figure 3. (a) Volutepriming; and (b) diffuser priming.(a) (b)

Volute priming Diffuser priming

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Advantages of volutepriming

The advantage of volute primingis its solids-handling ability. Thepump has a single cutwaterallowing solids to be passedwithout clogging. When usedwith a trash-handling impeller theunit will effectively pass largesolids. This design is alsomaintenance-friendly. In someunits the rotating end can bereplaced as a unit. There is noneed to remove suction ordischarge piping to work on thepower end. This can be used tominimize downtime by keeping aspare rotating end. The drawbackof volute priming is that wear atthe cutwater usually means thecasing needs to be replaced.

Diffuser priming

Diffuser priming is similar tovolute priming in concept. A diffuser contains multiplestationary vanes or ‘peelers’ thatpeel away the air and watermixture during the primingphase. The multiple stationaryvanes each act like a cutwater involute priming. These stationaryvanes are cast into a replaceablering that surrounds the impeller.The impeller and diffusercombination are mounted insidea pump casing just as in the caseof volute priming.

The priming cycle occurs exactlythe same way: an initial quantityof liquid is added to the casing.The pump is started and a vacuumis created at the eye of theimpeller. The multi-vane diffuser

peels the air/water mixture fromthe impeller where it is separatedin the discharge side of the casing(Figure 3b). Vacuum is created inthe suction line and atmosphericpressure forces liquid into thepump tank.

Although this design has limitedsolids-handling capability due tothe diffuser surrounding theimpeller, it does have severaladvantages over volute priming.

Advantages of diffuserpriming

1.Pump maintains efficientpriming for a longer period oftime due to multiple peelers.

2.Lower radial loads due tobalanced design extend seal,shaft and bearing life.

3.Replaceable diffuser allowsreturn to original performancewithout expensive casingreplacement.

With these advantages thediffuser priming method canachieve higher efficiencies thanvolute priming.

Utilizing self-primingpumps

It is important to note that self-priming pumps will have specialconsiderations that end suctionpumps will not. For example, asmall vacuum leak in the suctionline will prevent the unit frompriming. The pump willcontinuously pull air from theleak instead of the suction lineuntil all the priming liquid in the

casing is evaporated. This is acommon cause of priming failureas the leak can be very small orinvisible to the naked eye but willprevent priming.

Another common cause ofpriming failure for both types ofpriming methods is the inabilityto vent air from the discharge lineduring the priming cycle - forexample, because of a closeddischarge valve. The pump is notcapable of compressing the airduring the priming phase; it must be allowed to escapethrough the discharge and vent toatmosphere.

The self-priming pump shouldalso be located as close as possibleto the source. It is best to belocated directly above the sumpwith as few elbows as possible toreduce friction. Once the unit isprimed and pumping it willrequire net positive suction head(NPSH), as with any centrifugalpump. Without sufficient NPSHthe unit will cavitate. Thereforelifts through long suction lines or lifts with obstructions will be plagued by cavitation-typeproblems. It could be the case thatone problem is solved only tocreate another. It is these specialconsiderations that must beexamined before the installationis cast in concrete. Many ownershave puzzled over problemapplications because one of thesefactors has been neglected.

In conclusion

Self-priming pumps have beenproviding economical and reliableservice to their owners for manyyears. They offer an alternative tovertical or submersible pumps insump applications, and can helpwhere overhead space is limited.They can be found in virtually any industry, from farms topetrochemicals. They solve thevery basic problem of how to getliquid to the pump.

CONTACTJohn KanuteGoulds Pumps (A Subsidiary of ITTIndustries)500 East Centre StAshland, PA 17921, USA.Tel: +1-570-875-6104Fax: +1-570-875-2657E-mail: John.kanute@itt.comwww.itt.com

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Figure 4. Typical self-priming centrifugalpump on industrial

sump service.