Around 11 pm, an 8″ elbow of a hydrotreating (HDT) unit at a refinery transforming heavy products (heavy diesel, fuel oil) into medium distillates (gas oil, heating fuel oil, diesel, kerosene) broke, in freeing hydrogen and producing an explosion and fire. The operator alerted the site’s residential neighbours to close their windows and remain indoors, while indicating the absence of any particular risk. Fire-fighters controlled the situation in 3½ hours. One worker was slightly injured during the shutdown manoeuvres. The hydrotreating unit was stopped. Property damage was major, amounting to an estimated USD 30 million.

During the previous maintenance down time, the exchanger connection lines had been disassembled; they contained elbows that were geometrically identical yet of a different metallurgical composition. During reassembly, the elbow made of carbon steel was installed in the hotter zone, where plans called for installing a 5% chrome alloy. The weaker part broke due to corrosion by hydrogen at high temperature 3 months after service restart.

This accident could have been avoided by adopting the same metallurgy for interchangeable parts when designing the units. For existing installations, the operator identified this same risk regarding pipe standard changes and introduced a part position control system for reassembly operations.

On 23rd March, 2005, an explosion in this same refinery had caused 15 deaths and 170 injuries (ARIA no. 29598).