Around 10:30 a.m., in a company specialised in the manufacture of active ingredients for the pharmaceutical industry, an explosion of solvent vapours occurred in a drain in the industrial effluent network. This explosion was immediately followed by a second explosion.

The internal intervention personnel stopped the manufacturing process and secured the site with the retention basin. The internal emergency plan was initiated. The network was flooded with water from the site’s fire prevention and extinguishing network to avoid any risk of a new explosion.

The firefighters were called but did not have to intervene. At 11:56 a.m., the situation was brought under control and the individuals present (83 employees, 21 external contractors) were able to resume their activities.

No injuries or damage were reported. Endoscopic testing of the effluent pipes did not reveal any damage.

Causal analyses

The explosions were caused by the ignition of solvent vapours (heptane) contained in the effluents circulating in the network. These effluents came from the synthetic products drying workshop. During the vacuum drying of an active ingredient containing heptane, solvent passed through the liquid ring of the vacuum pumps. Following technical problems, the liquid rings sealing these pumps had been operating for several weeks in an open circuit (wastewater operation) with water from the network. The water leaving these rings was discharged into the network involved in the explosions. Vacuum pumps normally operate in a closed circuit.

At the time of the accident, work was being carried out by an external company. A plumber installed a PVC trap on one of the outlets of the duct containing the effluents from the vacuum pumps. Following difficulty with the assembly, he took the initiative to heat the part with a blowtorch, thus deviating from the framework established in the work order. The hot spot created by the torch set off the explosion. The work permit did not identify the risks of an effluent containing a flammable solvent in significant concentration circulating in the system near the work site.

The presence of heptane in the gutter at a concentration between its LEL and UEL was explained by poor condensation of this solvent. Due to a design problem, the condensation system upstream of the vacuum pumps did not trap enough heptane from the drying product. The water circulating in the vacuum pumps was filled with heptane which, as it does not mix with water, remained on the surface and resulted in the release of vapours.

The design of the solvent condensation systems and the vacuum pumps had not been validated by a multidisciplinary working group (new work, production, maintenance, HSE, etc.). The risk associated with the decantation of heptane, after passing through the vacuum pumps, had been underestimated.

Measures taken

Immediately after the accident, the operator diverted the water from the vacuum pumps to a gutter, suitable for receiving solvents, after collecting the supernatant liquid in a container.

In addition, the facility operator:

• modified the installations in order to:
  • improve the recovery of solvents through condensation during the drying process;
  • allow the vacuum pumps to operate in a closed circuit;
• reminded the external companies that it is prohibited to deviate from the framework of the work orders;
• revised its selection criteria regarding external companies, giving priority to personnel with chemical hazard training.