Analysis of the effect of explosions in unconfined environments

Industrial risk is defined as an accidental technological event that can have serious consequences for people, property and the natural environment inside and outside the site. Industrial risk is considered major when the criteria of low frequency and high severity are combined. This means that risk analysis can be carried out using both deterministic and probabilistic approaches, based on physical, functional and statistical modeling. This implies that the scenarios that could lead to an accident are established in advance during the design phase. Deterministic analysis consists mainly in assessing the development, propagation and consequences of each event selected, in order to confirm that the acceptance criteria for these consequences are met. Probabilistic analysis aims to measure the occurrence and probability of the hazardous phenomenon or accident.

This study focuses on the deterministic analysis of the consequences of explosions, with particular emphasis on overpressure effects. Thermal effects are not the subject of this article.

Accidental explosions are associated with energy releases that produce large quantities of expanding gas. Indeed, most past incidents involving gas cloud explosions point to the presence of a fuel leak as a potential cause. If a quantity of fuel is accidentally released into the atmosphere and mixes with the air, a cloud of flammable gas may result. If this cloud encounters a source of ignition, it will under certain conditions develop an intense explosion and blast. However, the initial gas may be in the form of compressed gas when stored, resulting in a loss of containment. In all cases, the rapid expansion of the gas leads to a blast or pressure wave, which can have far-reaching consequences for the environment. The most important property of an explosion is the blast. The mechanical energy of the explosion generates a blast wave that travels at a certain speed through the surrounding air. Overpressure is the result of two contradictory phenomena: the intensification of pressure due to combustion and the reduction of pressure due to gas expansion.

The shape of the blast wave depends on the type of explosion. Before the arrival of the blast wave, the pressure is at ambient pressure P ₀ . For an ideal blast wave, the overpressure increases instantaneously to a value P ₀ + ΔP and then decreases more slowly to negative values, reaches a minimum and finally returns to ambient pressure. This type of blast wave, known as a shock wave, corresponds to a detonation. In the case of a deflagration, the pressure rise is less rapid and the maximum pressure lower than the detonation pressure. However, the blast wave has a positive...