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. 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. 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. A 10 gpm pump would require a pressure tank with 10 gallons of drawdown, etc.

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). Using the term hydro-pneumatic to describe a conventional tank is a misnomer. All pressure tanks used in the groundwater industry are hydro-pneumatic meaning they contain water (hydro) and air (pneumatic). In this article, we will use the terms "captive air" and "conventional" to differentiate the two types of tanks.

A 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. For a 30-50-pressure switch, the tank would have a 28-psi pre-charge. 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. Pre-charge pressure in captive air tanks is always checked with the tank's water chamber empty.

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. If it is set too low, the tank will not have as much drawdown as it would with the pre-charge set properly. Boyle's law confirms this as described below.

Conventional tanks are sealed vessels that do not have a membrane to separate air from water, as do captive air tanks. 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. Several schemes are commonly practiced to minimize air absorption, and replenish air, without which a conventional tank would become water logged, defeating its purpose and function.

To help reduce amount of air absorbed into the water, a plastic float a little smaller than diameter of the tank can be fitted. These are flexible enough to be rolled into a cylinder and inserted into a pipe fitting in the side of the tank. Even with a float however, it is necessary to replenish the air, to maintain proper air/water ratio for optimal tank performance.

Drawdown 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. 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 a captive-air tank, a 30/60 pressure switch setting and 28 psi pre-charge yields a drawdown ratio of 0.3. This means 30% of the tank's total volume is available as drawdown. An important concept is 70% of the tank's total volume is compressed air that is available to push out the 30% of water.

Compare this to amount of air available in a non pre-charged conventional 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. 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. Without a pre-charge, a conventional tank running at 30/50 has a drawdown factor of 10%. To deliver 22 gallons of drawdown would require a 220-gallon conventional tank without a pre-charge, vs. a 73-gallon captive air tank.

But 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.

In practice, it is possible to increase pre-charge pressure in conventional 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. However, regarding the latter, most hydro-pneumatic tank manufacturers put the air release fitting at or above center on the tank. 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.

Off-the-shelf air compressor systems are controlled by a water level probe and separate pressure switch. By optimizing height of the water level probe it is possible to maximize drawdown yielding ratios comparable to those of captive air tanks.

Next month we will cover specifics of optimizing water level in conventional tanks and describe in detail various ways of air charging them. Till then.