In the United States, more than 2.4 million pumps, which consume more than 142 billion kWh each year, are used in industrial manufacturing processes. At an electricity cost of 5 cents per kWh, energy used for fluids transport costs more than $7.1 billion per year.
Even one pump can consume substantial energy. A continuously operated centrifugal pump driven by a fully loaded 100 hp motor requires 726,000 kWh per year. This costs more than $36,000, assuming average electricity costs of 5 cents per kWh. Even a 10 percent reduction in operating costs saves $3,600 per year.
Surveying Pumping SystemsPumps larger than a minimum size and with significant operating hours should be surveyed to determine the following:
- Establish a baseline for your current pumping energy
consumption and costs.
- Identify inefficient pumps.
- Determine efficiency measures.
- Estimate the potential for energy savings.
The survey team should gather pump and drive motor nameplate information and document operating schedules to develop load profiles, then obtain head/capacity curves (if available) from the pump manufacturers to document the pumping system design and operating points. The team also should note the system flow rate and pressure requirements, pump style, operating speed, number of stages and specific gravity of the fluid being pumped.
If possible, the team also should measure the flow rate as well as the suction and discharge pressures. Finally, the team should note conditions that are associated with inefficient pump operation, including indicators such as:
- Pumps with high maintenance requirements.
- Oversized pumps that operate in a throttled condition.
- Cavitating or badly worn pumps.
- Misapplied pumps.
- Pumping systems with large flow-rate or pressure variations.
- Pumping systems with bypass flow.
- Throttled control valves to provide fixed or variable flow
- Noisy pumps or valves.
- Clogged pipelines or pumps.
- Wear on pump impellers and casings that increase clearances between
fixed and moving parts.
- Excessive wear on wear rings and bearings.
- Improper packing adjustment that causes binding on the pump
- Multiple pump systems where excess capacity is bypassed or excess
pressure is provided.
- Changes from initial design conditions.
- Changes that shift the pump performance from the original system
curve such as distribution system cross-connections, parallel main lines, or
changes in pipe diameter or material.
- Low-flow rate, high-pressure end use applications. For example, an
entire pumping system may be operating at high pressure to meet the
requirements of a single end use. A booster or dedicated pump may allow system
operating pressure to be reduced.
- Shutting down unnecessary pumps. Re-optimize pumping
systems when a plant’s water use requirements change. Use pressure switches to
control the number of pumps in service when flow rate requirements vary.
- Restore internal clearances.
- Replace standard-efficiency pump drive motors with NEMA premium
- Replace or modify oversized pumps. Trim or change the pump impellers
to match the output with system requirements when the pumping head exceeds system
requirements. Consult with the vendor to determine the minimum impeller diameter
for a pump casing.
- Meet variable flow-rate requirements with an adjustable-speed drive or multiple pump arrangement instead of throttling or bypassing excess flow.
Pump Selection ConsiderationsIf, based on the results from your in-plant survey, it becomes necessary to replace a pump, several actions should be taken. First, take care to accurately identify of the process flow rate and pressure requirements. Measure the actual head and flow rate, and develop a system curve. Second, it is important to select a pump with high efficiency over the expected range of operating conditions. Specify electric motors that meet the NEMA premium full-load efficiency standards. Use lifecycle costing techniques to justify acquiring high-efficiency pumps and designing efficient systems.
When it comes to understanding pumping system requirements, users need to remember 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).
End-Use RequirementsThe design pump capacity, or desired pump discharge in gallons per minute (gal/min), is needed to accurately size the piping system, determine friction head losses, construct a system curve and select a pump and drive motor. Process requirements may be met by providing a constant flow rate (with on/off control and storage used to satisfy variable flow-rate requirements), or by using a throttling valve or variable-speed drive to supply continuously variable flow rates.
The total system head has three components: static head, elevation (potential energy), and velocity (or dynamic) head. Static head is the pressure of the fluid in the system, and this is the quantity measured by conventional pressure gauges. The height of the fluid level can have a substantial impact on system head. Dynamic head is the pressure required by the system to overcome head losses caused by flow-rate resistance in pipes, valves, fittings and mechanical equipment. Dynamic head losses are approximately proportional to the square of the fluid flow velocity, or flow rate. If the flow rate doubles, dynamic losses increase fourfold.
For many pumping systems, total system head requirements may vary. For example, in wet well or reservoir applications, suction and static lift requirements may vary as the water surface elevations fluctuate. For return systems such as circulating water pumps, the values for the static and elevation heads equal zero.
You also need to be aware of a pump’s net positive suction head requirements. Centrifugal pumps require a certain amount of fluid pressure at the inlet to avoid cavitation. A rule of thumb is to ensure that the suction head available exceeds that required by the pump by at least 25 percent over the range of expected flow rates.
Finally, environmental conditions such as ambient temperature and humidity, elevation above sea level, and whether the pump is to be installed indoors or out need to be considered when selecting a pump.
This article was provided by the Hydraulic Institute, the U.S. Department of Energy and Pump Systems Matter.
The Hydraulic Institute, Parsippany, N.J., is an association of pump producers in North America. It serves member companies and pump users by developing industry standards, providing education and training, and serving as a forum for the exchange of industry information. For more information from the Hydraulic Institute, call (973) 267-9700 or visit www.pumps.org.
The DOE’s Industrial Technologies Program, through partnerships with industry, government and non-governmental organizations, develops and delivers advanced energy efficiency, renewable energy and pollution prevention technologies for industrial applications. For more information from the DOE’s Industrial Technologies Program, call (877) 337-3463 or visit www.eere.energy.gov/industry.
Developed by the Hydraulic Institute, Pump Systems Matter is an educational initiative created to assist North American pump users gain a more competitive business advantages through strategic, broad-based energy management and pump system performance optimization. For more information about Pump Systems Matter from the Hydraulic Institute, call (973) 267-9700 or visit www.pumps.org.
Sidebar:The properties of the fluids being pumped can significantly affect the
choice of pump. Key considerations include the following criteria.
Acidity/Alkalinity (pH) and Chemical Composition.Corrosive and acidic fluids can degrade pumps and these conditions should be considered when selecting pump materials.
Operating Temperature.Pump materials and expansion rate, mechanical seal components and packing materials need to be considered with pumped fluids that are hotter than 200oF (93oC).
Solids Concentrations and Particle Sizes.When pumping abrasive liquids such as industrial slurries, selecting a pump that will not clog or fail prematurely depends on particle size, hardness and the volumetric percentage of solids.
Specific Gravity.The fluid-specific gravity is the ratio of the fluid density to that of water under specified conditions. Specific gravity affects the energy required to lift and move the fluid, and must be considered when determining pump power requirements.
Vapor Pressure.A fluid’s vapor pressure is the force per unit area that a fluid exerts in an effort to change phase from a liquid to a vapor, and depends on the fluid’s chemical and physical properties. Proper consideration of the fluid’s vapor pressure will help to minimize the risk of cavitation.
Viscosity.The viscosity of a fluid is a measure of its resistance to motion. Since kinematic viscosity normally varies directly with temperature, the pumping system designer must know the viscosity of the fluid at the lowest anticipated pumping temperature. High-viscosity fluids result in reduced centrifugal pump performance and increased power requirements. It is particularly important to consider pump action suction side-line losses when pumping viscous fluids.