VIKING GAS TRANSMISSION CO

Incident Narrative

AN ORIGINAL EQUIPMENT MANUFACTURER (OEM) FITTING ON A 4X6 ANDERSON-GREENWOOD RELIEF VALVE LOCATED ON A RECIPROCATING NATURAL GAS COMPRESSOR DISCHARGE PIPING, FAILED ON MARCH 20, 2013 AT THE ADA, MN COMPRESSOR STATION, CAUSING THE UNINTENDED RELEASE OF NATURAL GAS. EMPLOYEES FROM THE ADA COMPRESSOR STATION WERE NOTIFIED OF THE BLOWING GAS AT APPROXIMATELY 6:29 A.M. BY A NEIGHBOR TO THE COMPRESSOR STATION. THE RESPONDING EMPLOYEES WERE ABLE TO IDENTIFY THE PROBLEM AND SHUT DOWN THE UNIT AND THE VENTING GAS BY 7:02 A.M. A REVIEW OF THE STATION CONTROLLER LOG AT THE ADA COMPRESSOR STATION INDICATED THAT THE OEM FITTING FAILED AT 00:17 A.M., WHICH ALLOWED GAS TO VENT FOR 6 HOURS AND 45 MINUTES. THE EVENT WAS REPORTED TO THE NRC IN ANTICIPATION OF GAS LOSS EXCEEDING THE 3,000 MCF THRESHOLD LEVEL. A NEW FITTING WAS INSTALLED TO REPLACE THE FAILED FITTING AND THE COMPRESSOR WAS RESTORED TO NORMAL SERVICE THE SAME DAY. THE FAILED FITTING WAS SENT TO SHERRY LABS FOR A FAILURE ANALYSIS. THE FAILURES ANALYSIS CONCLUDED THE RELIEF VALVE FITTING FRACTURED THROUGH THE 1/2"" DIAMETER THREADS OF ONE END BY UNIAXIAL BENDING FATIGUE. THE PRIMARY CAUSE OF FAILURE APPREARED TO BE USE OF A MATERIAL WITH INSUFFICIENT FATIGUE RESISTANT FOR THE APPLICATION. THE FAILED FITTING WAS A OEM STANDARD SUPPLIED FITTING, WHICH IS PART OF THE PILOT SENSING SYSTEM. THE OEM HAS DESIGNED A NEW HIGH STRENGTH, STAINLESS STEEL COMPONENT WHICH WILL REPLACE THOSE EXISTING PILOTS WITH SIMILAR RELIEF PILOT INSTALLATIONS. THIS WORK WILL BE COMPLETED IN 2013. OVER-PRESSURE WAS NOT A CONTRIBUTING FACTOR TO THE FITTING FAILURE.