Judgement of explosion hazard

How to judge explosive atmospheres?

This chapter examines the scope and implementation of Directive 1999/92/EC (ATEX 153) "on minimum requirements for improving the health protection and safety of workers at risk from explosive atmospheres". 

Seven questions are used to assess whether there is a risk of a potentially explosive atmosphereforming, whether the additional measures taken are sufficient to prevent the occurrence of a hazardous explosive atmosphere, whether the area is to be divided into zones and whether ignition of the area is prevented. This process is shown schematically.

Scheme risk management

Explosive atmospheres risk management

Explosion hazards can occur in all companies where flammable substances are used. These substances include consumables, intermediate products, finished products and residuals from the daily work process. An explosion occurs if both a mixture of fuel and air (i.e., sufficient oxygen) within the explosion limits and an ignition source are present. In the event of an explosion, workers are endangered by the uncontrolled effects of flames and pressure in the form of heat radiation, flames, pressure waves and flying debris, as well as by harmful reaction products and by the consumption of oxygen from breathing air.

Examples of the creation of an explosive atmosphere

In almost all industries and in the most diverse processes and operations,explosive atmospheres can cause risks. Some examples are:

• Chemical Industry;
• Landfills, Road and Civil engineering works;
• Power Plants, Waste Disposal Plants;
• Gas Companies;
• Wood Processing Industry;
• Paint Spraying;
• Agriculture;
• Metal Processing Companies;
• Food And Feed Industry;
• Pharmaceutical Industry;
• Refineries;
• Recycling Companies.

Petrochemical plant

In accordance with Article 1 of Directive 1999/92/EC, the Directive does not apply to: 

• Areas directly used for and during medical treatment of patients;
• The use of gas appliances in accordance with the requirements of Directive 90/396/EEC;
• The use of explosives or chemically unstable substances;
• Extractive industries covered by Directive 92/91/EEC or Directive 92/104/EEC;
• Transport by land, water and air where the relevant regulations by international agreements (such as ADNR, ADR, ICAO, IMO, RID) are followed.

Employer responsibility for safe working atmospheres

Article 3 of European Directive 1999/92/EC (ATEX 153) states that an employer must prevent the formation of explosive atmospheres and protect workers against explosions. The text reads as follows: 

"With a view to preventing […] and providing protection against explosions, the employer shall take technical and/or organizational measures appropriate to the nature of the operation, in order of priority and in accordance with the following basic principles:

• the prevention of the formation of explosive atmospheres, or where the nature of the activity does not allow that,
• the avoidance of the ignition of explosive atmospheres, and
• the mitigation of the detrimental effects of an explosion so as to ensure the health and safety of workers.

These measures shall where necessary be combined and/or supplemented with measures against the propagation of explosions and shall be reviewed regularly and, in any event, whenever significant changes occur."

In order to prevent the occurrence of explosive atmospheres, the assessment of explosion risks must first consider whether a hazardous explosive atmosphere can arise and ignite under the given conditions.

Careful consideration must be given to the likelihood and persistence of hazardous explosive atmospheres, the likelihood of ignition sources being present and becoming active, the installations, substances used, methods of operation and their possible interactions, and the scale of the expected consequences.

The assessment must be carried out for each process, for each installation and for changes in processes or installations. The assessment of new or existing installations must be based on the following conditions of use:

• normal conditions of use, including maintenance operations;
• commissioning and decommissioning;
• operational faults and foreseeable malfunctions;
• reasonably foreseeable misuse. 

Explosion risks must always be assessed as a whole. Of importance are:

• used work equipment;
• structural conditions;
• used substances;
• working and operating conditions;
• possible interaction either mutual or with the working environment.

In addition, spaces that are connected or can be connected to potentially explosive areas through openings must be included in the assessment of explosion risks.

Assess the explosion risks

When the explosive atmosphere contains various flammable gases, vapors, mists or dust, this can greatly enhance the effect of the explosion. This must be taken into account when assessing explosion risks. 

Systematic methods are suited for assessing explosion risks in work processes or technical installations. Systematic means working in an orderly, objective and logical manner. Possiblesources of a hazardous explosive atmosphere and potentially concurrent ignition sources must be considered.

In practice, it is usually sufficient to systematically identify and assess the explosion risk with a series of specific questions. Other risk assessment methods, such as FMEA (Failure Modes and Effects Analysis), HAZOP (Hazard and Operability Study) or ETA (Event Tree Analysis) are only useful when identifying ignition sources in highly complex technical installations.

The explosion hazard assessment must be done irrespective of whether ignition sources are present or likely to be active.

The following four conditions must be met simultaneously for any explosion with dangerous consequences to occur:

• A high degree of dispersion of flammable substances;
• Concentration of the flammable substances in air within their combined explosion limits;
• Hazardous quantities of explosive atmospheres;
• An active source of ignition.

Seven questions to measure the explosion risks

To test whether an explosion risk actually exists and whether measures to protect against an explosion risk are required, the first four questions must be answered:

1. Are there flammable substances present?
2. Can an explosive atmosphere be formed by distribution in air?
3. Where can an explosive atmosphere occur?
4. Can a hazardous explosive atmosphere occur?

If answers show that a hazardous explosive atmosphere occurs, then measures must be taken to prevent it as much as possible!

The following three questions are used to determine whether the measures reduce the explosion risk to an acceptable level. This step should be combined with a choice of precautions from chapter 4 and be repeated until an appropriate solution has been found.

5. Has a dangerous explosive atmosphere been adequately prevented?  
6. Into which areas can explosion hazardous areas be classified? 
7. Has ignition of an explosive atmosphere been adequately prevented?

If the answer to the last question is 'yes', no further action is required. If it turns out that ignition of an explosive atmosphere cannot be prevented, constructive and organizational protective measures must be taken to minimize the harmful effects of an explosion.

Question 5 asks for "adequate" prevention of hazardous explosive atmospheres. This question can only be answered with "yes" if the technical and organizational measures in place are such that, taking into account all operating conditions and reasonably foreseeable malfunctions, the occurrence of an explosion is not to be expected.

Are there flammable substances present?

A prerequisite for the occurrence of an explosion is that flammable substances are present during the process. This means that at least one flammable substance is used as a base or auxiliary substance, as a residual, intermediate or end product, or can be formed in the event of a normal operational failure. Only if flammable substances are present, do we need to look further for possible explosion hazards.

Can explosive atmospheres be formed by distribution in air?

Whether the presence of flammable substances will form an explosive atmosphere depends on the ignition hazard of the mixture formed. If the required degree of dispersion is achieved and the concentration of the flammable substances in air is within its explosive limits, an explosive atmosphere is present. In the case of gas or vapor, there is always sufficient dispersion. 

In answering this question, the following properties of the substances and their possible state during processing have to be taken into account:

• lower and upper explosion limit;
• flash point;
• processing and ambient temperature;
• concentrations formed or already present;
• processing and use of liquids (e.g. spraying, evaporation, high pressures);
• presence or formation of dust/air mixtures and dust accumulation;
• particle-size distribution (particularly grains smaller than 0,5 mm), humidity, smoldering point.

The formation of a dust cloud during filling and transport

In practice, the explosion limits for dust are not as useful as those for gases and vapors. Dustconcentration can change significantly by suspended dust particles or by their precipitation. In this way suspended dust particles can cause an explosive atmosphere.

Where can explosive atmospheres occur?

If explosive atmospheres are likely to occur, it is necessary to determine exactly where they occur in order to limit the risks. To determine this, the properties of the substances and the specific conditions within installations, process technology and environment must also be taken into account.

For gases and vapors, the density compared to air is important. The density of most gases is greater than air. The gases sink down, mixing with the air present and ending up in wells, channels and shafts. Some gases such as hydrogen and methane are much lighter than air. These gases tend to disperse quickly into the atmosphere, unless they are trapped.

Even slight air movements (natural draughts, people walking around, thermal convection) can significantly accelerate the mixing of gas and air.

For liquids and mists, the evaporation number, the size of the evaporation surface and the processing temperature are important. Pressurized liquids can be released into the environment by spraying or vaporizing forming explosive mists.

In the case of dust, possible accumulation of dust on horizontal or slightly inclined surfaces must be taken into account. Dust in filters, in conveyor systems, at transfer points or in dryers can also cause explosive atmospheres.

In general, the following specific conditions should be considered:

• filling and emptying;
• release of substances at valves, slides, and joints;
• aeration and venting conditions;
• places where ventilation is not possible, such as underground wells, ducts and shafts.

Can hazardous explosive atmospheres occur?

In places where the occurrence of explosive atmospheres requires special precautions to protect the health and safety of the workers concerned, the explosive atmosphere is classified as a hazardous explosive atmosphere and the places are regarded as hazardous places.

Even a small amount of flammable liquid can lead to a large amount of flammable vaporonce evaporated. For example, 1 liter of liquid propane corresponds to 260 liters of propane in gaseous form. Diluted to the lower explosion limit, this creates an explosive atmosphere of 13,000 liters.

Whether an explosive atmosphere is actually a hazardous explosive atmosphere depends on the volume of the explosive atmosphere and the harmful consequences of an explosion. As a rule, however, an explosion is always accompanied by extensive damage and therefore any explosive atmosphere is considered hazardous.

Is the prevention of hazardous explosive atmospheres adequate?

If hazardous explosive atmospheres can occur, measures must be taken to prevent them as muchas possible. Possible measures are described in prevention of explosion hazard in conjunction with the organisational measures in chapter 5 organisational measures. 

Measures taken to prevent the formation of an explosive atmosphere must be examined in terms of their effectiveness. All the different operating conditions and all malfunctions (including rare ones) must be taken into account. Only when it is certain that the formation of a hazardous explosive atmosphere has been prevented can further measures be dispensed with.

Is the ignition of an explosive atmosphere adequately prevented?

If the formation of hazardous explosive atmospheres cannot be completely eliminated, measures must be taken to prevent active sources of ignition. The more likely the occurrence of hazardous explosive atmospheres, the more import this becomes. Possible measures to prevent active ignition sources are described in prevention of explosion hazard.

If the concurrent presence of both a hazardous explosive atmosphere and active sources of ignition cannot be ruled out completely, preventive constructive measures as described in prevention of explosion hazard are also necessary. In all other cases, appropriate mitigating measures must be taken.