Tech Topics: Pump Selection - Part 3: Performance Charts
December 1, 2007
This is the third is 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 vs. 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 performance information is available to you from the manufacturer’s performance curve for this pump, as we discussed a couple of months ago. 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, and the third column lists the delivery pressure in PSI. This value is the pressure available to fill the pressure tank and shut off your pressure switch. You should choose a pump that has enough capacity to provide more than enough pres-sure to shut off the pressure switch, and to allow for a drop in the pumping level and/or the eventual deterioration of the pump’s performance. If you have a 20/40 pressure switch, choose a pump that gives you 50 psi at the desired flow rate and pumping level.
The row of numbers across the top beginning with 20 and ending with 1,000 is the vertical 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 level of the pressure tank and pressure switch. In other words, how far does the pump have to push the water up to get it up to the tank pressure switch? Once you’ve pressurized the tank, the tank provides the working pressure to get the water to the fixtures in the house.
Remember when selecting a pressure switch to take into consideration the loss of pressure due to vertical lift 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 showerhead 30 feet above the tank just before the pump comes on at 20 psi (30 ft. x 0.433 = 13 psi, and 20 psi – 13 = 7 psi). You should choose a 30/50 or 40/60 pressure switch in such an applica-tion to provide enough pressure at a second-story showerhead for a satisfactory shower.
The numbers in the boxes on the chart represent the GPM available at the various combinations of pumping level and discharge pressure. For ex-ample, 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 120 feet, you only would develop the 50 psi necessary to shut off the pressure switch after the flow of water out of the pressure tank stopped. The shut-off pressure is shown on the bottom line. You can see that the total shut-off pressure available at a 120-foot pump-ing level is 54 psi. Therefore, at zero flow (shut off) you could, with a brand-new pump that performs up to the manufacturer’s published curve, build enough pressure to satisfy the pressure switch. However, you have not built into your system any reserve for a drop in the water level in the well or in the pump’s performance.
This chart also shows 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, you would be able to provide 10 gpm continuously at 20 psi with this pump. However, if you need 30 psi to sat-isfy an upstairs showerhead, and 10 gpm is your design flow rate, this pump would not do the job.
Jet Pump Perfomance ChartsPerformance charts for jet pumps are similar to those for subs, but contain additional information having to do with a jet pump’s suction lift capa-bility. Figures 2 and 3 show shallow well and deep well performance charts for a 1⁄2-HP convertible jet pump. Notice that the total suction lift for the shallow well configuration maxs out at 25 feet because its lifting capability is limited by atmospheric pressure to that height. Please refer to our March 2007 article for a refresher on how pumps lift water; you can find this article at www.nationaldriller.com. (Click on “Editorial Archives” and open the March 2007 tab where you will find the article.)
In the deep well configuration, the jet assembly is down in the well and pushes the water up to the surface, so lift is not limited by atmospheric pressure. Figure 3 shows that this same 1⁄2-HP pump can lift (actually push) from 60 feet and still provide 5 gpm at 30 psi. The one additional col-umn 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 the necessary minimum 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 our April and May 2007 articles in this publication, also available online.
In summary, the ability to properly use performance curves and charts is critical to selecting the right pump for the job, both in terms of getting the job done right, and also in doing it efficiently. With a little practice, the process of selecting pumps will become second nature.
Next month, we will attempt to unravel the mysteries of NPSH, net positive suction head. ’Til then ….