There are two reasons for having a pressure tank in a pumped water system. The first is to give the pump motor a minute or two to dissipate the extra heat energy generated each time the pump starts, and the second 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 stored in a pressure tank is called the drawdown. Drawdown is the amount of water that can be drawn from the tank between the time the pressure switch cuts out, turning off the pump, and it cuts back in, turning on the pump. Tanks are sized with enough drawdown to allow the pump to run for at least one minute between cycles, as recommended by the motor manufacturers. For instance, 10-gpm residential water system pump would require a pressure tank with 10 gallons of drawdown.
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 specify a conventional tank is a bit of a misnomer. All pressure tanks used in the ground water 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 (see Figure 1 on p.16) has a rubber or plastic membrane separating the water from the air, and must be pre-charged to a certain pressure, usually 2 psi below the pressure switch cut-in point. 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 the pre-charge pressure in the tank to 18 psi or 38 psi, respectively. Pre-charge pressure in captive-air tanks always is checked with the tank’s water chamber completely 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 completely 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 were the pre-charge set properly. Boyle’s law confirms this as described below.
Conventional tanks (see Figure 2 on p.16) simply are sealed vessels that do not have a membrane to separate the air from the water, as do captive-air tanks. Because the air pressure in a conventional tank is higher than the vapor pressure of the water in the tank, air is absorbed into the water, and it is necessary to replenish the air on a regular basis. Several schemes are commonly practiced to minimize this air absorption, and to replenish the air, without which a conventional tank would become completely waterlogged in short order, defeating its purpose and function.
To help reduce the amount of air absorbed into the water, a round plastic float a little smaller than the diameter of the tank can be fitted. These are flexible enough to be rolled into a cylinder, and inserted into one of the pipe fittings in the side of the tank. Even with a float, however, it is necessary to replenish the air to maintain the proper air/water ratio for optimal tank performance. Our September 2011 article will describe how to maintain the proper air/water ratio.
With the hassle of having to deal with maintaining the proper air charge in a conventional tank, why do contractors still use conventional tanks in residential systems instead of pre-charged tanks? There actually are several reasons – some more legitimate than others in my opinion. If you have iron or sulfur dioxide in your water, a conventional pressure tank can help oxidize and reduce these contaminants since there is air in contact with the top surface of the water. Installing conventional tanks brings in service work.
Many contractors were burned by some of the captive-air tanks that came onto the market in the 1980s. Many of these tanks proved to be disasters in terms of reliability. The quality of the captive-air tanks on the market today makes this reason obsolete.
The fourth reason simply is the comfort level that some contractors have doing pump work the same way, day in and day out, and installing the same products they always have installed and perhaps their fathers installed. Some would call this product loyalty, and it can’t be faulted, except for the missed opportunities that it perpetuates. Enough said.
Drawdown FactorThe drawdown of a pressure tank, whether it be captive-air or conventional, can be calculated using a formula known as Boyle’s Law, which we will cover in detail next month. Boyle’s Law takes into account the amount of pre-charge and the cut-in and cut-out settings of the pressure switch to come up with a ratio of total tank capacity to drawdown. In a captive-air tank, a 30/50 pressure switch setting and 28-psi pre-charge yields a drawdown ratio of 0.3. This means that 30 percent of the tank’s total volume is available as drawdown. An important concept to understand is that 70 percent of the tank’s total volume is compressed air that is available to push out the 30 percent of water.
Compare this number to the amount of air available in a non pre-charged conventional tank. In a conventional tank, without a pre-charge, 75 percent of the tank’s volume must be filled with water just to compress the air to 50 psi when it initially is filled. That leaves only 25 percent for air to force out the water, 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 just 10 percent. 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-percent air to 30-percent water? If it were possible to control the water level in a conventional to the same level it would be in a captive-air tank, the drawdown would be the same for both.
In practice, it is possible to increase the pre-charge pressure in conventional tank. By using a compressor system with a water-level probe, or a spring-loaded air-release valve located one-third 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 the 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 percent and 30 percent, depending on whether or not you use a spring-loaded air-release valve and on the 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 the height of the water-level probe, it is possible to maximize the drawdown yielding ratios comparable to those of captive-air tanks.
Next month, we will cover the specifics of optimizing the water level in conventional tanks and describe in detail the various ways of air-charging them. ’Til then ….