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Most textbooks say the onlytrue self-priming pumps are positive displacement

pumps. While that may be a truestatement, it does not reflect on howthe term is used in the pump industry.Self-priming pumps are divided intotwo classes. ‘Wet prime’ refers topumps that need to have an initialcharge of water put into them in orderto begin the priming process. ‘Dryprime’ refers to those pumps thatdepend on an auxiliary device toevacuate the air from the pump andsuction line such as a vacuum pump orcompressed air through a venturi. Noinitial liquid needs to be added to dryprime pumps to begin the primingprocess. While we talk of priming thepump, (the act of adding an initialcharge of liquid to the wet prime pumpcase) the real goal is to be able to fillthe suction line with liquid withoutthe need for a troublesome foot valve.That is the job of the self-primingpump.

Wet prime self-priming pumps are assimple as straight centrifugal pumpsand follow most of the same rules ofoperation and installation. They havethe added feature of being able todisplace the air in the suction lineinitially or if the liquid suction leg islost. Some have an easily servicedsuction check valve to help preventthe loss of the suction leg when thepump shuts down.

Most wet prime self-priming pumps onthe market today operate in much thesame way. The spinning impellerthrows some of the initial charge of

liquid out of it causing a negativepressure in the eye of the impeller.Atmospheric pressure acts on thesource and pushes an equal amount ofliquid up the pipe trying to balanceout the pressure in the suction line.Modern wet prime self-priming pumpshave a hopper area on the dischargeside of the volute scroll where thedisplaced air can bubble out of theliquid and escape out the dischargepipe or air release line/device. Theheavier liquid finds its way back intothe tips of the impeller either throughthe volute throat or through a primingreturn channel. The return watercontinues to mix with more air in theimpeller and is thrown out leavingmore void for atmospheric pressure topush in more liquid. This processcontinues until the suction line hasbecome completely full of liquid andthe impeller is displacing only waterand the pumping action begins.

Performance

Many wet prime self-priming pumpsare built robustly and are veryforgiving. However, choosing theproper pump is the most critical factorin getting good life and service out of apump. Most manufacturers advertisewhat performance can be expected sothe customer can choose the properpump for the job. It is important toknow the priming limits of the pumpalso. Most contractor pumps arefurnished on engines and have arelatively large speed range to meetvarious pumping conditions. Caremust still be taken that the pump canprime at the chosen speed. As a rule ofthumb, higher speeds will allow higherlifts and faster priming times.

Even though a pump can reprime if itloses prime because the water leveldrops below the suction hose inlet, it is

Getting the best out of yourwet prime pumpSelf-priming pumps are divided into two classes ‘wet’ and ‘dry prime. Thisarticle applies primarily to wet prime pumps. However, many of the pointsabout installation and operation discussed by Dave Meister, director ofengineering for Gorman-Rupp will also apply to dry prime pumps. Choosingthe proper pump is the most critical element in getting good life and serviceout of a pump. Other factors include correct installation, priming limits andre-priming considerations.

f e a t u r e s e l f p r i m i n g p u m p s

Figure 1. Pump curve.

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desirable to match the inflow to thesump to the capacity of the pump.This is especially true of liquidscontaining abrasives. When the pumploses prime, the abrasive solids cansettle back into the pump housingbecause of the no flow condition.There, they are repeatedly recir-culated through the pump during thepriming cycle causing excessive wear.When the flow cannot be matched,the use of float switches reduces thisproblem by allowing the pump to stopbefore it can lose prime.

Placement

Placement of the pump near the liquidsource is one way to assure the bestpriming and pumping performance.The Figure 1 (pump curve), Figure 2(priming limits) and Figure 3a/3b(pump set-up) shows the comparisonof two pumping scenarios. The pumpshould operate on the system lineshown. If placed on a 20 ft suction lift(Figure 3a) the maximum flow will beabout 285 gpm. Interpolating thepriming limits the pump will needabout 1550 rpm to prime the 20 ft. lift.But at 1550 rpm it will be pumping ina cavitation zone on the suction break

of the pump. The pump will likelyexperience serious cavitation damageto the impeller and eventual primingproblems. If the pump is slowed downto match the curve and loses thesuction leg it will not be able to primethe high lift.

However, if the pump is placed so thelift is only 10 ft (Figure 3b) it couldfunction properly all the way out to410 gpm. Since it only has to prime 10 feet, the speed could be sloweddown to get as little as 80 gpm and itwould still prime the 10 ft lift ifnecessary.

The initial priming of a pump isusually not a problem because thedischarge line is empty and the aircoming out of the suction line caneasily escape through the opendischarge. However, if there is adischarge check valve placed near thedischarge of the pump, provision mustbe made to vent the air once there isback pressure on the discharge checkvalve. A small open bypass back to thesump is one option. If it is submergedthe pump is less likely to lose its primeif debris should get hung up in thesuction check valve. Using a valvethat automatically closes when thepump primes is a better route to takewhere the liquid has a lot of solids thatwill eventually plug a small line that iscontinually bypassing.

The term ‘re-prime’ has become animportant part of many pumpspecifications. When a pump is shutoff and the suction check fails orsomething becomes lodged betweenthe valve and the seat, it is like any siphoning plumbing trapconfiguration. All the water will leavethe pump housing until it reaches the top of the eye of the impeller and the siphon effect is broken. In aself-priming pump this reduced level is called the re-prime level. A check valve is great to hold thesuction leg full so it is ready to de-liver the next time the pump turns on. However, on unattended servicethe pump should be able to ‘re-prime’with this minimal amount of liquid in it if there is a check valvemalfunction.

Getting the mostflow

Over sizing a suction line is commonpractice in order to get the most flowout of a pump, especially if it isoperating to the right hand side of thecurve or the suction line isexceptionally long. Over sizing thesuction line increases the priming time.If you know the time with the normalsuction diameter, multiply it by thenew diameter squared and divide theproduct by the normal diametersquared. For example, a 6" pump with8" suction will take 1.78 times as longto prime the same lift and length ofpipe.

Avoid long horizontal and/or humps insuction lines that can trap air pockets.The suction inlet should always be thehigh point in the suction plumbing.

Obviously, air leaks can hinder primingand if bad enough could prevent apump from ever becoming primed.Small leaks may allow the pump toprime but between cycles allow thesuction leg to drop causing the pump toprime every time it starts. Duringpumping, when the air in the suctionline moves toward the higher vacuumin the eye of the impeller, it expandsand thus chokes off some of the flow sothe full potential of the pump is neverrealized.

Placement of the suction line in the pitis more important to pumping than it isto priming. Usually during priming thevelocity coming into the suctionentrance is slow enough that solids arenot immediately sucked up, nor will itsuck itself tight to the sump unless it isin very close proximity. However whenfull flow starts, both scenarios become apossibility. Suction lines should beplaced far enough from pit bottoms toavoid this problem. The suction inletmust also have enough water coveringit to prevent vortexing duringpumping. Severe vortexing can causethe pump to lose prime. Where solidsare present, even trash handling pumpsshould have a strainer with openingsslightly smaller than the smallestopenings in the pump.

Figure 2 Priming limits.

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Maintaining good face clearances isimportant in applications where thepriming limits are being stretched. Asimpeller tips wear and face clearancesincrease, priming times get longer andmaximum lifts decrease. Pumps withexternal adjustments make thismaintenance task easy enough that itshould not be neglected.

Like all centrifugal pumps, self-primingpumps can heat up if allowed to runwithout air or water going throughthem to keep the liquid fromoverheating. Likewise there is no safe

way get into any pump while it isoverheated. Even though the pumpcannot prime, the casing and linescould be pressurized with hot liquid orsteam. Always allow a hot pump to coolto ambient temperature beforeremoving any access into the pump ordisconnecting any plumbing. ■

CONTACTThe Gorman-Rupp Company305 Bowman St.Mansfield, OH 44903Phone: +1- 419-755-1207Fax: +1-419-755-1404Email: grsales@gormanrupp.comWebsite: www.gormanrupp.com

Figure 3a/3b. Pump set up.

In summary• Choose a pump to meet both priming and performance criteria.

• Install as close to the water source as possible.

• Provide for air escape - open discharge, release line back to sump, releasevalve.

• Avoid oversize or long suctions.

• Try to match flow or use a float control.

• Eliminate suction leaks.

• Allow hot pumps to cool before opening.