The need for an Air Valve
An air valve is a mainstay of any pumping or plumbing system. The ever present air valves in a pipeline are found there for good reason. An air valve is present to drive air out of the pipeline so that a continuous smooth flow of fluid is maintained through it. There are primarily three ways that air enters the pipeline:
– Firstly, during the start, the pipeline already contains air. This air must be driven out before the pipeline begins to fill. Once the pipeline begins to fill with water, the air is pushed downstream. Mechanical apparatus such as hydrants and faucets are positioned downstream to help exhaust this air. However these are not 100 percent effective and a lot of air will remain in the pipeline at the high points of the system.
– Secondly, about 2 percent by volume of water is dissolved air. This air gets released from the water due to an increase in temperature or a drop in pressure. This would also occur at the high points of the system due to the elevation.
– Thirdly, some amount of air enters through the fittings, valves, pipe joints, etc when vacuum conditions are applied.
All this air that gets trapped into the pipeline needs to be removed. Air trapped at high points may cause a headloss to the flow. Sometimes these air pockets can stop all flow. They are also harmful to the pipeline and its machinery. Hence, air valve needs to be used to get rid of all this trapped air. An air valve allows air to leave but not enter back. A variety of automatic air valves are used in pipelines.
Types of air valve used
There are three basic types of air valves. The air valve for our system is chosen from amongst them.
– Air release valve: Air release valves are most commonly used as air valve in a pipeline. It releases air under pressure and can be placed at various points of the system. This type of an air valve consists of a float to sense the presence of air and a mechanism for linkage of the float to the mechanism for opening of the orifice for release of the air.
– Air vacuum valve: Usually placed downstream in a pipeline or at high points. This kind of air valve is generally used for driving out the air just after startup once the pipe begins to fill.
– Combination air valve: This air valve combines the features of both air release and air vacuum valves and is the best choice for use at high points. It is also safe to use combination valve wherever possible as it functions as either of the valves and thus doesn’t leave the pipeline unprotected.
The positioning of an Air valve in a pipeline
The positioning of an air valve in a pipeline is critical to its proper functioning. The right type of air valve must be placed at the right position. A few guidelines ensures that the right air valve is placed at the right place:
1. High point: combination valve.
2. Long horizontal run: air release valve at fixed intervals.
3. Long descent: combination valve at fixed intervals.
4. Long ascent: vacuum valve at fixed intervals.
5. Decrease in up-slope: vacuum valve.
6. Increase in down-slope: combination valve.