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Pumps for Power


Topic last reviewed: 1 February 2014
Sectors:  Downstream, Midstream, Upstream

Pumps are used throughout the petroleum and natural gas industry for different exploration and production processes as well as oil and gas transportation and refining applications. Typical pump applications include:

  • Crude oil transfer (truck loading or transfer to pipeline);
  • Secondary recovery (well water flooding [seawater or freshwater injection], chemical injection);
  • Glycol dehydration;
  • Produced water disposal;
  • Blow out preventer (BOP pumps);
  • Hydraulic fracturing;
  • Well servicing;
  • Amine sweetening;
  • Water cooling systems;
  • Fire protection;
  • Lean oil circulation;
  • Refinery and gas plant process fluids;
  • Water disposal for refineries and gas plants;

Closed drain systems;

  • Knockout drum pump-out (vessel emptying); and
  • Liquefied natural gas (LNG).

This topic does not include submersible pumps and gas lift pumps which are covered in the energy efficient activation topic. Pumps come in an extensive range of sizes and multiple types. Pumps are more generally classified into two groups based on the way they add energy to a fluid: positive displacement pumps and centrifugal pumps. Positive displacement pumps pressurize the fluid directly, while centrifugal pumps (also called “roto-dynamic pumps”) speed up the fluid and convert this kinetic energy to pressure.

Common Issues with Pumping Systems

Problems in individual pumps and pumping systems are typically a result of poor design and improper system operation. Often, pumping system problems occur from improper pump selection and operation, later producing considerable maintenance needs and loss of energy efficiency. Examples of these problems include inefficient operation, poor flow control, internal recirculation and high maintenance. Due to the mechanical nature of pumps, they are also subject to wear, erosion, cavitation, and leakage.

A pump’s efficiency can degrade as much as 10% to 25% before it is replaced.  Efficiencies of 50% to 60% or lower are quite common in degraded pumps [Reference 1].  However, because these inefficiencies are not readily obvious, opportunities to save energy by repairing or replacing components and optimizing systems are often overlooked.  A starting point to tackle system problems is to perform a systems assessment, which reviews the operation of a pumping system, often using certain tools to help identify inefficiencies in the system, determine improvement measures and estimate potential energy savings.

Pumps are often operated over a wide range of flow conditions. A key to improving system performance and reliability is to fully understand system requirements (peak demand, average demand, and the variability of demand) in a daily and yearly basis. This information should be used to match flow and pressure requirements more closely to system needs. In matching pumping equipment to loads, pumps should be sized to maximize the amount of time that a pump operates at or near its best efficiency point (BEP) [a]. A more detailed description of the efficiency improvement opportunities and types of system improvements that can be made is discussed below.

Efficiency Improvement Opportunities

The following section describes key improvement opportunities in pumping systems to assist operators and engineers in reducing energy use and prolonging the durability of pumping systems:

Oversized Pumps

Oversized pumps are often a result of conservative engineering practices where a margin of safety is included to compensate for pump sizing or service requirements have changed (e.g., decrease in production rate). This can be a result of poor understanding of the flow requirements (peak flow, average flow and variability) or uncertainties in the design process. The tendency to design pumps larger than the system requires may also occur due to anticipated expansions in system capacity and potential fouling effects.

Issues with oversized pumps frequently develop because the system is designed for peak loads while normal operating loads are much smaller, which means excess flow energy is being applied into the system. This excess flow energy translates into increasing operating costs due to the additional energy used to run a larger pump. It also generates unnecessary wear on components such as valves, piping, and piping supports, which eventually increases maintenance requirements. These costs are often overlooked during the system specification process. Since many of these operating and maintenance costs are avoidable, correcting an oversized pump can be a cost-effective system improvement.

Indications of oversized pumps include:

  • Use of high bypass flow rates;
  • Throttled flow control valves;
  •  Frequent replacement of bearings and seals;
  • Excessive flow noise; and
  • Intermittent pump operation.

An obvious solution is to replace the pump/motor assembly with a downsized version; however, this is costly and may not be feasible in all situations [Reference 2]. Instead of replacing the entire pump/motor assembly, alternative measures may be evaluated including:

  • Replacing the impeller of the existing pump with a smaller impeller (applies to centrifugal pumps only);
  • Reducing the outside diameter of the existing impeller (applies to centrifugal pumps only);
  • Installing an adjustable speed drive (ASD) to control the pump if flow varies over time; and
  • Adding a smaller pump to reduce the intermittent operation of the existing pump (i.e. pony pumps).

 Efficient Piping Configurations

In systems dominated by friction head [b], there are multiple energy and money-saving opportunities that work by reducing the power required to overcome the friction head. The frictional power required depends on the flow rate, fluid pressure, pipe size (diameter), overall pipe length, fittings installed (valves, junctions, etc.), pipe characteristics (surface roughness, material, etc.), and properties of the fluid being pumped.

Optimizing the configuration of the pumping system involves several steps, which include determining a proper pipe size, designing a piping system layout that minimizes pressure and selecting fittings with low pressure drops. Determining the proper pipe size involves weighting the initial cost of the pipe against the cost of pushing fluid through it. Although larger pipes create less friction loss for a given flow rate, they have higher material and installation costs. Although piping system layouts are usually subject to space constraints, there are often chances to minimize unnecessary pressure drops by avoiding sharp bends, expansions, and contractions and by keeping piping as straight as possible.

A key configuration improvement is to establish a uniform velocity flow profile upstream of the pump. Poor flow profiles are a result of inefficient piping configurations that promote uneven flow and/or turbulent flow, which diminishes pump performance. Consistent velocity profiles can be achieved by making sure a straight run of pipe leads into the pump inlet. If an elbow must be placed just upstream of the pump due to space restrictions, a long radius elbow should be selected. To correct any disruption in flow in the elbow, a flow straightener, such as a baffle plate or a set of turning vanes should be installed with an elbow. A flow straightener creates a more even velocity profile but consideration must be taken to ensure that the pressure drop across the straightener does not cause cavitation.

In addition, suction and discharge piping close to the pump should be properly supported by hangers. Properly supporting the piping near the pump allows the pipe reaction to be carried by the pipe hangers rather than by the pump casing itself, thus reducing strain on the pump. Moreover, proper support of the piping near the pump stiffens the system, which can reduce system vibrations.

Parallel Pumping Systems

An alternative to having a single pump to fulfill the system’s requirements is to use several smaller pumps arranged in parallel. Systems with wide demand changes may cause a single pump to consistently operate far from its best efficiency point (BEP), which can result in higher operating and maintenance costs. Using a combination of multiple pumps to meet varied flow requirements can allow each pump to operate more efficiently than a single larger pump. Multiple pump arrangements have advantages of flexibility, redundancy, and the ability to meet changing flow needs efficiently in systems with high static head [c]. However, this efficiency benefit will depend on the pump curves, the system curve, and the demand change that is being met.

Typically, pumps in parallel systems are identical. When the same models are used and their impeller diameters and rotational speeds are identical, the pumps deliver equal flow rates, balancing the load. Using different-sized pumps could result in a situation where the largest pump dominates the system, forcing other pumps to operate far away from their best efficiency point (BEP) or below their minimum flow ratings. If a different-sized pumps parallel system is the only option, the pumps’ performance curves should be carefully reviewed to ensure that all pumps operate efficiently and no pump operates below its minimum flow requirement.

Although parallel operation can be advantageous for static head-dominated systems, it is not as effective in friction-dominated systems. Parallel operation in friction-dominated systems can cause significant fluid friction losses, reduce the flow rate provided by each pump and alter the efficiency of each pump. These factors cause an increase in the energy required to transfer a given fluid volume. For friction-dominated systems, the efficiency penalties of multiple parallel pump operation mean that it is generally advantageous to use adjustable speed drives (ASDs) to meet varied flow demand, rather than parallel pumps.

It is recommended that the minimum number of pumps required are operated at any given time. One exception might involve storage applications with high “peak period” demand charges. Multiple pumps should be selected with head-versus-capacity performance curves that rise at a constant rate when these pumps approach no-flow or shutoff head[d].

Controlling Flow

 

Variations in load (demand changes) may be managed by controlling flow which can be achieved by any of these four methods [Reference 2]:

  • Bypass lines: a line that connects the pump discharge side to a low pressure area, usually the pump's suction tank or line, in order to regulate flow in the system while avoiding deadheading [e]. A major drawback of bypass valves is that they are an energy-inefficient flow control option. The power used to pump the bypassed fluid is wasted. Hence, this flow control option is generally not recommended. Bypass control may be used in conjunction with throttling valves and/or multiple pump arrangements.
  • Throttle valves: valves placed in the discharge side of the pump to provide flow control by chocking the fluid which increases upstream backpressure, consequently reducing pump flow. Throttle valves are usually more efficient than bypass valves, because they maintain upstream pressure that can help drive fluid through parallel branches of the system while the valve is shut. However, forcing the pump to operate against a high backpressure is inefficient since this backpressure is typically higher than the pressure associated with the pump’s BEP. Thus, usage of throttle valves is generally not recommended. Throttling valves may be used in conjunction with bypass control and/or multiple pump arrangements.
  • Multiple pump arrangements (as discussed in the previous section “Parallel Pumping Systems”).
  • Adjustable speed drives (ASDs): work by changing the speed at which a pump operates, thereby changing the flow through the pump. ASDs are generally used independently of the control methods listed above, although bypass lines may be necessary to ensure that a minimum flow is maintained through the pump during periods of low load.

Adjustable Speed Drives (ASDs)

ASDs are generally the most efficient method for controlling pump flow. Adjustable speed drives (ASDs) are an efficient flow control option that allow pump speed adjustments over a continuous range. These drives are generally classified as mechanical drives (fluid or eddy current) and variable frequency drives (VFDs). VFDs are the most frequently specified type of ASD, and pulse-width-modulated VFDs are the most commonly used [Reference 3].

VFDs are solid-state electronic motor controllers that adjust the frequency and voltage of the power supplied to an alternating current (AC), allowing it to operate over a wide range of speeds. The frequency and motor speed is adjusted by using external sensors that monitor flow, liquid levels or pressure, and which a signal to the controller (VFD) to adjust the motor speed to match process requirements. VFDs allow for precise speed and flow control. Furthermore, using VFDs to reduce pump speeds decreases the deterioration of the pumping system, in terms of lower bearing loads, and reduced shaft deflection. Moreover, soft -starting [f] reduces thermal and mechanical stresses on windings, couplings, and belts. These factors translate into higher pump reliability and lower pump maintenance costs.

In centrifugal pump applications with no static head, system power requirements vary with the cube of the pump speed. Therefore, small decreases in speed or flow can significantly reduce energy use. For example, reducing the speed of a pump by 20% (translates to a 20% reduction in flow) can reduce input power requirements by approximately 50% [Reference 3]. It is for this reason that ASDs should always be considered to control flow in friction-dominated (low static head) systems.

Maintenance

Maintenance is time-consuming and costly, but essential to minimize costs and downtime. It is important to consider how critical the pump system is to the overall operation, what the consequences of failure would be, how severe the service application would be, and the sensitivity of the pump design to operating conditions. Energy and maintenance costs will account for over 50-95% of pump ownership costs, while initial costs account for less than 15% of pump life cycle costs [Reference 4]. The main areas to look for pump wear are [Reference 5]:

  • cavitation or internal recirculation;
  • pump impellers and casings that increase clearances between fixed and moving parts;
  • wear rings and bearings; and
  • packing adjustment on the pump shaft.

Deciding when to do maintenance requires a balance between cost and system performance [Reference 6]. The following are potential options:

  • Do nothing until something goes wrong. Not recommended unless it is a non-critical application;
  • Periodic strip-down. Relies on good information regarding the pump and its application to determine when maintenance should occur; or
  • Condition-based maintenance. Regular on-line monitoring of pump conditions but requires investment in time and monitoring equipment.

The use of high quality material (steel, rubber, seal, bearings) reduces maintenance costs and improves efficiency.

Summary

A summary of the energy savings methods. Techniques to lower pump energy consumption are shown in Table 1.

 

Table 1. Techniques to Lower Pump Energy Consumption [Reference 4] 

[a] BEP is described as the point at which a centrifugal pump is operating at its highest efficiency, transferring energy from the prime mover to the system fluid with the least amount of losses.

[b] Friction head (usually in units of feet) is the amount of energy used to overcome resistance to the flow of liquids through the pumping system.

[c] Static head represents the net change in height, in feet, that the pump must overcome. It applies only in open systems.

[d] Shut-off head is the total head corresponding to zero flow on the pump performance curve.

[e] Deadheading occurs when the pump's discharge is closed either due to a blockage in the line or a closed valve. This brings the pump to its maximum shut-off head, causing the fluid to be recirculated within the pump and resulting in overheating and possible damage.

[f] Soft starting is a generally achieved by an auxiliary device in AC electric motors that temporarily reduces the load and torque in the powertrain and electrical current surge of the motor during startup.

 

Technology maturity

Commercially available?: Yes 
Offshore viability: Yes 
Brownfield retrofit?: Yes 
Years experience in the industry: 21+ 

Key Metrics

Range of application:
Oversized pump capacity reduction can be applied to any and all pumping systems. Piping configuration improvements can be applied to any pumping system, but are particularly helpful for systems dominated by friction losses. Friction losses depend on the flow rate, pipe diameter, overall pipe length, surface roughness, and properties of the fluid being pumped. So for example, a system pumping a highly viscous fluid through small pipes will suffer significant friction losses. Parallel operation is most appropriate for high static head systems, i.e. systems with height variations between components or pressurized upstream equipment. Adjustable speed drives can be used in systems with high friction losses or a need for precise flow control. ASDs can be used in both centrifugal and positive displacement pumps.
Efficiency: Efficiency improvements for oversized pump capacity reduction, piping configuration improvements, and parallel operation vary by system. Pumps should be sized to run within the best efficiency band (65-100% of the load) [Reference 8]. For adjustable speed drives, system efficiency is the product of the variable frequency drive efficiency, the motor efficiency at its load point, and the pump efficiency (ηsystem = ηVFD x ηMotor x ηEquipment). VFD efficiency decreases with decreasing motor load, but for most pumping applications, the VFD efficiency should be between 53% and 97% [Reference 3].
Guideline capital costs: For oversized pump capacity reduction, piping configuration improvements, and parallel operation, improvement costs vary by system. For adjustable speed drives, the purchase and installation costs for VFDs range from about $400 for a 1 horsepower (746 W) motor application to nearly $70,000 for a 1000 horsepower (0.75 MW) motor [Reference 9]. Payback period for these drives can range from just a few months to less than three years for 25- to 250- horsepower models [Reference 10]. When designing and installing a new pumping system, the capital cost of an ASD can often be offset by eliminating control valves, bypass lines, and conventional starters.
 
Guideline operational costs:
For oversized pump capacity reduction, piping configuration improvements, and parallel operation, improvements should not significantly affect operations, but may reduce maintenance costs. Maintenance Costs can represent one-third of the life cycle costs for a medium-sized industrial pump [Reference 11]. For adjustable speed drives: The maintenance cost for a VFD is about $500 each year. It is assumed that it will not need any repairs over the project’s 8-year life.
 
GHG reduction costs: Energy savings, most likely in terms of electricity, will result from improving performance efficiency pumping systems, which translates into life cycle reductions of greenhouse gas emissions in most cases from reduced energy consumption. Exceptions would be where electricity is strictly derived from renewable sources, which is not frequent in most operations.
Time to perform engineering and installation: For oversized pump capacity reduction, piping configuration improvements, parallel operation, installation time varies by system. For adjustable speed drives, installation can take from 10 to over 70 labor-hours [Reference 10].
Typical scope of work description:

There are significant opportunities to improve the reliability, performance, and efficiency of pumping systems in the oil and gas industry. This section discusses basic steps that can help in identifying and implementing pumping system improvement projects:
1. Start by using a “systems approach” to analyze pumping systems[Reference 2]. This involves the following types of interrelated actions:

  • Examine current conditions and operating parameters;
  • Determine present and estimate future production conditions that may affect the system;
  • Develop load duty cycles and sample and analyze operating data;
  • Assess potential system designs and improvements;
  • Avoid conservative designs that only consider peak operational conditions;
  • Determine the most technically and economically feasible options, considering all of the components in the system;
  • Assess which design will maintain efficiency for a long time, and give low maintenance costs.

2. Implement the best option (parallel systems, adjustable speed drives, piping configuration improvements, etc.) Installation considerations for adjustable speed drives (ASDs) are:

  • Programming drive controllers to avoid operating pumps at speeds which may result in equipment or systems resonances;
  • Installation of a manual bypass to keep the motor operating at a fixed speed if the ASD should fail;
  • Installation of a single ASD to control multiple pump motors;
  • Use of caution when reducing the flow velocities of slurries.

3. Assess energy consumption with respect to performance. Survey the priority pumps in the facility and conduct efficiency tests on them to continue to monitor and optimize the system, trying to maintain peak performance. Software tools such as DOE’s Pumping System Assessment Tool (PSAT) provide estimates of optimal efficiency [Reference 12].

Decision drivers

Technical: Oversized pump capacity reduction: evaluate any systems where any of the following conditions are present:use of bypass flow control; throttled flow control valves; frequent replacement of bearings and seals; excessive flow noise; intermittent pump operation
Piping configuration complexity: potential improvements to piping configurations are indicated in any systems where the existing piping system is complex or has excess fittings, valves, junctions or pipe length
Pipe diameter: the cost of increasing pipe diameters should be analyzed relative to its impact on energy cost for all new systems. Existing systems may benefit from evaluation if system capacity has been expanded since original design, annual run time is significant and/or inadequate flow is delivered by the pumping system
Load type: piping configuration improvements are appropriate for systems with constant or variable loads
- Parallel operation
Low friction head: parallel pump operation is a viable and effective control strategy for systems with low friction heat compared to static head.
Load type: parallel pumps operation is only required in systems with variable loads
- Adjustable speed drives
Low static head: ASDs are ideal for circulating pumping systems in which the system curve is defined by dynamic or friction head losses
High horsepower (greater than 15 to 30 hp): The higher the pump horsepower, the more cost-effective the ASD application
Load type: Centrifugal loads with variable-torque requirements, such as centrifugal pumps, have the greatest potential for energy savings. ASDs can be cost-effective on positive displacement pumps, but the savings will generally not be as great as with centrifugal loads
Operational: Oversized pump capacity reduction: May reduce maintenance issues associated with wear to pump parts and piping systems
Piping configuration improvements: Simplifying piping configurations and reducing pressure drop across systems typically improves pressure and flow control at end users. May also reduce maintenance issues associated with wear to pump parts and piping systems
Parallel operation: Improved pressure and flow control compared to single large pump, but less accurate and variable control than ASDs. Can be designed to provide some level of redundancy in the event of pump failure.
Adjustable speed drives: Improved process control: small variations in pressure and flow can be corrected more rapidly by a VFD than by other control forms due to monitoring feature of the VFDs. There is less likelihood of flow or pressure surges when the control device provides rates of change which are infinitely variable [Reference 2]. The converter will have losses, thus ventilation requirements is an important issue. The life expectancy of the converter is generally directly related to the temperature of the internal components, especially capacitors. The converter may require installation in more protective environment than the motor control gear it replaces. Electronics are less suitable to cope with corrosive and damp locations than conventional starters [Reference 13]. In general, ASDs are cost-effective only on pumps that operate for at least 2,000 hours per year at average utility rates [Reference 14].
Maintenance: Maintenance is time consuming and costly but essential to minimize costs and downtime.
Commercial: High utility rates: Reduced expenses from high utility energy charges provide a more rapid payback on any pump system improvements
Availability of efficiency incentives: electric utility incentives for reducing energy use or installing energy-saving technologies are sometimes available and will reduce payback periods
Environmental: Environmental benefits are associated with greenhouse gas emissions reduction from decreased energy consumption.
Economic rule-of-thumb: Identify flow rates that vary 30% or more from the BEP and systems imbalances greater than 20%. Those systems will likely benefit from efficiency improvement measures discussed here.

Additional comments

The advantages and disadvantages of ASDs are summarized below [Reference 15]:

Advantages:

Energy savings - between 30% and 50% have been achieved in many installations by installing ASDs;

  • Improved process control;
  • Increased reliability - decreased mechanical impact from soft-starts;
  • Decreased maintenance costs:
    •  Increased equipment life;
    • Elimination of excessive throttling;
  • Built-in soft starting of pumps

Disadtantages:

  • Less efficient at 100% rated motor speed;
  • Possible winding insulation breakdown - recommended use of inverter-rated motors;
  • Harmonics - may lead to problems with vibrations and noise;
  • Possible voltage reflected wave from long lead lengths;
  • Initial investment cost - payback from lower energy consumption.

Additional comments

The advantages and disadvantages of ASDs are summarized below [Reference 15]:

Advantages:

Energy savings - between 30% and 50% have been achieved in many installations by installing ASDs;

  • Improved process control;
  • Increased reliability - decreased mechanical impact from soft-starts;
  • Decreased maintenance costs:
    •  Increased equipment life;
    • Elimination of excessive throttling;
  • Built-in soft starting of pumps

Disadtantages:

  • Less efficient at 100% rated motor speed;
  • Possible winding insulation breakdown - recommended use of inverter-rated motors;
  • Harmonics - may lead to problems with vibrations and noise;
  • Possible voltage reflected wave from long lead lengths;
  • Initial investment cost - payback from lower energy consumption.

References:

  1. US Department of Energy (DOE), 2005. “Test for Pumping System Efficiency. Pumping Systems Tip Sheet #4.” September. Accessed online at DOE Energy Efficiency and Renewable Energy website
  2. US Department of Energy (DOE) and Hydraulic Institute, 2006. “Improving Pumping System Performance. A Sourcebook for Industry.” Second Edition. May. Accessed online at DOE Energy Efficiency and Renewable Energy website
  3. US Department of Energy (DOE), 2012. “Adjustable Speed Drive Par-Load Efficiency. Energy Tips. Motor Systems.” Fact Sheet. November. Accessed online at DOE Energy Efficiency and Renewable Energy website
  4. Good Practice Guide 249: Energy Savings in Industrial Water Pumping Systems, prepared by ETSU for the Department of Environment, Transport and Regions, US, September 1998
  5. Comment on draft Pumps for Power paper by Yugo Santos, Marta, October 7, 2013
  6. Study on Improving the Energy Efficiency of Pumps, European Commission, February 2001.
  7. Schlumberger (SLB), 2011.”Variable Speed Drives. Optimize Production Potential.” Accessed online
  8. Energy-Related Best Practices: A Sourcebook for the Chemical Industry, Chapter 7, Pumps and Motors, Iowa State University, 2005
  9. VFDs.com - Variable Frequency Drives Retailer, 2013. Accessed online at
  10. California Energy Commission (CEC), 2000. “Variable Frequency Drives.” Accessed online
  11. Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems, Executive Summary, a collaboration between the Hydraulic Institute, Europump, and the US Department of Energy’s Office of Industrial Technologies (OIT), January 3001
  12. US Department of Energy (DOE), 2010. “The Pumping System Assessment Tool (PSAT).” Fact Sheet. August. Accessed online at DOE Energy Efficiency and Renewable Energy website
  13. Hydraulic Institute, Europump and US Department of Energy, 2004. “Variable Speed Pumping. A Guide to Successful Applications.” Executive Summary. May. Accessed online
  14. US Department of Energy (DOE), 2007. “Adjustable Speed Pumping Applications. Pumping Systems Tip Sheet #11.” January. Accessed online at DOE Energy Efficiency and Renewable Energy website
  15. ECOVA, 2010. “Variable Frequency Drives.” Presentation for Total Resource Efficiency Education (TREE). Accessed online

References: 

  1. US Department of Energy (DOE), 2005. “Test for Pumping System Efficiency. Pumping Systems Tip Sheet #4.” September. Accessed online at DOE Energy Efficiency and Renewable Energy website
  2. US Department of Energy (DOE) and Hydraulic Institute, 2006. “Improving Pumping System Performance. A Sourcebook for Industry.” Second Edition. May. Accessed online at DOE Energy Efficiency and Renewable Energy website
  3. US Department of Energy (DOE), 2012. “Adjustable Speed Drive Par-Load Efficiency. Energy Tips. Motor Systems.” Fact Sheet. November. Accessed online at DOE Energy Efficiency and Renewable Energy website
  4. Good Practice Guide 249: Energy Savings in Industrial Water Pumping Systems, prepared by ETSU for the Department of Environment, Transport and Regions, US, September 1998
  5. Comment on draft Pumps for Power paper by Yugo Santos, Marta, October 7, 2013
  6. Study on Improving the Energy Efficiency of Pumps, European Commission, February 2001.
  7. Schlumberger (SLB), 2011.”Variable Speed Drives. Optimize Production Potential.” Accessed online
  8. Energy-Related Best Practices: A Sourcebook for the Chemical Industry, Chapter 7, Pumps and Motors, Iowa State University, 2005
  9. VFDs.com - Variable Frequency Drives Retailer, 2013. Accessed online at
  10. California Energy Commission (CEC), 2000. “Variable Frequency Drives.” Accessed online
  11. Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems, Executive Summary, a collaboration between the Hydraulic Institute, Europump, and the US Department of Energy’s Office of Industrial Technologies (OIT), January 3001
  12. US Department of Energy (DOE), 2010. “The Pumping System Assessment Tool (PSAT).” Fact Sheet. August. Accessed online at DOE Energy Efficiency and Renewable Energy website
  13. Hydraulic Institute, Europump and US Department of Energy, 2004. “Variable Speed Pumping. A Guide to Successful Applications.” Executive Summary. May. Accessed online
  14. US Department of Energy (DOE), 2007. “Adjustable Speed Pumping Applications. Pumping Systems Tip Sheet #11.” January. Accessed online at DOE Energy Efficiency and Renewable Energy website
  15. ECOVA, 2010. “Variable Frequency Drives.” Presentation for Total Resource Efficiency Education (TREE). Accessed online