A blowdown is the act of releasing / purging natural gas from a section of pipeline so work can be done safely. A loud noise occurs when the natural gas, which is compressed into the pipe at very high pressure, escapes through the opening. As the natural gas rushes through the blowdown valve, a gas plume extends upward of 30 to 60 meters (100 to 200 feet). This gas release is similar to letting air out of a car tire: the most forceful rush of air occurs at the very beginning, then the flow gradually slows down. The first 30 to 60 minutes of the blowdown are the loudest, but the entire blowdown may last up to three hours. After the natural gas is released, a funnel-shaped air expeller is placed on top of the blowdown valve opening. This machine is used to draw any remaining gas out of the pipe to make it absolutely safe for activities such as welding.