Specifying pumps the smart way
Engineers in charge of very large projects routinely have used a lifecycle cost analysis (LCA) to help decide among the alternatives available to them.
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Engineers in charge of very large projects routinely have used a lifecycle cost analysis (LCA) to help decide among the alternatives available to them. Wise decisions in expensive, long-lived projects are well worth the additional data collection, estimations, and calculations required by LCA.
In contrast, engineers who specify pumps for building service have traditionally not used LCA. All too often, they used a simple payback period analysis, or even worse, they went along with a general contractor's choice of the pump with the lowest initial cost. However, LCA concepts are not difficult to understand or apply, and they can help an engineer make better decisions on behalf of the building owner.
Simply stated, the lifecycle cost is the sum of all costs associated with owning the pump over its service life.
LCC = Ci + Cin + Ce + Co + Cm + Cd + Cenv + Cdis
LCC = Lifecycle cost
Ci = Initial cost, including the pump and fittings
Cin = Installation, commissioning, training costs
Ce = Energy cost over the pump's service life
Co = Operations, labor costs
Cm = Routine maintenance and repair
Cd = Costs incurred due to pump downtime
Cenv = Cost of remediating or preventing environmental damage
Cdis = Disposal cost
Centrifugal pumps tend to have a long service life, so the time value of the cost elements must be considered; most analyses calculate the net present value at some assumed interest rate.
The LCA is a simple spreadsheet exercise. The real difficulty lies in estimating the cost elements. Routine maintenance and downtime costs are difficult to estimate, unless there is a similar system already in operation and its maintenance history is available. Changes in energy cost over the next 10 to 20 years are hard to predict, and for smaller, less expensive systems the LCA may seem like “overkill.” Despite this, incorporating LCA ideas into the pump selection process will make for better choices.
Proper selection of a centrifugal pump is the first step toward equipment longevity, but it's not that easy. The design flow and head requirements of a system are obtained from estimations, rules of thumb, and safety factors, which often result in excess flows. On occasion, systems are not built as designed, creating either insufficient or excess flows. The actual flow rate of the system determines where the selected pump is operating on its performance curve, which ultimately determines the pump's expected life and energy use. It's likely (possible) that the pump is operating away from both the design criteria and its best efficiency point (BEP).
Operating with flows well above or below BEP lowers the pump's efficiency and results in excessive radial forces on the shaft. Lower efficiency will increase the pump's operation costs, and increased radial forces can lead to premature bearing, mechanical seal, or shaft failure. Operating centrifugal pumps with flows well above BEP can create two additional situations that could shorten the life of a pump.
First, the net positive suction head requirements of the pump increase. If the available net positive suction head is less than required, cavitation may occur, which could damage the impeller or casing. Secondly, fluid velocity within the pipes increases, which can lead to excessive noise, vibration, and erosion. These conditions can cause premature failure of additional components within the system, as well as the pump. Typical recommendations for pump selection limit the flow from 66% to 115% of BEP for the trimmed impeller diameter. This flow range will provide the most economical and trouble-free operation throughout the pump's life without adversely affecting the other components. It is important to keep the pump operating at the design condition.
Some pump manufacturers provide computer-based selection applications that show exactly where the pump will operate in a given system. Some also include the ability to estimate energy costs for a given load profile. A “load profile” is the relationship between percentage of full flow versus time. Some industrial pumps may operate at the same point on their curve, but pumps in building service are more commonly used in systems that vary flow. During periods of lower flow, energy costs are greatly reduced. Variable speed drives are increasingly popular in building service pumps, and properly applied, they can reduce energy costs and decrease the LCC while maintaining proper system design characteristics and extending pump life.
Proper pump installation is the second area that requires attention to attain optimal pump life. Always follow the manufacturer's recommendations in the installation manual for each classification of pump in the system. Base mounted, in-line, end-suction, and double-suction pumps all have their own installation nuances that require attention. Add pressure gauges on both the discharge and suction sides of the pump to allow for verification of the pressure differential of the pump. And finally, don't underestimate the added value of piping the pump correctly.
Suction and discharge piping should be properly supported to avoid any strain on the pump flanges. Large forces or moments on flanges can distort the casing causing premature failure of shafts, seals, or bearings. Also, take into account any thermal expansion or contraction of the piping and pump when calculating flange forces and moments. Avoid mounting elbows directly to the suction flange. Elbows supply an uneven flow to the impeller that can result in a shortened pump life. Ensure that the suction piping will deliver uniform, non-swirling flow by installing a length of straight pipe five to ten times its diameter before the pump. This will allow the pump to operate under ideal suction conditions, which will extend the life of the pump. A long radius elbow with an eccentric reducer or just a suction diffuser will straighten out flow and minimize pump suction difficulties.
Accurate shaft alignment will reduce shaft and bearing loads and result in less downtime and lower repair costs. Plus, newer style couplers can tolerate a large range of misalignment.
Commissioning should not be overlooked. Component failures often are caused by system problems that should have been recognized and corrected during the commissioning process. At this stage, all of the uncertainties during the design of the system have gone away, and the actual performance of the pump and system can be measured.
Verify the pump flow after the system is hydronically balanced throughout all circuits. If the pump is oversized, trimming the impeller reduces the flow and corresponding horsepower requirements. This can lower energy costs significantly. Throttling a valve on the discharge of the pump is another method to reduce flow, but this should be analyzed in detail. A decision should be based on an economic analysis comparing throttling and trimming costs on the expected life of the pump.
Also, check local energy codes regarding throttling losses; trimming the impeller may be the only solution if throttling losses are high. And it can be the best solution for saving energy and money for the owner.
Pump maintenance also plays a pivotal role in the life of a pump. Proper bearing lubrication is the most important step for extending the life of a pump. Follow the pump and motor manufacturers' maintenance recommendations on bearings and lubrication. Use the specified grease or oil and add a sufficient amount to get the job done—adding too little or too much will shorten the bearing life.
Finally, keep data on the pump's operating hours, bearing temperatures, noise levels, and lubrication schedules. This will help monitor the bearings' condition and can be used to prevent emergency shut downs. Predictive monitoring will reduce costs and allow replacement of bearings during a routine maintenance schedule.
Kutin is senior product specialist in the Commercial HVAC Pump group for ITT Residential & Commercial Water. He has experience with HVAC system design, applications, and centrifugal pumps.