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Source: Manual of Individual Water Systems, EPA-570/9-82-004, U.S. Environmental Protection Agency, Office of Drinking Water, Washington, DC, October 1982, p. 99.

Source: Manual of Individual Water Systems, EPA-570/9-82-004, U.S. Environmental Protection Agency, Office of Drinking Water, Washington, DC, October 1982, p. 99.

FIGURE 2.14 Chart for determining pressure storage tank volume and pump size. Pressure tank volume provides 15-minutes of storage. (Source: J. A. Salvato Jr., "The Design of Pressure Tanks for Small Water Systems," J. Am. Water Works Assoc., June 1949, pp. 532-536. Reprinted by permission. Copyright © 1949 by the American Water Works Association.)

FIGURE 2.14 Chart for determining pressure storage tank volume and pump size. Pressure tank volume provides 15-minutes of storage. (Source: J. A. Salvato Jr., "The Design of Pressure Tanks for Small Water Systems," J. Am. Water Works Assoc., June 1949, pp. 532-536. Reprinted by permission. Copyright © 1949 by the American Water Works Association.)

The pump capacity given on the curve is equal to 125 percent of the maximum hourly demand rate. The maximum hourly demand is based on the following but should be determined for each situation:

Average water use per day

Average daily rate =-1-based on annual water use

1440 min/day in gpm

Average maximum monthly rate = 1.5 x average daily rate

Maximum hourly demand rate = 6 x average maximum monthly rate or

9 x average daily rate Instantaneous rate (pump capacity) = 1.25 x maximum hourly demand rate or

11.25 x average daily rate

The pressure tank is assumed to be just empty when the pressure gauge reads zero. Figure 2.14 can also be used for larger or smaller flows by dividing or multiplying the vertical and horizontal axes by a convenient factor. The required pressure tank volume can be reduced proportionately if less than 15-minutes of storage is acceptable. For example, it can be reduced to one-third if 5-minutes storage is adequate, or to 1/15 if 1-minutes storage is adequate. Also, if the water consumption in Figure 2.14 is 1/10 of 6,500 gpd, that is 650 gpd, the corresponding pressure tank volume would be 1/10 of 2,800 gallons, or 280 gallons. The pump capacity would be 1/10 of 34 gpm, or 3.4 gpm. But if all water is used in 12 hours, as in a typical residential dwelling, double the required pump capacity, which in this case would be 6.8 gpm. The larger pump is usually provided in small installations for faster pressure tank recovery and to meet momentary demands that are more likely to vary widely than in large installations. See previous text and Table 2.11. Also see Figures 2.10 and 2.11. An example for a larger system is given under "Design of Small Water Systems," this chapter.

The water available for distribution is equal to the difference between the dynamic head (friction plus static head) and the tank pressure. Because of the relatively small quantity of water actually available between the usual operating pressures, a higher initial (when the tank is empty) air pressure and range are sometimes maintained in a pressure tank to increase the water available under pressure. When this is done, the escape of air into the distribution system is more likely. Most home pressure tanks come equipped with a pressure switch and an automatic air volume control (Figure 2.15), which is set to maintain a definite air-water volume in the pressure tank at previously established water pressures, usually 20 to 40 psi. Air usually needs to be added to replace that absorbed by the water to prevent the tank from becoming waterlogged. Small pressure tanks are available with a diaphragm inside that separates air from the water, thereby minimizing this problem. Some manufacturers, or their representatives, increase the pressure tank storage slightly by precharging the tank with air. With deep-well displacement and submersible pumps, an excess of air is usually pumped with the water, causing the pressure tank to become airbound unless an air-release or needle valve is installed to permit excess air to escape.

In large installations an air compressor is needed, and an air-relief valve is installed at the top of the tank. A pressure-relief valve should also be included on the tank. See Figure 2.16.

Where a well yield (source) is inadequate to meet water demand with a pressure tank, then gravity or in-well storage, an additional source of water, or double pumping with intermediate storage, may be considered. Intermediate ground-level storage can be provided between the well pump and the pressure-tank pump. The well pump will require a low-water cutoff, and its capacity must be related to the dependable well yield. The intermediate storage tank (tightly covered) should have a pump stop-and-start device to control the well pump and a low-water sensor to signal depletion of water in the intermediate storage tank. A centrifugal

Pressure gauge

Air volume control

Snifting valve

Pressure gauge

Air volume control

Snifting valve

Diaphragm

To pump suction chamber

Pressure tank (c)

Diaphragm

Pressure tank (c)

Pressure tank

To pump suction chamber

Pressure tank

Float

Pressure tank

Float

Float

Air line to pump—| Pressure gauge socket -

Flexible, watertight tube

Air line to pump—| Pressure gauge socket -

Flexible, watertight tube

^ Float rod

Pressure tank

Float

^ Float rod

FIGURE 2.15 Pressure-tank air volume controls: (a) shallow-well type for adding air; (b) deep-well type for air release—used with submersible and piston pumps; (c) diaphragm-type in position when pump is not operating (used mostly with centrifugal pump). Small air precharged pressure tanks with a diaphragm to separate air and water are replacing air-volume controls. (Source: Pumps and Plumbing for the Farmstead, Tennessee Valley Authority, Agriculture and Engineering Development Division, November 1940.)

hO CO

Air relief valve

Water level at maximum operating pressure

Water level at minimum operating pressure -

-Tank supports-

fclMWA^!AW

Intake 2" plank

Note: Use special rubber hose fitting between pump and pressure tank for quiet operation.

-Drain

Air relief valve

Water level at maximum operating pressure

Water level at minimum operating pressure -

Intake 2" plank

Note: Use special rubber hose fitting between pump and pressure tank for quiet operation.

-Tank supports-

fclMWA^!AW

Discharge

VAvm/mjmm\m

-Drain

FIGURE 2.16 Typical large pressure tank installation.

pump would pump water from the intermediate tank to a pressure tank, with a pressure switch control, and thence to the distribution system.

Low-rate pumping to elevated storage, a deeper well to provide internal storage, or an oversize pressure tank may be possible alternatives to intermediate ground-level storage, depending on the extent of the problem and relative cost.

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