NORTHERN NATURAL GAS CO — Gas transmission and gathering — incident on 2012-08-03

Gas transmission and gathering incident in MASON CITY, CERRO GORDO County, IA. Reported cause: EQUIPMENT FAILURE. Reported consequences: 0 fatalities, 0 injuries, 4300 NATURAL GAS released, $500 reported property damage. Operator-reported incident data submitted to PHMSA, generally within 30 days. Records may be supplemented or corrected and do not represent final agency causal findings.

Report number: 20120085.

Operator ID: 13750.

Reported incident date: 8/3/2012 10:13.

Location detail: 14.158

Cause detail: MALFUNCTION OF CONTROL/RELIEF EQUIPMENT

Cause detail: ON AUGUST 3, 2012, AT APPROXIMATELY 10:13 A.M. LOCAL TIME NORTHERN NATURAL GAS COMPANY'S OPERATIONS COMMUNICATION CENTER, IN OMAHA, NE, RECEIVED A REPORT FROM THE CERRO GORDO COUNTY SHERIFF'S DEPARTMENT OF GAS BLOWING TO ATMOSPHERE AT THE MASON CITY, IOWA TOWN BORDER STATION. AT NEARLY THE SAME TIME NORTHERN'S GAS CONTROL RECEIVED A SCADA HIGH, HIGH INLET GAS PRESSURE ALARM FOR THE FACILITY. THE SHERIFF REPORTED THAT LAW AND FIRE DEPARTMENT PERSONNEL WERE ON-SITE AND THAT THEY WERE IN THE PROCESS OF BLOCKING OFF THE ROAD, BUSINESS ROUTE HIGHWAY 18 THAT GOES BY THE TOWN BORDER STATION, AND EVACUATING SEVERAL STORES IN THE IMMEDIATE AREA. APPROXIMATELY 140 PEOPLE WERE EVACUATED FROM BUSINESSES NEAR THE TOWN BORDER STATION. NORTHERN TECHNICIANS ARRIVED ON-SITE AT APPROXIMATELY 10:35 A.M., AND FOUND THE STATIONS INLET 4-INCH FISHER EZ RELIEF VALVE WAS VENTING. NATURAL GAS TO THE TOWN BORDER STATION IS DELIVERED FROM THE MASON CITY IAB72002 PIPELINE, WHICH HAS A MAOP OF 800 PSI. GAS PRESSURE INTO THE TOWN BORDER STATION IS CONTROLLED BY PRESSURE CONTROLLING REGULATORS OF NATURAL GAS FROM IAB72002 AND RELIEF VALVE PROTECTED PIPING, AT A DELIVERY GAS PRESSURE OF APPROXIMATELY 330 PSI INTO THE STATION, WHICH HAS A 500 PSI MAOP. THE STATION INLET RELIEF VALVE IS SET AT 490 PSI TO PROTECT THE STATIONS 500 PSI MAOP. THE STATIONS INLET PRIMARY REGULATOR RUN WAS ISOLATED AND THE SECONDARY REGULATOR INLET RUN WAS PLACED IN-SERVICE. THE RELIEF VALVE CONTINUED TO SEEP AND IT WAS ALSO ISOLATED TO ALLOW INSPECTION. THE STREET WAS REOPENED AT APPROXIMATELY 11:10 A.M. AND BUSINESSES WERE NOTIFIED THAT THE CONDITION HAD BEEN REMEDIATED. ON INSPECTION OF THE PRIMARY REGULATOR IT WAS FOUND THAT SMALL PIECES OF WELDING SLAG AS WELL AS SMALL PIPELINE DIRT AND DEBRIS WERE PREVENTING A POSITIVE SEAL ON THE REGULATOR. THIS ALLOWED THE GAS PRESSURE TO CONTINUE TO INCREASE UNTIL THE RELIEF VALVE SET POINT WAS REACHED AND THE RELIEF OPENED RELEASING GAS TO ATMOSPHERE. IN ADDITION, TO SIMILAR SMALL AMOUNT OF DEBRIS FOUND IN THE RELIEF VALVE, A WASHED OUT AREA ON THE PILOT SEAT WAS FOUND WHICH LIKELY CONTRIBUTED TO THE RELIEF VALVE NOT FULLY SEALING OFF WHICH LIKELY CONTRIBUTED TO THE RELIEF VALVE CONTINUING TO RELEASE GAS TO ATMOSPHERE EVEN WHEN THE PRIMARY REGULATOR WAS SHUT IN AND THE BACK-UP REGULATOR WAS PLACED IN SERVICE. THE PRIMARY REGULATOR AND THE RELIEF VALVE WERE INSPECTED, CLEANED AND REPAIRED, PILOTS REBUILT AND TESTED. REGULATOR STRAINERS DOWNSTREAM OF THE INLET REGULATION WERE ALSO CHECKED AND CLEANED OF MINIMAL DEBRIS. THE REPAIRED REGULATOR AND RELIEF SETUP WAS PLACED BACK IN-SERVICE AROUND 4:25 P.M. RECORDS INDICATE THAT THE LAST DATE OF INSPECTION ON THE REGULATORS AND RELIEF VALVE WAS COMPLETED ON JULY 18, 2012. THIS FACILITY IS NORMALLY A BACK-UP GAS SOURCE TO THE UTILITY AND ONLY FLOWS WHEN NECESSARY IN THE HEATING MONTHS TO SUPPLEMENT GAS SUPPLIES TO THE WEST SIDE OF MASON CITY, IOWA; HOWEVER, THE FACILITY HAD BEEN PLACED INTO FULL SERVICE ON AUGUST 2, 2012, AT THE REQUEST OF ALLIANT ENERGY SO THAT MODIFICATIONS TO THEIR SYSTEM, ELSEWHERE IN THE CITY, COULD BE COMPLETED. NOTIFICATIONS WERE MADE TO THE NATIONAL RESPONSE CENTER, TICKET NUMBER 1019884, TO THE IOWA UTILITY BOARD IN DES MOINES, IOWA AND THE PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION IN KANSAS CITY. THERE WAS NO FIRE, NO EXPLOSION AND NO INJURIES. THIS REPORT IS BEING FILED AS THE QUANTITY OF GAS RELEASED EXCEEDED THE REPORTING LEVEL THRESHOLD AND THE SIGNIFICANT NUMBER OF PEOPLE EVACUATED. 10/09/2012 FILED SUPPLEMENT/FINAL PER JIM BUNN REVIEW WITH MINOR CHANGE TO LONGTIDUE READING AND CHANGING THE APPARENT CAUSE TO EQUIPMENT FAILURE. SUPPLEMENTAL/FINAL - UPDATED REPORT BY INDICATING TRANSMISSION IN E5F PER PHMSA REQUEST ON REPORT 20120085, DATED 8/23/2012.

PHMSA trend classification: not significant, not serious.

PHMSA standardized cause: EQUIPMENT FAILURE — MALFUNCTION OF CONTROL/RELIEF EQUIPMENT.

PHMSA indexed costs: $13,979 reported total cost, $11,211.202 in 1984 dollars, $14,741.242 in current-year dollars.

PHMSA trend flags, standardized causes, and indexed costs are analytical fields added to operator-reported incident data. They do not represent final agency causal findings.