Accident Description:An explosion and fire led to the fatal injury of seven employees when a nearly forty-year-old heat exchanger catastrophically failed during a maintenance operation at the Tesoro refinery in Anacortes, Washington. The CSB’s investigation found an immediate cause of the tragedy to be long-term, undetected High Temperature Hydrogen Attack (HTHA) of the steel equipment, which led to the vessel rupture. Tesoro, like others in the industry, used published data from the American Petroleum Institute (API), called the Nelson Curves, to predict the susceptibility of the heat exchangers to HTHA damage. The CSB found these curves unreliable because they use historical experience data concerning HTHA that may not sufficiently reflect actual operating conditions. For example, a CSB computer reconstruction of the process conditions in the exchangers determined that the portion of the carbon steel exchanger that failed likely operated below the applicable Nelson curve—indicating it was “safe.” The CSB found that recommended practices of the API do not require users to verify actual operating conditions in establishing operation limits of the equipment or to confirm that the materials of construction selection will prevent the damage. An inspection strategy that relied on design operating conditions rather than verifying actual operating parameters contributed to the accident. The CSB determined that inspections for such damage are unreliable because the microscopic cracks can be localized and difficult to identify. The CSB noted that while API has identified high- chromium steels that are highly resistant; these were not installed by Tesoro.
Accident Description:On the evening of January 12, 2009, two refinery operators and two contractors suffered serious burns resulting from a flash fire at the Silver Eagle Refinery in Woods Cross, Utah. The accident occurred when a large flammable vapor cloud was released from an atmospheric storage tank, known as tank 105, which contained an estimated 440,000 gallons of light naphtha. The vapor cloud found an ignition source and the ensuing flash fire spread up to 230 feet west of the tank farm. On November 4, 2009, a second accident occurred at the Silver Eagle Refinery in Woods Cross, Utah, when a powerful blast wave, caused by the failure of a 10 inch pipe, damaged nearby homes. The CSB investigation team found that the examination of the ruptured pipe segment and adjacent piping clearly indicated wall thinning had occurred in the piping component. The elbow adjacent to the pipe segment that failed was noted to have an original thickness of 0.719-inch. A 2007 thickness measurement of the elbow indicated a wall thickness of 0.483-inch, indicating years of thinning had taken place. The adjacent straight-run segment that failed was found to have a wall thickness as low as 0.039-inch and there were no records of any previous inspection. The CSB found that in this investigations, as with other refinery investigations, mechanical integrity programs at refineries repeatedly emphasize inspection strategies rather than the use of inherently safer design to control the damage mechanisms that ultimately cause major process safety incidents.