Every catalogue datasheet contains a pump curve. It looks like a simple graph, yet it answers the most important question: will this pump deliver my flow at my head, and how efficiently?
The axes
The horizontal axis is flow (Q), normally in m³/h. The vertical axis is head (H) in metres. The main line is the pump's Q-H curve: as flow increases, the head the pump can generate falls.
1. Find the duty point
Mark your required flow on the horizontal axis and go up to your required head. If the point lies on or just below the curve, the pump can do the job. In practice the pump works where its curve crosses your system curve, which rises with friction losses.
2. Check the efficiency
The efficiency curve shows a peak, the best efficiency point (BEP). A well-chosen pump runs close to the BEP, ideally between about 70% and 110% of the BEP flow. Far from it, the pump is less efficient, runs hotter and wears faster.
3. Check the power
The power curve gives the shaft power needed at your duty. The motor should cover that power with a margin, and for pumps whose power rises with flow the motor must cover the highest flow you will ever run.
4. Check the NPSH
NPSHr is the suction head the pump needs to avoid cavitation. The NPSH available in your installation must be higher, with a margin, or the pump will be noisy and damaged by vapour bubbles.
Five common mistakes
- Selecting at the very end of the curve instead of near the BEP
- Forgetting the friction losses in the pipework
- Ignoring the 50 Hz or 60 Hz difference: curves change with speed
- Oversizing the pump "to be safe", which wastes energy
- Not checking NPSH for suction lift or hot water
Tolerances
Published curves carry manufacturing tolerances defined by standards such as ISO 9906 or GB/T 3216. Our selection reports state the tolerance level used so that the figures can be checked.
Send us your duty and we will plot it against the catalogue curves and send you the datasheet. Contact CNP Pump Global.
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