Thermal Fatigue Probable Cause of Enterprise Products Explosion: CSB

The U.S. Chemical Safety Board released its final report on the June 27, 2016, explosion and fire at the Enterprise Products Pascagoula Gas Plant in Pascagoula, Miss., with recommendations to two trade associations to include information on the potential for minor leaks and catastrophic failure of brazed aluminum heat exchangers.

The U.S. Chemical Safety Board released its final report on the June 27, 2016, explosion and fire at the Enterprise Products Pascagoula Gas Plant in Pascagoula, Miss., with the board finding the probable cause was thermal fatigue in a brazed aluminum heat exchanger (BAHX). The board issued recommendations to two trade associations and local emergency responders in the report.

The explosion occurred when a major loss of containment in a heat exchanger released methane, ethane, propane, and several other hydrocarbons that ignited, causing a series of fires and explosions. The site was shut down for almost six months afterward, CSB reported.

"More than 500 gas processing facilities operate across the country, and the use of similar heat exchangers is common," said CSB Interim Executive Kristen Kulinowski. "Extending the life cycle of equipment at these facilities requires more robust inspection protocols. Operators shouldn't take the risk of waiting to find a leak because, as this case demonstrates, that leak could result in a catastrophic failure."

The recommendations to the American Petroleum Institute and the GPA Midstream Association ask both of them to include information on the potential for minor leaks and catastrophic failure of brazed aluminum heat exchangers in a BAHX standard and a technical bulletin, respectively. The board recommended that the GPA Midstream Association develop a database for operators to submit BAHX operational data for collaborative industry learning and analysis.

According to the safety board, the Enterprise Plant receives raw natural gas via a pipeline from the Gulf of Mexico and separates the material into two products: natural gas liquids, which serve as a feedstock to the chemical industry, and a natural gas fuel stream, primarily composed of methane. A key piece of equipment used in the process is the brazed aluminum heat exchanger, which allows for the transfer of heat between two different process streams while keeping the streams separate. CSB's investigation determined the heat exchanger failed due to thermal fatigue, and its report details how thermal fatigue occurs between aluminum parts of a BAHX. "As the exchanger is heated or cooled, the tightly connected parts expand or contract. If the parts change temperatures at sufficiently different rates, the expansion and contraction can be disproportionate. Over time, this process weakens the metal, and ultimately causes cracks, which can lead to the escape of hydrocarbons. Typically, when a leak is found, it can be repaired with minimal expense or consequence before a major loss of containment occurs. Assuming that leaks will be discovered and can be repaired prior to a catastrophic failure is referred to as a 'leak-before failure' assumption. Thermal fatigue is a known factor to BAHXs, and there is industry guidance on recommended limits for maximum cyclic temperature fluctuations during operation and rates of cooling or heating during startup and shutdown. However, the CSB found this guidance was not robust for the diverse operations and environments where BAHXs operate," according to the board's release about the report.

It noted that at the Enterprise plant, process data for the exchangers show its BAHXs were repeatedly subjected to temperature changes that exceeded industry-recommended practices, and over a 17-year period, four different BAHX heat exchangers at the plant were repaired nine times.

This incident and four other BAHX failure events at other facilities "illustrate that relying on a leak-before-failure assumption is not adequate," CSB pointed out. "Operators of midstream gas plants need a more robust assessment and risk management plan that considers thermal fatigue to prevent the risk of sudden and catastrophic rupture of BAHX."

"A number of midstream gas plant operators have reported that the limits and rates in existing industry guidance may not be realistic. Our report encourages a meaningful dialogue among BAHX manufacturers, gas processors, and repair technicians. The CSB concluded that more realistic and updated guidance is needed to improve the safe use of BAHX," said CSB Investigator William Hougland.

The board's recommendation to the Jackson County Local Emergency Planning Committee asked that its members work to explicitly define the methods for communication updates (such as using social media, local news outlets, etc.) to ensure members of the public can get current safety information -- it recommends developing a more robust and engaged community alert network, one that includes social media and the ability to expand opportunities to interact with the community throughout an incident.

The board also released on Feb. 13 an interactive 3D model of the heat exchanger used at the Enterprise Plant to enhance understanding of how this type of heat exchanger operates and its vulnerability to thermal fatigue.

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