There are a quite a few articles on the cost benefits of operating a pump in an efficient manner, but how much efficiency can we expect to get from a given pump design? This issue deals with centrifugal pump efficiency and how specific speed-(Ns)* and flow are the most significant factors in determining the maximum pump efficiency for a given pump style and fluid type.
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Dale B. Andrews - Editor |
Printable Version |
*see our Aug 2004 issue for a discussion of Ns.
Efficiency is a measure of how much of the power input to the pump shaft is converted to useful hydraulic output by the pump. Hydraulic power (Pw) is the power that would be used by a 100% efficient pump.
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Overall efficiency is the ratio of the hydraulic power to the shaft input power. Put into more practical terms, the difference between the input power and the hydraulic power is power that is available to damage the pump. This power is manifested in the form of recirculation, heat, noise, and vibration. The higher the power, the more closely the operation should be held to the best efficiency point.
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Efficiency losses can be broken down into two major categories: losses associated with off-BEP operation and losses intrinsic to the overall pump design.
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Efficiency loss due to off-BEP operation is primarily the result of recirculation and fluid blockage caused by a mismatch between the pump's design flow and the actual flow. As the flow rate moves away from BEP, there is an increasing differential between the inlet vane angle and the approaching flow angle. Similar losses occur between the impeller vane exit and the volute or diffuser. The result is increased recirculation within the impeller passages and between the impeller and casing.
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In last month's issue, I mentioned that efficiency varies as a function of specific speed. Specific speed reflects the geometric design of the pump. The geometric design is a major determinant of the maximum efficiency a centrifugal pump can produce.
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Chart 2
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Chart 2 shows the various losses in a pump as a function of specific speed (Ns). Low Ns pumps have small flows and high disk friction losses (3). Disk friction is created by the shearing forces acting on a fluid film trapped between closely spaced surfaces (like an impeller and a casing) that are moving relative to each other. As Ns increases, disk friction as a percentage goes down, but hydraulic kinetic energy losses (5) in the casing and inlet increase. The constant mechanical(1) and impeller(2) losses are not entirely accurate, but these losses are small compared to the frictional and kinetic influences. This chart is not accurate for estimating efficiencies because it assumes a constant flow. However, its representation of the relative relationships between efficiency influencing factors at various specific speeds is reasonably accurate, and it serves as a useful initial approximation. |
Up to around 2000 Ns disk friction is the largest single source of efficiency loss. The impact of Ns on disk friction can be seen in these images. Note the relatively large flow passage areas and relatively small impeller to stator surface area between the high ~10,000Ns 38,000 GPM impeller on the right, and the low 1000 Ns 1800 GPM impeller on the left. Both impellers are approximately the same diameter. The high Ns impeller rotates in close proximity to its casing only at the blade tips. The low Ns impeller rotates in close proximity to its casing along the entire surface of the front and back shrouds. |
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Chart 3 |
The flow at a given specific speed is also a major determinant of maximum efficiency. Chart 3 shows the estimated maximum efficiency for centrifugal pumps at various specific speeds and flow rates on water. This chart provides reasonable efficiency estimation for a single-stage overhung design with an enclosed impeller and tight wear ring clearances. Significant reductions in efficiency occur when handling viscous fluids and some slurries. Such efficiency reductions are more pronounced for low specific speed pumps due to the higher disc friction component, but that is a topic for another day. |
The maximum efficiency obtainable is depends on the geometry and flow. The geometry chosen should take into consideration all aspects of an application. A 3600 rpm pump with wear rings that handles slurry will have only a short term efficiency advantage over a lower speed, lower Ns slurry design. Maintenance costs will soon eclipse any capital cost benefit associated with the smaller, higher speed pump. Basing pump selection primarily on suitability for the service is usually more cost efficient. Obtaining the maximum efficiency is usually a secondary consideration.
Charts 2 & 3:
Steppanoff- Centrifugal and Axial Flow Pumps - 1957 John Wiley and Sons |
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