Reverse osmosis (RO) hasbecome a significant processused in the purification of

water. Selecting the correct pump is asignificant step in a well-designed andproperly operating system. Althoughthe pump is a relatively small part ofthe overall cost of an RO system,expenses resulting from downtime and inefficiency quickly can diminishthe productivity of the system andyour return on investment. Due to the corrosive atmosphere and highdemands on these sea water ROsystems, special attention is required in the pump selection process. Thereare several key factors that will help to ensure optimum pump and system performance.

Pump CompatibilityPump compatibility is a key element in the selection process. Due to the fact that seawater is a corrosive liquid, it may contain a variety of microorganisms and the watertemperature may vary, the materials of construction of your pump arecritical. Some locations and waterconditions may tolerate a liquid-endconstruction of nickel aluminumbronze. Other locations may demandmore corrosion-resistant materialssuch as 316SS and Duplex StainlessSteel. Each installation must bereviewed on its own merits, but having these pump options assures the best fit to the installation.

Pump EfficiencyPump efficiency is a key element in keeping the costs of providing fresh water at an affordable price.Generally, positive displacement (PD)plunger pumps operate in the 80–90percent efficiency range, whilecentrifugal pumps are between 30–60percent efficient. With the lowerefficiency pumps, the addition of anenergy recovery system is required.When these energy recovery systemsare coupled with the higher efficiencyPD pumps, you can achieve maximumoutput and return on your investment.

Pump SpeedLower revolutions per minute (rpm)operation is highly recommended whenworking with corrosive liquids such asseawater. Higher rpm operation willfurther aggravate the corrosion-erosion wear process initiated by

seawater. Lower rpm also helps toreduce the risk of cavitation.

Ease of InstallationEasy and compact installation is aconsideration. Many RO systems areon board cruise ships, naval vessels,freighters, off shore platforms or athotels and resorts. Space and weightare two primary considerations.Convenient, easy connection of inletand discharge plumbing is preferred.Installation is easier when systemaccessories conveniently attach to thepump head or immediately in the inletand discharge lines. This minimizesdelays in valve response time andfacilitates periodic visual checks of thesystem performance. The pump shouldbe adaptable with booster pump feedas most often the seawater is liftedfrom a well or through the ship hullvalve or sea chest.

Accessories for Your SystemSystem accessories are nearly asimportant as the pump itself. Withoutproper pressure relief installed in thesystem, the pump is subject to deadhead

Water Quality ProductsSeptember 2003 29www.wqpmag.com

TECH UPDATE By Darla Jean Thompson, CAT PUMPS

Selecting a Pump forSeawaterReverseOsmosis

Making the Right DecisionBy Wendi Hope King, WQP Staff

As with any water treatment application, the right product needs to beselected in order to perform at its best. The pump selection for desali-nation projects is no exception. Key to any project’s success will be thequality of the pump and the service you receive from the manufacturer.

The most common types of pumps used in desalination projectstoday are plunger pumps, according to Iver Schmidt, president ofSK Watermakers, Inc. “Multistage centrifugal pumps can be usedbut are not as efficient and generally require more maintenancebecause of the higher speed of operation.”

Many times it may be price that motivates a particular purchase, butthere are many other things to consider such as the product’s quali-ty, the manufacturer’s service or the “lifetime” of the product. Hereare some tips to guide you through the pump selection process.

• Required maintenance. For Whitney W. Irons, president of MatrixUtility Corp., Matrix Desalination, Inc., trouble-free operation isthe most important aspect to consider when making the pur-chase decision. “We look for pumps that run without significantongoing maintenance,” he says.

Other tips for getting longer life out of a pump include running it reg-ularly to prevent corrosion, change the seals, flush with fresh waterand change the oil monthly. Plunger/pony shafts should be keptclean and free of salt and corrosion to avoid seal failure, adds Irons.

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C&IA commercial & industrial article

About the AuthorDarla Jean Thompson is the director of marketing and advertisingat CAT PUMPS. CAT PUMPS is a manufacturer of high pressurereciprocating plunger pumps and pumping systems up to 320gpm, 7,000 psi. For more information, call 763-780-5440;techsupport@catpumps.com; www.catpumps.com.

Water Quality Products September 200330 www.wqpmag.com

and destruction should the system failor become blocked. Pulsation isdamaging to the RO membranes. Apulsation dampener will minimizepulsation and prolong the life of theentire system. A suction stabilizer orcaptive acceleration tube will minimizethe inlet pulsations and reduce the riskof cavitation. A pressure gauge isessential in the system as a monitor ofsystem performance. As soon aspressure drops or begins to fluctuate, a maintenance inspection should takeplace. Prolonged operation with worncomponents can result in more seriousand costly repairs.

Service and SupportPump serviceability is a significant factorin selecting an RO pump. Deliveringfresh water requires a continuous-dutysystem and downtime is not tolerated.Periodic maintenance should beminimal, easy to perform and requirefew tools. Easily removed pump valvesand seals, and replaceable heads arepreferred. Good technical support andreadily available parts are essential.Selecting a manufacturer that cantechnically support your pump onceit’s in operation is not only a comfortbut a necessity. Knowing your pumpcan be serviced within 24 hoursanywhere around the world is a benefitfor you and your RO system.

Proven DependableDependability is an absolute necessity.RO is an application where a supplier’sreputation for dependability weighsheavily in the selection process. Withlives and livelihoods depending onfresh water delivered by your system,you do not want to be experimentingwith an unproven pump. You will want to select a pump manufacturerwho has experience with thisapplication and has a history of dependable performance. WQP

Making the Right Decisioncontinued from previous page

• Costs. Aside from the up front costs of purchasing the pump, con-sider the “hidden costs.” “We look for spare parts availability andcost,” Irons says. “You can buy an inexpensive pump but spend afortune on spare parts.” He recommends checking for life cyclecost over a 10-year period.

• Size selection. Once you have determined the size pump you willneed, think about purchasing a larger one. “A larger pump willbring down the rpm,” Schmidt says. “Pay particular attention tothe suction side (air removal, suction dampness, etc.).”

• Materials of construction. You know what solutions you are work-ing with. Make certain the pump’s materials are compatible with allsolutions and treatments it will come into contact with.

• Customer service. The service a manufacturer provides couldmean the difference between a project’s success and its failure.Despite a well-constructed product, you may need to contact themanufacturer in the future. Select the manufacturer that is willingto work with you on purchasing questions; future maintenance,repair or spare parts needs; and any other service needs you mayhave down the road.

• Price. Consider the price after determining the previously mentioned items. It is sometimes faster to jump on the price, but you could pay more in the long-term. WQP

continued from previous page

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