Engineered Systems - October 2008 - (Page 32)

HydroTech BY JOHN KUEMPEL JR., P.E. SWISS CHEESE Pump Impeller Don’t let cavitation put holes in your performance. This is the real deal. This pump impeller was removed from a six-month-old condenser water filtration installation. The bronze casting is almost completely gone. The impeller blades are deteriorated like swiss cheese. The pump flange casting even has complete erosion through ½ in.-thick material. Pump selection must be particularly careful when the pressure and temperature of the water entering the pump can reach the vapor pressure of the fluid. When the absolute pressure at the pump surface approaches the vapor pressure, vapor pockets form in the impeller passages. As the pressure increases, these vapor pockets collapse and are destructive to the surrounding materials. The collapse of vapor pockets is called cavitation. Cavitation is noisy and sounds likes marbles in the pump. It is easy to identify. Extreme cases left uncorrected can cause severe destruction ”— just look at the pictures. Typically, if the pump is incorrectly selected, the only solution is to switch to a different pump. In some cases, the pump flow can be reduced by closing a discharge valve, but system performance may be compromised. To prevent cavitation, pumps should be selected with the correct net positive suction head required (NPSHR). This pressure, published by the pump manufacturer, is the point above which there will be pressure to prevent formation of vapor pockets. NPSHR will vary with a given pump for different flows and pump speeds. It is typically included on the pump performance curve. Systems where particular attention to pump selection to prevent cavitation is necessary include condenser tower applications, steam condensate return pumps, steam boiler feed pumps, and other open systems. Closed heating and cooling piping systems are generally not a problem as the fill pressure can be set to increase pump suction above the required NPSHR. The next time you have a pump that sounds like it has marbles in it — remember these pictures. ES Kuempel is vice president of design-build for DeBra-Kuempel, an EMCOR company providing HVAC, electrical, plumbing, automation, service, and maintenance in the Cincinnati and Dayton Ohio area. 32 En gi neer ed S y stem s October 2008

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Engineered Systems - October 2008 Contents Editor’s Note Letters Back2Basics HVAC Challenge Case in Point Commissioning Building Automation HydroTech HVACR Designer Tips Special Insert: Engineer's Technical Library Fall 2008 Living Wild! Chilled Beams in Chicago Phoenix Convention Center: On The Rise Cold-Climate HVAC Challenges Humidification Strategies For Data Centers & Network Rooms Special Advertising Supplement: Tabb Talk Computers & Software Issues & Events Products Literature & Catalogs Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - October 2008

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