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Pump Basics 2: Pump Curves
Wed, October 10, 2018
When a centrifugal pump is designed, the manufacturer will test the operation of the pump to the theoretical design. Pumps are tested for flow, head (pressure differential), power consumption, efficiency and minimum allowable suction pressures, also known as Net Positive Suction Head Required (NPSHR). Generally, as the pump handles greater flows, the pressure differential it creates decreases. We plot this flow-to-head relationship to create a pump curve. As the illustration here shows, pump curves can operate at different RPM’s and with different impeller diameters. Sample curves based on specific diameters create what we call the family of curves. It’s a quick way to see what the pump will do for any given set of impeller diameters.
As you can see from the illustration, as flow increases, the horsepower requirement usually (but not always) increases. The horsepower requirement is the sum of the power needed by the pump to perform the work (Water HP), plus the losses due to inefficiencies of the pump. This is known as Brake HP, and this is what we see on the pump curve. You don’t need to figure out the BHP. It’s already shown on the curve for that given impeller diameter and that speed.
Speaking of pump speed, if a 4-pole motor should run at 1800 RPM, why do the manufacturers show 1750, 1765, 1740, etc? Well, in a 60 Hertz world, the polarity and electrical current flow change every 1/60th of a second. It is this polarity change that causes the rotor in the motor to move. Not all the polarity changes move the rotor completely though, and we end up with a certain amount of what we call slippage, causing the motor to turn a bit more slowly. The manufacture selects a reasonable amount of slippage for a motor and uses that RPM to run the pump performance tests.
Next time: Why is the pump differential pressure stated in feet of head?
((Missed part 1 of our Pump Basics series? Click here to revisit our review of impellers and volutes in centrifugal pumps!))
Tags: bell & gossett , Bornquist , centrifugal pumps , Education , engineering , horsepower , impeller , motors , pump curves , volute
When a centrifugal pump is designed, the manufacturer will test the operation of the pump to the theoretical design. Pumps are tested for flow, head (pressure differential), power consumption, efficiency and minimum allowable suction pressures, also known as Net Positive Suction Head Required (NPSHR). Generally, as the pump handles greater flows, the pressure differential it creates decreases. We plot this flow-to-head relationship to create a pump curve. As the illustration here shows, pump curves can operate at different RPM’s and with different impeller diameters. Sample curves based on specific diameters create what we call the family of curves. It’s a quick way to see what the pump will do for any given set of impeller diameters.
As you can see from the illustration, as flow increases, the horsepower requirement usually (but not always) increases. The horsepower requirement is the sum of the power needed by the pump to perform the work (Water HP), plus the losses due to inefficiencies of the pump. This is known as Brake HP, and this is what we see on the pump curve. You don’t need to figure out the BHP. It’s already shown on the curve for that given impeller diameter and that speed.
Speaking of pump speed, if a 4-pole motor should run at 1800 RPM, why do the manufacturers show 1750, 1765, 1740, etc? Well, in a 60 Hertz world, the polarity and electrical current flow change every 1/60th of a second. It is this polarity change that causes the rotor in the motor to move. Not all the polarity changes move the rotor completely though, and we end up with a certain amount of what we call slippage, causing the motor to turn a bit more slowly. The manufacture selects a reasonable amount of slippage for a motor and uses that RPM to run the pump performance tests.
Next time: Why is the pump differential pressure stated in feet of head?
((Missed part 1 of our Pump Basics series? Click here to revisit our review of impellers and volutes in centrifugal pumps!))
Tags: bell & gossett , Bornquist , centrifugal pumps , Education , engineering , horsepower , impeller , motors , pump curves , volute
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