MIDWESTERN GAS TRANSMISSION CO

Incident Narrative

ON 02/08/2019 AT 23:05 HOURS, THE MGT-PORTLAND COMPRESSOR STATION EXPERIENCED A SHUTDOWN ON THE #1 COMPRESSOR. THE FIELD MECHANIC RESPONDED TO A CALLOUT AND STARTED THE #5 COMPRESSOR TO CONTINUE WITH THE GAS PROCESSING. UPON MAKING REPAIRS TO THE #1 COMPRESSOR AND STARTING THE UNIT BACK UP, THE FIELD MECHANIC ISSUED A SHUTDOWN COMMAND TO THE #5 COMPRESSOR. AS PER NORMAL OPERATIONS, WHEN A COMPRESSOR SHUTS DOWN, TELEMETRY CLOSES THE SUCTION VALVE AND BLOWS DOWN THE COMPRESSOR. UNKNOWN TO THE FIELD MECHANIC, THE VALVE ACTUATOR WHICH CLOSES THE #5 COMPRESSOR SUCTION VALVE WAS UNABLE TO PERFORM THE CLOSE FUNCTION. THE #5 COMPRESSOR SUCTION VALVE REMAINED OPEN. THE COMPRESSOR BEGAN TO BLOW DOWN AS PROGRAMMED, BUT WITH THE SUCTION VALVE REMAINING OPEN, NATURAL GAS CONTINUED TO RELIEVE OUT THE BLOW DOWN VENT. THE FIELD MECHANIC DID NOT SEE THE SUCTION VALVE HAD REMAINED OPEN AND THE CONTROLLER HAD A CLOSED VALVE INDICATOR ON HIS SCADA DISPLAY AS THE DISCONNECTED ACTUATOR HAD ACTIVATED THE LIMIT SWITCH DISPLAYING THE VALVE AS CLOSED IN SCADA. THE FIELD MECHANIC DID NOT HEAR THE GAS RELIEVING THROUGH THE VENT LINE AS THE BLOW DOWN HAS A SILENCER INSTALLED. DURING STATION ROUNDS ON 02/09/2019 AT 09:00, THE FIELD OPERATOR FOUND THE GAS RELIEVING THROUGH THE COMPRESSOR VENT LINE. THE GAS LOSS WAS CALCULATED AT 4.65 MMCF AND WAS REPORTED TO THE NRC ON 02/09/2019 AT 1100 HRS. THE ORIGINAL NRC REPORT NUMBER IS 1237270. THE 48 HOUR NRC REPORT NUMBER IS 1237760. ACTION TAKEN: AN LEL TRANSMITTER TO ALERT THE CONTROL CENTER OF VENTING GAS FROM THE BLOW DOWN HAS BEEN INSTALLED TO ASSIST THEM IN DETERMINING IF GAS IS VENTING TO ATMOSPHERE. THE SUCTION VALVE ACTUATOR WAS REPAIRED AND THE REMAINING STATION ACTUATORS WERE INSPECTED WITH NO ISSUES FOUND. SUPPLEMENTAL REPORT WAS CREATED DUE TO THE NRC REPORTING DATE IN PART A WAS INCORRECT IN THE FIRST REPORT. 3/18/2019 - SUPPLEMENTAL CREATED TO CORRECT NRC ORIGINAL REPORT NUMBER AND DESCRIPTION OF CONTROL VALVE BECOMING DISENGAGED FROM VALVE. THE MOUNTING NUTS HAD VIBRATED LOOSE ON THE MOUNTING STUDS OF THE VALVE ACTUATOR. WHEN THE NUTS VIBRATED LOOSE, THE ACTUATOR SLID OFF OF THE SHAFT WHICH DISENGAGED THE KEY ON THE SHAFT. WHEN A CLOSED COMMAND WAS SENT TO THE VALVE, THE ELECTRIC OPERATOR CLOSED AS IT SHOULD, BUT DID NOT MOVE THE VALVE. IT MADE THE CLOSED LIMIT SWITCH ON THE ELECTRIC OPERATOR, BUT BECAUSE IT WAS DISENGAGED FROM THE GEAR BOX SHAFT IT DID NOT OPERATE THE SUCTION VALVE. THE CONTROLS SAW THE CLOSED LIMIT ON THE SUCTION VALVE AND ALSO ON THE DISCHARGE VALVE AND IT THEN OPERATED AS IT SHOULD ALLOWING THE VENT VALVE TO OPEN TO BLOW DOWN THE COMPRESSORS. THE REPAIR WAS MADE BY INSTALLING THE ELECTRIC OPERATOR BACK ON THE SHAFT AND REPLACING THE NUTS. THE OPEN AND CLOSED LIMITS WERE THEN RESET AND TESTED AS PER PROCEDURE. INSTALLATION OF THE ACTUATOR WAS CONDUCTED BY A CONTRACTOR AND THE ACTUATOR HAD PERFORMED CORRECTLY POST INSTALLATION. MAINTENANCE HAD BEEN PERFORMED ON THE VALVE AND THERE WERE NO ISSUES FOUND DURING THE MAINTENANCE ACTIVITY. ALL ACTUATORS HAVE BEEN INSPECTED AT THE COMPRESSOR STATION TO VERIFY NUTS WERE TIGHT. 3/19/2019 SUPPLEMENTAL: UPDATED ROOT CAUSE TO INCORRECT OPERATION DURING INSTALLATION. THE BOLTS, IF TORQUED, SHOULD NOT HAVE VIBRATED LOOSE. THERE WAS NOT A FAILURE OF THE ACTUATOR, AS IT PERFORMED AS DESIGNED. THERE WERE NO BROKEN PARTS TO THE ACTUATOR.