In a food processing plant, the dome of an extraction filter opened accidentally at 3:10 p.m. during an alginate extraction operation. The noise alerted the employees who discovered the filter open and the dome held in place by only 11 of the 96 screws. The other screws had been blown out. The roof, false ceiling, piping and jib crane had been damaged. The installation was secured and locked out.
The damaged filter, having a volume of 11,450 litres, had been commissioned in 1986 and customarily operated at a pressure of 5 bar.
The Inspection Authorities for classified facilities went to the site on 16/07 and noted that the operator was unaware of the regulations applicable to this type of pressure vessel. Several problems were noted:
- a lack of regulatory control and no operational file,
- repair work had been performed in 2011 without inspection by an authorised body,
- operators not authorised to operate the equipment,
- filter operating regularly at excess pressure (5.5 bar instead of 5 bar) and no safety device in place
- no metrological monitoring of the ancillary devices (pressure gauges, pressure switch),
- lack of written operating procedures, notably for the manual disassembly and assembly of the dome, performed 50 times/year on average,
- screws tightened with an impact wrench (regular force) and a filter configuration that does not allow for uniform tightening owing to reduced accessibility at the back of the dome,
- no traceability of reassembly operations,
- lack of instructions, and documents setting the limit operating conditions,
- no maintenance program,
- corrosion and wear on the threaded fasteners, which had never been greased. Since the number of screws found was less than the theoretical number, it was also assumed that all the screws were not in place at the time of the accident.
The Inspection Authorities for classified facilities required that the operator have an expert assessment conducted by a third party to determine the precise causes of the accident and to issue an opinion on whether or not the equipment can return to service. This expert evaluation confirmed that the dome opened as a result of an overpressure associated with the poor condition of the bolts resulting from the poor operating conditions mentioned above. Also, the body noted a deformation of the dome’s lower flange leading to a contact defect between the 2 flanges and substantial degradation of the nut and screw threads. It considered that the device could not be put back into service without additional inspections (flatness of the 2 flanges, non-destructive testing of the welds, etc.).
