When purchasing centrifugal wastewater pumps, several key factors should always be considered for a successful application. The essential factors for any individual pump installation are performance and service life. Performance is affected by design specifications (capacity and total dynamic head) and pump efficiency. Service life is the length of operating time before major components must be replaced. When centrifugal pumps come as part of an entire pre-engineered pump station, other important factors become equally important, including capital cost and maintenance requirements.
Performance
Performance is the key ingredient of any centrifugal pump purchase. Most suppliers of pumps make available published pump curves and other relevant data to demonstrate how the pump should perform at given operating conditions. These curves help ensure that the pump is used for the proper application and is appropriately sized. If the pump is improperly applied, it can fail rapidly. Improper application also can result in excessive stresses, cavitation or accelerated deterioration of a pumps internal components. Therefore, it is always important to analyze the pump curves provided by suppliers. Published pump curves also can be used to determine a pumps efficiency under various operating conditions.
Service Life
Performance is only valuable if service life is long. Because all pumps contain wearing parts, higher quality materials of construction make a pump more durable. Likewise, proper routine maintenance preserves these components, extending service life. To make an informed decision when analyzing a pumps construction, it may be helpful to understand what increases wearing and contributes to pump failure.
Radial thrust is the most significant cause of stress on internal components. In a centrifugal pump with a single volute, near uniform pressures act on the impeller when the pump is operated at its design capacity and best efficiency point (BEP). The kinetic activity within centrifugal pumps and varying flows common in wastewater applications make radial thrust inevitable because the pressure at the periphery entrance of the volute is not always constant. Under other flow conditions, pressures are not uniform, causing a radial thrust on the impeller. The thrust causes the pump shaft to bend slightly (and may cause the impeller to shift unevenly). As radial thrust increases - resulting in shaft bending - problems with bearings and mechanical seals are likely. Continued operation under such conditions accelerates the deterioration of mechanical and hydraulic performance and ultimately, pump failure results.
Because all centrifugal pumps experience some radial thrust, operators and maintenance personnel should understand what certain pump components do and how they should be maintained. These components include the pump shaft, volute design, mechanical seals, impeller and bearings.
Pump Station ConsiderationsCapital Costs
When making buying decisions for pump stations, map out your potential for "hidden" costs by figuring the total installation cost. Total installation cost is the sum of all expenses for equipment, services and materials necessary to complete and operate an entire pump station. The cost variables dependent upon the type of station selection include: (1) excavation, (2) amount of concrete, (3) ancillary equipment to handle pumps, (4) equipment and gear for personnel. A proper analysis of these figures will provide a more accurate total of the actual capital investment.
Maintenance Requirements
Routine maintenance is important for extending the life of a wastewater pump. Therefore, ease of maintenance and accessibility to the pump is a key consideration for pump stations. Ask yourself these questions: How does the pump station installation simplify maintenance concerns? Does the station facilitate easy access to the pump, or at least provide adequate sensors, instrumentation or telemetry to indicate problems? What are the potential confined space considerations and necessities, if any?
Regardless, potential confined-space requirements saddle municipal facilities with additional paperwork and thorough training. Workers also must be suited with appropriate safety equipment, which can be costly to purchase and maintain. Obviously, it is more feasible to minimize confined space entry when possible. There is universal agreement among safety specialists that it is better to eliminate hazardous work conditions through engineering than it is to deal with hazards head-on.
Finally, always choose from a manufacturer or supplier that is reliable. The ability to provide parts is something an operator or maintenance technician will depend upon in the future. A responsible manufacturer or distributor with detailed records and knowledge of each particular installation can make a dramatic difference in expenses and convenience in times of poor pump performance.
Conclusion
Responsible equipment selection and analysis, as well as proper routine maintenance, are important factors in determining the cost and service life of centrifugal pumps. These points should be considered with other standard guidelines to realize cost savings in not just the short term, but also over the long term. When time is taken to understand the variable factors that contribute to the actual costs of installing and operating centrifugal pumps, an excellent return on investment - not to mention less hassle - is almost always guaranteed.
About the Author:
Mr. Weis, with nearly 45 years of pumping engineering experience and holder of more than 35 patents, is Senior Engineering Consultant with Smith & Loveless Inc., Lenexa, Kan.
