The lower part of a waste feed chute on a furnace (capacity: 28 tonnes/hr) at an incineration plant exploded. A technician was violently thrown and sustained serious injuries. Water under 3 bar of pressure cooled the base of the chute, which was exposed to radiation from the combustion grate (1000°C). To improve circulation, the chute was composed of 20 independent boxes with 2 metal sidewalls (16 mm on the inside, 8 mm outside) spaced 10 cm apart, isolated from the water circuit by 2 (inlet/outlet) valves and fitted with an air exhaust vent. With the boxes dilating when exposed to heat, stress cracks generated water leaks that had to be repaired during furnace downtime. The most recent cracks, detected in boxes no. 18 on 2 boilers, were relatively inaccessible. The operator decided to drain both suspect boxes and insulate them with concrete (creating a thermal screen). The furnace had been restarted 14 days prior.

The explosion occurred at the level of the box filled with concrete, which was heavily damaged; both the torn outer sheet metal and interior sheets compressed around the furnace proved that the explosion originated inside the box whose vent valve had remained shut. A more extensive observation showed that interior sheet metal had also been torn, as flames escaping from the furnace burned the technician. Initial analyses revealed that the explosion was undoubtedly related to sudden water vaporization within an enclosed volume.

After the operator’s analysis, the most likely accident cause was determined to be a vaporization phenomenon tied to the sudden intake of water into the box. Anomalies observed on the installation had been reported to management, yet the potential onsite hazard was not adequately perceived from these reports.

This accident followed a series of near-accidents, for which the operator failed to gauge the importance, as well as a series of technical modifications of facilities performed by operating staff without any external technical support and without a preliminary risk evaluation.

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