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Pump performance curves show the relationship
at a given motor speed between required pump capacity and:
1. Total head developed by the pump
2. Pump efficiency
3. Brake horsepower (horsepower required at pump shaft)
4. NPSH required
Most pump curves are based on water performance with water having a viscosity
of 30 SSU and specific gravity of 1.0. |
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The above curve shows a pump selected
at peak efficiency for 180 GPM @ 120 ft. of head. The BHP required at
pumping point is 7.5 but 10 would be recommended to run-out. The NPSH
required is approximately 9 feet. The impeller diameter would be cut at
approximately 6”. |
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Pumps are selected to operate as close
to the best efficiency point as possible. However, in many process chemical
applications, operation to the best efficiency point is normal, given
the varying viscosities and gravities of many chemicals. The use of different
impeller diameters, varying pump speed and discharged line control valves
can aid in reaching the proper pumping performance. |
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Viscous fluids tend
to reduce the capacity, head, and efficiency while increasing the brake
horsepower required. Centrifugal pumps may be used for viscosities up
to 1000 - 1500 SSU. Above this limit rotary pumps are used. See chart
11 (page 170). |
