Potentially explosive atmospheres: conditions and parameters
What makes a space truly “potentially explosive”? Is it simply the presence of combustible materials, or is there more to the equation? Imagine a room where pressure rises silently past a critical threshold, turning an everyday space into a potential hazard. For safety inspectors and engineers, understanding the precise conditions that classify an area as explosive—and knowing the design safeguards to mitigate these risks—can be the difference between routine operation and disaster. Dive into the nuanced factors that define hazardous zones and explore the calculations, structural requirements, and safety protocols essential for maintaining control in potentially volatile environments.
When can a room be classified as having potentially explosive atmosphere?
A primary criterion for designating a room as a potential source of explosion hazard is when pressure of over 5 kPa can build in it following an explosion.
When assessing the explosion risk in a room, safety professionals must consider the worst-case scenario regarding the impact of an explosion. This involves evaluating the most hazardous type of substance present and the maximum quantity that could participate in an explosion.
Requirements for potentially explosive atmospheres
Classifying a room as a potentially explosive atmosphere triggers additional requirements:
Paragraph 221.
1. a lightweight roof, made of at least flame-retardant materials and weighing no more than 75 kg/m² in projection, counting without roof support structure components such as beams, trusses and girders, should be used over the explosive room.
(2) Paragraph 1 shall not apply to a room in which the total area of pressure-relieving (explosion-proof) devices such as partitions, dampers and openings glazed with ordinary glass is greater than 0.065 m²/m³ room volume.
(3) Walls separating a potentially explosive room from other rooms should be able to withstand a pressure of 15 kN/m² (15 kPa).
Paragraph 222.
(1) A potentially explosive room should be situated on the top floor of the building. This requirement does not apply to buildings in enclosed areas.
(2) Other locations of the premises referred to in paragraph (1) shall be permitted, provided that appropriate explosion-proof installations and equipment are used, as agreed with the relevant Regional Commander of the State Fire Service.
3. if at least one of the buildings has a room at risk of explosion, then the distance between their external walls should not be less than 20 m.
Paragraph 237.
(1) A passageway, hereinafter referred to as an “escape passage”, of a length not exceeding. the length of the building, shall be provided in the premises from the furthest point where a person may be present to an emergency exit to an escape route or to another fire zone or to the outside of the building:
(a) in ZL fire zones – 40 m;
(b) in PM fire zones with a fire load density exceeding 500 MJ/m² in a building with more than one storey above ground – 75 m;
(c) in PM fire zones with a fire load in excess of 500 MJ/m² in a building with more than one storey above ground and in PM fire zones in a building with one storey above ground irrespective of the size of the fire load – 100 m.
(2) In a room at risk of explosion, the length of the escape passage referred to in paragraph 1(2) and (3) shall not exceed 40 m.
Paragraph 245.
Staircases designed for evacuation from the fire zone:
a) ZL II in a low-rise building (N),
b) ZL I, ZL II, ZL III or ZL V in a medium-rise (SW) building,
(c) PM with a fire load density greater than 500 MJ/m² or containing a potentially explosive room in a low (N) or medium rise (SW) building
should be enclosed and closed with smoke-proof doors and equipped with smoke prevention or smoke removal devices activated automatically by a smoke detection system.
How to calculate the explosion pressure build-up
The increase in room pressure ΔP caused by an explosion involving homogeneous flammable gases or vapours with molecules made up of carbon, hydrogen, oxygen, nitrogen and halogen atoms is determined by the equation:
Increase in room pressure ΔP, caused by an explosion involving combustible substances not listed above:
Where:
- ΔP – the increase in room pressure caused by an explosion involving combustible substances [Pa],
- mmax– maximum mass of flammable substances forming an explosive mixture that can be released in the room under consideration [kg],
- ΔPmax– maximum pressure rise at explosion of a stoichiometric gas- or vapour-air mixture in a closed chamber [Pa].
- W – coefficient of the explosion reaction course,
- V – volume of the air space of the compartment, being the difference between the volume of the compartment and the volume of the installations, equipment, closed packaging, etc. contained therein. [m³],
- ρ – density of flammable gases or vapours at flash point and normal operating conditions [kg/m³],
- Cst – stoichiometric concentration by volume of flammable gases and vapours:
- B – stoichiometric ratio of oxygen in the explosion reaction:
Where:
- nc, nH, nO, nCO2– the number of carbon, hydrogen, halogen and oxygen atoms in a gas or vapour molecule, respectively,
- qsp – heat of combustion [J/kg],
- P0– atmospheric pressure, equal to 101325 [Pa],
- ρp– density of air at temperature T [kg/m³],
- cp – specific heat of air, equal to 1.01*103 [J/kgK],
- T – air temperature [K].