I'm not too familiar with ARV's however I know the basic principle behind how ARV's work. However, what exactly are the "Air and Vacuum Flow rate" and the "Automatic Air discharge"? I noticed each is plotted on a graph, but what exactly does it represent? Also, in selecting an ARV, how do these two parameters affect your choice in selecting an ARV. This may sound like a stupid question, but please bear with me.

I think you're talking about simple float valves, which have a Cv that varies as the float nears the seat and gradually closes the valve.  The flow rate varies with that Cv and with the pressure difference, which is limited for a vacuum.

ARVs are used for releasing tramp gas from liquid reservoirs.
I have very little experience with them.

I do have experience with their inverted twin, automatic drain valves, used for releasing tramp liquid from gas reservoirs.  I've never seen one that was more than a year old; typically they get stuck closed long before that and are replaced with bleed orifices or timers and solenoid valves.

Please revert with a more exact information on which type of valve  and application you are refering to.

If we are talking about an air relief valve(outlet)and vacuum (air inlet valve to avoid vacuum) type of valve for out and inlet of air on top-points of for instance water mains the brief answer is following:

1. Air outlet flow rate is air outlet when filling up the main with water. Capacity is normally given as a curve of amount of air depending on pressure difference.
2. Air inlet is a similar curve for air inlet at negative pressure difference.
3. Automatic air release is the trippel-function, a separate higer placed float within the valve, able to operate a smaller inbuilt separate valve to release smaller amounts of air from an else waterfilled and pressurized pipeline.

Note! for all capacity curves for this type of valves a given amount of air will compared to the factual area of the valve opening give a certain velocity.

If you compare this velocity you will find that certain producers give a very high maximum velocity (sound velocity in air is of course the max theoretical obtainable) and others more conservative data.

Your anser is then that to compare valves you have to compare areas more than curve capacity and Cv, unless you have checked the calculation parameters for the Cv and max capacity parameters by comparing velocities.

Other important issues are checking possible blocking of float by high velocities of air outlet by filling, and quality and and construction (blocking) of the 'automatic air escape'.

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