Air Release in Piping Systems
Read this online lesson as well as any related source materials.
Free air can be found in most piping systems and fittings. When the line is filled incompletely, air pockets will be left at high points, and at a variety of fittings along the way. A pressure change up or down will cause the release of dissolved air, or air can enter the system by the vortex action of the pump, at the intake. All openings, connections, and fittings can allow air to enter the system.
Why are ARVs Needed?
Air release valves (ARVs) are used to release the air from air pockets that are formed in pipelines. Air pockets will form at the high points of a fully pressurized pipeline. ARVs are necessary for pipeline efficiency and also for water hammer protection. Air in the water lines can cause many problems, such as speeding up the process of corrosion. A reduction of flow is also created by air that has been trapped at the bends, tees and other fittings in the system. Flow can even stop due to an “air lock” in the water line. This usually happens in chemical feed lines at the water plant. If ARVs are not installed, uncontrolled air releases may occur, causing pressure surges in the system, which can increase the chances of hazards from cavitation. Also, if air is left in a water main, it will eventually cause customers’ meters to read improperly, thus charging the customer for air that is going through the meter, and not water.
The “universal type” of ARVs from Crispin are designed to allow the automatic discharge of large amounts of air from the water main while it is being filled, and also to allow air to enter the water main when the water line is being emptied. It will also allow the air to escape the water main when it is in operation and under pressure. This occurs with a compound lever system functioning in conjunction with a large and small orifice in one integral body casting. As the liquid rises into the valve, air escapes through the large orifice to the atmosphere. Liquid entering the valve raises the float and lever system, carrying with it the pressure plunger and the main valve. When the liquid has raised the float to its limit, the main valve and pressure plunger will rest against their seats- then the valve is closed and no liquid can escape.
If accumulating air rises into the valve while the line is in operation and under pressure, it will displace the liquid at the top of the valve body and the float will drop as the liquid recedes. As this occurs, the pressure valve will open, permitting the escape of the accumulated air, after which the liquid level will rise and the valve will close.
Types of Air Release Valves
There are three basic types of air release valves;
• Kinetic Air Release valves
• Automatic Air Release valves
• Combination Air Release valves
Kinetic Air Release Valves with Large Orifice
This type of valve is used to release air at high flow rates during the filling of the line and to allow air to enter the line at high flow rates during its emptying.
The Kinetic Air Release valve will function when the line is not under pressure. It is designed so that when the line is under pressure (water flowing) the valve will stay closed.
Automatic Air Release Valves with Small Orifice
This type of valve is used to release air during water flow, while the system is under pressure. When air bubbles appear in the valve, the ball will drop, causing air to be released. When the water rises again, the ball will be lifted, causing the valve to close.
Combo Air Release Valve
This type of valve incorporates an automatic and kinetic air release valve into one unit.
Air release valves are located on rising mains from the pumps to release and admit air. They are located at peaks throughout most systems. ARVs should be located at transition points in the pipe slope, especially before and after steep slopes in the system. It is recommended that ARVs be located every 500m or 1500 ft. along pipeline sections of long uniform slope.
In this lesson we learned that air release valves (ARVs) are very important in the water industry. ARVs are used for the release of air pockets that form in water lines. Air pockets will form at the high points of a fully pressurized pipeline. These valves will help to improve water line efficiency and help to slow down water hammer. Air enters water line in many ways causing problems for the distribution system operators. The process of corrosion can occur with large amounts of air in the water line. This is something that we do not want to encourage. Flow is reduced if there is a great deal of air in the water line. Air in the line will also increase the chances of hazards from cavitation. If there are no ARVs, the air will eventually get out of the lines by going through customer meters.
ARVs work in a two-fold manner. They discharge air from the line, while it is being filled, and yet, will let air into the line as it is being emptied. Also, this valve will allow air out of the line when it is in operation and under pressure. There are three basic types of ARVs; 1- Kinetic ARV, used to release air during high flow rates, 2 - Automatic ARVs which also release air, and 3- Combo ARVs which incorporate both previous types into one single device.
ARVs are located on rising mains from the pump and in the peaks throughout most systems. ARVs should be located at transition points in the hydraulic gradient; especially before and after steep slopes in the distribution system.
Plast-o-matic Valves, Inc.
Universal Air Release Valves
Operator Training Handbook - AWWA
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