Propane is usually stored in tanks that vary in size from 120 gallons up to 1,000 gallons. When properly filled, these tanks contain about 80 percent liquid. The remaining 20 percent of space above the liquid contains propane vapor. (It is this vapor that burns in your appliance when mixed with air.) The vapor space in the propane tank also provides room for the liquid propane to expand if it is exposed to warmer temperatures. If a tank is overfilled, (that is, beyond 80 percent capacity), there won't be enough vapor space to accommodate the expanding liquid if the cylinder is exposed to warmer temperatures. If overfilled, any of the following hazardous conditions could occur:

1. The pressure relief valve may open, discharging propane to relieve the pressure.
2. Propane liquid could enter the piping system, resulting in higher than normal pressures to the appliances.
3. If the tank becomes full of liquid and the pressure relief valve fails to open, the tank could rupture, resulting in serious injury or property damage.