Note: Please see the print edition of National Driller for the figures that accompany this article.
This is the third in a series on pump selection and pump performance curves and charts. In the last two articles, we discussed factors that should be considered when selecting a ground water pump, when to use a jet pump verses a submersible, how to convert from feet of head to psi, how to read performance curves, how performance curves are generated and how to estimate the cost of operation of a pump. This month, we will look at performance charts, another way to evaluate the performance characteristics of a ground water pump.
Figure 1 shows a typical performance chart for a 1/2-Hp, 10-gpm submersible pump. This same information is available to you from the manufacturer's performance curve for this pump, but here, some of the calculations necessary for proper sizing have been made for you, and the information is presented in tabular form.
The first two columns show the model number and Hp, while the third column lists the delivery pressure in psi. This value is the pressure available to shut off your pressure switch, and should be at least 5 psi above the shut off setting of the switch unless you want your pump running continuously to meet the demand. In other words, if you are using a 20 - 40 pressure switch, make sure you have at least 45 psi available at your demand flow rate and pumping level in the well.
The row of numbers across the top - beginning with 20 and ending with 1,000 - is the distance from the pumping level in the well (not the level at which the pump is located, but the level of the water in the well when the pump is running) to the pressure switch. Remember, when selecting a pressure switch, take into consideration the loss of pressure to delivery points physically above the switch. A 20 - 40 pressure switch located in the basement will give you 7 psi at a second story shower head 30 feet above the tank just before the pump comes on at 20 psi (30 feet X 0.433 = 13 psi. 20 psi - 13 = 7 psi).
The numbers in the boxes represent the gpm available at the various combinations of pumping level and discharge pressure. For example, this 10-gpm pump can deliver water from more than 200 feet. But if you are pumping into a pressure tank with a 30 - 50 pressure switch, from a pumping level of 140 feet, you might never develop enough pressure to shut off the pressure switch. Shut-off information is contained in the bottom row of numbers in terms of feet of head (258 for this pump) or psi for shut off at various pumping levels. You can see that the total shut-off pressure available at a 140-foot pumping level is 50 psi. If the water level in the well drops at all from the level for which you sized this system, your pressure switch would never turn off the pump.
Another way to use this chart is to see at what pressure you can provide a particular flow rate for a given pumping level. Let's say our pumping level is 120 feet. If the system demand were 10 gpm, we would be able to provide 10 gpm continuously at 20 psi with this pump. Once the demand goes away, the pressure would build up to a maximum of 59 psi, which would turn off the 30 - 50 pressure switch.
Performance charts for jet pumps are similar to those for subs, but contain additional information having to do with a jet pumps suction lift capability. Figures 2 and 3 show shallow well and deep well performance charts for a 1?HP convertible jet pump. Notice that the total suction lift for the shallow well configuration only goes to 25 feet because its lifting capability is limited by atmospheric pressure to that height. (Please refer to our February 1999 article in this publication for a refresher on how pumps lift water.)
In the deep well configuration, the jet assembly is down in the well and pushes the water up to the surface, so it is not limited by atmospheric pressure. Figure 3 shows that this pump can lift (actually push) from 60 feet, and still provide 5 gpm at 30 psi. The one additional column on the right side of the deep well chart shows minimum operating pressure for the injector. This is the amount of pressure required to operate the injector measured at the pump. If you are using a pressure switch and it is set above this number, you are covered. If you are pumping into an open tank, you will have to throttle the discharge to provide this amount of backpressure. To select a jet pump, determine the suction lift (feet) and discharge pressure (psi) required, and choose a pump that provides the capacity (gpm) you need. (For a review of how jet pumps work, please refer to my March and April 1999 articles in National Driller.)
In summeay, the ability to use performance curves and charts properly is critical to selecting the right pump for the job, both in getting the job done right and in doing it efficiently. With a little practice, the process will become second nature.