Sep 14, 2015

Importance of the Pre-charge of Pressure Tanks.


A pressure tank consists of an air and water chamber that are divided by a membrane. The air in the air chamber is compressed dry air.
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A pressure tank consists of an air and water chamber that are divided by a membrane. The air in the air chamber is compressed dry air. This so called pre-charge is a very important factor in the operation of a system with a pressure tank. It acts like a spring and stores the energy in a tank that is used to supply pressurized water when the pump is not running.

The membrane that separates the air and water chamber is an integral part of an expansion tank. If there was no membrane, the air would dissolve in the water and eventually the tank would not work anymore as the entire tank would be filled with water, otherwise known as water-logged.

The solubility of air in water depends on pressure and temperature. The higher the temperature, the lower the solubility of air in water. The higher the pressure, the higher the solubility of air in water. As the temperature of the water in a pumping system can be considered constant, the pressure is the dominant factor that determines the solubility of air in the system. When the water enters the pumping system it contains a certain amount of dissolved air and this amount will remain constant throughout the whole system. As the pump increases the water pressure, the solubility of air in water increases and effectively this pressurized water can be considered under saturated with regards to its air content. The moment the pressurized water is in direct contact with the pre-charged air, this air tends to dissolve in the under saturated water, trying to bring it back up to a saturated state, which in turn results in a loss of air pre-charge.

A loss of pre-charge is directly affecting the efficiency of the system as the pre-charge is the only setting at a tank to adjust its operating parameters. The pre-charge setting is used to adjust the amount of water in the tank during operation. There is no water in the tank as long as the system pressure is lower than the pre-charge. As soon as the system pressure is higher than the pre-charge, water will enter the tank and compress the air in the air chamber which is thereby increasing the pressure in the air chamber equally to the system pressure. Due to this we can calculate the percentage of water in a tank with a certain pre-charge at any system pressure.

By adjusting the pre-charge we can manage the pressure differential between pre-charge and maximum pressure and thereby the maximum amount of water in the tank at maximum pressure, which is also referred to as acceptance. This is done to limit the maximum elongation of the membrane. This is needed as the membrane would otherwise burst if stretched too much.

This pressure differential and thereby the membrane elongation can also be increased by lowering the lower pressure set point, which in this case is the pre-charge. It should not become obvious that as a tank loses air either by an external leak or by an imperfect sealing between the air and water chamber it will eventually fail.  This is due to the membrane stretching more and more due to the increasing pressure differential. The best way to avoid this is to design tanks in a way that ensure a permanent separation between air and water and avoid any external leaking.

GWS diaphragm pressure tanks use a high grade butyl diaphragm that is precision mounted to create permanent separation of the air and water chambers.  We also incorporate patented double sealed water connections and air valve assemblies to double ensure there is no air leakage from the tank.  It is because of these design features that GWS tanks are considered maintenance free and can last where other tanks fail.