The Well

The Well

Obviously, to meet the various capacity requirements discussed above, the well must be adequate. This is basically the driller's job. But the pump installer must be at least conversant with the usual measures of well performance so he can discuss them intelligently with the customer and/or driller. In this connection, the pump installer should secure a well log whenever possible. This is a written "profile" of the well, detailing its depth, rate of production, type of casing, etc.

The most common well characteristics include the following:

DIAMETER:

It might seem obvious that a 4-in. submersible pump can't fit in a 3-in. well. Yet occasionally an installer will order a new submersible from his wholesaler without checking the well diameter. It's also a good idea to check well straightness or plumbness if a submersible is being installed to replace a jet. A well that's adequate in size but crooked may not permit installation of a submersible.

DEPTH TO WATER:

Also known as the "standing water level" or "static water level", this is the number of feet from the surface to the water level when no water is being withdrawn and the well has reached equilibrium. In other words, this is the highest level water reaches in the well, and of course the pump or drop pipe must be set considerably below it.

PUMPING LEVEL:

Once the pump starts, the water level tends to fall because, presumably, the pump is removing water faster than it can flow into the well. However, a steady-state point is eventually reached at which water flows into the well at the same rate at which it is being pumped out. The water level at this point is known as the pumping level.

DRAWDOWN:

The difference between the depth to water or static water level and the pumping level is known as drawdown. This is not to be confused with "submergence," which is the depth below the pumping level (at full drawdown) where the pump or foot valve is located.

As a practical matter, there should always be adequate submergence to ensure proper pump operation. For the example given earlier of a modest vacation home with 5 outlets, this system would need pump capacity of 5 gpm. For a larger home with 2½ baths (8 outlets), kitchen sink and dishwasher (2 outlets), washing machine and laundry tub (2 outlets) and two outdoor bibs, 14 gpm would be required.

The accompanying table on page 36 shows the average rate of flow for various household fixtures. It's evident that unless all or most fixtures are used simultaneously — which is improbable — the "outlet count" method of sizing works.

Farms:

Sizing water systems for farms — especially where livestock is involved — presents a different problem. In this case both total demand and peak demand must be figured. The table at right shows total demand requirements for various types of livestock. The system should be sized to accommodate this demand within two hours (120 minutes). To the gpm thus arrived at must be added the needs of the farm household, plus all water outlets in the farm building.

To illustrate: Take a dairy farm with 70 milking cows, 30 dry cows and a flock of 500 chickens. From the table at right, total demand for livestock would be 35 x 70 gpd (milking cows) plus 30 x 15 gpd (dry cows) plus 5x 6 gpd (chickens) or 2,930 gpd. Since the system must be sized to deliver this demand in two hours, required capacity is 2,930 ÷ 120 = 24.4 gpm.

Assume also that there are 12 outlets in the farmhouse and an additional four in the milk house. Then the total capacity would be:

Livestock: 24.4 gpm

Farmhouse: 12.0 gpm

Milk House: 4.0 gpm

Total capacity: 40.0 gpm

The pump would have to be sized to deliver this flow rate.

Commercial Establishments:

There are no hard and fast rules for sizing commercial jobs because requirements vary so widely. The best procedure is to determine total daily requirements and then estimate the approximate time span over which this demand must be met. The table on page 36 (lower) shows average daily water needs for numerous nonresidential applications.

Schools, as we have seen, must meet this demand in half an hour or less. Motels, on the other hand, can extend it over a two-hour period. In the absence of any specific information, it is best to size nonresidential jobs by dividing total daily demand by two hours or 120 minutes.