10
Safety
Focus on ATEX
What is Potentially Explosive Atmosphere Certification and why you may need it!
Paul Lazor, Sales Director, Extronics
www.extronics.com
The Explosive Atmospheres Directive (ATEX 137) is a European Union Directive which requires employers to protect workers from the risk of explosive atmospheres. The ATEX Directive was ratified and its implementation mandatory from the 1st of July 2003 and was subject to various stages of implementation during the following three years and finally came into force for all workplaces on 30 June 2006.
Despite the fact that the ATEX Directive has been in force for many years it is evident that there are many organisations that do not fully understand the Directive requirements!
ATEX is the name commonly given to the framework for controlling explosive atmospheres and the standards of equipment and protective systems used in them. It is based on the requirements of two European Directives.
1. Directive 99/92/EC (also known as ‘ATEX 137’ or the 'ATEX Workplace Directive’) on minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres.
2. Directive 94/9/EC (also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’) on the approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres.
In summary, many manufacturing and processing industries generate potentially dangerous substances which are any substances used or present at work that could, if not properly controlled, cause harm to people as a result of a fire or explosion. They can be found in nearly all workplaces and include such things as solvents, paints, varnishes, flammable gases, such as liquid petroleum gas (LPG), dusts from machining and sanding operations and dusts from foodstuffs.
Previously there has been no mandatory obligation to use certified equipment (or indeed to classify an area as potentially explosive, however in the event of an accident companies were liable to prosecution), European Directive 137 (The protection of workers from potentially explosive atmospheres) makes it mandatory under European law to assess for an explosion risk and classify the area accordingly.
Once an area is classified as potentially explosive, a risk analysis will normally dictate that only electrical and mechanical equipment that is suitably certified can be installed. Directive 137 has increased the amount of ‘Classified or Zoned’ areas, and hence increased the demand for certified equipment. The ATEX Directive (94/9/EC) has forced manufacturers to gain certification of electrical and/or mechanical products that are intended for use in a potentially explosive atmosphere. Products without the appropriate certification are not legally allowed to be placed or offered on the European market after July 1st 2003.
As a result of the combination of these two Directives many manufacturers and workplaces have been forced to deal with issues with which they are unfamiliar, some organisations still operating in ignorance of the law or who have been operating essentially illegally are now addressing their obligations all be it late in the day. The following article deals with the basic codes, concepts and methodology of explosion protection.
What is an Explosion?
An explosion is any uncontrolled combustion wave. In order to create an explosion there has to be fuel (for example an explosive gas such as hydrogen), and an oxidizer (such as the oxygen in air) and a source of ignition energy (for example, a hot surface or an electrical spark). These three items are commonly referred to as ‘the fire triangle’ and are represented as below.
Figure 1. Fire Triangle
In addition to this, two additional facets are required; something to mix the fuel and the oxidizer (such as the turbulence created in a gas leak under pressure) and containment.
It is however common industrial practice to use the term ‘explosion’ for both confined and unconfined combustion.
For any mixture of a combustible gas or vapour with an oxidizer there is critical ignition energy. If one releases less than that critical amount of energy into the mixture, there will not be a self propagating explosion. Some combustion may occur transiently, but the combustion wave will not grow and become self propagating. If one releases at least the critical amount of energy, the combustion wave will pass through the incipient stages of growth and become self propagating as a plane wave, resulting in an explosion.
At a critical concentration called the most easily ignited concentration (MEIC), the amount of energy required to cause ignition is minimal. If the ignition experiment is conducted under conditions where it is assumed that all the energy injected into the gas/ vapour cloud is used in the combustion process, the critical energy at the MEIC is called minimum ignition energy (MIE). As the concentration is varied from the most easily ignited concentration the amount of energy required to cause ignition increases, until at certain points, the mixture is no longer explosive. These points (derived by experiment) are referred to as the lower explosive limit (or LEL) at the lower concentration limit, and the upper explosive limit (or UEL) at the higher concentration limit.
The LEL and UEL are not inherent properties of a combustible mixture. Their values depend on the nature of the experiment by which they are determined, especially the size of the vessel and the energy available from the ignition source. No prudent person controlling concentration to reduce the risk of an explosion would operate much higher than 50% of the LEL except under very carefully controlled conditions. In most situations the limit is set at 25% or lower.
Effects of Oxygen Enhancement, Temperature, and Pressure
Oxygen enrichment increases the heat release within the combustion zone of the developing wave front and therefore decreases the required initial energy contribution from the ignition source. The most easily ignited concentration of oxygen and vapours or gases ignites at about one hundredth the minimum ignition energy of the most easily ignited concentration of the same vapour or gas in air. Because the flame velocities are considerably higher, the pressure rise in an explosion proof enclosure may also be much higher. No means of explosion protection considered safe for atmospheric mixtures should be considered safe in oxygen enriched mixtures without careful examination.
The qualitative effect of increasing temperature is relatively easy to estimate. Every material has a spontaneous ignition tempera¬ture, SIT (or AIT, autogenous ignition temperature may occur without a spark or flame. If the product is heated well above its flash point, a
temperature is reached at which the product will ignite spontaneously, without any external source of ignition, provided sufficient oxygen is present. This is called autogenous ignition and the temperature at which it occurs is the auto-ignition temperature of the product) at which it will ignite spontaneously. Obviously, if the temperature of a mixture is raised, the amount of electrical energy required will decrease, reaching zero at the AIT.
The effect of pressure is understandable if one considers that when pressure increases the number of molecules per unit volume increases. The heat release per unit volume will consequently increase, and the ignition energy required causing the incipient flame sphere to grow to its critical diameter decreases. Similarly, decreasing pressure decreases the amount of energy released in the combustion zone and increases the required electrical ignition energy. This relationship has been verified experimentally over many atmospheres of pressure change. Doubling the pressure of a gas decreases the ignition energy to approximately 25% of its former value.
Historical Background of explosion protection
The first hazardous area was discovered in the early coal mines. This area held a double hazard: methane gas (firedamp) and coal dust. Methane gas is absorbed into the pores of coal. When the coal is mined the methane exudes, a process that takes a relatively short time. To be completely free of methane, coal has to be stored for a period of up to 1000 hours.
When miners worked an 8 hour shift pattern, the mined coal would be left in the shaft until the next day, during which time the methane would start to exude into the air in the shaft. The methane would collect in pockets at the roof of the mine and form an explosive layer. The miners returning for the next shift would carry with them the means of igniting the gas, hat mounted candles, and hand carried oil lanterns. The resulting ignition of the methane would in itself not necessarily be fatal for the miners. It was the secondary ignition of coal dust, thrown up into a cloud by the methane explosion, that resulted in a more violent and deadly detonation.
The first method used to remove the methane hazard was for a person to crawl along the mine floor holding a lighted lantern in their outstretched hand. This procedure would ‘safely!’ ignite the methane layer and burn it off before the miners started work. The person performing this task was known as the ‘fireman’ and it soon became apparent that there were very few volunteers for this hazardous job. This human form of Gas Detection resulted in prisoners being offered short jail terms if they would volunteer for the position.
With the advent of forced ventilation in the mines, the
hazards were reduced by the dilution of the methane with fresh air so that it was below its explosive limit. When electrical equipment was first introduced into the mines, there were some explosions due to electrical sparking. However, it was discovered that totally enclosed motors were able to contain explosions without transmitting them into the surrounding external atmosphere. This concept was transferred to the design of other electrical equipment; locating it inside substantial cast iron enclosures with tight fitting joints.
June/July 2010
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