NORTHERN NATURAL GAS CO — Gas transmission and gathering — incident on 2018-09-19

Gas transmission and gathering incident in Not Within a Municipality, CASS County, NE. Reported cause: EQUIPMENT FAILURE. Reported consequences: 0 fatalities, 0 injuries, 3869 NATURAL GAS released, $10 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: 20180101.

Operator ID: 13750.

Reported incident date: 9/19/2018 8:15.

Location detail: 15.31

Cause detail: MALFUNCTION OF CONTROL/RELIEF EQUIPMENT

Cause detail: ON SEPTEMBER 19, 2018, OPERATIONS PERSONNEL WERE MAKING PLANED PRESSURE ADJUSTMENTS TO THE OMAHA 3RD BRANCH LINE TO RAISE PRESSURE FROM 725 TO 775 PSIG. THE MAOP IS 800 PSIG. WHEN PRESSURE REACHED 750 PSIG, THE RELIEF VALVE BEGAN TO VENT. OPERATIONS PERSONNEL ISOLATED THE RELIEF VALVE AND THE VENTING STOPPED. VENTING OCCURRED FOR APPROXIMATELY 5 MINUTES. APPROXIMATELY 3,869 MCF OF GAS WAS RELEASED. THE CAUSE OF THE RELIEF VALVE TO VENT BELOW SET POINT WAS VIBRATION CREATED BY THE V250 CONTROL VALVE AND THE FACILITY PIPING. THE ENERGY TRANSFERRED IN THE FORM OF VIBRATION OVER TIME LOOSENED THE LOCKING NUT ON THE PILOT SET POINT ADJUSTMENT STEM. THIS ALLOWED THE STEM TO BACK OUT, CHANGING (LOWERING) THE SET POINT. CONTRIBUTING CAUSES MAY HAVE BEEN THE INCREASED CONTROL VALVE NOISE LEVEL DUE TO A CHANGE IN OPERATING PRESSURES (INCREASED PRESSURE DROP DURING CONSTRUCTION ACTIVITIES) AND/OR INSUFFICIENT TORQUE ON THE LOCK NUT WHEN PREVIOUSLY TESTED. ADDITIONAL IMPACT TO THE RELIEF VALVE ASSEMBLY WAS DUE TO THE IMPROPER DESIGN OF THE VENT PIPING AND SUPPORT STRUCTURE, WHICH ENHANCED THE SIDE THRUST WHEN THE RELIEF VALVE OPENED.

PHMSA trend classification: not significant, not serious.

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

PHMSA indexed costs: $10,931 reported total cost, $10,198.313 in 1984 dollars, $12,533.776 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.