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Efficiency Characteristics of Centrifugal Pumps Guide prepared by the EU SAVE Pump Study Group (Contractor: ETSU, AEAT PLC, (United Kingdom); Participants: CETIM (France), David T. Reeves (United Kingdom), NESA (Denmark), Technical University Darmstadt (Germany)

This section gives a quick overview of the fundamentals of choosing and using a pump for best efficiency. This is not an exhaustive guide, but is designed to be just sufficient for nontechnical personnel to get a better understanding of the technical background to this work. Other publications (such as the Enersave guide) are recommended as giving much fuller explanations. The following applies to most types of centrifugal pumps only.

Part load characteristics of pumps Pumps are always defined by the basic Pump characteristics below (figure A3.1). They show the relationship between head, power and efficiency against flow. It is important to see just how "peaky" the efficiency might be, showing that running at a duty (head and/or flow) below rated duty is likely to lead to a significant reduction in pump efficiency. The Best Efficiency Point (BEP) of a pump is ideally at the rated duty point.

Figure (A3.1) – Centrifugal pump characteristics Running at reduced flow, or indeed above rated flow, will accelerate pump wear and might give operational problems, (figure A3.2).

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Figure (A3.2) – Onset of adverse effects when operating a pump away from its peak efficiency flow In practice pumps are usually found to be over-rated for the duty, since either the demand varies, and/or the system designer has been prudent and over-sized the system. This problem is generally over-come by throttling the flow with a valve. But deliberately restricting the system flow is far inferior to better matching of the pump to the actual system requirements. This is because it often causes additional wear, (figure A3.2), and reduces system efficiency, (figure A3.3).

Figure (A3.3) – Illustration of the effect on efficiency of throttling a pump

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One pump - different duties Reducing the diameter of the impeller will make an existing pump run more efficiently at a lower flow without throttling, (figure A3.4). Manufacturers commonly offer the same pump housing (volute) with a range of impeller diameters because of this. Manufacturers may also offer different styles of impeller to cope with different duties.

Figure (A3.4) – Pump characteristics showing various impeller diameters Often a manufacturer will offer the same pump with different motor options to allow the one pump to be used over a much wider range of duties. For instance, changing from the most common 4-pole motor to a faster 2-pole motor will enable the same pump to deliver twice as much peak flow and 4 times the head. (The effect of running a pump with 4 and 2 pole motors is the same as what happens when running at 50% and 100% speeds as shown in figure A3.5). Variable Speed Drives allow a pump to operate efficiently over a wide range of speeds and hence duties, and so are very good for saving energy, (figure A3.5). They are particularly useful in systems where there is a wide variation in demanded flow.

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Figure (A3.5) – Effect of speed reduction on pump characteristics What this all means is that the same basic pump can serve different duties depending on both the diameter of impeller fitted and the speed of the motor chosen. The power consumption will vary with these parameters and with other factors such as viscosity. It is therefore not useful, and indeed would be very misleading, to characterise pumpsets by power rating. The size of motor required therefore needs to be determined for each application.

Selecting a pump When selecting a pump, a manufacturer will use "tombstone" curves, which show their ranges of pumps to cover a range of duties (figure (A3.6). Ideally, the duty you want will be roughly 20% below the maximum flow shown on the tombstone, which corresponds to the BEP of the selected pump (each tombstone is built up from the individual pump curves in figure A3.4). But for economic reasons they have to restrict the number of pumps that they offer. This means that even a manufacturer of particularly efficient pumps may lose out, when quoting efficiencies in competition with less efficient pumps whose BEP just happens to be nearer the requested performance. The worked example in the box makes this clearer.

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Figure (A3.6) – “Tombstone” curves for the selection of pumps by duty. In addition, pumps wear over time (figure A3.7), but their efficiency can be maintained by refurbishment figure (A3.8).

Figure (A3.7) – Effect of wear on pump characteristics

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Figure (A3.8) – Average wear trends for maintained and unmaintained pumps

Worked example This worked example shows the key issues that arise when selecting a pump on the basis of efficiency.

BEP (B) BEP (A) = 82%

= 86% 83%

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Desired (Quoted) Duty Point

82

80% Pump B

80% 77% Pump A

Head

Actual (Installed) Duty Point

Flow

Pump Efficiency

Pump A Pump B

Peak (BEP)

82%

At desired (quoted) duty point

81.5%

86% 80%

At actual (installed) duty point

79%