why doesn't the compressed air blow the water backwards into the supply line, and blow the shut off valve?
(Kendall Park, NJ)
when they winterized the sprinkler system, why doesn't the compressed air blow the small amount of water in the inlet pipe (coming from the basement) backwards and totally blow the inlet pipe and inlet shut off valve?
Kishor, compressed air will always try to flow from higher pressure to low.
When you pressurize a water line with compressed air, the air will push on the water, and move the water towards the point of least resistance.
Typically, that point of least resistance is the open tap at the end of the run, not the supply side of the water line.
Water moves out of the open tap as the air is driving it there. The compressed air "wants" to get from the pressurized environment into the open, where it dissipates back to ambient air pressure.
Hope this helps,