Portable Pump Selection
TOOLS & EQUIPMENT
SELECTING THE RIGHT portable pump for your specific needs is no easy task. Not only have the uses for these pumps grown, but their capabilities have changed dramatically. Because of the broad selection of pumps available today, you should evaluate certain key criteria in your selection process, including range of flow (gpm), pressure (pounds per square inch, or psi), and probable lift conditions (in feet), which will all affect the pump’s performance.
Portable pumps are often described using a chart to illustrate maximum flow and maximum pressure. While these charts can be useful as a quick reference, the true picture of a pump’s performance is demonstrated by a head (psi)/capacity curve. This curve depicts the mathematical relationship between flow and pressure and the factors that determine them, such as horsepower and rpm. All manufacturers have curves for their products, and they should be made available to customers on request.
Figure 1 shows an example of a head/ capacity curve. The vertical axis measures the discharge pressure, which is normally shown in psi or feet of head (ft.). The horizontal axis measures flow or capacity in gallons per minute (gpm). The heavy black curve depicts the pressure sustained when a certain number of gallons per minute are flowing at full throttle.
For example, at 450 gpm the pressure will be 65 psi; conversely, if the pressure reads 65 psi, the pump will be flowing at 450 gpm. As long as the throttle remains in the full position, the pump operates somewhere on this curve. The exact point of operation is determined by the amount of restriction on the discharge: Opening or closing a discharge valve or nozzle will move performance to the right or left along the curve.
Usually, a discharge pressure gauge is supplied with the pump. This gauge reads the pressure at the pump discharge and can be used in conjunction with the curve to determine the flow. Maximum flow, or the farthest right point on the curve, is seldom used. Many curves show this as maximum flow at zero. This point should only be used as a reference to help picture a curve, because zero psi means the water would, theoretically, not have the pressure to escape the pump.
This also holds true for maximum pressure, which is shown as the farthest left point on the curve. Often shown as maximum pressure at zero gpm, this would mean, theoretically, that no water is flowing. Again, this should be considered merely a reference point.
Attempting to operate a pump at or near these points presents other problems as well. The maximum pressure is also the point of maximum shaft deflection. At maximum pressure, the shaft will attempt to bend, significantly shortening the life of the pump bearings.
Maximum flow normally occurs when suction lift is zero. To actually operate at this point, a flooded suction (where the pump is below water level, or water is entering the pump with positive suction pressure) would be required.
To find the ideal range of operation, look to the point immediately to the right of the middle of the curve. This area is known as the BEP, or Best Efficiency Point. At the BEP, the internal forces of the pump are relatively balanced. Operating at this point ensures that the pump and engine are working at their highest potential, thus maximizing the life of the equipment.
Because portable pumps are generally used in applications requiring a variety of flows and pressures, operating below the curve is sometimes desired. (Remember, the curve depicts maximum performance, so you cannot operate above it.) This is accomplished by reducing the speed or rpm of the engine. The curve then shifts downward (see Figure 2). Once the engine rpm is adjusted, movement along the curve is caused by changes in the discharge opening.
The final piece of information usually shown on a pump curve is a suction lift line. Suction lift lines represent the maximum flow and pressure under which the pump can successfully deliv-, er water, considering the following fao tors:
- the distance the pump is above the! water level (lift), and
- the diameter and length of the suction hose.
Most curves picture the pump’s abili-. ty to lift as an angled vertical line. As lift increases, flow decreases. If the desired performance point is to the left of the line, the pump will operate satisfactorily. If the desired performance point is to the right of the line, the pump will cavitate and fail to operate properly.
The lift has little effect on discharge pressure. For example, a 12-foot lift would only reduce the discharge pressure by 5 psi compared with operating with a flooded suction. Once the pump is started a vacuum is created, which draws water into the pump. A perfect (but not practical) vacuum would theoretically lift water just under 34 feet. A centrifugal pump will usually be able to sustain a lift of 20 to 22 feet under normal conditions.
Understanding these critical points, on the curve will help you determine which pump best suits your needs. As you review a particular pump’s curve, ask yourself the following questions: What is your specific range of required pressures (psi)? What is your specific range of required flows (gpm)? How far away from the source of water are you (lift)?
Each of these factors impacts a pump’s performance. Because of the complexity of these factors, the only true way to see the complete picture is to read the curve. Be sure to ask pump manufacturers for curves and compare them carefully during your selection process.
