Reading Pump Curves - Veolia

1 Equipment 471 Reading Pump Curves By James McDonald, PE, CWT Originally Published; CSTN September/October 2004 The performance of a centrifugal pump can be shown graphically on a characteristic curve. A typical characteristic curve shows the total dynamic head, constant horsepower, efficiency, and net positive suction head all plotted over the capacity range of the pump. The figure below shows a typical pump curve as furnished by a manufacturer. It is a composite curve which tells at a glance what the pump will do at a given speed with various impeller diameters from maximum to minimum. Constant horsepower, efficiency, and Net Positive Suction Head Required (NPSHR) lines are superimposed over the various head Curves .

2 It is made up from individual test Curves at various diameters. Equipment 472 A breakdown of the individual components of the example pump curve incudes: Total Head (feet H2O): The Y-axis shows the Total Head expressed in feet of water. Total Head is the total dynamic discharge head minus the total dynamic suction head or plus the total dynamic suction lift. In plain English, read the pressure gauges before and after the pump. If the pump is sucking water up from a tank, add the two numbers. If the pump has a pressured supply or is lower than the tank it is pumping from, subtract the suction pressure from the discharge pressure.

3 Remember that 1 psig equals feet of water. Total Head = Hd + Hs (with a suction lift) Total Head = Hd Hs (with a suction head) Gallons per Minute (gpm): The X-axis shows the gallons per minute for the pump. Impeller Diameter: The nearly horizontal lines that slope downward on the right side are for each size impeller diameter (inches) available for this pump. Changing the impeller diameter can have a significant effect on the performance of the pump. Constant Horsepower: The diagonal Curves that slope down to the right are constant horsepower. These constant horsepower Curves indicate the motor sizes available for the selected pump model.

4 When the desired flow rate (gpm) and Total Head intersect between two constant horsepower lines, always select the higher horsepower. Pump Efficiency: The U-shaped Curves illustrate pump efficiency. Pump efficiency is the ratio of the liquid horsepower delivered by the pump and the brake horsepower delivered to the pump shaft. 100% efficiency is never realized because of mechanical and hydraulic losses incurred in the pump. The example curve shows the maximum efficiency for this pump to be in the 60 percents. The goal, of course, is to pick a pump where the system requirements allow it to operate at the highest efficiency. Net Positive Suction Head Required (NPSHR): The vertical, dashed lines indicate NPSHR in feet of water.

5 NPSH is an analysis of the energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump. NPSHR is the positive head in feet absolute required at the pump suction to overcome pressure drops within the pump and maintain the liquid above its vapor pressure. Pump Curves are not only used to pick the correct pump for an application, but can also be used to estimate flow rates for an already installed system. Equipment 473 Many pump Curves are available on the internet. By knowing the model, discharge pressure, suction pressure, and impeller size, the flow rate can easily be determined from the curve.

6 Resource: Goulds Pump Manual GPM5, 1988, Gould pumps , Inc.