In the United States, pumps used for a multitude of purposes consume some 142 billion kWh annually. At a cost of five cents per kWh, this translates to more than $7.1 billion per year in operation costs.
A single continuously operated centrifugal pump driven by a fully loaded 100-hp motor will use 726,000 kWh and cost about $36,000 per year. A 10 percent reduction in operating costs can produce $3,600 in savings annually.
For wastewater treatment professionals, the lesson is that effective maintenance practices and a survey of existing pumping systems are essential for keeping treatment plant pumps operating well. These measures help in detecting problems, scheduling repairs, and avoiding early failures.
Regular maintenance also reveals deteriorations in efficiency and capacity, which can occur long before a pump fails. For example, wear ring and rotor erosions can be costly problems that reduce wire-to-water efficiency by 10 percent or more. Beyond energy savings, downtime can prove extremely costly when it affects critical processes.
Preventive actions
Preventive maintenance includes coupling alignment, lubrication and seal maintenance and replacement. Mechanical seals must be inspected periodically to ensure that there is no leakage, or that leakage is within specifications. Mechanical seals that leak excessively usually must be replaced.
A certain amount of leakage is required to lubricate and cool the packing seals, but the packing gland needs to be adjusted if the leakage exceeds the manufacturer’s specifications. The packing gland must be replaced if it has to be tightened excessively to control leakage. Overtightening causes unnecessary wear on the shaft or its wear sleeve and increases electric power use. Routine maintenance of pump motors, such as proper lubrication and cleaning, is also vital.
Predictive actions
In addition to planned maintenance, predictive maintenance helps minimize unplanned equipment outages. Sometimes called condition assessment or condition monitoring, this practice has become easier with modern testing methods and equipment. Methods that apply specifically to pumping systems include:
Vibration analysis. Trending of vibration amplitude and frequency can detect an impending bearing failure and reveal voltage and mechanical imbalances that could be caused by impeller erosion or coupling problems. Changes in vibration over time are more meaningful than a single snapshot of the vibration spectrum.
Motor current signature analysis. Sometimes called dynamic analysis, this technique reveals deteriorating insulation, rotor bar damage, electrical system unbalance, and harmonics. It can also pick up system problems such as malfunctioning control valves that cause flow rate disturbances. Again, tracking of the signature over time is more valuable than a single measurement.
Lubrication oil analysis. This technique applies only to large, oil-lubricated pumps and is an expensive procedure. Oil analysis can detect bearing problems caused by metal particles or chemical changes that result from overheating, and seal problems caused by pumped fluid in the oil. It also gives guidance on proper oil-change intervals.
Periodic efficiency testing. Many progressive operators test wire-to-water efficiency and keep records to spot trends.
Surveying pumping systems
Pump system surveys should begin with the gathering of a broad range of information, including details from the drive motor nameplate, operating schedules, and head/capacity curve specifications (if available) that highlight design and operating points. A survey also should assess the system’s flow rate and pressure requirements, pump style, operating speed and number of stages.
A thorough survey also should check suction and discharge pressures and look for conditions commonly associated with inefficient pump operation. These include:
• High maintenance requirements
• Oversized pumps that operate in a throttled condition
• Throttled control valves that provide fixed or variable flow rates
• Cavitating, badly worn, noisy, clogged or misapplied pumps
• Pumping systems with large flow rates, pressure variations or bypass flow
• Impeller and casing wear, which increase clearances between fixed and moving parts
• Excessive wear on wear rings and bearings
• Improper packing adjustment that causes binding on the pump shaft
• Multiple pump systems where excess capacity is bypassed or excess pressure is provided.
In addition, engineers and other maintenance personnel need to review changes in the pump system that vary greatly from initial design conditions. Alterations to distribution system cross-connections, parallel mainlines, pipe diameter and pipe material can negatively affect the original system curve.
After a careful review of all these characteristics, professionals can better identify the sources of pumping system inefficiency and take a variety of energy- and cost-saving measures. These can include shutting down unnecessary pumps and re-optimizing pumping systems by using pressure switches to control the number of pumps in service when flow rate requirements vary.
Another technique entails reducing system operating pressures using a booster or dedicated pump when an entire pumping system may be operating at high pressure to meet the requirements of a single end use.
Looking deeper
Other efficiency procedures for consideration are the restoration of internal clearances, replacement of standard efficiency pump drive motors with NEMA Premium motors, switching oversized pumps with properly sized units, and trimming or changing pump impellers to ensure that outputs match system requirements.
Since most pump systems last more than 15 years, excessive costs related to inefficiencies can accumulate if left unchecked. Optimum design and operation efficiency are in the best interests of treatment facilities, which rely heavily on pumping systems for the operation of critical processes.
Treatment operators have access to excellent sources of information about pumping systems and their proper care. These include:
The Hydraulic Institute (HI). The HI is the largest association of pump producers and suppliers to the pump industry in North America and is a global authority on pumps and pumping systems. Its mission is to serve as a forum for the exchange of industry information, while providing value-added services to member companies and pump users worldwide. Visit www.pumps.org.
Pump Systems Matter (PSM). This initiative was created by the HI to help North American pump users gain a competitive business advantage through strategic, broad-based energy management and pump system performance optimization. The mission is to provide tools and collaborative opportunities to integrate pump system performance optimization and efficient energy management practices into normal business operations. Visit www.pumpsystems matter.org.
U.S. Department of Energy (DOE). The department’s Industrial Technologies Program (ITP), through partnerships with industry, government and nongovernmental organizations, develops and delivers advanced energy efficiency, renewable energy, and pollution prevention technologies for industrial applications. Best Practices is a part of ITP and offers resources on ways to reduce energy and maintenance costs. These include training workshops, software tools, sourcebooks, case studies and tip sheets. A Pumping System Assessment Tool aids in assessment of pumping system efficiency and in estimating energy and cost savings. Visit www.eere.energy. gov/industry.
Other helpful references include:
• ANSI/HI Pump Standards, Hydraulic Institute, 1997-2005.
• Pump Life Cycle Costs: A Guide to LCC Analysis of Pumping Systems, Hydraulic Institute & Europump, 2001.
• Extend Your Motor’s Operating Life, HI/PSM/DOE Tip Sheet, 2006.
• Test for Pumping System Efficiency, HI/PSM/DOE Tip Sheet, 2006.
This article is adapted from Improving Pumping System Performance: A Sourcebook for Industry, developed jointly by the U.S Department of Energy and the Hydraulic Institute (HI).
