How do you calculate purge times? Ours is 15min which seems a little excessive.

We use 6 mins as our purge time, I have also heard of 5 mins being used but never less. Being an operator and not an engineer, I was told and may have read in a design book, that boilers are designed and modeled as follows. After 1 min purge 90% of air has been replaced, hense after 2 mins another 90%. So after 2 mins 99.00% has been replaced. After 5 mins 99.999 % has been replaced, plus 1 minute for safety factor. Boiler design engineers would know for sure.

Purge time depends on the size of your boiler and the size of turbine (ie. axial compressor - air flow through the unit).
larger boiler = longer purge
however
larger turbine = shorter purge.
Itll be an engineered calculation based on the above.

The purge times is set by the number of air exchanges necessary to purge off any redisual combustion gases that might be trapped. I belive the code is for 10 exchanges which times are related to the size of the boiler/HRSG and air flow either from fans or the combustion turbine.

correction, volume of air exchanges need to be up to a factor of 5...NOT 10 that i have mentioned.