A pump is a machine that moves a given volume of liquid a given distance in a period of time. Pumps take energy from an external source (motor, turbine, etc.) and develop it in the form of discharge pressure or total dynamic head. A curve can be developed to measure a pump's performance. Tests to establish pump performance curves are run on clear, cold water with a specific gravity of 1.0, at 60°F.
To establish a centrifugal pump performance curve, the pump is run in conjunction with a gauge, a valve, and a flowmeter located on the discharge side. Running the pump with a closed valve on the discharge line, yields the maximum pressure, or feet of head, which the pump will develop at zero flow. The valve is then opened to allow a small flow, and the pressure and flow rate are noted. The required horsepower (or kilowatts of electricity) is also noted. This process is continued over the full operating range of the pump. Plotting the data points reveals the head-capacity and horsepower curves (Figure 11-1).
The flow rate from the pump is determined by the performance and friction curves of the plumbing connected to the pump. The horsepower curves indicated on a performance curve represent the power required when pumping water, or any fluid within the same specific gravity and viscosity as water. Corrections for different specific gravities are, however, quite straightforward—horsepower is directly proportional to the specific gravity of the fluids being pumped. Performance curves are used to determine the flow rate, head (pressure) developed, and horsepower required by a pump having a specific diameter impeller and operating at a given, constant speed.
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