As industry invests tremendous amounts of effort to reduce energy consumption, the U.S. Department of Energy (DOE) and the Hydraulic Institute (HI) have focused on improving efficiencies of pumps, motors and pump systems.
One way end users can reduce energy consumption—in some cases, by 10 percent or more—is the use of highly engineered structural-composite pumps and impellers.
Upgrading existing pumps with qualified structural-composite components can reduce energy use and improve return on investment (ROI) in five key ways.
1. Composite materials extend pump life because they are inert, corrosion-resistant and withstand cavitation better than metallics such as bronze.
An evaluation of the performance curve of any pump will show a huge drop in efficiency once a metallic impeller begins to wear, cavitate and/or corrode.
Even if the starting efficiency of a pump appears sufficient when a pump is new, tests demonstrate rapid deterioration of efficiency and performance as a pump continues to operate more inefficiently. Engineered structural-composite impellers and rings can help prevent these heavy losses of efficiency and performance. Pump inefficiency contributes heavily to increasing energy, maintenance and repair costs.
In some cases, structural composites can provide five to seven times the lifespan of metallic pumps and impellers in corrosive applications.
2. Structural composites provide a reduction in energy costs through pump optimization.
In many cases, a facility purchases a pump for one specific performance, but once the pump is put into service, system requirements cause it to operate at another point entirely different from the pump’s original design point.
When a pump operates away from the original design point or best efficiency point (BEP), problems occur, including higher loading; excessive noise and vibration of the pump; shaft oscillation; cavitation; and premature wear and failure of the mechanical seals, bearings, rings, sleeves and impellers.
In extreme cases, the pump shaft will break right behind the impeller from the excessive radial forces that occur when a pump operates away from the original design point.
Operating a pump away from the BEP has a detrimental effect on efficiency and performance, and it wastes energy and money. Operating the pump in this way also severely reduces pump life.
The larger the pump, the more energy is wasted. These problems can be resolved with structural-composite pumps and/or impellers and rings that have been engineered for a facility’s system requirements. This improves the pump’s efficiency, reliability and longevity.
Many industry professionals do not realize the cost and impact of low efficiency. As an example, a motor powering a pump with a severely deteriorated impeller could easily lose 50 percent of its original efficiency.
If the motor was drawing 75 kilowatts (kW) at the original efficiency of 80 percent and now the pump is operating at an efficiency of 40 percent, there would be an approximate loss of half of the original efficiency, which would equate to $38,880 per year at $0.12 per kilowatt-hour (kWh) (37.5 kW loss x 8,640 hours x $0.12/kWh = $38,880 per year).
Even if the pump was operating only 10 percent away from the BEP, the approximate loss would be $7,776 per year plus additional maintenance expenses (7.5 kW loss x 8,640 hours x $0.12/kWh = $7,776).
Structural-composite pumps are designed specifically for the operating point in the plant, factory or system, making them more efficient than many traditional pumps.
The operating point in the system becomes the BEP, saving the user thousands of dollars each year. Because the pumps operate at the BEP, they have less radial and axial movement, resulting in a longer lifespan.
Structural-composite pumps and impellers are completely machined (as opposed to being cast or molded) on the outside as well as the inside using state-of-the-art computational fluid dynamics (CFD) techniques, which make them even more efficient.
Depending on where the metallic impeller is operating on the pump curve, some composite pumps and impellers can increase efficiency by 5 to 15 percent.
3. Completely machined structural-composite impellers experience less vibration and lower radial and axial movement as a result of better mechanical and hydraulic balance.
Engineered structural-composite impellers are completely machined from a center position out of a solid block of structural composite on both the inside and the outside.
As a result, the vanes are all equally spaced and the impellers are well-balanced and remain well-balanced for the life of the pump, leading to increased longevity and enhanced performance as a result of less vibration.
A very similar type of technology has been employed for propeller design and manufacture. Propellers that are machined on five-axis machining centers not only produce less vibration, but they also pay for themselves in fuel savings.
4. Composite impellers are lightweight and reduce shaft deflection.
Structural-composite impellers and casing rings are only 15 percent the weight of traditional metallic materials. The lower weight reduces startup load and shaft deflection, which allows the rotating element to run with tighter clearances between the rings and the impeller.
The tighter ring clearance reduces leakage, and the reduction in shaft deflection enables bearings, sleeves, mechanical seals and rings to last much longer, which saves on repair and maintenance costs.
5. Composite impellers and casing rings reduce leakage and prevent pump “wash-out.”
Because of the excellent lubrication qualities of many structural-composite materials and because of the low coefficient of friction, the ring clearances in operation are less than metallic rings and allow less leakage between the impeller casing rings and the impeller wear ring.
The smoother surfaces and less leakage between the casing rings and impeller wear ring result in a 2 to 3 percent increase in efficiency. Additionally, the structural-composite rings seal against the pump casing similar to a gasket, preventing the fluid from wearing or corroding the landing areas under the casing rings.
Estimated Operational Savings
Because of the many benefits listed above, structural-composite impellers and casing rings can outlast metallic impellers and ring sets, especially in corrosive environments.
Table 1 demonstrates the estimated operational savings that structural-composite materials can provide.