When the Well Runs Dry
Others are not so lucky. Their wells may produce plenty of water in the wet season but slow down during the dry season to where the pumping level drops to the inlet of the pump. Still others may have slow-producing wells that only yield a few gallons per minute, making it necessary to turn the pump off when the well draws down, leave it off long enough for the well to recover and turn it back on again once it has recovered.
There are tens of thousands of these stingy wells in the United States doing a fine job of providing water for their owners. In this article, we will recap the special considerations necessary to make these wells work satisfactorily.
Storage - The first consideration is providing enough water storage capacity to assure that the family does not run out of water in the house. A typical pumped water system would include a pressure tank with enough storage capacity to allow the pump to run for at least one minute between pump on-off cycles to allow for motor cooling. That means a 10-gallon-per-minute (gpm) pump should have a pressure tank with a minimum of 10 gallons of draw down at the pressure switch range you are using. For larger pumps, the run time should be increased to provide adequate cool down time for the motor. Depending on the amount of water the well produces, you may get away with just increasing the size of the pressure tank.
The Water Systems Council suggests providing for peak demand for a period of 7 minutes. If you have a well that will support a 10-gpm pump and a peak demand of 13 gpm, the 3-gpm difference can be provided by the pressure tank. In this example, 3 gpm times 7 minutes equals 21 gallons of additional storage that must come from the pressure tank. As mentioned above, you would size a pressure tank with at least 10 gallons of draw down for a 10-gpm pump to allow a minimum of 1 minute of pump run time between cycles. Adding the 21 gallons of additional capacity needed to meet peak demand, you'd need a total of 31 gallons of draw down. A 119-gallon pressure tank operating at a 30/50 differential will provide 36 gallons of draw down, more than enough to do the job.
What if the peak demand far exceeds the capacity of the well? In this case, you will need to provide a significant amount of storage capacity, either in a very large pressure tank, which could get prohibitively expensive, or a non-pressurized storage system with a booster pump and separate pressure tank. If you keep your old copies of National Driller, check out the April 2000 article for more information on open storage tanks and booster systems.
Controlling the pump - The problem for submersible pumps when the well runs dry is their overheating. Submersible pump motors depend on the flow of water past the motor for cooling, and if the pump is left on when the well runs dry, it will overheat, melt the insulation on the windings and probably quit running altogether. Depending on the size of the motor, it may or may not have thermal overloads built into the case, but even if it does, enough damage can be caused prior to the thermal overloads switching it off to shorten the life of the motor significantly. So, how do we tell the motor to turn off when the well runs dry before it sustains any damage?
Low-pressure cut-off pressure switches - Several manufacturers make pressure switches with automatic low-pressure cut-off features. These switches provide a basic level of protection preventing the pump and motor from being damaged by flow problems. However, this type of protection is not foolproof. It will not shut the pump off if the flow stops during the fill cycle when the system pressure is between the cut-in and cutout settings of the pressure switch. The scenario is as follows; the system demand stops while the pump is running to refill the pressure tank, and the well runs dry before the pressure tank is filled. What happens is the check valve between the pump and the pressure switch keeps the pressure on the house side of the check valve where it was when the well ran dry. Because the pressure switch is on the house side, it doesn't know the pump isn't pumping and won't know until someone turns on a faucet and drops the house side pressure low enough for the low pressure cut-off to do its thing. For this reason, I do not recommend low-pressure cutout switches for dry-run protection.
Probes - The oldest method for providing run-dry protection is to install a water level probe down in the well just above the pump and wire it into the pump control circuit so it turns the pump off when the water level drops below the probe. You will want to install another "pump-on" probe above the "pump-off" probe to tell the pump the well has recovered enough to turn on again. The distance between the two probes will depend on the capacity of the well and pump but should be long enough to let the well recover for at least 15 minutes. Probes work well in theory, but they require frequent servicing to maintain their reliability. For this reason, I do not recommend them for run-dry protection unless no other options are applicable to your particular situation.
If you are in an area where there is more than a remote likelihood of the well running dry or of something else happening to slow or stop the flow from the pump, you will be better to choose an electronic pump protector, which is the subject of my next article.