HOW DOES AIR GET INTO A SEALED GAS SYSTEM - TESTING VS. THEORY

摘要

There are situations that occur when customers and servicemen have difficulty lighting pilots on appliances after the natural gas or propane system has been shut off or following an out-of-gas call in a propane system. The inability to light the pilot light is believed to be due to air getting into the gas system. In some circumstances customers or servicemen will purge the gas line in order to get the "air" out of the lines. Sometimes this results in flammable gas entering the structure resulting in an explosion, flash fire or fire, which causes property damage, serious injuries, and sometimes death. In the propane industry, it is often claimed that after an out-of-gas situation, air is drawn back into the propane tank with moisture, which results in the formation of rust and the resulting oxidation of the propane odorant ethyl mercaptan. In this scenario it is claimed the propane gas was odorless at the time the line was purged by the unsuspecting serviceman or homeowner, which results in a significant explosion. The question arises as to how air can enter a closed gas system, particularly given the presence of safety valves in appliance gas control valves. To answer these questions, pressures and temperatures were measured inside empty propane tanks for several days. The data was used to determine what pressure changes occur inside the gas system due to temperature variations. In addition, testing was conducted on a number of appliance gas control valves to determine how any negative pressures created in a closed gas system affected the movement of air or gas through control valves, and ultimately whether air can get into the system. This paper includes descriptions of the testing and summaries of the various results. Calculations were compared to the actual testing. The conclusions spell out what can generally be expected in gas systems in terms of air infiltration into the system.