BOATKEEPER

A Water Pump PrimerFrom Pacific Fishing, March 2001By Terry Johnson, University of Alaska Sea Grant, Marine Advisory Program4014 Lake Street, Suite 201B, Homer, AK 99603, (907) 235-5643, email: rftlj@uaf.edu

The need to move fluids from one place toanother is universal on boats. Water pumpsclear bilges and flush heads and holdingtanks, supply house water to sinks and show-ers, circulate engine coolant, and providepressurized water for washdown and refrig-eration. Other pumps transfer fuel, drain oil,compress refrigerants, and pressurize hydrau-lic systems. This report addresses waterpumps only. A separate issue of Boat- keeperdiscusses bilge pumps specifically.

Water pumps fall into three generaltypes: center-suction centrifugal pumps, vari-able-volume flexible-impeller and vanepumps, and positive displacement pumps,including power and manual diaphragmpumps and piston, gear, rotary, and lobepumps. Vane, gear, and rotary pumps nor-mally are used for moving oils and fuels,and lobe pumps are used to move cosmet-ics and solids.

How They WorkAll pumps use atmospheric pressure to pushthe fluid into the pump housing to fill thevacuum created when fluid or air alreadyin the housing is expelled by the action ofthe pump. If the pump is capable of start-ing up dry and sucking fluid up an inlet lineit is considered self-priming. If its inlet hasto be immersed or if the fluid has to bepoured into it prior to starting, it is non-self-priming.

Centrifugal water pumps use an electricmotor to drive a hard plastic or metal rotor.The blades are set in a pattern such thatwhen the rotor turns water is sucked into thecenter of the chamber and spun outward tothe periphery of the housing and throughthe housing outlet. Center-suction centrifu-gal pumps (like the common electric bilgepump) are non-self-priming and must be sub-merged to work. Some types of centrifugalsare self-priming; they have a suction inletabove the centerline of the pump housing.

Centrifugal pumps are somewhat inef-ficient but are versatile and relativelysimple, cheap, and easy to maintain and re-

pair. They are moderately resistant to clog-ging and damage from debris in the water.The rotor does not contact the housing sothere is no friction or damage from runningdry, although the lip seal that prevents wa-ter from entering the motor may soon over-heat and cause the motor to burn up if it isnot lubricated and cooled by water. Theyare subject to drastic reduction in flow rateas a result of head pressure, which is dis-cussed further in the report on bilge pumps.Most engine coolant circulating pumps arecentrifugal, an application where neitherdebris nor head pressure is a problem.

The variable-volume flexible impellerpump has an impeller with neoprene or ni-trile blades revolving inside a housing that

is circular except for a ramped cam in theliner adjacent to the outlet port. As the im-peller spins, each blade leaves the cam,unfolds to full length at the inlet port, andentraps a small quantity of water. As it ap-proaches the side of the housing with theoutlet port it is folded back by the cam andcompresses the trapped water, forcing it outthrough the port. Commonly called a“Jabsco,” these pumps are also made byJohnson, Sherwood, and other companies.

Flexible impeller pumps are self-prim-ing but should not be run dry more than afew seconds or the impeller blades may burnand break. They serve as engine coolingseawater pumps, but a sea strainer is nec-essary to protect the impellers from beingdamaged by debris. Head macerator pumpscombine a flexible impeller (suction side)with a centrifugal impeller (discharge side)and set of cutting blades to lift water intothe toilet bowl, chop waste, and discharge itto the holding tank with a single motor drive.

The flexible impeller pump differs fromthe vane pump, which has spring-loadedrigid vanes that slide in and out throughgrooves in the rotor hub. The rotor is offsetin the housing, and the vanes retract ratherthan bend as they contact the near side ofthe liner. Vane pumps tend to be more du-rable and more expensive than flexible im-peller pumps, and are generally used topump oils or other fluids that would be dam-aging to flexible impellers.

Positive-displacement pumps work onthe principle of changing the interior vol-ume of the pump chamber, increasing it tosuck fluid in, then decreasing it to force thefluid out the other side. Although differentmechanisms are employed in differentpumps, nearly all are self-priming and useatmospheric pressure to charge the chamber.

As the name implies, diaphragm pumpsuse one or more flexible rubber-like dia-phragms attached to a lever or cam arrange-ment which, when activated, sucks waterthrough the inlet and pushes it through theoutlet. A set of flapper valves keeps the fluid

from going the wrong direction. Bellowspumps work the same way except for a dif-ferent housing shape.

With no internal restriction, single dia-phragm pumps can pass fairly large lumpswithout choking, although small solids likescrews and even hair can damage or disablethe flapper valves. They are ideal for emp-tying holding tanks, but have limitations asbilge pumps because they have to be mountedabove maximum bilge-water level, and be-cause of the potential for debris damage.The larger capacity permanently installedmanual bilge pumps use double diaphragms.

Most house water pumps have a singlediaphragm in a single housing with three,four, or five small chambers and a wobbleplate actuated by a revolving shaft. This de-sign is compact and quiet, and provides amore even flow than a single chamber dia-phragm pump but is very sensitive to solidobjects in the water.

Piston pumps work in much the samefashion as the diaphragm and bellows typesexcept that a rigid piston is used instead ofa flexible diaphragm. Manual heads (toi-lets) use piston pumps.

Pump SelectionPumps are selected on the basis of the fol-lowing criteria: type of fluid to be moved(is it pure water, does it contain debris, oils,corrosive chemicals or non-macerated sew-age?), capacity required, lift required, spaceconstraints, whether it is likely ever to be rundry, and whether it needs to be self-priming.

Most flexible impellers are neoprene,which is cheap and flexible but quicklydestroyed by all sorts of chemicals, fromantifreeze to detergents, diesel fuel, andurine. Nitrile may be more expensive butis also more resistant to chemicals and oils.The last four digits of the impeller’s modelnumber tell the material: 0001 is neopreneand 0003 is nitrile (0004 is viton, a morechemical resistant plastic used to pump ac-ids and solvents). Centrifugal pump impel-lers generally are immune to chemical andoil damage because usually they are bronzeor plastic, although their lip seals may havechemical tolerance problems. Diaphragmand bellows pumps may be susceptible todamage by chemicals.

Although some pump types—large vol-ume diaphragm pumps and centrifugalpumps—are less subject to debris damagethan others, all pumps should be protectedby intake strainers, and diaphragm pumpswith multiple small chambers or valvesshould have a good in-line screen or filter.

Vane and flexible-impeller pumpsshould not be run dry. The others can berun without water for extended periodswithout damaging impeller or diaphragm,but motors and shaft seals may overheat.Motors are designed so that, if the mecha-nism becomes jammed, the overheatingcondition won’t start a fire.

Centrifugal and displacement pumpsreact differently to head pressure (resistanceon the discharge side). Flexible impeller,vane, and positive-displacement pumps willcontinue to try to pump the same volumeof fluid—drawing more amperage as theydo so—until resistance overcomes thepower of the pump and a fuse pops or themotor overheats and stops. This means thatin applications where flow can be stoppedat the discharge, such as on a washdown

system with garden hose nozzle, the pumpmust be fitted with an automatic pressureswitch. Centrifugal pumps will continue torun but flow will diminish to the point ofceasing altogether if resistance is sufficient.

Only centrifugal pumps are designed tobe submerged. Other types of pumps mayoperate at or below the level of the fluid theyare pumping as long as the fluid is containedin the plumbing. Positive-displacementpumps can suck fluids to heights of threeto ten feet or more, so they can be mountedwell above the level of the fluid, and canbe mounted in any position. In some appli-cations, self-priming capacity can be en-hanced by addition of a check valve in thesuction line to retain fluid in the line.

Flow capacity and the ability to pushwater uphill are important factors in select-ing a pump. They are addressed in the re-port on bilge pumps.

Operating and Maintenance TipsElectric Pumps:

• Ensure that voltage is adequate. Volt-age drops diminish pump output dra-matically.

• Except for submersibles, mountpumps where they will stay dry. Cor-rosion kills electric pumps.

• Keep electrical wiring and connec-tions out of the bilge, dry, and sealedagainst moisture.

• Use an in-line fuse or breaker to pre-vent overheating and possibly a fireif the pumping mechanism jams.

Saltwater Pumps:

• Use an inlet grate and sea strainerthat can be cleaned to prevent debrisdamage.

• Use an appropriate-size through-hullfitting complete with ball valveseacock.

• If the pump is mounted below thevessel’s waterline put an anti-siphonloop in the line.

Flexible Impeller Pumps:

• Use the correct impeller and checkit periodically for damage. Keepspares handy. Write down the model

number. Some Jabsco and Johnsonsplined shaft impellers are inter-changeable but Sherwood impellersdon’t interchange with either be-cause they fit a keyed shaft.

• If the pump is stored for a long pe-riod, remove the impeller. To win-terize, loosen the end cap and drainthe water to prevent freeze damage.

• When replacing a flexible impellerit doesn’t matter which way thevanes face; they will align them-selves correctly as soon as the pumpoperates.

All Types of Pumps:

• Use appropriate smooth-wall hosematerials in the correct diameter.Keep hose runs as short as possibleand minimize bends to allow a freeflow of discharge water.

• In house-water applications, use acollector tank to minimize surgingin lines and reduce the amount ofcycling. ✦

Thanks to Ron Evans of Great WesternPump Co. in Seattle for help with this report.