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Fast and preliminary method for identifying the portions of a plant that must undergo a quantitative explosion risk assessment (Part 1/2)

By: Baldassare Genova (P.E.) – National Fire Brigade – Ministry of the Interior (Rome) Italy

The occurrence of explosions in industrial plants handling hazardous substances, especially those with a major accident hazard, can pose a serious risk to the surrounding population as well as to assets, infrastructure and the environment.

For this reason, the “so-called” Seveso III Directive (Directive 2012/18/EU), requires that a risk assessment, not limited to the explosion risk, be carried out for these plants.

In this article, a fast and preliminary method is proposed for identifying the portions of a plant that must undergo a quantitative risk assessment (QRA); the method is especially addressed to plants in which explosive materials are present.

This first article will illustrate the main principles of the method to select plant units that should undergo a quantitative assessment with an increased level of detail. Part 2 will deal with explosion risk assessment quantified in terms of probability of occurrence and associated severity.

It is outmost important to highlight that the decision process aimed to the identification of critical areas requires attention considering that authorities having jurisdiction should verify the demonstrations given by the plant owners in safety reports/safety cases of the installations, in many cases being a very large industrial plant with multiple units with a certain degree of complexity. Proposed approach may grant benefits to both the end-users and to authorities. For the first as a tool to evaluate their entire plant with a graded and staged approach, for the latter as a tool to verify the approach that should be preliminarily focused to those area presenting a significant fire and explosion risk level.

Chemical plants are complex environments where potential hazardous material and processes are often

present. Work-owners must consider the risks related to substances and process conditions and individuate potential accident scenario and consequences as to grant working activities are performed in safe and secure ambiances to safeguard operators’ life and environment.

The principal methodologies to be compliant with the Seveso III directive are 30 years old (the first dated back to 1980) and updated in the early ’90s. Their development derives from the need for insurance companies to evaluate adequate insurance fees quickly. The most used are the Dow F&EI [1] and the Mond Index methods [2], both thought to be explicitly applied to the oil & gas industry. While for explosive substances plants at the moment the most current approaches are those based on consequence assessment with specific correlations or calculation tools, dealing mainly with the severity of the outcomes rather than on the risk as a factor composed by probability and magnitude that may be different considering all the units of such large installations.

The method draws its origins also on the previous work by [3].

The methodological approach

The proposed method consists of a succession of distinct phases, the exemplification of which is shown in the diagram reported in Figure 1. This method could find his opportunity also in initial inherent safety studies during incipient design stages of new installations or existing premises modifications.

The following paragraphs describe the steps that make up the method.


Divide the plant into separate installations

The first step of the method is to divide the plant into a number of separate (or independent) units. This is a complex process and can be the subject of discussion and shared choices among experts. Please note that the criteria below can only be considered a guideline for the definition itself. An important criterion that helps define what constitutes an independent installation is one that takes into account the loss of containment that affects it, which, if it occurs, must not result in the release of significant quantities of the substance from other installations.

Consequently, two installations are considered separate or independent if they can be isolated from each other within a very short time following an accident.

Calculate A for each installation

The intrinsic hazard of an installation depends on the substance hold-up, the physical and toxic properties of the substance and the typical process conditions. The indicative number “A” is calculated as a measure of the intrinsic hazard of an installation. It is a dimensionless number defined, in general, as [4]:




Q is the quantity of substance present in the installation [kg]

Oi is the factors for the process conditions

G is the limit value [kg]


Figure 1. Representation of phases

Calculation of Q (hold-up)

The hold-up present in an installation is the amount contained within it, in which possible developments in the process of desired or undesired events, including possible loss of control, must be considered. The following rules apply:

  • Preparations and mixtures can be distinguished into two different types, namely (1) a dangerous substance in a non-hazardous solvent, and (2) a mixture of dangerous substances;
    • if a hazardous substance is dissolved in a non-hazardous substance, only the amount of the hazardous substance needs be considered
    • if a mixture of several hazardous substances has defined chemical-physical and toxic properties, it should be treated in the same way as if it were a pure substance;
  • if hazardous substances are stored as small packaging units at a site, and containment losses are likely to occur simultaneously for a large number of packaging units, the total amount of substance stored at that site should be considered. Examples are the storage of explosives or fireworks, and the release of toxic combustion products during a fire.

The process conditions factor, Oi

The factors for process conditions apply only to toxic and flammable substances and reference should be made to [1]. For explosives, the different factors are unified and become O1=O2=O3=1.

Limit Value, G 

The limit value, G, is a measure of the hazardousness of the substance based on both the physical and the toxic/flammable/explosive properties of the substance. For explosive substances, the limit value is the quantity (in kg) that releases an energy equivalent to 1000 kg of TNT (the explosion energy of TNT is assumed to be 4600 kJ/kg).

Calculation of the indicative number Ai

The indicative number Ai of an installation for a substance i is calculated as:



Qi the quantity of explosive present in an installation [kg]

Gi the limit value of substance i [kg]

Various substances and process conditions may be present in an installation. In this case an indicative number, Ai,p is calculated for each substance i, and for each process condition, p. The indicative number, A for an installation is calculated as the sum of all indicative numbers:

Define calculation points

The calculation points are defined as the target sites along the plant boundary that must be checked against the explosion risk.

The distance between two adjacent sites must not be greater than 50 m.

Calculate S for each installation and each calculation point

The selection number, S, gives a measure of the danger posed by an installation in relation to a specific site and is calculated by multiplying the installation's own target number A by a factor typical for explosive and flammable substances of (100/L)3. Therefore, the selection number is given by:


Where L is the distance between the installation and the specific site, measured in [m], assuming a minimum value of 100 m. The selection number must be calculated for each installation at a minimum of 8 sites on the plant boundary. The selection number must be calculated for the entire boundary line of the plant, even if the plant adjoins a similar plant.

If the plant adjoins a water surface, the selection number must be calculated on the bank opposite the plant.

In addition to the calculation on the plant boundary, the selection number S must be calculated for each installation on the site in the nearest existing or planned residential area to the plant.

Select installations for detailed QRA

An installation is selected for QRA analysis if:

  • the calculated installation selection number (S) is larger than 1 (one) at a site on the boundary of the installation (or on the shore of the body of water opposite the installation) and larger than 50% of the maximum selection number at that site or
  • the calculated installation selection number is larger than one at a site in the nearest existing or planned residential area to the installation.


The fast and preliminary method proposed in the present article allows to identify the portions of a plant that must undergo a quantitative risk assessment (QRA) in a major accident hazard context. The proposed workflow is linear and the requested parameters can be easily calculated.

The workflow can be selected as a basis for the development of a risk indexing method, which could be

applied to all industrial realities, not only limited to large explosive employing plants, similarly to the methods already employed for fire initial risk assessment and risk screening (such as those derived from the Dow & Mond index method for industries operating with flammable substances).


  1. Dow Chemical Company, Fire & explosion index DOW (F&EI), 6Th Edition, AIChE, 1987
  2. Imperial Chemical Industries (ICI), The Mond Index, Second Edition. 1985
  3. Genova, M. Silvestrini, Dinamica delle reazioni esplosive, Hoepli, 2005
  4. TNO (2005) Purple book