Differences Between an Air Release Valve and a Degassing Valve

Quite simply, the difference between an air release valve and a degassing valve are volume and repeatability.

An Air Release Valve is open when the system is depressurized. At start up, the liquid process forces most of the air in the pipeline to high points, where the ARV allows it to escape. Once liquid rises, the valve closes and will not allow anything -- gas or liquid -- to pass in or out. It will not vent gas as it occurs during daily operations. It will reopen when the system is depressurized and liquid level drops.

A Degassing Valve is also open when the system is depressurized, but due to its small vent orifice is not well suited to expelling a large volume of air at system start up. The DGV's strength is the fact that it will open automatically -- even when the system is pressurized -- to periodically allow small amounts of gas to vent. Once the gas vents, the valve automatically closes as long as liquid is present.

These are the issues you need to focus on to help decide if your applications requires one or the other…or in many cases, both valves:

1. If the application requires that a large volume of air be vented at one time, such as when the system is fired up in the morning, it calls for an air release valve. According to Andy Ryan, Technical Services & Marketing Director at Plast-O-Matic, “an air release valve should be installed on virtually every system.” He explains, “sooner or later you have to turn it off. When you start it up again, you want to get all the air out so that the system will run at peak efficiency.”

2. If the application is prone to outgassing or otherwise routinely forming small air pockets that must be vented throughout the day, it calls for a degassing valve.

3. Because most piping systems will benefit from installation of an air release valve, the designer should consider using both the ARV and DGV in applications prone to outgassing.

Another important thing to note is that both valves need to be located at high points in the system, or wherever air or gas will routinely accumulate. Keep in mind that neither valve will “collect” gas as it passes by.

Some Helpful Links for this Tech Brief:

Understanding Flow Rate, Pressure Drop, and C_v Factor.

Effects of Water Hammer and Pulsations

Online Pressure Drop Calculator

-- Rick Bolger, Plast-O-Matic Valves, Inc.

For additional information, please return to the Plast-O-Matic Tech Info Center.