NORTHERN NATURAL GAS CO — Gas transmission and gathering — incident on 2012-03-10

Gas transmission and gathering incident in WATERLOO, BLACKHAWK County, IA. Reported cause: EQUIPMENT FAILURE. Reported consequences: 0 fatalities, 0 injuries, 3607 NATURAL GAS released, $0 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: 20120032.

Operator ID: 13750.

Reported incident date: 3/10/2012 12:40.

Cause detail: MALFUNCTION OF CONTROL/RELIEF EQUIPMENT

Cause detail: ON MARCH 10, 2012, NORTHERN'S OPERATIONS COMMUNICATION CENTER & GAS CONTROL RECEIVED A CALL FROM A NEIGHBOR NEAR THE WATERLOO, IOWA COMPRESSOR STATION STATING THAT HE THOUGHT HE COULD HEAR GAS BLOWING AT THE COMPRESSOR STATION. GAS CONTROL IMMEDIATELY CONTACTED THE WATERLOO, IOWA COMPRESSOR PLANT TEAM ON-CALL PERSON. THE ON-CALL PERSON WAS AWARE THAT ON THURSDAY, MARCH 8, 2012, THE TEAM PERSONNEL HAD INSPECTED AND TESTED COMPRESSOR UNIT # 2'S DISCHARGE RELIEF VALVE AND HAD RESET IT FROM A PRESSURE OF 1,030 PSIG TO A SET POINT PRESSURE OF 999 PSIG. DUE TO CONCERNS THAT UNIT DISCHARGE PRESSURE PULSATIONS COULD IMPACT THE RELIEF VALVE AND CAUSE IT TO BLOW PREMATURELY, THE TEAM ALSO HAD TESTED THE PULSATION IMPACT ON THE RELIEF ON MARCH 8, 2012, BY PUTTING UNIT 2 ON-LINE AND RUNNING THE DISCHARGE PRESSURE TO 958 PSIG, WHICH IS 2 PSIG BELOW THE 960 PSIG MAOP, AND THEY DID NOT HAVE ANY PROBLEMS WITH THE RELIEF VALVE BEING ACTIVATED. AS THE ON-CALL PERSON WAS AWARE OF THE PREVIOUS WORK AND CHANGES TO THE RELIEF VALVE HE WAS FAIRLY CERTAIN THAT THIS COULD BE THE CAUSE OF THE BLOWING GAS SO HE REQUESTED THAT GAS CONTROL REMOTELY SHUTDOWN UNIT # 2. UPON ARRIVAL AT THE PLANT THERE WAS NO GAS BLOWING. A REVIEW OF THE HISTORICAL PRESSURE TRENDS ON THE LOCATION HMI INDICATED THAT THE RELIEF VALVE STARTED TO VENT AT APRPOXIAMTELY 946 PSIG, WHICH IS WELL BELOW THE 999 PSIG SET POINT, AND ALSO WELL BELOW THE MAOP OF 960 PSIG. WHEN INSPECTING AND CHANGING THE SET POINT LOCATION PERSONNEL WERE AWARE THAT THE RELIEF VALVE WAS EQUIPPED WITH AN ANDERSON GREENWOOD AGCO PRESSURE SPIKE SNUBBER.IT WAS BELIEVED THAT THIS WOULD PREVENT PREMATURE ACTIVATION OF THE RELIEF VALVE. ADDITIONAL REVIEW INDICATED THAT THE UNITS WERE BEING OPERATED AT A REDUCED RATE OF SPEED AND LOAD AND THIS UNDOUBTEDLY CONTRIBUTED TO DISCHARGE PRESSURE PULSATIONS, SURGES AND SPIKES THAT WERE OUTSIDE OF THE OPERATIONAL CAPABILITY OF THE AGCO PRESSURE SNUBBER TO DAMPEN. ON MONDAY, MARCH 12, 2012 THE TEAM INSPECTED, RESET AND TESTED THE RELIEF VALVE AT A SET POINT OF 1,030 PSIG AND THE UNIT WAS PUT BACK INTO OPERATIONAL SERVICE. THE LOCATION IS DISCUSSING WITH THE EQUIPMENT VENDOR WHETHER THERE IS OPTIONS ON OTHER DEVICES THAT THEY MAY HAVE FOR INSTALLATION ON THE RELIEF VALVE THAT WOULD ALLOW FOR OPERATION AS AT A PRESSURE SET POINT OF 999 PSIG. THIS REPORT IS BEING FILED AS IT WAS DETERMINED THAT THE TOTAL GAS RELEASED BY THE RELIEF VALVE OPENING EXCEEDED THE THREE MILLION CUBIC FEET REPORTING THRESHOLD. NO FURTHER ACTIONS AS PENDING OR ARE REQUIRED AT THIS TIME SO THIS REPORT IS BEING CONSIDERED AS THE ORIGINAL/FINAL REPORT FOR THIS EVENT. 10/09/2012 A SUPPLEMENTAL/FINAL REPORT WAS FILED ON REPORT # 20120032-15474 DATED 03/29/2012 AS A FOLLOW UP TO RECOMMENDATIONS FROM A PHMSA REVIEW. ITEMS 6, 19A, PART C ITEM 3 AND 4 WERE REVISED. SUPPLEMENTAL/FINAL: UPDATE REPORT PER PHMSA REQUEST PART E5F BY INDICATING TRANSMISSION ON REPORT 20120032 DATED 3/29/2012.

PHMSA trend classification: not significant, not serious.

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

PHMSA indexed costs: $10,479 reported total cost, $8,689.283 in 1984 dollars, $10,799.198 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.