Because of the volume of flammable and explosive substances typically present in refineries,
scenarios tend to include fires and explosions with potentially high consequences if not adequately
controlled. In particular, production of hydrocarbon products leads to a high presence of flammable
compounds onsite. Not surprisingly, therefore, nearly 80% of the events studied involved a fire or
explosion (see Figure 4 below). In addition, a significant amount of toxic substances may be present
such that refineries are also exposed to the risk of potential toxic releases. Many crude oils contain a
significant percentage of hydrogen sulphide that is eventually separated from the crude and usually
processed to produce sulphur for the marketplace. Other processes require the presence of sulphuric
acid or hydrofluoric acid (for alkylation) or ammonia (to remove nitrogen from the crude feedstock).
In fact, over one third of the refinery accident events involving corrosion have also generated toxic
releases. Toxic releases to the soil were slightly higher in relation to toxic releases to water and air,
probably resulting from a number of accidents stemming from tank and underground pipe failures
included in the database.
Releases were most often hydrogen and hydrocarbon compounds including process gases, naphtha,
crude oil and various types of fuels. (See Figure 5 on the next page). The largest release was
estimated to be around 100,000 tonnes of crude oil followed by 50,000 tonnes of fuel. Hydrogen
sulphide was the toxic gas released more often than any other (16 cases). Fewer than 10% of
accidents involved releases of other toxic gases such as hydrogen fluoride, carbon monoxide and
sulphur dioxide. The highest (known) release of a substance toxic to human health was 15 tonnes of
furfural, followed by 1 tonne of sulphur dioxide.