Facts about Propane 1. Propane is a liquid in the cylinder, but is burned as a gas. Propane gas must be tapped from the top of the cylinder. As the liquid propane cools, the pressure of the vapor above it drops. Overheating liquid propane will cause dramatic, and potentially catastrophic, increase in the pressure of the vapor above it.
Most commercial cylinders have a pressure relief device.
If this opens the cylinder will not explode, but it could vent the entire contents of the cylinder. Liquid propane is not only flammable, it's an effective solvent. A propane cylinder could leak, and it's best to assume it does leak. Propane is denser than air and can settle in basins or run along the ground to a source of ignition, then flash-back. It could also drain into a sewer and cause an underground explosion hazard.