Pump Selection - Part 2: Performance Curves
Last month, we began this mini-series on pump selection with a look at the simplest of pump curves - those showing only pressure and flow, as typically are seen for residential submersible pumps. When the capacity and horsepower requirements of a pumping system go up, additional information often is provided, such as pump efficiency, horsepower and NPSH (net positive suction head). This month, we will begin to look at these additional factors.
But first, let's see how pump curves are created. The pump curves we see in pump manufacturer's catalogs are a visual representation of test data collected in their labs. To collect this data, a technician connects the pump to a test station containing a pressure gauge, a valve to regulate the flow and a flow meter to measure the flow rate (See Figure 1).
To plot a pump's curve, the technician starts with a piece of graph paper. The horizontal axis represents the flow rate, and the vertical axis shows pressure. In the United States, flow is measured in gallons per minute, and pressure is measured in pounds per square inch (PSI), or feet of water. The first point on the curve is generated by closing the regulating valve all the way to 'dead head' the pump and measuring the pressure on the gauge. Next, the valve is slowly opened until say, 5 gpm shows on the flow meter, and the resulting pressure is recorded on the graph paper. This procedure is repeated until the regulating valve is wide open, and the pump is operating at open discharge. The resulting curve is the simplest form of a pump curve.
As mentioned earlier, there are three other factors that could be considered when selecting a pump - efficiency, horsepower and NPSH. These are graphically represented (as shown on Figure 2) as the efficiency curve, the horsepower curve and the NPSH curve. The NPSH curve will be discussed in detail in this column in the February 2004 issue of National Driller.
The efficiency curve shows the relative cost to run the pump. It is generated by measuring the amount of energy consumed by a pump at various flow rate points, relating that to the amount of work done (GPM and NPSH) at each point, and calculating an efficiency number. Pump efficiency always is important but is particularly important when you are pumping a lot of water. If you have several pumps to choose from for a particular job, the more expensive pump might pay for itself in a short period of time through energy savings if it is more efficient.
Horsepower curves are essential in selecting the proper pump motor and can be used in the same way as the efficiency curve to determine the relative operating cost of a particular pump. For example, when we take electric motor efficiency into account, one horsepower equals approximately three quarters of a kilowatt (750 watts). To estimate the annual operating cost of a pump, find the horsepower at the operating point and multiply by the number of hours per year you expect the pump to run. This times 0.75 gives the number of kilowatt-hours of electricity consumed by the pump in a year. Multiply this number by the cost of a kilowatt, and you have an estimate of the annual cost of the energy to run the pump. For example, in Figure 2, at 70 gpm the motor is drawing 3 HP. If we expect to run the pump 5,000 hours per year, this pump would use 11,250 kW hours per year (3 HP x 5,000 hrs. x 0.75 = 11,250). At 20 cents per kW hour, the annual operating cost for this pump is $2,250 for electricity.
Now, if you could find another, more efficient pump that only used 2.5 HP to pump the required amount of water, the annual cost of electricity will be 2.5 x 5,000 x 0.75 x 0.2 = $1,875 per year, a savings of $375 over the first pump. Factor in the cost difference of the two pumps, and you might find the more efficient, more expensive pump would cost less over the life of the pump.
Next month, we will continue this mini series on pump curves with a look at composite curves and pump charts. 'Til then... . ND