Sizing a Residential Water System
Once you know this information, you can determine which type of pump best serves your application — a straight centrifugal pump, a shallow well jet pump, a deep well jet pump or a submersible pump. If the water level from which you are pumping, be it a lake, storage tank or well, is 15 feet or less, you can use a straight centrifugal. If it is 20 feet or less, you can use a shallow well jet pump. If it is no more than 100 feet, a deep well jet can be used. When you have more lift than 100 feet, a submersible pump generally is the best choice, and many pump installers prefer to use submersibles for every pumping level.
If your customer lives in the boonies, you might even choose pump that uses a positive displacement cylinder like a windmill or a hand pump. The final selection will depend upon the availability of power, the amount of water needed and the pressure required.
Once you have determined which type of pump to use, you need to size the pump. Pumps are rated based on flow rate in terms of gallons per minute (gpm) and pressure in terms of pounds per square inch (psi) or feet of head. Knowing these factors will allow you to size a system to meet the peak demand of the water system.
Flow Requirements of a PumpIn order to design the complete water system, however, you will need to know the water usage patterns of your customer in terms of gallons per minute (gpm) and the production capacity of your well. There are several ways to estimate the water usage of a residential application, which we will detail in a moment. The gpm-production capacity of a well is available from the well driller if it is a new well or from experience if it is an older well and the well test data is not available.
If you find that a 30-gpm pump is required to meet the peak flow demand but the well only produces 25 gpm on a sustained basis, some storage capacity must be added. We’ll talk more about storage systems in the coming months.
Peak flow - In a residential application, peak flow usually occurs when there are showers running and cooking going on. This would likely be in the morning or around suppertime. Table 1 shows that two showers, a washing machine and a sink, all going at the same time, would be using from 12 gpm to 20 gpm, depending on the age of the fixtures. Note - Instead of sizing the pumping system to old fixtures, you would do well to suggest to your customers that they install water-conserving fixtures, like low-flow showerheads and water-conserving toilets.
It normally is not necessary to include the usage requirements for lawn sprinkling and yard watering because their usage can be timed around the household requirements. However, if they exceed the demand of the household, size the system to the yard watering requirements.
An alternative method of determining flow requirements, suggested by the Water Systems Council, is to simply count all of the fixtures and faucets in the home. The total number of fixtures gives you the gpm required by the pump. For instance, a two-bathroom home might add up like this: each bathroom - three fixtures (1 each for the bath tub/shower, sink and toilet); two for the kitchen (sink and dishwasher); one for the washing machine and one for a water softener (uses water during backwash). Total = 10 fixtures, 10 gpm. Again, if the requirements of the outside watering system exceed these numbers, size the system on the outside requirements.
The bottom line – Whichever system you use, be conservative in your sizing. You will never hear a customer complain, “My system provides too much water”.
Pressure – As we mentioned above, pressure is measured in terms of psi (pounds per square inch) or feet of head. To help visualize the relationship between psi and feet of head, picture this: Take an empty piece of pipe — any diameter — about three feet long, plug the bottom and put a pressure gage in the plug. Stand it up straight and fill the pipe until the pressure gage reads 1 psi. This is a hypothetical exercise, but if you had an accurate enough pressure gage, you would see that 2.3 feet of water standing in the pipe produces 1 psi on the gage. That is the relationship between psi and feet of head — 1 psi = 2.3 feet of head. If you lowered the water level to 1 foot, you would see 0.433 psi on the gauge. Both terms are used in the pump industry, and you can convert from one to the other by using the factors: 1 psi = 2.3 feet of head or conversely, 1 foot of head = 0.433 psi.
Another good way to remember this relationship is to picture a water tower. If you have a 100-foot-high water tower in your town, what would be the pressure in terms of psi at the bottom of the tank? The answer — 43 psi, about the pressure you’d like to have coming into your house. Plant that picture in your brain, and you will never forget how to convert from feet of head to psi. A 100-foot water tower equals 43 psi. The feet of head number is larger than the psi number.
With this brief introduction to the terms used in working with pressure, next month we will continue our series on residential water system sizing with a look at how to determine the pressure requirements. ’Til then ... .