Explosion in a waste tank of a pharmaceutical plant

An explosion occurred on a fixed-roof tank at a pharmaceutical plant. The tank belonged to a waste treatment unit as part of an antibiotic production process. A contractor was conducting hot work (with a disc saw) at a pipe located near the roof of the tank. The aim was to add a second feed line to the tank, allowing the delivery of additional waste. The existing feed
line had been cleared up and disconnected from the tank, without closing it by a blind flange.
This hot work ignited the flammable gases present inside the tank. The explosion blasted the roof of the tank 20 m away and caused one fatality and three sever injuries.
Fire fighters arrived on the scene 15 min after the explosion and extinguished the resulting fire.

Under normal operations, the tank collected a liquid fermentation residue composed of water and micro-organisms assumed dead. To homogenise the effluents, the initial design included an air injection circuit at the tank bottom and a suction circuit at the level of its vent. This air circulation system was shut down 10 years prior to the accident, because considered redundant: a mixing baffle had already been installed at the tank bottom – but this resulted in the formation of an anaerobic atmosphere.
The post-accident analysis discovered that this anaerobic fermentation of both the liquid phase and a solid deposit was responsible for generating flammable gases (mainly hydrogen and methane).

INITIATING CAUSE: the anaerobic fermentation of both the liquid phase and a solid deposit in the thank was responsible for generating explosive gases (especially hydrogen and methane).

IGNITION SOURCE: sparks generated while the subcontractor was working adjacent to the air duct triggered the ignition and the explosion of the flammable mixture. The duct had been temporarily disconnected but not sealed.

ROOT CAUSE According to the initial design, the tank was equipped with an air circulation system aiming at ensuring liquid homogeneity. The design had been changed after years of operation. The air circulation system had been replaced by a mechanical mixer, but the resulting additional hazards related to the formation of flammable gases by anaerobic reactions had not been recognised.

Several deficiencies were observed regarding works organisation, namely: lack of precision in written procedures governing hot works and poor understanding of procedures among employees and subcontractors, plus inadequate subcontractor expertise: they did not plugged the air duct.

waste treatment tank, antibiotic production

The subcontractor performing cutting works was killed by the explosion. Three other subcontractors and a plant employee were severely burnt and injured by the shock wave ( 40, 166, 198 and 120 days of sick leave).

According to a preliminary evaluation, the accident generated a € 2.6 million loss:
• € 1.8 million of equipment, structure and production losses;
• € 0.8 millions for response and restoration of the establishment.

A detailed investigation was carried out by the judicial authority, supported by the Regional Environmental Agency and technical experts.
The recorded deficiencies led the Regional Environmental Agency to order closure of a portion of the plant for 2 weeks. In July 2012, upon completion of the judicial investigation, charges were brought against 15 plant personnel, including the foreman and HSE (Health, Safety & Environment) Manager.

(from the Seveso II Directive points of view):

IDENTIFICATION OF POSSIBLE ACCIDENTAL EVENTS, SAFETY ANALYSIS AND RESIDUAL RISK:
The risk assessment did not identify all ATEX areas inside the establishment, as expected from the operator. The classification, size and location of a particular zone depend on the probability of an explosive atmosphere to appear and its persistence if so. The classification needs to take into account not only the present hazardous substances, but also the possible unwanted formation of other dangerous substances.

PERSONNEL TRAINING:
The work permit procedure was not adequately applied during the maintenance operation and work permits forms were not correctly filled by both contractor and operator. In particular, the safety measures required for ‘hot’ maintenance operations, such as the closing of openings, were not adopted.

OPERATIONAL CONTROL AND MAINTENANCE PROCEDURES:
the written procedure was unclear and not easily understandable by the staff, particularly as concerns the delivery of authorization to the contractor by the Direction/SMS responsible, as noted above.