Thermal fatigue was likely the cause of the explosion and fire at Enterprise Products Partners’ Pascagoula, Miss., gas plant late in the evening of June 27, 2016, a US Chemical Safety Board investigation concluded on Feb. 19.
The failure of the plant’s brazed aluminum heat exchanger (BAHX) led to the release of methane, ethane, propane, and other hydrocarbons that then ignited, CSB’s final report on the incident said. The resulting explosion and fires ultimately shut the site down for 6 months, it noted.
“More than 500 gas processing facilities operate across the country and the use of similar heat exchangers is common,” CSB Interim Executive Kristen Kulinowski said. “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.”
EPP’s Pascagoula plant receives raw natural gas through a pipeline from the Gulf of Mexico and separates it into natural gas liquids and a natural gas fuel stream. The BAHX allows for the transfer of heat between two different process streams while keeping the streams separate, the report said.
Thermal fatigue can occur between a BAHX’s aluminum parts, which expand or contract as the exchanger is heated or cooled, the report said. “If the parts change temperature at sufficiently different rates, the expansion and contraction can be disproportionate,” it stated. “Over time, this process weakens 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 loss of containment occurs, the report said. “Assuming that leaks will be discovered and can be repaired prior to a catastrophic failure is referred to as a ‘leak-before failure’ assumption,” it pointed out.
Assessing industry guidance
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, the report said.
At the Pascagoula gas plant, process data for the exchangers show that the BAHXs were repeatedly subjected to temperature changes that exceeded industry-recommended practices, it said. This increases stresses on the connections within the heat exchangers as the aluminum parts push against and pull apart from each other. At EPP over a 17-year period four different BAHX heat exchangers were repaired nine times, the report said.
It said the 2016 incident, as well as four other BAHX failure events at other facilities, illustrate that relying on a leak-before-failure assumption is not adequate. Operators of midstream gas plants need a more robust assessment and risk management plan that considers thermal fatigue to prevent the risk of a BAHX’s sudden and catastrophic rupture, it said.
“A number of midstream gas plant operators have reported that the limits and rates in existing industry guidance may not be realistic,” Investigator William Hougland said. “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.”
CSB also issued recommendations to the American Petroleum Institute and the GPA Midstream Association to share information related to failure hazards of BAHX’s from thermal fatigue.
Contact Nick Snow at [email protected].
