exceed a pressure set point, the monitor valve moves to control the downstream pressure. Events of September 9, 2010 An Uninterruptible Power Supply (UPS) system at the Milpitas Terminal provides temporary power for the SCADA and control equipment during a power outage before emergency generators start delivering backup power. On March 31, 2010 the UPS failed to function properly. On April 1-2, 2010 PG&E installed three temporary mini-UPS units to provide temporary backup power. During the afternoon of September 9, 2010, PG&E personnel were working on the Milpitas Terminal UPS system. At 5:22 p.m., the SCADA center alarm console displayed over 60 alarms within a few seconds, including controller error alarms and high differential pressure and backflow alarms from the Milpitas Terminal. These alarms were followed by pressure alarms on several lines leaving the Milpitas Terminal, including Line 132. PG&E later determined that the error alarms were related to power supply unit power fluctuations. Records of SCADA alarms and pressure readings indicate valves opening and pressure increasing at 5:23 p.m. Pressure readings on Lines 132, 101, and 109 increased from 370 pounds per square inch gage (psig) to 380 psig in about 90 seconds. The highest pressure recorded at an upstream location closest to Segment 180 just prior to the failure was 386 psig. After the rupture and explosion occurred, about 95 minutes passed before PG&E isolated the rupture to stop the flow of gas. This contributed to the severity and extent of property damage, according to an investigative report by the NTSB (NTSB Report).
exceed a pressure set point, the monitor valve moves to control the downstream pressure. Events of September 9, 2010 An Uninterruptible Power Supply (UPS) system at the Milpitas Terminal provides temporary power for the SCADA and control equipment during a power outage before emergency generators start delivering backup power. On March 31, 2010 the UPS failed to function properly. On April 1-2, 2010 PG&E installed three temporary mini-UPS units to provide temporary backup power. During the afternoon of September 9, 2010, PG&E personnel were working on the Milpitas Terminal UPS system. At 5:22 p.m., the SCADA center alarm console displayed over 60 alarms within a few seconds, including controller error alarms and high differential pressure and backflow alarms from the Milpitas Terminal. These alarms were followed by pressure alarms on several lines leaving the Milpitas Terminal, including Line 132. PG&E later determined that the error alarms were related to power supply unit power fluctuations. Records of SCADA alarms and pressure readings indicate valves opening and pressure increasing at 5:23 p.m. Pressure readings on Lines 132, 101, and 109 increased from 370 pounds per square inch gage (psig) to 380 psig in about 90 seconds. The highest pressure recorded at an upstream location closest to Segment 180 just prior to the failure was 386 psig. After the rupture and explosion occurred, about 95 minutes passed before PG&E isolated the rupture to stop the flow of gas. This contributed to the severity and extent of property damage, according to an investigative report by the NTSB (NTSB Report).