PRIVATE WATER SYSTEMS EDUCATION SYSTEM

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Pressure Tank The most common type of storage is the pressure tank. As a pressure tank is filled with water, the air that was inside the tank is compressed. As more and more water is added to the tank, the pressure inside the tank grows as the air is forced to occupy a smaller and smaller volume. Then when the water is used, the compressed air pushes the water out of the tank under pressure. Note that as the water level drops in the tank, the volume that the air is able to occupy grows, and the pressure it exerts decreases.


Tanks If the tank is filled until the air occupies about 1/3 of the total volume, then the pressure that the water will leave the tank under is about 40 pounds per square inch (psi). Assume you have a 42 gallon pressure tank, after 6.5 gallons of water has been used, the pressure is lowered to 20 pounds. The pump then automatically starts and forces 6.5 gallons of water back into the tank and the pressure is back up to 40 psi.

When more water is being used than the pump can deliver, the pressure will drop to under 20 pounds as the pump and the tank work together to deliver water. Pressure tanks come in a variety of sizes.

Since the useable capacity of a pressure tank is only about 20% of its total capacity, it is not a good method for storing large quantities of water. Its main attraction is the ability to store enough water to supply small demands, thus reducing wear on the pump by keeping the number of starts and stops low.

To increase the useable capacity of a pressure tank, instead of using only the air that is trapped inside the tank, it is possible to precharge a tank. A tank is precharged by forcing more air into the tank to supplement the air already inside.

Water-Logged Tanks Given enough time, air-absorption by the water can lower the pressure in the tank such that the pump will start after removal of only a few pints of water instead of 6.5 gallons. When this happens, the tank is said to be water logged. The use of air controls are a means of controlling air-absorption.

Constant Air devices are part of the pressure tank. One type uses a movable separator, or float, to prevent air absorption. Others use as elastic air container to increase the pressure of the air, thus preventing air absorption. Constant air devices provide more available water than add-air or release-air devices.

Add-air devices are available in three types. An air volume control valve situated on the side of the tank allows air to be added or released. The float-operated type of add-air device is for systems with shallow-well pumps. The diaphragm type is for jet pump systems, and the displacement air control is for any system with an above ground pump (pictures of each of these are forthcoming). Air-release devices are release excess air continuously added by certain types of pumps; deep-well piston and submersible pumps usually add air as they pump water. Much more detail concerning air volume controls in pressure tanks and other types of storages is available in the Control section from the menu.

Needed Tank Volume

For common water systems, a pressure tank about 10 times the pump capacity is adequate. It the tank is the precharged type, six times the pump capacity is adequate. For water systems requiring pumps larger than 25 gpm, consult the supplier for recommended tank sizes.

The storage capacity of a pressure tank is usually small when compared to the daily water used. Most tanks use any 10% to 30% of their volume for usable storage.

Do not depend on the pressure tank to meet peak use rate. For a few minutes, the pressure tank can supply water at any rate demanded, but as soon as the "pump on" capacity is reached, system flow rate is reduced to the pump's capacity.

Example

10 gpm pump, 100-gal pressure tank of which 30 gal is useable storage. Only two minutes of a peak demand at 15 gpm uses up the 30 gal; then the pump comes on and supplies 10 gpm.

The water system should be able to supply peak use rate for 1 hour for a home (2 hrs for a farm), which is much beyond what the pressure tank can provide. Meet peak demand needs with intermediate storage if necessary.

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