In each area in which there is a process that causes a raising of temperature of flammable substances and their evaporation, such as hydrocarbons, can produce a leak of gas or of vapor which, in contact with flames or sparks caused by electrical processes, may explode. This event can happen both during normal operations and in case of breakdown or accident. Talking about these events, it is usually to cite a refinery as an example because it is, perhaps, the place where an explosion can easier happen. But the same conditions can occur in other chemical plants, or even in simple and seemingly innocuous places, such as, a painting room or a room where charge forklift batteries. Therefore, within any facility or establishment where can be, even for limited periods of time, the presence of flammable gases or vapors in percentages that could cause an explosion, it is important to determine the hazardous areas and design electrical systems in order to eliminate the possible causes of an explosion.

The identification of hazardous areas

The identification of hazardous area in a chemical or petrochemical plant must be done by highly qualified personnel as process engineers. The most hazardous situations are where combustible gases or dusts may be present during normal operation or due to a fault. The classification depends on the type of substances present inside the plant and on how long and in which percentages they are there. Last January, the revision of CEI EN 60079-10-1 standard (CEI 31-87 - January 2010) was published including the classification of areas where the explosion hazard is due to the presence of gas, vapors or mists. At the same time, the CEI EN 60079-10-2 standard (CEI 31-88 - January 2010) has been published also for the classification of hazardous areas for the presence of combustible dusts. This rule, which replaces the previous CEI EN 60079-1 (CEI 31-30) standard, is based on analytical assessments that consider principles such as the real ventilation of the environment, the concentration of potentially explosive mixtures, the timing of mixtures permanence calculated in relation to the Lower Explosive Limit (LEL).

At the end of this analysis, every dangerous place has to be classified into one of following three zones considering: 

Every other area of the plant is considered a SAFE AREA. 

The various zones are defined according to the following probability values:

The classification of dangerous areas does not differ substantially from the previous standard but introduces some new concepts, such as the use of equipment with the highest protection level (EPL).

The rule considers several parameters to determine if there is a real danger of explosion due to the presence of gas, vapor or mist. First of all, must be defined the quantity of hazardous substances and the existence of a source of emission.

Then, it’s necessary to follow the procedure for the classification of hazardous areas suggested by the guide:

1. Identify the substances and their relevant data, such as the Lower Explosive Limit of the mixture (LEL).

2. Identify the emission sources, checking first of all the possibility to eliminate or reduce them as much as possible.

3. For each source is necessary to determine the degree of emission (continuous, first, second).

4. For each place must be defined the temperature values and the ventilation characteristics witch may be high, medium or low.

5. At the end, must be defined the type of zone using the following table and calculate the hazardous area extension. 

The extension of the dangerous zone depends also on the mode of issue, in particular:

• State of emission (one-phase gas or vapor, liquid or liquefied gas, etc.)
• Velocity of emission.

The gases and vapors tend to spread in the air occupying the entire space.

Therefore, gases spread into the air both for their weight and the ventilation. The changes introduced by the new standard brought the CEI SC 31J to decide on the revision of CEI 31-35 Guide, which is on working.