QUESTAR PIPELINE COMPANY

Incident Narrative

SUPPLEMENTAL BEING FILED TO CORRECT THE CAUSE OF THE INCIDENT. DISCHARGE PRESSURES FROM THISTLE CREEK COMPRESSOR STATION WERE OPERATING NEAR THE RELIEF POINT OF THE THERMAL RELIEF VALVE FROM THE DISCHARGE GAS COOLERS. THE RELIEF VALVE OPENED AND BEGAN TO RELIEVE, BUT DID NOT RESEAT. THE SITUATION WAS IDENTIFIED BY AN OPERATING EMPLOYEE NEXT MORNING. THE RELIEF VALVE WAS ISOLATED, MANUALLY RESEATED, AND TESTED TO BE WITHIN 5% OF ITS SETPOINT. INITIALLY, A REPORT WAS MADE TO THE NRC BECAUSE IT WAS THOUGHT THAT THIS EVENT WAS AN UNINTENTIONAL RELEASE OF GAS NEAR THE 3 MMSCF REPORTING THRESHOLD. THE ACTUAL VENTED GAS WAS APPROXIMATELY 2.69 MMSCF, WHICH IS BELOW THE REPORTING THRESHOLD; HOWEVER, QUESTAR PIPELINE COMPANY IS FILING THIS REPORT TO FOLLOW-UP WITH THE REPORT MADE TO THE NRC. QUESTAR PIPELINE ENGINEERING SERVICES COMPLETED AN ANALYSIS OF THE THERMAL RELIEF LINE AND CONCLUDED THAT IT IS NOT NEEDED, BECAUSE THE DISCHARGE GAS COOLER WILL NOT BE BLOCKED FROM THE MAIN STATION DISCHARGE RELIEF VALVE DURING AND ESD OR UNIT ISOLOATION. CONSEQUENTLY, THE ISOLATION VALVE TO THE THERMAL RELIEF VALVE OF THE GAS COOLER HAS BEEN LOCKED CLOSED.