Years ago, in some dank basement, a contractor installed a large air vent near the end of a steam main. He did this because he knew that steam and air are both gases, but steam is lighter than air, so the two won’t mix. He also knew that the empty steam main was going to start out filled with air (which is only natural, right?), and that once it left the boiler, the steam would shove the air ahead of itself and down the main, just like a big, hot plunger.
The contractor sized the air vent to release the air from the pipe at a certain rate so that the steam could move faster than he could run. The design velocity for steam in a space-heating system can be as high as 60 miles per hour. The only thing standing in the steam’s way is air.