20 Pump Curves
Reading a centrifugal pump performance curve
Knowing how to read a centrifugal pump performance curve is an essential skill when selecting a centrifugal pump. To help users interpret a typical curve chart, Tony Dashwood, Technical Director at Crest Pumps, explained what the different elements referenced are.
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or each of their products, pump manufacturers will supply a performance curve detailing information, which includes flow, head, power absorbed, efficiency and NPSH required. Some essential information will be shown on all curves, such as the pump model, speed and suction, and discharge sizes.
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1. DUTY POINT The duty point is where a centrifugal pump operates at maximum efficiency. It is determined by the flow rate and the total head at the respective pump speed – both of which will be defined by the application the pump is required for. Shown as the red lines in our example (figure 1), flow can be read from the horizontal (X) axis, and head is read from the vertical (Y) axis. In our case, the duty point required is 170m³/hr at a head of 36m.
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2. IMPELLER TRIM Impeller trim refers to variances in impeller diameter. In this case, three impeller trims are shown – 210mm, 200mm and 190mm diameters. 3. EFFICIENCY We can see in our example that the duty point sits very nicely at the BEP (Best Efficiency Point). Unless there are exceptional circumstances, a pump should be selected that will operate at a point between 70% and 120% of its BEP. 4. NPSHr (NET POSITIVE SUCTION HEAD REQUIRED) Shown on the right-hand side of our example, reading down the vertical line, we can see that the pump at the duty point requires an NPSH of 3m. Generally speaking, NPSHr curves do not vary much between impeller trims which is why we see only one NPSHr curve, although three sizes of impellers are indicated on the curve. 5. POWER REQUIRED Reading further down the left-hand vertical axis, we can read the pump requires a power of 37.5kW at the duty point (for a 210mm impeller). However, the motor selected to drive the pump should take into account potential variations in the duty point, so it is common practice to choose the motor power for end-of-curve conditions. 6. SHUT OFF HEAD The performance curve will also show the maximum Quarter 2 2021
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head the pump can achieve at closed valve conditions. In this example, we can read that the shut off head for a 210mm impeller is 55m. When looking at pump curves, you may also come across tombstone or composite curves (figure 2). These are produced as a figure 2 guide to see the performance of a complete range of pumps. They are similar in their format and used to see which particular pump will suit the duty required, with specific details found on the individual pump performance curve. chevron-circle-right www.crestpumps.co.uk www.bpma.org.uk
