At around 6.20 p.m., one of the 2 supply pumps of the petroleum spirit hydrogenation unit (HDT) stopped unexpectedly, causing a fall in the unit’s loading flow, pressure build-up in a downstream pump and opening of the circuit relief valve towards the safety flare for the destruction of part of the gaseous hydrocarbons. A pressure surge occurred 20 min. later on an in-service leak plugging system located on a line upstream of an exchanger. A petroleum spirit leak was then detected. A safety perimeter was set up. The leak was isolated and the HDT unit was stopped to ensure security.

During this stoppage, one of the cracking furnaces was supplied with make-up butane to compensate for the fall in ethane. The butane vaporiser was started up using manual topping up with vapour. However, the vaporiser was pressurised quickly and the addition of vapour was not sufficient to ensure complete and stable vaporisation of the butane. Accordingly, liquid butane arrived in the furnace and its immediate vaporisation caused a local pressure build-up. The seal of the check valve bonnet upstream of the dilution vapour/hydrocarbon mixing point lost its watertightness. A flaming leak appeared. The melting of a fuse triggering an alarm in the control room and the confirmation by an outside technician allowed the operator to decide on an emergency stoppage of the furnace, putting a stop to the fire, which had lasted 1 min. until 8.15 p.m.

The spirit leak at the level of the in-service leak plugging system lasted 9 min. during which 400l of hydrogenated spirit consisting of 30% benzene were spilled. The flaming gas leak, occurring 1.5 hours later, lasted 1 min. The operator estimated the quantity of butane burned at 50kg. The flaring necessary to ensure safety lasted 13h 40 min. The accompanying emissions were estimated by the operator at 1.7t of nitrogen oxides (NOx), 3.4t of volatile organic compounds (VOC) and 127kg of dust. The HDT unit was restarted 4 days later. The furnace was restarted 45 days afterwards.

The stoppage of one of the 2 HDT unit supply pumps caused the series of events. The 2 charging pumps were in operation. One of them operated empty, undoubtedly due to a lack of load absorbed by the other pump, thus causing it to switch off. Then, regarding the spirit leak, the sealing paste used for the in-service leak plugging system was unsuitable for the composition of the fluid conveyed. The presence of toluene in the line had not been mentioned in the risk analysis prior to installation of the in-service leak plugging system.

The operator replaced the in-service leak plugging system and bolstered the pre-installation risk analysis regarding the choice of the type of sealing paste. The vaporiser starting guide was also revised to ensure satisfactory kinetics for its start-up, before starting the furnace.