DEFINE DRAWDOWN IN WATER INSTALL

By optimizing height of the water level probe it is possible to maximize drawdown yielding ratios comparable to those of captive air tanks.

Off-the-shelf air compressor systems are controlled by a water level probe and separate pressure switch.

DEFINE DRAWDOWN IN WATER INSTALL

Since you are forced to install the air release valve in the fitting provided by the tank manufacturers, the drawdown factor ends up being between 10% - 30% depending on whether you use a spring-loaded air release valve and exact location of the air release fitting on the tank. However, regarding the latter, most hydro-pneumatic tank manufacturers put the air release fitting at or above center on the tank. By using a compressor system with a water level probe, or a spring-loaded air release valve 1/3 up the side of the tank, you get comparable drawdowns to captive air tanks. In practice, it is possible to increase pre-charge pressure in conventional tank. a 73-gallon captive air tank.īut who says you can't pre-charge a conventional tank? Remember the ratio of air to water in a captive-air tank operating at a 30/50 pressure range - 70% air to 30% water? If it were possible to control water level in a conventional to the same level as in a captive air tank, the drawdown would be the same for both.

To deliver 22 gallons of drawdown would require a 220-gallon conventional tank without a pre-charge, vs. Without a pre-charge, a conventional tank running at 30/50 has a drawdown factor of 10%. That leaves only 25% for air to force water out, which is why it takes a much larger conventional non-pre-charged tank to deliver the same drawdown as a captive air tank. In a conventional tank, without a pre-charge, 75% of the tank's volume must be filled with water to compress the air to 50 psi when it is initially filled. An important concept is 70% of the tank's total volume is compressed air that is available to push out the 30% of water.Ĭompare this to amount of air available in a non pre-charged conventional tank. This means 30% of the tank's total volume is available as drawdown. In a captive-air tank, a 30/60 pressure switch setting and 28 psi pre-charge yields a drawdown ratio of 0.3. Boyle's Law takes into account amount of pre-charge and cut-in and cut-out settings of pressure switch to set a ratio of total tank capacity to drawdown. In this article, we will use the terms 'captive air' and 'conventional' to differentiate the two types of tanks.ĭrawdown Factor The drawdown of a pressure tank, whether captive air or conventional, can be calculated using a formula known as Boyle's Law which was covered in detail in March 2000 National Driller article. 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.

Define drawdown in water