GREAT LAKES GAS TRANSMISSION CO

Incident Narrative

DURING A ROUTINE WALK-THROUGH OF THE COMPRESSOR UNIT 303 BUILDING, A STATION TECHNICIAN WAS ABLE TO SMELL GAS IN THE COMPRESSOR BUILDING. THE TECHNICIAN HAD THE UNIT SHUTDOWN AND UNIT PIPING EVACUATED OF GAS. THE PIPING WAS THEN REPRESSURIZED TO A POINT WHERE THE GAS LEAK COULD BE HEARD AND DETECTED USING A PORTABLE LEAK DETECTOR. THE LEAK WAS EMANATING AT A FILLET WELD THAT CONNECTED A WELD-O-LET TO THE DISCHARGE HORN PIPING OF THE UNIT GAS COMPRESSOR. THE PIPING WAS THEN DEPRESSURIZED TO ALLOW FOR FURTHER ANALYSIS. A MAGNETIC PARTICLE EXAMINATION OF THE FILLET WELD REVEALED A CIRCUMFERENTIAL CRACK AROUND PART OF THE BASE OF THE WELD. DUE TO UNCERTAINTY OF THE INTEGRITY OF THE PIPE (CONTAINING THE WELD-O-LET) AND AN ASSOCIATED FLANGE, IT WAS DECIDED TO REMOVE AND REPLACE THE AFFECTED PIPE SEGMENT AND FLANGE. THE REMOVED MATERIAL WAS SENT TO A LABORATORY FOR ANALYSIS, WHICH DETERMINED THAT THE FRACTURE MECHANISM CAUSING THE THROUGH-WALL CRACK ON THE 30-INCH HEADER PIPE (DISCHARGE HORN) WAS HIGH CYCLE, LOW STRESS FATIGUE.