Entrapped air inside the water and wastewater network can result in serious operational problems. Many minimizing the effects of entrapped air. Entrapped air can cause equipment damages, fitting leakage, and faulty instruments reading. Improper installation of the line can cause some of these problems, the failure of air releasing can cause most of these problems. Using automatic air valves to de-aerate the lines will prevent entrapped air related problems.
There are different sources for the pressurized air inside the pipelines. At the beginning stage and during the installation of pipes. The pipes are full of air. Once the lines are operational. Part of this air will be drained out from downstream or hydrates, and the remaining will be trapped at the pipe high points. The second source is water. Water contains approximately 2% of dissolved air under normal conditions. The increase of temperature or decrease in pressure will cause the air to go out of the water and enter the equipment such as pumps, fittings, and valves.
The operation and efficiency of the system can be severely affected by the entrapped air. Figure 1 showing an air pocket collect at the high point. The air pocket in figure 1 has a height of H along the pipe high point. Such an air pocket will cause head loss. The air pocket in figure 1 will cause a head loss of H. the presence of a large number of air pockets can cause a huge head loss enough to stop the flow. Air pockets can cause a sudden change in the velocity of the flow. The passing of flow through a restriction such as a control valve. A dislodged air pockets can cause a surge or water hammer. Water hammer can damage the equipment or lessen the fitting, which may cause corrosion and reduce the service life of the system. Manual releasing of entrapped air using manual vents or fire hydrants can be effective at the early stages and before starting up the line. During the operational phase of the line. Automatic air valves should be installed to avoid the problems associated with the entrapped air.
Basic types of Air valves
According to the American water work association. There are three basic types of air valves: air-release, air/vacuum, and combination air valve. It is essential to understand the function and limitations of each type.
- Air release valve: Air release valves typically produced in size of ½ in (13mm) through 3 in (76 mm). The valve has a small orifice to release the air. The size of the orifice ranges from 1/16mm (1.6 mm) to ½ in (13 mm). This valve equipped with a float to sense the presence of air. The float is connected to the orifice valve by a mechanical arm. Under full pressure, the float will move the mechanical arm will open the orifice to release the air. This type of air valves suitable for releasing a limited capacity of air. Therefore, most of the pipeline's location will require an air release valve and Air/Vacuum valves for exhausting a large volume of air.
- Air/Vacuum valve: this type of valve installed at the downstream of pumps and at the high points of the line to release a large volume of air during the filling and starting up of the line. Also, it will allow the air to reenter the system to avoid the loss of vacuum and to allow the draining of the pipe. It is important to note that during the operation period of the line. The float will be held tight by the pressure of the pipe. And it will not release the accumulated air. Therefore, the air release valve should install to release the accumulated air.
- Combination air valve: this type of valves combines the function of air release valve and air/vacuum valve.it is a good choice for the high points. A combination of air release orifice and air/vacuum port are put together in one assembly or one valve. For small size valves. Lesser than 8 in (200 mm). The float and lever mechanism are contained in a single body, as shown in figure no: 4. for larger sizes. The valve consists of an air release valve and air/vacuum valve combined together, as shown in figure no: 5.