At 11:45 pm on a petrochemical site, 20 t of Isobutane (iC4) and 100 kg of Hydrogen Fluoride (HF) were released from a corroded 6″ nominal bore carbon steel pipeline on a Hydrofluoric Acid Alkylation Plant. The release created a flammable gas cloud that fortunately did not ignite. The gas alarms sounded, together with pressure alarms (pressure drop). The company following its own emergency procedures dealt with the incident ; the plant was shut down and depressurised. The HF was transferred to the emergency dump vessel ; the release lasted 84 minutes. No injuries were sustained and no environmental damage reported.

The pipeline was part of a thermal relief line from a pressure relief valve (PRV), which discharged back to the process. The process conditions were 10 bar, 72° C, iC4 with 0.3% HF and no free water. The pipeline ran from the PRV down to a horizontal pipetrack approximately 10m in the air. The site of the failure was close to the tie point back to the process. The failure was in the Normally No Flow (NNF) line and was due to internal corrosion of the pipeline. The subsequent inspection showed significant internal scale, which in some places completely blocked the line. It appeared that the line was subjected to a hydrotest using water 2 years before the incident. The line might have contained scale from previous operations, which became wet as a result of the hydrotest. The drying procedures rely on any water in the system being driven to low points by the circulation of hydrocarbons, where the water is collected and subsequently drained. However, this relies on flow through the plant and linework. Since the failed line was on the discharge of a pressure relief valve (PRV), there would be no flow. It is therefore suspected that the line was still wet when taken back into service. The presence of this water, together with the 0.3% Hydrogen Fluoride (HF) in the Isobutane (iC4) process stream, is thought to have caused the accelerated corrosion and scaling.

The operator reviewed its risk based inspection methodology, the hydrotest and drying procedures as well as the levels of corrosion, scaling and pressure relief system of the plant.