TENNESSEE GAS PIPELINE COMPANY

Incident Narrative

ON SUNDAY MARCH 7TH, AT 11:24 EST THE HEBRON WITHDRAWAL RELIEF VALVE (313C-101) AT TENNESSEE GAS PIPELINE'S (TGP) COMPRESSOR STATION 313, ACTUATED PREMATURELY. AT THE TIME OF EVENT, THE TGP OPERATOR, WORKING FROM COMPRESSOR STATION 313, WAS PERFORMING HIS HOURLY CHECKS HEARD THE RELEASE OF GAS, INITIATED THE INVESTIGATION AND CONTACTED THE OPERATIONS SUPERVISOR AT 11:25 AM. AFTER CONSIDERING SAFE APPROACH PROTOCOLS, AND CONFIRMING PRESSURE WAS BELOW ESTABLISHED MAOP, THE TGP OPERATOR ISOLATED RELIEF VALVE AT APPROXIMATELY 11:35 EST. LINE PRESSURE AT THE TIME OF EVENT WAS 1049 PSIG AND SET PRESSURE OF RELIEF VALVE WAS 1250 PSIG. DUE TO REDUNDANT OVER PRESSURE PROTECTION, NO FACILITIES WERE TAKEN OUT OF SERVICE DUE TO THIS INCIDENT. AT 12:30 EST, AN INTERNAL EMERGENCY RESPONSE LINE (ERL) CALL COMMENCED AND DURING THAT CALL, TGP PERSONNEL ANNOUNCED THAT IT HAD DETERMINED THE RELEASE VOLUME EXCEEDED 3,000 MSCF. AT 12:51 EST, TGP NOTIFIED THE NRC OF THE EVENT AND ESTIMATED THE TOTAL GAS LOSS AT GREATER THAN 3,000 MSCF. THE TOTAL UN-INTENTIONAL GAS LOSS WAS LATER DETERMINED TO BE 10,651MSCF AND REPORTED TO THE NRC AS A 48-HR UPDATE. AFTER ISOLATION, TGP OPERATOR PERFORMED A VISUAL INVESTIGATION FOR THE IMMEDIATE CAUSE OF MALFUNCTION AND FOUND NO EXTERNAL ISSUES WITH RELIEF VALVE. TGP OPERATOR THEN REMOVED THE RELIEF VALVE FROM SERVICE AND CONDUCTED ROOT CAUSE ANALYSIS. WATER WAS DISCOVERED INSIDE THE VALVE HOUSING WHICH ALLOWED THE FORMATION OF HYDRATES INSIDE THE RELIEF VALVE'S PILOT CONTROL DEVICE, WHICH SUPPLIES PRESSURE TO THE TOP OF THE VALVE TO KEEP IT IN THE CLOSED POSITION. WITHOUT THE PRESSURE FROM THE TOP OF THE RELIEF THE VALVE THE VALVE SIT LIFTED ALLOWING THE GAS RELEASE. EXCESSIVE WATER, COUPLED WITH AN UNPRECEDENTED AMOUNT OF FREEZE AND THAW CYCLES IN THE PAST MONTH, ALLOWED FOR THE RELIEF VALVE TO ACTIVATE. THE VALVE WAS SERVICED BY AN OEM APPROVED VENDOR FOR COMPLETE REBUILD AND INSPECTION TO ENSURE THERE WAS NO MECHANICAL DAMAGE TO THE VALVE'S INTEGRAL PARTS. RELIEF VALVE WAS REINSTALLED AND BACK IN-SERVICE ON 3/22/2021. FOLLOWING THE EVENT, THE FREQUENCY OF METHANOL DRIP INSPECTIONS CHANGED FROM WEEKLY TO EVERY OTHER DAY FOR THOSE LOCATIONS WITH A HISTORY OF HYDRATE PRODUCTION DURING COLD AMBIENT TEMPERATURES.