NORTHERN STATES POWER CO OF MINNESOTA

Incident Narrative

CREW WAS ON SITE TO REPAIR A LEAKING FLANGE AT A REGULATOR STATION. THE CREW WAS IN THE PROCESS OF REMOVING A BYPASS LINE AND RETURNING THE SYSTEM TO NORMAL. THIS PROCESS INVOLVED REOPENING A BLOCK VALVE AND PUTTING IT BACK INTO SERVICE. WHEN DOING THIS, THE SENSE LINE INADVERTENTLY REMAINED IN THE CLOSED POSITION. THIS LEAD TO THE PRESSURE BEING MEASURED ON THE TRAPPED GAS IN THE BYPASS LINE, NOT THE DOWNSTREAM PRESSURE. THE CREW THOUGHT THEY HAD RETURNED THE SYSTEM TO NORMAL. AFTER APPROXIMATELY 30 MINUTES, THE GAUGE HAD DROPPED ONLY 1/4 LB. THE CREW EXPECTED THE PRESSURE TO DROP MORE RAPIDLY. THEY DID SOME TROUBLESHOOTING AND CHECKED THE GAUGE IN THE VAULT (WHICH WAS MONITORING DOWNSTREAM PRESSURE) AND SAW THE GAUGE READ 1LB. THEY REALIZED IMMEDIATELY THAT THEY WERE NOT READING THE DOWNSTREAM PRESSURE, BUT IN FACT THE TRAPPED PRESSURE IN THE SENSE LINE. THE CREW OPENED THE TRAP AND THE PRESSURE DROPPED FROM 18LBS TO 1LB INSTANTLY. THE SYSTEM WAS SHUT DOWN, RISER VALVES TURNED OFF, SYTEM RE-PRESSURIZE AND THEN PERFORM RELIGHTS. XCEL ENERGY REVIEWED APPLICABLE POLICIES AND PROCEDURES WITH FRONT LINE EMPLOYEES INVOLVED WITH THIS EVENT. XCEL ENERGY IS DEVELOPING ADDITIONAL TRAINING THAT IS FOCUSED ON WHAT SPECIFIC WORK REQUIRES A WRITTEN PROCEDURE.