Booster Systems for Slow Producing Wells - Part 2
Last month, we said that even a 1 gpm well is not a bad well if you can store its output for later use to meet peak demand because 1 gpm over 24 hours totals 1,440 gallons, which is way more than a typical household uses in a day. Further, we talked about some of the storage options available, such as increasing the capacity of your pressure tank by changing the cut-in, cut-out pressure switch settings, or adding more pressure tanks. Finally, we discussed using a non-pressurized storage tank to hold the water and a second pump, a booster pump, to provide the pressure for the household. We now turn our attention to the pumps and controls required for such a system.
Flow-inducing shrouds - If you are using a submersible well pump, you should install a flow-inducing shroud over the motor to ensure adequate motor cooling. One can be fabricated from a piece of thin-wall 4-inch PVC pipe about the same length as the sub and motor. With a hacksaw, make three longitudinal cuts 4 inches long evenly spaced around one end. Use a hose clamp to clamp the shroud to the pump just above the inlet screen. Use plastic tape to seal the upper end or the shroud where it passes over the wire guard. A perfect seal is not required.
Submersible applications - In any slow-producing well application, it is essential to use some sort of dry well protection device to prevent the pump from running when the water level in the well drops below the inlet of the pump. A dry-running submersible pump will destroy itself and/or its motor in short order. For subs, electronic devices such as Symcom's Pump Saver or Franklin Electric's Pumptec monitor the power going to the pump motor and shut off the pump when they sense the pump beginning to run dry.
Jet pump applications - Since air entering a jet pump system causes it to stop pumping and necessitates re-priming the system, it is important to take precautions to ensure the water level in the well does not drop below the inlet of the foot valve. To prevent air from entering the foot valve in a slow producing well, try the following. Because a pump cannot lift water more than 34 feet maximum due to the limitations of atmospheric pressure, installing a 34-foot section of suction pipe below the injector will preclude the possibility of the water level being drawn down below the foot valve. As the well draws down, increasing the suction head on the injector, its pumping capacity slows down until it matches that of the well. With the foot valve set at 34 feet below the injector, even in a well with no production, air cannot enter the foot valve because the injector is not be able to lift the last foot of water to empty the well.
The pump only needs enough head pressure capability to get the water from the pumping level in the well to the top of the tank, plus a safety margin of 10 percent or so to compensate for wear. The second pump, the booster, covers the pressure requirements of the residence.
If fire protection is a part of this system, the booster pump will need to be sized accordingly. The flow requirements of a fire protection system can vary from 20 gpm to 60 gpm for a residential application, depending on local codes. As a minimum, go with the flow rate recommended by the fire hose nozzle manufacturer or with what is required by local codes.
Using two booster pumps and an alternating relay - Let's say your county code requires a 2,500-gallon storage tank and 60 gpm of flow at 45 psi for fire protection. If you are pumping from a storage tank, I would recommend using an alternating pump system to provide 30 gpm for normal usage and 60 gpm for fire fighting. In such a system, you would install two identical booster pumps side by side at the storage tank. Each pump has the capability to meet the non-fire capacity requirements, and they can be operated simultaneously to meet the fire needs. In non-fire service, they alternate from one to the other using an alternating relay (see Figure 1). Each pump is on every other cycle of the pressure tank.
Figure 2 depicts a typical schematic for a cross-connected alternating relay. Cross connection in a two-pump fire system means the relay uses two pressure switches to either alternate the pumps or turn on both of them simultaneously. The first pressure switch is set at, say, 50/70 psi and alternates the pumps with each cycle as long as the pressure does not drop below 50 psi. When the system is in the fire mode, more water is demanded than one pump can deliver and the system pressure drops below 50 psi. At this point, the second pressure switch set at, say, 45/65 psi closes and turns on the second pump. Once the system pressure reaches 65 psi, pump #2 turns off, and when the pressure reaches 70 psi, pump #1 turns off.
A system like this has several advantages. First, the pumps last longer because they get extra cooling between cycles. Second, they save electricity because they are sized for normal, every day demand, not for fire demand. If a single larger pump were used, it would be overkill 90 percent of the time. Third, a two-pump system offers redundancy, which is particularly important in a rural application where lack of water may be more than just an inconvenience.
The bottom line - How do you decide which storage alternative is best in your unique situation? If your well almost does the job, discuss with the homeowner the option of lowering the cut-in and cut-out settings on the pressure switch, or using a 25- or 30-psi spread instead of the standard 20. Make sure they understand that they definitely will notice the pressure fluctuations in the shower.
If your decision is between using a larger pressure tank or an atmospheric storage tank, run the numbers. It may be cheaper to use a larger pressure tank or multiple pressure tanks than an atmospheric storage tank and a second pump. Whichever course you take, properly designed and constructed, you can turn a slow producing well into a very adequate water system.
Next month, we will turn our attention the electrical side of systems with a look at single-phase electricity. 'Til then ...