Gas Line Safety Proposal Excess flow valves in all high-pressure gas systems can reduce the number of natural gas explosions, according to the National Transportation Safety Board (NTSB). The valve automatically shuts off when the flow of gas through it exceeds a set rate, guarding against the rapid escape of gas when a line is broken. NTSB recommends that the U.S. Department of Transportation (DOT) require the valves when service lines are installed or existing lines are exposed during maintenance work. The recommendation is in the NTSB report on a Kentucky school double explosion and fire in which a student was killed. Thirty-seven other persons were injured, mostly fire fighters.