Just an anecdote. I was working the Geothermals in the California desert many many moons ago. We were runnin a ton of SS. Everyone was having purge problems. We'd all do the displacement calcs, O2 measurments etc., runnin the gas slow and form the bottom, double check the integrity of our dams, and then some. It helped but not elimitated. 1 hr, two hours we'd purge, dams as close as you could get em, and still fight the miserable thing for the rest of the day. Most of it being 12" or larger.
Then one old boy that had seen a thing or two I suppose, did it different. He purged for almost 10 hours. Stood around for most of the day waiting. Then towards the end of the shift he zipped his root in clean and pretty as you please run a cover, packed up his lunch box and hit the bus, finishing it the next day.
Go figure. Sometimes the science gets tossed out the window and you just need to know something.