Ex Protection

Explosive atmosphere

Explosion Protection

An explosive atmosphere is built up by a mixture of flammable gases, vapours, mists or dust with atmosphere air. If the mixture (flammable material/air) has the right ratio it can be an ignition source and create an explosion.

Factors for creating an explosion:

  • Air (or in fact oxygen in the air)
  • Flammable material (substance)
  • Ignition source

There are two main principles to avoid an explosion; primary and secondary precautions. Primary explosion protection can be achieved by (when an ignition source are present):

  • Using natural or forced ventilation to llimit the explosive concentration
  • Avoiding flammable materials (substance)
  • Using inert gas in the atmospphere (e.g. Nitrogen)

If still an explosive atmosphere can be created in a area it is only one way of avoiding axplosion, Ignition Control.

Type of Ignition Sources:

  • Hot surfaces
  • Flames and hot gases
  • Mechanically produced sparks
  • Electrical equipment
  • Transient currents
  • Static electricity
  • Lightning strikes
  • Electromagnetic waves
  • Optical radiation
  • Ultrasound
  • Chemical reactions
  • People (indirectly)

The Techniques of equipment protection for use in explosive atmospheres are just a matter of controlling (eliminating) possible ignition sources (secondary explosive protection).

Where do we find explosive atmospheres?

  • Metal surface grinding, especially aluminium dust and particles
  • Oil refineries, rigs and processing plants
  • Gas pipelines and distribution centres
  • Printing industries, paper and textiles
  • Aircraft refuelling and hangars
  • Chemical prosessing plants
  • Grain handling and storage
  • Sewage treatment plants
  • Surface coating industries
  • Underground coalmines
  • Woodworking areas
  • Sugar refineries
  • Vessels/ships
  • Power plants

Where a potential explosive atmosphere can occur, certain safety levels need to be taken into account regarding the possible danger of an explosion in this area. the areas therefore need to be devided into zones according to presence of the flammable materials.

Zone 0 Zone 1 Zone 2
A place where the explosive atmosphere of flammable gas is present continously or for a period of frequently A place where the explosive atmosphere of flammable gas is likely to occur in normal operation occasionally A place where the explosive atmosphere of flammable gas is not likely to occur in normal operation, but if it does occur it will only be present for a short period
Zone 20 Zone 21 Zone 22
A place where an explosive dust atmosphere is present continuosly or for a long period of time A place where an explosive dust atmosphere is likely to occur during normal operation A place where an explosive dust atmosphere is not likely to occur during normal operation, but if it does occur it will be present for a short period

Examples of the criteria for the mixture of flammable materials (gas) towards air in such a way that an explosion can occur are:

Mixture

Typical concentration of gases in the air where an explosion can appear
(% of gas in air):

  LEL Explosion UEL
Methane 4,4% 16,5%
Propane 1,7% 10,6%
Butane 1,4% 9,3%

Explosive Atmosphere

Atex Directive

Ignition Sources

Basic Electrical Techniques

Equipment Protection Level

IP-Rating