Wastewater treatment facilities, chemical and power-generation plants, electric and gas companies, municipalities, and commercial buildings all have a common prime mover—centrifugal pumps. Collectively, these pumping systems represent an enormous amount of electrical energy usage. Pump systems, on average, represent the highest energy usage of any type of rotating equipment employed in industrial and commercial facilities. For example, pumping systems account for 20 to 30 percent of electrical energy consumption in wastewater plants and up to 46 percent in municipal water systems. By comparison, chemical and allied product plants typically use enough electrical power in pump systems to run a small city.
Pump system designs are normally in the range of 65 to 85 percent at their best efficiency point (BEP). Because of oversizing and sometimes undersizing, pump systems typically operate well below their BEP. The excess energy usage is transmuted into vibration, heat and noise, all of which increase maintenance and energy costs.
When not optimized for best efficiency, pump systems drain maintenance budgets by decreasing the mean time between repairs. Pump systems represent one of the best ways to reduce overall plant operating costs.
The Underlying Cause of Pump Inefficiency
One reason so many pumping systems run significantly below their BEP is that organizations focus on purchase price rather than the total life-cycle cost (LCC) of the system. Figure 1 depicts the LCC of a standard 75-horsepower (hp) pumping system over 20 years of operation. While the initial purchase price and installation cost represent 17 percent of the LCC, 55 percent of costs are related to operations, and the 28 percent balance is maintenance costs. In severely oversized systems, the first cost of the pump can drop to 10 percent or less. Rather than base a purchasing decision on price alone, plant designers should evaluate a system’s total LCC and make decisions that will minimize energy and maintenance costs.
Another important factor affecting pump efficiency is the lack of system standards that guide the design of efficient pumping systems. As a result, engineering, procurement and construction (EPC) firms continue to use the same design approaches—“if-it-ain’t-broke-don’t-fix-it syndrome.” These routine configurations typically consist of a fixed-speed control valve system. This design approach has been mostly unchallenged. The design engineer often cites time and budget as the reasons other designs, such as not using a variable-speed drive or parallel multi-pump system in lieu of an oversized control valve system, are not considered.
If the design process uses more due diligence, engineering firms can easily overcome perceived barriers to achieve the lowest practical LCC. Furthermore, without a design standard, end users will lack the needed information to effectively challenge designers and prove that a system is poorly or incorrectly conceived. Even today, industry continues to design and install oversized pumping systems because of the lack of a standard. There is no good reason not to use pump and pipe design tools and techniques that optimize the selection of best pump, pipe and control valve combinations to achieve the lowest LCC.
Effects of Pump Inefficiency
For maximum efficiency, pumps should operate at or near their mechanical BEP. Optimally, pumps should not run at flow more than 10 to 15 percent outside of the BEP. When operating at excess capacity or greater than BEP, pumps may surge and vibrate, creating potential bearing and shaft seal problems while requiring excessive power. Cavitation may also occur, causing damage to pump components.
When pumps operate at reduced capacity, or lower than BEP, fixed-vane angles may cause eddy currents within the impeller, inside the casing and between the wear rings. The radial thrust on the rotor will increase, causing higher shaft stresses, increased shaft deflection, and potential bearing and mechanical seal problems. Radial vibration and axial shaft movement will also increase. Figure 2 shows the effects of operating away from the BEP on the pump.
The Need for Pump Systems Optimization
Systems optimization is the process of evaluating pumping systems to identify opportunities for improvements that will reduce energy consumption and improve reliability. Improving a single component—installing a more efficient motor, for example—will do little to improve overall system efficiency. Engineers who wish to implement systems optimization must evaluate how all pump components work together and determine how to make certain system changes to improve net efficiency.
Greater pump system efficiency achieved through systems optimization will improve reliability and lower operating costs by reducing wear and tear. This will decrease downtime and costs associated with lost production, maintenance and repairs, while extending equipment life. Costs associated with downtime routinely exceed energy and reliability costs combined.