Pumping systems
General Description [1]
Pumps are used widely in industry to provide cooling and lubrication services, to transfer fluids for processing, and to provide the motive force in hydraulic systems. In fact, most manufacturing plants, commercial buildings, and municipalities rely on pumping systems for their daily operation. In the manufacturing sector, pumps represent 27% of the electricity used by industrial systems. In the commercial sector, pumps are used primarily in heating, ventilation, and air-conditioning (HVAC) systems to provide water for heat transfer. Municipalities use pumps for water and wastewater transfer and treatment and for land drainage. Since they serve such diverse needs, pumps range in size from fractions of a horsepower to several thousand horsepower.
Cost-effective operation and maintenance of a pumping system require attention to the needs of both individual equipment and the entire system. Often, operators are so focused on the immediate demands of the equipment that they forget to step back and notice how certain system parameters are affecting this equipment.
A key to improving system performance and reliability is to fully understand system require-ments (peak demand, average demand, and the variability of demand) with respect to time of day and time of year. It is much simpler to design and operate systems with relatively consistent requirements than to have to account for wide variations in demand.
Pump selection starts with a basic knowledge of system operating conditions: fluid properties, pressures, temperatures, and system layout. These conditions determine the type of pump that is required to meet certain service needs. There are two basic types of pumps: positive displacement and centrifugal. Although axial-flow pumps are frequently classified as a separate type, they have essentially the same operating principles as centrifugal pumps.
Inefficient system operation can be caused by a number of problems, such as improper pump selection, poor system design, excessive wear-ring clearances, and wasteful flow control practices. Indications of inefficient system operation include high energy costs, excessive noise in the pipes and across valves, and high maintenance requirements.
Each centrifugal pump has a best efficiency point (BEP) at which its operating efficiency is highest and its radial bearing loads are lowest (except for pumps with concentric case designs). At its BEP, a pump operates most cost-effectively in terms of both energy efficiency and maintenance. In reality, continuously operating a pump at its BEP is difficult because systems usually have changing demands. However, selecting a pump with a BEP that is close to the system’s normal operating range can result in significant operating cost savings.
Potential Energy and Cost Savings Opportunities:
The below energy conservation opportunities or energy efficiency actions (EE ACTIONS) are provided as a partial list of potential savings opportunities in your plant. They are grouped as no-to-low cost, moderate cost, and long-term cost investments. Consider each for your plant and feel free to contact us for clarification or any assistance you may need in assessing specific projects.
No-to-low cost investment
EE ACTION: Conduct an In-Plant Pumping System Survey [2]
Pumps larger than a minimum size and with significant operating hours should be surveyed to determine a baseline for your current pumping energy consumption and costs, identify inefficient pumps, determine efficiency measures, and estimate the potential for energy savings. The U.S. Department of Energy’s (DOE) Pump System Energy Opportunity Screening worksheet will help you identify systems that merit a survey.
EE ACTION: Pump Selection Considerations [2]
When designing a pump system it is important to understand that pumps transfer liquids from one point to another by converting mechanical energy from a rotating impeller into pressure energy (head). The pressure applied to the liquid forces the fluid to flow at the required rate and to overcome friction (or head) losses in piping, valves, fittings, and process equipment. The pumping system designer must consider fluid properties, determine end use requirements, and understand environmental conditions. Pumping applications include constant or variable flow rate requirements, serving single or networked loads, and consisting of open loops (non-return or liquid delivery) or closed loops (return systems).
EE ACTION: Select an Energy-Efficient Centrifugal Pump [2]
Centrifugal pumps handle high flow rates, provide smooth, non-pulsating delivery, and regulate the flow rate over a wide range without damaging the pump. Centrifugal pumps have few moving parts, and the wear caused by normal operation is minimal. They are also compact and easily disassembled for maintenance. The design of an efficient pumping system depends on relationships between fluid flow rate, piping layout, control methodology, and pump selection. Before a centrifugal pump is selected, its application must be clearly understood.
EE ACTION: Test for Pumping System Efficiency [2]
A pump’s efficiency can degrade as much as 10% to 25% before it is replaced, according to a study of industrial facilities commissioned by the U.S. Department of Energy (DOE), and efficiencies of 50% to 60% or lower are quite common. However, because these inefficiencies are not readily apparent, opportunities to save energy by repairing or replacing components and optimizing systems are often overlooked.
EE ACTION: Proper Maintenance [2]
Effective pump maintenance allows industrial plants to keep pumps operating well, to detect problems in time to schedule repairs, and to avoid early pump failures. Regular maintenance also reveals deteriorations in efficiency and capacity, which can occur long before a pump fails. Wear ring and rotor erosions, for example, can be costly problems that reduce wire-to-water efficiency by 10% or more.
EE ACTION: Match Pumps to System Requirements [2]
An industrial facility can reduce the energy costs associated with its pumping systems, and save both energy and money, in many ways. They include reducing the pumping system flow rate, lowering the operating pressure, operating the system for a shorter period of time each day, and, perhaps most important, improving the system’s overall efficiency. Often, a pumping system runs inefficiently because its requirements differ from the original design conditions. The original design might have been too conservative, or oversized pumps might have been installed to accommodate future increases in plant capacity. The result is an imbalance that causes the system to be inefficient and thus more expensive to operate.
EE ACTION: Trim or Replace Impellers on Oversized Pumps [2]
As a result of conservative engineering practices, pumps are often substantially larger than they need to be for an industrial plant’s process requirements. Centrifugal pumps can often be oversized because of “rounding up,” trying to accommodate gradual increases in pipe surface roughness and flow resistance over time, or anticipating future plant capacity expansions. In addition, the plant’s pumping requirements might not have been clearly defined during the design phase. Because of this conservative approach, pumps can have operating points completely different from their design points. The pump head is often less than expected, while the flow rate is greater. This can cause cavitation and waste energy as the flow rate typically must be regulated with bypass or throttle control. Manufacturers can often provide trim correction charts based on historical test data.
Moderate investment
EE ACTION: Optimize Parallel Pumping Systems [2]
When multiple pumps operate continuously as part of a parallel pumping system, there can be opportunities for significant energy savings. For example, lead and spare (or lag) pumps are frequently operated together when a single pump could meet process flow rate requirements. This can result from a common misconception—that operating two identical pumps in parallel doubles the flow rate. Although parallel operation does increase the flow rate, it also causes greater fluid friction losses, results in a higher discharge pressure, reduces the flow rate provided by each pump, and alters the efficiency of each pump. In addition, more energy is required to transfer a given fluid volume.
EE ACTION: Reduce Pumping Costs through Optimum Pipe Sizing [2]
Every industrial facility has a piping network that carries water or other fluids. According to the U.S. Department of Energy (DOE), 16% of a typical facility’s electricity costs are for its pumping systems. The power consumed to overcome the static head in a pumping system varies linearly with flow, and very little can be done to reduce the static component of the system requirement. However, there are several energy- and money-saving opportunities to reduce the power required to overcome the friction component.
EE ACTION: Energy Savings Opportunities in Control Valves [2]
Pumping system control valve inefficiencies in plant processes offer opportunities for energy savings and reduced maintenance costs. Valves that consume a large fraction of the total pressure drop for the system or are excessively throttled can be opportunities for energy savings. Pressure drops or head losses in liquid pumping systems increase the energy requirements of these systems. Pressure drops are caused by resistance or friction in piping and in bends, elbows, or joints, as well as by throttling across the control valves. The power required to overcome a pressure drop is proportional to both the fluid flow rate (given in gallons per minute [gpm]) and the magnitude of the pressure drop (expressed in feet of head).
EE ACTION: Adjustable Speed Pumping Applications [2]
Most pumps operating today were selected to meet a maximum system demand, or potential future demands. This means that most pumps are oversized, rarely operating at their full design capacity. In addition, pumps are often installed in systems with multiple operating points that coincide with process requirements. A throttling valve is usually employed when the process flow requirement is less than the flow at the pumping system’s natural operating point.
EE ACTION: Control Strategies for Centrifugal Pumps with Variable Flow Rate Requirements [2]
In pumping applications with variable flow rate requirements, adjustable speed drives (ASDs) are an efficient control alternative to throttling or bypass methods. Due to drive inefficiencies, however, ASDs do not save energy in applications that operate close to fully loaded most of the time. It is important to understand the control parameters for each system, and to ensure that control algorithms operate pumps in the most economical configurations.
References
[1] exerpts from Improving Pump System Performance: A Sourcebook for Industry, National Renewable Energy Laboratory (U.S.), 2006, Second Edition, United States Department of Energy, Washington, D. C., pp. 122 (cited 9/13/10),
[2] Energy Tips – Pumping Systems, Industrial Technologies Program (U.S.), United States Department of Energy, Washington, D. C., (cited 9/13/10). To find more tip sheets go to EERE Publication and Product Library.
