Hi Benson,
HAMMER does not have a way to determine how much air accumulates in a pipeline under normal conditions, because this process happens quite slowly relative to the transients that HAMMER analyzes. However sizing guidelines from valve manufacturers should help you size the air valves to get rid of this air.
What HAMMER will help you do is size for the more critical case where a transient causes a large amount of air to be drawn into the pipeline. This air needs to be released in a controlled way - too fast and a large upsurge is the likely result as the air valve slams shut.
Regards,
Mal