NORTHERN NATURAL GAS CO

Incident Narrative

DURING A CONSTRUCTION PROJECT TO REPLACE A RELIEF VALVE, THE NEW VALVE VENTED APPROXIMATELY 6.4 MMCF IN NATURAL GAS. THE INVESTIGATION CONCLUDED THAT THERE WAS GAS SEEPAGE BY THE CONTROL VALVES THAT CAUSED THE DOWNSTREAM PRESSURE TO REACH 130 PSIG, RESULTING IN THE ACTIVATION OF THE NEW RELIEF VALVE. THIS BUILDUP IN PRESSURE WAS CONCLUDED TO OCCUR BETWEEN THE FIVE MINUTE SCANS BEFORE 0820 HOURS ON JUNE 5, 2014. IT WAS CONCLUDED THAT THE SEATS ON THE CONTROL VALVES WERE THE CAUSE OF THE VALVES NOT HOLDING, WHICH HAVE SINCE BEEN REPLACED. ADDITIONAL OUTSIDE CONDITIONS MAY HAVE CONTRIBUTED TO THE EVENT, BUT ARE NOT THE ROOT CAUSE. DURING THE INSTALLATION OF A RELIEF STACK TO THE NEW RELIEF VALVE, VIBRATION OF THE INSTALLATION MAY HAVE ADJUSTED THE SET POINT OF THE RELIEF VALVE. IT WAS CONCLUDED THAT THE VIBRATION OF THE RELIEF VALVE POPPING OPEN AND CLOSED DID WALK THE SET SCREW OUT FROM 0820 TO 1100 HOURS WHEN THE INVESTIGATIVE TEAM HEARD A DISTINCTIVE SOUND DIFFERENCE IN THE RELIEF VALVE VENTING. AFTER THE SOUND OF GAS VENTING CHANGED, A TECHNICIAN NOTICED THE SET SCREW FOR THE RELIEF VALVE ON THE GROUND AT 1100 HOURS. THE INVESTIGATIVE TEAM RE-INSERTED THE SET SCREW AND REPAIRED THE CONTROL VALVES BEFORE RETURNING THE DELIVERY POINT BACK INTO SERVICE.