Abstract: A pneumatically controlled system is provided for detecting and isolating a rupture in one or more lines of a parallel looped gas transmission network employing valve controlled equalizing lines at intervals. When the gas transmission network incorporates three or four mainlines the direction and magnitude of flow in the equalizing lines is sensed pneumatically and a pneumatic signal representing such flow is transmitted to a control module. The control module is effective to close selected mainline valves and equalizing line valves to isolate a ruptured pipeline section while permitting other valves to remain open thus continuing gas transmission in other portions of the transmission system.