Last edit: 03/08/2023
What causes an explosion is the combustion of gas. For this to occur, 3 elements are needed:
– Combustible: the gas
– Oxidizer: the air (oxygen)
– Ignition: can be of various kinds
The concentration of fuel and oxidiser influences whether combustion can take place or not. A range of fuel-combustion concentration is established within which combustion is possible. The lower and upper limits of this range are called the Lower Explosive Limit (LEL) and an Upper Explosive Limit (UEL) respectively. Below the LEL the fuel concentration in the mixture is not sufficient to allow the reaction to take place, above the UEL the fuel concentration is very high at the expense of a reduced oxidiser concentration, in which case too the reaction cannot take place.
Depending on the volumetric concentration of the air, the necessary energy that the ignition must supply in order to have combustion varies, we speak in this case of Ignition Energy and it depends on the fuel considered. Below is a graph showing the ignition energy required for propane and hydrogen as the air concentration varies.
Autoignition temperature and flash point
Two temperatures are defined which characterise flammable substances:
– Flammability temperature or Flash Point: this is the lowest temperature at which vapours are formed in such quantities that in the presence of oxygen (air) and an ignition source they give rise to the phenomenon of combustion (the gas becomes flammable and can therefore explode).
– Autoignition temperature: the lowest temperature at which the substance begins to burn spontaneously in the presence of oxygen, without an external source of ignition.
Every substance has its own flash point and self-ignition temperature. On the basis of the flammability temperature, the various flammable liquids are classified:
– Category A (Ti<21°C): petrol, alcohols; these are the most dangerous products as they are extremely flammable even at room temperature and must be kept away from possible ignitions;
– Category B (21 °C ≤ Ti ≤ 65 °C): diesel fuel;
– Category C (Ti > 65 °C): glycerine, bitumen.
Category A and B flammable liquids are in turn divided into (Classification of flammable substances according to Directive 67/548/EEC):
– R10 Flammable: Liquid substances and preparations having a flash point t > 21°C and < 55°C.
– R11 highly flammable: Liquid substances and preparations having a flash point t > 0 °C and < 21 °C.
– R12 extremely flammable: Liquid substances and preparations having a flash point T < 0 °C.
The Auto-ignition temperature determines the Temperature Class to which a Gas belongs. Table 5.5-A shows the Classes associated with the auto-ignition temperature values.
The temperature class
The Auto-ignition temperature determines the Temperature Class to which a Gas belongs. Table 5.5-A shows the Classes associated with the auto-ignition temperature values.
Gas group
Based on their hazardousness, related to the places where they are typically present or the minimum ignition energy, the gases are also divided into groups:
– Group I: Grisou – Mine gas (~80% Methane). M.I.E.=0.28mJ
– Group IIA: Propane, Gasoline, Methanol. M.I.E.=0.16mJ
– Group IIB: Ethylene. M.I.E.=0.06mJ
– Group IIC: Hydrogen, Acetylene. M.I.E.=0.02mJ
The gas is then identified by Temperature Class and Group. This information is shown on the CE marking of products used in a particular ATEX Zone.