1 How does an explosion occur?
The four points of the risk tetrahedron (Figure 1) describe the four conditions that all have to be present simultaneously for an explosion to occur:
- combustible hazardous material;
- an oxidising agent, e. g. oxygen;
- intermixing in the right ratio; and
- an effective ignition source.
Combustible materials comprise all those substances that can burn as a result of ignition in the presence of oxidising agent – this generally involving the oxygen that occurs naturally in the air. Typical examples are the solvents used in paints and coatings, adhesives and cleaning agents along with the dusts that are generated from coal, wood, metal and sugar. For an intermixing to take place between the combustible material and the oxidising agent there has to be an evaporation or vaporisation of fluids or a swirling up of dust. In the case of fluids this is usually determined on the basis of the flash point. This is the lowest temperature at which a fluid is able, under a set of standard conditions, to produce a sufficient amount of vapour to create a mixture that can be ignited. If the prevailing temperature is not safely below the flash point there is a chance that an explosive mixture will be formed. This does not apply to sprayed or atomised liquids – here the risk of explosion must always be assumed.
Solid materials only represent an explosion risk if they are present in a sufficiently fine state: as the grain size diminishes so the surface area of the dust will increase (Figure 2), thereby allowing the combustion process to proceed more swiftly and ultimately resulting in an explosive development. This is typically the case for particle sizes of less than 0.5 mm.
Such mixtures are generally designated as “explosive mixtures”. If the mixture in question is present under atmospheric conditions (ambient temperature of – 20 to + 60 °C, air pressure 0.8 to 1.1 bar, oxidising agent air) it is referred-to as an “explosive atmosphere”. As this tends to be the most common scenario, the following paper will focus exclusively on occurrences of this type. In the case of a hazardous explosive atmosphere they are present in dangerous quantities. A dangerous quantity is said to exist at a volume of ≥ 10 l or ≥ 1/10 000 volume capacity or when it occurs in close proximity to persons.
2 It’s all about the mixture
This applies not just to cooking but also to explosion protection. Merely the fact that a combustible material mixes with air is not of itself enough. A mixture of combustible substances with air can only explode when ignited provided that the correct mixing ratio is present – that is to say when the mix is somewhere between the lower explosive limit (LEL) and the upper explosive limit (UEL). Below the LEL the mixture contains too little fuel, it is too “lean”. Above the UEL it contains too much fuel, it is too “rich”and “only” burns off instead of exploding (Figure 3). The UEL does not apply to dust mixtures.
The final condition for an explosion is an effective ignition source. In practice the following six types of ignition source, out of the total of thirteen, play the greatest role:
- hot surfaces;
- flames and hot gases;
- sources of ignition caused by mechanical friction, impact and severance processes;
- electrical equipment;
- electrical compensating currents, cathodic corrosion protection; and
- static electricity.
3 Risk assessment
The most important tool for gauging and controlling the explosion hazard is the risk assessment process. The measures involved are subject to the order of priority outlined in table 1.
The best means of protecting against an explosion is not to have anything present that might explode, i. e. the formation of an hazardous explosive atmosphere is safely prevented. In other words, one of the three corners of the risk tetrahedron, namely “oxidising agent”, “combustible material” or “intermixing in the right ratio”, has to be absent. If this is not the case then the source of ignition has to be prevented, that means the fourth peak of the tetrahedron has to be excluded. If that cannot be reliably achieved measures have to be taken to prevent the propagation of an explosion and to limit its impact.
3.1 First level of measures
The first level of measures involves preventing the formation of an hazardous explosive atmosphere. The so-called “STOP principle” is in fact enshrined in the Hazardous Substances Ordinance (GefStoffV). The abbreviation STOP denotes the order of priority for the protective measures to be taken: substitution, technical protective measures, organisational protection measures and personal protection measures.
The primary protective measure is substitution. This involves investigating whether the combustible materials can be replaced, or substituted, with less dangerous substances. This could be fluids with a higher flash point or those that are non-flammable. Granules can be used in place of dusts, while working methods that entail the release of no or fewer substances, e.g., brushing instead of spraying, can offer greater protection.
If a sufficient degree of substitution is not possible then some thought must be given to whether measures such as the sealing of containers or equipment, the removal of dust deposits or the use of natural ventilation can reliably prevent an hazardous explosive atmosphere. In such a case – which is to be recorded in the risk assessment – further explosion protection measures will not be required. Any further measures must be monitored and assessed in order to ensure their effectiveness. They are therefore subject to testing obligations in accordance with Annex 2, Section 3 of the Operational Safety Ordinance (BetrSichV). Such measures may, e. g., include:
- Technical ventilation measures: the use of an extraction or room ventilation system can remove combustible substances from the air or at least reduce their concentration levels. The effectiveness of an extraction or ventilation system can, e. g., be monitored using gas detectors (for measuring the concentration of combustible gases and vapours) or flow sensors (for measuring the airflow).
- Inertisation: inertisation is a process whereby non-combustible substances – primarily gases like nitrogen and carbon dioxide – displace the oxygen to such a degree that an explosion can no longer take place. Inertisation can only be employed in enclosed plant sections and the process must be carefully monitored. Inert gases produce a suffocating effect.
Testing obligation in accordance with Annex 2, Section 3
of the Operational Safety Ordinance (BetrSichV):
Equipment operating in hazardous areas (Ex systems) are subject to regular inspection and special testing requirements apply in this case. The legal basis here is provided by the Operational Safety Ordinance (BetrSichV). The employer is required to define the type, scope and depth of the test, the inspection intervals and the professional qualifications of the persons participating in the testing process. This information is to be recorded in the explosion protection document.
The tests must be carried out
- prior to initial commissioning;
- after each modification subject to mandatory testing; and
- on a recurring basis.
The testing and inspection requirements are laid down in the Technical Regulations on Operational Safety (TRBS 1201) part 1 “Testing of equipment in potentially explosive atmospheres”.
3.2 Second level of measures
The second level of measures is aimed at preventing ignitions occurring. Where an hazardous explosive atmosphere cannot be safely avoided an assessment has to be made as to its frequency and duration of occurrence. Similarly for ignition sources it is necessary to establish how likely it is that such sources are present or can arise – as part of the process taking place, e.g., when cutting or grinding, or in the event of an equipment fault or malfunction – if they are active and effective.
Workplaces of this kind are identified by the D-W021 warning sign denoting an explosive atmosphere (Ex triangle). Exposed sources of ignition, such as smoking and open fires, are prohibited in these areas and unauthorised persons are not allowed to enter high-risk zones of this kind.
The long-established and well-proven practice of zoning helps to regulate the scope of the required ignition-source prevention measures. This same system is also used in the Technical Rules for Hazardous Substances (TRGS – 700s series). This division into zones is based on the frequency and duration of the occurrence of a hazardous explosive atmosphere. Here the zones are considered in relation to the operating period of the process in question – rather than to a timeframe of a week or a year. Zones 0, 1 and 2 apply in the case of hazardous explosive atmospheres that involve a mixture of air with combustible gases, vapours or mists, while zones 20, 21 and 22 are used for hazardous explosive atmospheres in the form of a cloud of dust in air.
Operators can then refer to the zone designation in order to derive the most appropriate protective measures – provided that the emergence of the hazardous explosive atmosphere and the occurrence of an effective ignition source are independent events. The relationship between the emergence of an hazardous explosive atmosphere, the resulting zone designation and the ignition source that is to be avoided is shown in table 2. The portfolio of examples presented in Social Accident Insurance (DGUV) Regulation 113-001 (Figure 4) serves as a useful aid to zoning.
If a firm opts not to employ the zoning system it first has to assume that an hazardous explosive atmosphere is permanently present throughout the entire working area (worst case sce-nario). This means that protective measures have to be adopted in the form of a zone 0/20 allocation. In individual cases a company can specify other measures as part of its risk assessment strategy provided that these ensure the same level of safety. Examples are activities described in TRGS 507 (“Surface treatment in rooms and containers”).
3.3 Third level of measures
The third level of measures involves the prevention of explosion propagation and/or the mitigation of the effects of an explosion. If, in spite of all the steps taken, it is not possible to safely preclude the possibility of an explosion occurring then certain design-related safety measures have to be put in place in order to minimise its potential impact. This means that containers and plant components are to be designed or equipped in such a way that no persons will be injured and that buildings and other structures will suffer the least possible damage in the event of an explosion. Such measures may, e. g., include:
- Explosion-resistant design: the container or installation is constructed to withstand the blast of an explosion.
- Explosion relief: here the explosion overpressure is dissipated by means of rupture discs, explosion valves or permanent openings to the atmosphere. The venting system must not pose a danger to any person.
- Explosion suppression: measures of this kind identify and suppress a developing explosion before it can achieve dangerous overpressure levels.
Measures also have to be put in place to prevent the propagation of an explosive blast to other vulnerable technical areas (explosion decoupling), such as by installing extinguisher barriers, rapid-action mechanical isolation, and rotary valves.
Operators are required to record their explosion risk assessment in an explosion protection document, this forming a special section of the documentation of the risk assessment. Further information on this is available in DGUV Information 213-106 “Explosion protection document”. Tests must also be recorded as laid down in the Operational Safety Ordinance (BetrSichV), see TRBS 1201 part 1 “Testing of plant and equipment operating in potentially explosive environments”.