Case study –  “Explosion risk document” in a chemical plant (ATEX)

The preparation of the “Explosion Risk Document” is a legal requirement (Article 294 of Legislative Decree no. 81/2008) to which the employer must comply in case he acknowledges that explosives atmosphere (ATEX) may be present in the workplace. These potentially explosive atmospheres may be generated by flammable chemicals in the air, under atmospheric conditions, in the form of gases, vapors, mists or dusts. In the chemical plant in question we could find flammable solvents (acetone, toluene, methanol, isopropanol) and dust (carbon black) used in production processes (production of paints), flammable gases such as hydrogen (which can be generated in the process of charging battery forklift trucks) and methane (used for heating): all these flammable substances can generate explosive atmosphere, so it is necessary to carry out:

  • the classification of areas where explosive atmospheres can develop
  • the assessment of the explosion risk

Many employers often believe that a simple classification of dangerous areas, indicating the distribution of the work zones in areas where explosive atmospheres may form (zones 0/20, 1/21, 2/22) is sufficient to fulfill what is required by law. In fact, the Legislative Decree no. 81/2008 specifies that the employer has the obligation to “…. assess the specific risks arising from explosive atmospheres, taking account at least the following elements (art. 290):

  • …probability and duration of the presence of explosive atmospheres;
  • probability ‘that ignition sources, including electrostatic discharges are present and could become active and effective;
  • characteristics of installations, systems, substances used, processes, and their possible interactions;
  • extent of foresseable effects … “.

This assessment is hence the result of a real risk analysis in which we needed to assess the likelihood and the extent of damage, according to the classic definition of risk Risk = Probability x Damage. In order to carry out the ATEX risk assessment, we proceeded with a preliminary study of the processes and stages of work: this study consisted in the execution of  inspections at the plant with a local representative (Site HSE Head), asking questions in order to identify

  • areas where ATEXs may form,
  • possible sources of ignition,
  • measures of prevention and protection measures already adopted,
  • presence of specific procedures,
  • training of employees,
  • warnign signals…

In the case of this chemical prodution site, it was already present a classification of the areas. It had been carefully studied in order to verify the completeness and if it had been recently updated. The ATEX classification of the areas provides an indication of the likelihood of the presence of a potentially explosive atmosphere in a given area: the next step is to determine the probability of ignition of these ATEXs and the probability of the presence of workers (in this context, an explosion risk is defined as a risk that can cause harm to people, so it is important to evaluate the presence in areas where an explosion can occur). The possible sources of ignition may be:

  • cigarettes
  • hot surfaces
  • welding
  • exothermic reactions
  • adiabatic compression
  • heat generated by friction
  • impact of falling objects
  • electrical installations (shock, heat produced by resistance, short circuit, fuses, switches)
  • electrostatic discharges

We paid attention to the adequacy of equipment and of the facilities: everything that was purchased after 30 June 2003 have to include the specific ATEX markings (the “category” of protection of individual atex equipment must correspond to the specific ATEX area classification). As regards the equipment purchased before 30 June 2003 the employer should “… ensure that the work equipment with their connecting devices, as well as the structure of the workplace are designed, constructed, assembled, installed, held in efficiency and used in such a way as to minimize the risk of explosion for workers .. ” (Appendix Leg.Decr. 81/2008as amended). So we need to evaluate the probability of creation of sources of ignition. This evaluation is performed considering the area in which ATEX equipment is installed, the degree of protection that it must ensure and therefore the necessary preventive measures to avoid that ignition sources are present during normal operation, possibly created by malfunctions.

In the present case, the risk assessment was conducted considering, through the technique of Fault Tree (fault tree), the combination of primary events that lead to achieving the unwanted event, namely the ATEX explosion. Determining the likelihood of such primary events and expected damage, it is possible to calculate the risk of explosion and evaluate it, according to a suitable criterion, to determine whether it is acceptable or not and if was necessary taking the appropriate measures of prevention and protection to reduce it.