![]() ![]() Because air pressure in a conventional tank is higher than vapor pressure of water in the tank, air is absorbed into the water, and it is necessary to replenish the air regularly. Boyle's law confirms this as described below.Ĭonventional tanks are sealed vessels that do not have a membrane to separate air from water, as do captive air tanks. If it is set too low, the tank will not have as much drawdown as it would with the pre-charge set properly. What happens if the pre-charge is not correct? If it is set higher than the pump cut-in pressure, the tank will run out of water before the pump turns on and there will be a momentary loss of system pressure. Pre-charge pressure in captive air tanks is always checked with the tank's water chamber empty. If you decide to use a different pressure switch setting, say 20-40 or 40-60, it will be necessary to adjust pre-charge pressure in the tank to 18 or 38 psi respectively. For a 30-50-pressure switch, the tank would have a 28-psi pre-charge. In this article, we will use the terms "captive air" and "conventional" to differentiate the two types of tanks.Ī captive air tank has a rubber or plastic membrane separating water from air, and must be pre-charged to a certain pressure, usually two psi below expected pressure switch cut in point to operate properly. ![]() All pressure tanks used in the groundwater industry are hydro-pneumatic meaning they contain water (hydro) and air (pneumatic). Using the term hydro-pneumatic to describe a conventional tank is a misnomer. There are two types of pressure tanks, captive-air tanks (also called pre-charged, diaphragm or bladder tanks), and conventional tanks, (also known as hydro-pneumatic, galvanized, ASME, and epoxy-lined tanks). A 10 gpm pump would require a pressure tank with 10 gallons of drawdown, etc. ![]() Tanks are sized with enough drawdown to allow the pump to run a minimum of one minute between cycles, letting motor cool, as recommended by motor manufacturers. The amount of usable water in a pressure tank is called drawdown, and is the amount of water drawn from the tank between the time the pressure switch cuts out turning off the pump, and cuts back in, turning the pump on. The primary function of a pressure tank in a pumped water system is to store water under pressure so the pump does not have to come on every time there is a small, intermittent demand for water. ![]()
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