HERE

HERE'S WHY
Propane is stored as a liquid under pressure inside the cylinder. On a hot day, liquid expands and pressure increases. To guard against excessive pressure, every cylinder is equipped with a pressure relief valve which opens automatically to release gas and then closes when pressure is reduced.
If the cylinder is UPRIGHT, a small amount of vapor is released.	If the cylinder is HORIZONTAL, a large amount of liquid propane is released.

Liquid propane is highly volatile. It vaporizes instantly at a rate of about 270 to 1. Put another way; if one cup of liquid propane is released, it instantly becomes 270 cups of pure propane vapor. Mixed with oxygen, it forms a combustible fuel which can readily be ignited by such ordinary sources as a cigarette or a spark from an electrical connection in your automobile. Unfortunately there have been many serious personal injury accidents over the years.

and now A.D. Runyon