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Differences Between Degassing Valves, Air Release Valves, and other Venting Valves in Liquid Piping Systems

Air or any other gas is both friend and foe in a pressurized liquid piping system. One one hand, the system designer needs to ensure that air will flow into a tank to prevent implosion when the tank is drained. In addition, air is sometimes required to prevent gravity induced siphoning. On the other hand, entrained air diminishes pump efficiency. And once it outgasses, air creates constrictions that can lead to water hammer, pulsations, dead heading...even catastrophic pump failure.

In the past, engineers relied on concoctions of standpipes and various manual or automatic shutoff valves to deal with air in piping systems. Few worked well; some even created additional problems. Fortunately, today's system designer is armed with a number of special-purpose valves to manage air effectively and effortlessly.

First, a look at valves designed to vent air out of a liquid piping system.

Air Release Valve -- Designed for rapid venting of a large amount of air at system start up, or when a tank is being filled. A true air release valve is normally-open but closes as soon as liquid rises in the system and lifts a float within the valve. The valve allows the rising liquid to force out air that occurs naturally in a piping system. Once the liquid rises and reaches pressure, the air release valve remains sealed until the system is depressurized. Depending on the application, it should be mounted at high points in the piping system, as well as the tank.

When considering the function of an air release valve, some might assume that a pressure relief valve can fit the bill. Yes, a pressure relief valve will vent air at a pre-set pressure point, but it will also vent precious liquid -- a pressure relief valve cannot tell the difference. Furthermore, a relief valve won't vent accumulated air at pressures below the set point. Unlike a pressure relief valve, an air release valve is open at atmospheric pressure, then closes automatically when liquid is present.

Degassing Valve -- Designed for periodically venting trace amounts of gas as it occurs. Like an air release valve, this normally-open valve closes once liquid rises in the system. But unlike an air release valve, a degassing valve will automatically re-open whenever additional pockets of gas rise in the valve, even when the system is pressurized. Once the gas is expelled, the presence of liquid closes the valve. The venting orifice on this type of valve is substantially smaller than a typical air release valve, so it is not suitable for rapidly venting a large volume of air. With that in mind, systems prone to continuous outgassing will benefit from both an air release valve and a degassing valve.

It is important to note that a degassing valve will not "trap" or collect gas as it passes by the valve, so it must be installed on a lead at high points in a pipeline where gas would otherwise collect naturally. Even miniature degassing valves built into dosing pumps are only capable of expelling accumulated gas, not gas entrained in liquid. In many cases an additional degassing valve is required elsewhere in the system.

Photo shows typical recommended combination use of both an Air Release Valve and a Degassing Valve at the high point in a piping system. The Air Release Valve is positioned first in line, at left in this pipeline.

You may have noticed above that air release and degassing valves are generally "normally-open" designs, in other words, unless liquid is present, both allow air to flow into a piping system. With that in mind, it might seem that these valves could provide the function of a vacuum breaker. But air release and degassing valves are designed specifically to expel air, and are not as effective for drawing air into a system. For that purpose, a normally-closed valve is much better suited.

Vacuum Breaker -- There are many types of vacuum breakers. The most versatile and effective are normally-closed valves that allow air to flow in one direction only -- into a tank or pipeline. A vacuum breaker is most commonly used to prevent a tank from collapsing when liquid is pumped out or drained from the tank. But the key to this valve is the fact that it is normally-closed, and shuts automatically as soon as the tank returns to atmospheric pressure, preventing vapors or harmful liquids from flowing out of a tank.

Another popular application for the vacuum breaker is to prevent siphoning. When pumping to a pipeline below tank level, the flow of liquid often creates a vacuum in the pipeline when the pump is turned off, which results in suction and can siphon the tank. By installing a vacuum breaker in a section of pipeline above the tank level, the vacuum condition will cause the valve to open and create an air pocket. The air pocket will stop the flow of liquid, and because this section of pipe is now above the level of the supply tank, it will no longer siphon.

To summarize the three valves as simply as possible:

It is not unusual for applications to require all three types of valves to help the system operate at peak efficiency. Depending on the complexity of the system and how prone it is to building up air/gas, some applications could require multiple usage for some or all of these valves.

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