Classification of hazardous areas due to the presence of gas

In this newsletter, we’ll talk about the classification of hazardous areas due to the presence of gases, vapors and mists. In the next...

In this newsletter, we’ll talk about the classification of hazardous areas due to the presence of gases, vapors and mists. In the next newsletter, we will discuss the effectiveness of ventilation in areas with danger of explosion and the comparison between the provisions of International, European and North America standards.


Around the world are followed the recommendations of IEC 60079-10-1 standard, now acknowledge and become the European standard EN 60079-10-1. This standard is relevant to the substances classes of gas, vapors and mists. The EN 60079-10-2 concerns the classification of hazardous areas for the presence of combustible dust. EN 60079-10-1 standard applies to all places in which substances in form of vapor or gas are present and can cause explosive mixtures along with air. Mainly, we’re referring to chemical or petrochemical plants, gas storage, natural gas decompression station, spray booths, fuel store and all those environments, which are the most hazardous environments, where there is the presence of substances that may create an explosive mixture in the form of gas, vapors or mists. In order to evaluate the danger and, therefore, the area classification, this standard is based on analytical assessments that consider some principles such as the real ventilation of the area, the concentrations of potentially explosive mixtures, the mixtures’ residence times calculated in relation to the LEL (Lower Explosive Limit) and the ventilation of the place. At the end of this analysis every dangerous place must be classified in one of the following three zones based on the frequency of the formation and retention of an explosive atmosphere:

Every other area of the plant is considered SAFE AREA.

The probabilistic values by which define the various areas are the following:


In order to clarify the concepts of the first standard EN 60079-10, issued in 1996, and give to technicians an analytical method for the area classification, the Guide IEC 31 -35 was issued in Italy in 1999, prepared by the SC 31J of the Italian Electrotechnical Committee. This guide, which over the years has been amended on the basis of changes applied to the standard, is currently being implemented at European level.


The guide considers several parameters in order to determine if, indeed, there is a danger of explosion due to the presence of gas, vapor or mist. First of all, it’s necessary to define the amount of hazardous substances in sufficient amount and the existence of a source of emission. Subsequently, it’s necessary to follow the procedure proposed by the guide 31-35 for the classification of hazardous areas: 

A. identify the substances;

B. identify the emission sources;

C. determine the emission degree for each source;

D. identify the ambient temperature and the the ventilation efficacy;

E. fix the type of zone.


First of all, it’s required to identify what are the substances in the environment to be classified and their features. For this purpose, the Guide has an Appendix GA in which there is a table outlining all flammable or combustible substances and their significant characteristics, the formulas relevant to the Lower Explosive Limit of the mixture (LEL), the density of gas and the formula to convert the LEL% vol. in LEL in kg/m3.


Once you have identified the substances, it’s necessary to identify the emission sources, verifying the opportunity to eliminate or reduce them as much as possible. The emission sources are the key (hole) or areas (opening) from which the inflammable substance in the form of gas or vapor are introduced into the environment. Emission sources are generally those parts of plant such as tanks, pipes, vessels, valves, joints, inspection hatches from which flammable gases or vapors can exit during normal operation or as a result of foreseeable faults, wear or malfunction. Are not to be considered emission sources those parts that, while containing flammable substances, are structured so that they can not emit them into the environment in absence of catastrophic events, such as a fully welded pipe or an airtight container. In absence of emission sources, the place can not be classified as hazardous area unless it’s adjacent to another site not properly closed. Based on the above, the emission sources are divided into three main categories:

  • structural Emissions;
  • emissions due to failure;
  • system components not considered sources of emission. 

Structural Emissions

Structural emissions may occur during normal operation of the system, from the components discontinuity points of the containment system of hazardous substances. These parties may be:

  • connecting flanges of the piping;
  • joints between equipment parts and machinery components;
  • breathers of safety valves.

The calculation of this type of emissions is very difficult, because the losses are negligible in the case of a new components or equipment that have undergone recent maintenance, but can become significant over time due to the usage conditions and external environmental influences. GB Appendix shows in some tables the statistics data of structural emissions and losses in safety valves. The formulas in the same appendix allows the calculation of the substances volume, their time of persistence and the extension of hazardous area relevant to each issue, thus allowing the classification of each area.

Emissions due to failure

The sources of emissions due to failure are:

  • flanges;
  • valves;
  • centrifugal pumps;
  • centrifugal or alternative compressors;
  • small connections.

The GB Appendix of the Guide lists the holes sections that may occur in the emission sources indicated. These are the classic sources of emission of second degree. The evaluation of the holes size that may occur in case of failure is very difficult and depends on the materials used for seals and the accuracy and frequency of maintenance operations.

System components not considered sources of emission

According to the guide, in a facility all those parts which may emit hazardous only due to catastrophic events, unforeseen at the design stage, are not considered emission sources. Are not considered sources of emission:

  • containers and welded pipes;
  • the connection of containers or pipes by means of sealed connection devices;
  • containers of substances with closed lids that do not generate emissions and that meet certain safety conditions;
  • double seals applied to rotating shafts fitted with devices which exclude significant losses in case of malfunction.


Once identified, the sources of emissions should be classified, and may be done in three steps:  Continuous degree: when they emit continuously or for long periods flammable substances in the atmosphere. These emission degrees are for example internal parts of a processing machine, a tank, a pipe or the outside part of an open tank containing flammable liquid or a breather pipe.

First degree: when they issue regularly or occasionally explosive substances in the atmosphere during the normal operation. For example, a source that emits a flammable substance in large quantities for less than 20 minutes every 24 hours can be considered of first degree. For longer period, the source should be classified as continuous.

Second degree: when there are no emissions during normal operation but may occur infrequently and for short periods (e.g.: less than 5 minutes every 24 hours). These emissions are typically caused by unwanted events such as failures, opening of safety valves, worn gaskets, etc..

For now, we stop here. In the next newsletter we will continue to speak about the areas classification taking into account the ventilation degree.