Physical fundamentals and definitions

An explosion is the sudden chemical reaction of a flammable substance with oxygen with the simultaneous release of high energy. Flammable substances may be present in the form of gases, vapors, mists or dusts. An explosion can only take place if three factors come together:

  • Flammable material (in ignitable quantities)
  • Oxygen (in the air)
  • Ignition source
An explosion can only take place if these three factors come together: An explosion can only take place if these three factors come together:

Certain characteristic properties of these materials are to be observed for safety considerations. The flash point of a flammable liquid is the minimum temperature at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid (at normal atmospheric air pressure). If the flash point of a flammable liquid is well above the maximum temperatures that arise, an explosive atmosphere cannot be formed. The flash point of a mixture of various liquids may be lower than that of the individual components. In addition to the boiling point, the flash point of a liquid serves to classify a liquid as highly flammable, easily flammable, and flammable liquids in the Gefahrstoffverordnung (Ordinance on Hazardous Substances) and Betriebssicherheitsverordnung (Ordinance on Industrial Safety and Health).

Classification of flammable liquids:

Designation of the flammable liquidAt flash point and boiling point °C
Highly flammableFlash point < 0 °C and boiling point < 35°C
Easily flammableFlash point < 0 °C and boiling point > 35°
or 0 °C < flash point < 21°C
Flammable21 °C < flash point < 55°C

To form an explosive atmosphere, the flammable substance must be present in a certain concentration.

Explosion limits Explosion limits

If the concentration is too high (rich mixture) or too low (lean mixture), no explosion occurs. Instead, there is just a steady-state combustion reaction or none at all. It is only in the range between the lower and upper explosion limit that the mixture reacts explosively when ignited. The explosion limits depend on the ambient pressure and the proportion of oxygen in the air.

Explosion limits of selected gases and vapors:

Substance designationLower explosion limit [Vol. %]Upper explosion limit [Vol. %]
Acetylene2.3100 (self-decomposing!)
Ethylene2.432.6
Gasoline~ 0.6~ 8
Benzol1.28
Heating oil/diesel~ 0.6~ 6.5
Methane4.417
Propane1.710.8
Carbon disulfide0.660.0
Hydrogen4.077.0
Excerpt from the table "Sicherheitstechnische Kenngrößen, Band 1: Brennbare Flüssigkeiten und Gase" (Safety characteristics, vol. 1: flammable liquids and gases) by E. Brandes and W. Möller as well as by T. Redeker and G. Schön – (6th addendum)

Depending on the speed of combustion, one speaks of deflagration, explosion or detonation. An atmosphere is described as hazardous or explosive if there is danger to human life or property by an explosion. An explosive atmosphere of even just a few liters can be dangerous in an enclosed space.

Ignition source

Ignition of an explosive atmosphere can be caused by various sources:

  • Hot surfaces
  • Flames and hot gases
  • Mechanically generated sparks
  • Electrical installations
  • Electrical equalizing currents, cathodic corrosion protection
  • Static electricity
  • Lightning
  • Electromagnetic waves (high-frequency)
  • Optical radiation
  • Ionizing radiation
  • Ultrasonics
  • Adiabatic compression and shock waves
  • Exothermal reactions

Preventing explosive atmospheres (Primary Explosion Protection)

The term primary explosion protection refers to all precautions that prevent a hazardous explosive atmosphere from forming.

This can be achieved by:

  • avoiding flammable substances (replacement technologies)
  • inerting (addition of nitrogen, carbon dioxide etc.)
  • limitation of the concentration by means of natural or technical ventilation

Avoiding ignition of explosive atmospheres

If the danger of explosion is not excluded or excluded only partially by measures for preventing the formation of an hazardous explosive atmospheres, then measures must be taken that avoid the ignition of the explosive atmosphere.

The required safety level of these measures depends on the possible danger potential in the installation location. The hazardous areas are therefore divided into danger zones according to the probability of an explosive atmosphere being formed.

In the USA and some other countries, hazardous locations are classified into Classes and Divisions. For locations classified in this way, requirements must be met concerning the apparatus which are approved for use in these locations. Also defined is how adherence to these requirements is to be verified.

Mitigation of the explosion effects (Constructive Explosion Protection)

If hazardous explosive atmospheres cannot be safely avoided and their ignition cannot be excluded, then measures must be taken which limit the effect of explosions to a safe degree, e.g. by means of:

  • Explosion pressure resistant construction
  • Explosion relief devices
  • Explosion suppression by means of extinguishers

The principle of integrated explosion protection requires that the explosion protection measures be taken in a specific sequence.

Explosion protection measures Explosion protection measures