The basic physic principles 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, vapours, mists or dusts. Explosion can only occur, when three factors come together:

1. Flammable material (in ignitable quantities)
2. Oxygen (in the air)
3. Ignition source

An explosion can only occur, when these three factors come together

Certain characteristic properties of these materials are required for safety considerations. The flash point of a flammable liquid is the minimum temperature at which a liquid gives off vapour in sufficient concentration to form an ignitable mixture with air near the surface of the liquid (at normal air pressure). If the flash point of a flammable liquid is well above the maximum temperatures that arise, an explosive atmosphere can not 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 liquids as highly flammable, easily flammable, and flammable liquids in the Council Directive 98/24/EC “risks related to chemical agents”.

Classification of flammable liquids:

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

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 Vapours:

Extract 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 – (6 th addendum)

Depending on the speed of combustion, we speak 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 litres 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
• equalizing currents, cathodic corrosion protection
• static electricity
• lightning
• electromagnetic waves (high-frequency)
• optical radiation
• ionising radiation
• ultrasonics
• adiabatic compression and shock waves
• exothermal reactions

Preventing explosive atmospheres (Primary Explosion Protection)

The term primary explosion protection refers to all precautions, which prevent a hazardous explosive atmosphere from being created.

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 cannot be completely or only partly avoided by measures of preventing the formation of an hazardous explosive atmosphere, 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 zones, according to the probability of an explosive atmosphere being formed.

In the USA and 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. In addition, it is also necessary to prove that these requirements have been met.

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 following explosion protection measures in a certain sequence.

Basic principles of explosion protection