TENNESSEE GAS PIPELINE COMPANY

Incident Narrative

ON 02/26/2018; TECHNICIANS ARRIVED AT UNMANNED COMPRESSOR STATION 241 FOR ROUTINE TASKS AT WHICH TIME THEY HEARD GAS VENTING TO ATMOSPHERE THROUGH THE STATION'S BLOWDOWN SILENCER. THE TECHNICIANS ASSESSED THE ATMOSPHERE SAFETY OF A-BUILDING BY EVALUATING THE STATUS OF ITS GAS MONITORING SYSTEM WHICH WAS DETERMINED TO BE SAFE. THEY PROCEEDED TO ENTER A-BUILDING AND ASSESSED THE HMI NEAREST TO THE BUILDING ENTRANCE AND OBSERVED NORMAL RUN STATUS FOR UNIT 1A. CONTINUING THEIR INVESTIGATION TO FIND THE SOURCE OF THE VENTING GAS, THE TECHNICIAN THEN APPROACHED UNIT 1A AND OBSERVED VISUALLY AND AUDIBLY THAT IT WAS NOT RUNNING, EVEN THOUGH THE HMI HAD INDICATED IT WAS. FOLLOWING THE NOISE OF THE VENTING GAS, A 1 1/2-INCH VENT VALVE ON UNIT 1A WAS FOUND TO BE IN THE OPEN POSITION AS OPPOSED TO ITS NORMAL CLOSED POSITION. THE TECHNICIAN CLOSED THE VALVE MANUALLY TERMINATING THE RELEASE OF NATURAL GAS AT 07:20. OUR INCIDENT INVESTIGATION FOUND A LOSS OF ""CLOSE"" SIGNAL FROM A LOCAL PLC TO THIS VALVE CAUSED THE VENT VALVE TO OPEN AS DESIGNED. WE ALSO DETERMINED THAT A WIRING ISSUE CONCERNING THE UNIT'S BYPASS VALVE OPERATOR CAUSED AN ISSUE WITH THE PLC PROGRAM AND A SOFTWARE LOCK-UP OF THE PLC PROCESSOR ( MAJOR FAULT OF PLC PROCESSOR ) RESULTING IN DE-ENERGIZED OUTPUTS CAUSING VENT VALVE TO OPEN WHILE SUCTION AND DISCHARGE VALVES ALSO REMAINED OPEN. WE ALSO DETERMINED THAT AN ALARM TO INDICATE A LOSS OF LOCAL COMMUNICATION WAS NOT ENABLED WITHIN THE PCL LOGIC THEREFORE GAS CONTROL DID NOT RECEIVE NOTICE OF LOSS OF LOCAL COMMUNICATION. POST INVESTIGATION WIRING REPAIR WAS COMPLETED 02/26/2018 AND UNIT 1A WAS RETURNED TO SERVICE ON 02/26/2018. IN ADDITION CHANGES TO PLC LOGIC AND STATION ALARM WERE COMMISSIONED BY 02/27/2018. TGP ALSO DETERMINED THAT THE UNINTENTIONAL RELEASE OF GAS STARTED ON 02/25/2018, AT 23:30 HOURS.