Coffee Break Training: Hazardous Materials: Breakaway Devices for Fuel Dispenser Hoses [open pdf - 318KB]
"From time to time a careless motorist will drive away from a fuel dispenser without first removing the nozzle from the automobile's fill opening. In that case, there is potential for a fuel dispenser to continue delivering product through an open orifice. To minimize the chance of unwanted flow, the model fire codes require dispenser hoses for Class I and II liquids to be equipped with a listed breakaway device that is designed to retain liquid on both sides of the breakaway point. These devices are required to be installed and maintained in accordance with the manufacturer's instructions."
Coffee Break Training - Fire Protection Series No. FP-2011-12
United States Fire Administration: http://www.usfa.fema.gov/