Tech Topics: Air-charging Techniques for Small Conventional Tanks
April 16, 2008
Controlling the amount of air in a conventional pressure tank is crucial in providing an efficient and reliable pumped water system. Last month, we described the compressor-based air-charging systems, which use water-level probes to control the air charge in larger conventional tanks. In smaller residential systems, simpler, less expensive air-charging devices are common. The type of charging system used depends on whether the water system uses a suction-type pump like a shallow-well jet pump or a pressure pump, like a submersible pump or deep-well jet pump.
Jet Pump SystemsFigure 1 shows a jet pump system. An air-volume control (AVC) is mounted on the side of the tank, and plumbed to the suction side of the jet pump with a small tube. When the pump turns on, the reduced pressure created in the AVC draws in a small charge of air, which then is released into the tank when the pump shuts off. Because of the design of the valve, it controls the water level by drawing air from the tank or outside atmosphere, depending on the level of water in the tank. These valves are very effective, but must be kept clean to function properly.
Submersible Pump SystemsFigure 2 is a schematic of a submersible pump system. It consists of a bleeder orifice (Figure 3), a snifter valve (Figure 4), and an air-release valve (Figure 5). The bleeder orifice, installed in the drop pipe approximately 10 feet below the well head, acts as a drain port. It is designed to seal when the pump is running and to open up when the pump shuts off. At the end of each pumping cycle, the water in the pipe between the check valve and the bleeder orifice drains out through the bleeder, opening back onto the well.
The snifter valve, typically mounted on a tapped check valve between the wellhead and the tank, lets air into the drop pipe so that the water can be released through the bleeder orifice when the pump shuts off. A snifter valve looks like a valve stem on a tire, but it has a very weak spring, or no spring at all, to function properly in this application.
At the beginning of the next pumping cycle, the air in the drop pipe, between the bleeder orifice and the snifter valve, is pushed into the pressure tank ahead of the water. More air than is needed is added each pump cycle to replenish the air absorbed into the water, so an air-release valve is needed to purge the excess air from the tank.
Air-release valves come in several configurations, and are mounted on the side of the tank in the fitting provided by the tank manufacturer. They simply are a float valve, similar to a toilet bowl float valve, which opens when the water level is below the float to let air escape and closes when the water level raises and pushes up on the float.
One of the common failure modes in this type of air-charge system is clogged components. Iron algae, calcium or other minerals can foul the tiny openings in snifter valves, rendering them inoperable. The problem can easily be remedied by cleaning or replacing the core. Make sure the replacement core is designed for this application, with a weak spring, or no spring. A standard automobile or bicycle core will not work.
As with snifter valves, contamination is an air-release valve’s worst enemy. Minerals or iron bacteria can clog the air orifice or jam the float, rendering the air-release system inoperable, leading to a waterlogged tank, or a tank with excessive air, which could allow air to enter the plumbing system in the house.
With the added maintenance, extra cost and possible reduced drawdown of a conventional tank vs. captive-air tank, why would someone choose the former over the latter? One good reason is the beneficial side of its biggest drawback – the fact that a conventional tank absorbs air into the water. Air is an excellent oxidizer of iron, hydrogen sulfide and other dissolved contaminants, so the use of a conventional tank may eliminate the need for a separate oxidation contact tank or chemical treatment to fix contaminated water.
The choice of pressure tank and other system components is important to the long-term operation of a water system and to the satisfaction of your customer. As I have said in the past, use the best components you can find, and install them in accordance with local and national codes and practices. Nothing is more important to the success of your business than a bunch of happy customers.
Next month, we will look at using multiple pressure tanks for larger pumped water systems. ’Til then …. ND