Explosion and fire on a piping of a desulphurisation unit

At the time of the event, the desulphurisation unit was shut down for scheduled cleaning of a cooling tower with reintroduction of a gas mixture into the system at a high hydrogen content.
A 8″ heat-insulated carbon steel pipeline carrying a hydrocarbon/hydrogen mixture suddenly ruptured, releasing its content (it was located located along an upper rack). This triggered an explosion whose pressure surge was evaluated by the site operator at approx. 60 mbar at a distance of 10 m.
Since the leak was being fuelled, it followed a fire which affected other pipelines as well as the exhaust valves connected to the flare system. This explains the persistence of several secondary fire outbreaks, finally extinguished 8 hours later. The quantities released were estimated by the operator to be less than 2 tonnes of liquid hydrocarbons and 50 kg of hydrogen.

The INITIATING CAUSE was a leak on a pipe carrying hydrogen and hydrocarbons.

The rupture was due to external corrosion localised on the pipeline supporting structure. Over a section approx. 45 cm long on the line’s lower half-circumference, the residual pipe wall thickness was in average 3 mm thick, while the original average thickness was 6.5 mm. In some area the wall was think down to 1.2 mm. The rupture occurred in two stages: an initial break, followed by propagation of the opening because of the high pressure of gases circulating inside the pipeline.

A ROOT CAUSE is not provided by the source of this event. Nevertheless, from the corrective measures adopted, it can be deduced that the plant did not have effective inspection procedures able to detect degradation of the type occurred in the event. External corrosion is usually easy visible. However, it is very probable that the ruptured pipe was surrounded by thermal isolation layers. These not only hide any surface aggradation, but they may also as well contribute to the corrosion because of retainment of moistures.

pipe

The desulphurisation unit was stopped for a programmed clean-up of an aero-cooling with circulation of a hydrogen-rich mixture.

No victims were reported. The economic consequences were appraised at €500,000 of property damage and €2 million in production losses.

Local authorities asked for deep investigation on the accident causes and imposed conditions for restarting of the activity

The main fire was extinguished by the internal fire department after 50 minutes. As hydrogen leak was continuous, the fire extended to other pipes resulting in some secondary fires which were controlled after 10 hours.

The plant operator adopted the following measures, aiming at avoiding the reoccurrence of the event.

The risk assessment was reviewed, for the gas lines with corrosion hazard, either under the heat insulation or at the supporting points. In the new risk assessment, the possibility of a pipe burst has to be taken into account under specific operative conditions: service pressure above 25 bar; presence of gaseous or mixed fluids; diameter exceeding 2″; presence of hazardous fluid (H2, C2, C3, H2+H2S).

Moreover, the plant operator also ensured the adequate implementation procedures and good state of heat insulation. Finally, a special examination was also carried out on pipes not being stored on skids.