I am installing a new pipe line and would like to incorporate air release valves into the system.  The problem is I don't know how often I need these valves.  If anyone knows of a formula or book or website or anything that has this information it would be very helpful.  Thanks in advance.

VPL has given you the answer -you need one valve at each high point. That is if you have a main transmission pipeline. If you have a distribution pipeline with connections then generally you can allow the pipe to vent through connections and you don't need air valves. Air valves need maintenance.

For a main pipeline you also need to ensure that the air moves to the air valve. Typically pipes should be laid at a minimum gradient of 1 in 500 when rising to an air valve and 1 in 500 when falling from an air valve.

On long lengths you may also need air valves at changes in gradient - generally spacing should not exceed 1 to 1.5 km.

Having decided where to locate the vales you will need to decide what size to make them. There are three requirements

1) to vent large volumes of air during filling
2) to allow air into the pipe during emptying (some pipe materials can collapse under negative pressure i the pipe is drained faster than the valve will admit air into the pipe.
3) to vent small volumes of air that come out of solution during service.

To cater for these requirements it is common to use double orifice air valves that contain a large orifice to allow air evacuation and air entry during filling and emptying and a small orifice that allows venting of air accumulation during service.

To size the air valves you need to consider the filling rate and the rate that the air will need to be evacuated during filling. If you have a pipe material that is susceptible to collapse under vacuum you will also need to consider the maximum drain down rate and the rate you need to allow air into the pipeline to limit the sub atmospheric pressures.

The Vent-o-Mat design guides will give you the methodology but beware Vent-o-mat are in the business of selling their valves and if you follow their guides you will end up with larger valves than some of us would consider necessary.

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