Explosion risk assessment and the problems of ensuring safe production in explosion hazard conditions

Andrzej Wolff, PhD, Eng. | Atex Wolff i Wspólnicy sp.j. (Grupa Wolff)

Part 1. Risk and problems resulting from the provisions of the Regulation of the Minister of Economy of 8 July 2010 regarding minimum requirements for occupational safety and health, related to potentially explosive atmosphere in the place of work (ATEX 137 Directive).

The legal basis for production in explosion hazard conditions is determined by the EU ATEX 137 Directive (EN 99/92/EC). It was implemented in the Polish law by virtue of the Regulation of the Minister of Economy of 8 July 2010 regarding minimum requirements for occupational safety and health, related to potentially explosive atmosphere in the place of work (Journal of Laws of 2010 No. 138, item 931). Both of the legal acts define the basic duties of employers in that regard.

The duties of the manufacturers of equipment and protective systems intended for operation in explosion hazard zones are determined in the Directive of the European Parliament 94/9/EC of 23 March 1994 (ATEX 95 Directive). The directive was implemented in the Polish law by the Regulation of the Minister of Economy of 22 December 2005 regarding basic requirements for equipment and protective systems intended for use in potentially explosive atmosphere (Journal of Laws of 2005 No. 263, item 2203).

The ATEX 137 and ATEX 95 Directives are highly related to the respective Polish Regulations.

Interpretation of the provisions of the Regulation regarding the minimum requirements results in many difficulties and ambiguities. Below presented are the problems resulting from the author’s experience in interpreting the provisions of the Regulation, with which we cope when preparing Explosion Risk Assessment Documents (ERAD) and Explosion Protection Documents (EPD).

Below presented are the problematic provisions of the Regulation and comments explaining doubts.

Paragraph 1.

The Regulation determines the minimum requirements for occupational safety and health, in a work environment where due to the reasons related to the characteristics of the place of work, equipment or substances/mixtures used an explosive atmosphere may originate.

The term “minimum requirements” is defined in the Regulation only in a descriptive manner, which in practice means that it is the employer (user/owner of the system) who has to determine the level of the minimum requirements. Theoretically this is a legible provision, but in practice the employer/system owner may face and faces a series of problems related to interpretation of this rather vague idea. The Regulation does not determine precise criteria to establish minimum requirements. Setting out such requirements for the given production line (system or plant) is based not only on technical factors but also on financial ones. Determination of technical factors (ensuring safe production conditions) requires an adequate explosion risk assessment, for which extensive experience and responsibility are needed. Focusing on financial factors (how much is it going to cost and what funds we can assign) is a real criterion but also a risky one. Finding a compromise between the adequate technical level and financial resources is not easy.

Fulfilment of the minimum requirements is in practice performed mainly by ensuring process and explosion safety. However, this does not contribute to larger production (profits) but requires financial expenditures.

Paragraph 2.

The paragraph defines basic terms: equipment, hazard zones (and non-hazard zones), flammable substances, explosive atmosphere, normal equipment/protective system operation, workers, and other.

Explosive atmosphere is defined as a mixture with air in atmospheric conditions of flammable substances in the form of gases, vapours, mists or dusts, in which combustion propagates spontaneously to the whole mixture after inflammation.

The definition refers to standard atmospheric conditions (temperature between -25°C and +60°C, pressure between 0.8 and 1.1 bar, air with normal oxygen content – usually 21% by volume). Now, what shall an employer/system owner do when the process conditions of a system are much different from the atmospheric ones? In industry we deal with many process carried out under pressure, in vacuum, at high or low temperatures.

This does not mean that the employer is exempt from the duty of fulfilling explosion protection requirements in such conditions. The employer must ensure safe work in any conditions! The Regulation does not, however, provide any conduct recommendations.

Paragraph 4.1.

The paragraph reads that in order to prevent explosion and provide protection against explosion effects, the employer must apply technical and organisational protective measures adequate to the type of activities. When determining the protective measures, fulfilment of the following objectives must be ensured first of all:

  • prevention of explosive atmosphere origination (A),
  • prevention of ignition in an explosive atmosphere (B),
  • limitation of the negative effect of an explosion in order to ensure workers health and safety (C).

Such defined objectives are clear, but their practical implementation faces a lot of problems, e.g. how to avoid an explosive atmosphere in industrial processes when unavoidably the air includes oxygen. This refers to a majority of industrial processes.

There are, of course, some possibilities – providing inert atmosphere (mainly nitrogen) or carrying out a process in the atmosphere of flue gases. However, inert atmosphere is naturally limited, as it requires complete tightness of the used machines and equipment (which means costs). Similar problems relate to the use of flue gases. Such solutions are applied in some processes (coal milling at power plants or cement plants, or in refining and chemical processes), but this is a rather small percentage of industrial processes.

The situation is better in the case of settled dust due to, for example, lack of tightness of devices and process equipment, or making some operation in open system (e.g. dosing). In such cases effective vacuuming may significantly improve the working conditions.

As regards B, the situation is much better. Industry has various technical and organisational solutions limiting the origination of explosive atmosphere ignition or preventing such ignition – magnetic separators (eliminate ferromagnetic pollution in the stream of transported substances), electrostatic earthing (eliminates spark/stream of spark discharge), earthing of devices, lighting arrester system, spark-proof casings, etc.

Explosion risk assessment takes into account the probability and time of explosion atmosphere origination (R1), but also the probability of presence and activation of ignition sources (R2). This is clear. Yet, it is not clear how to determine the probability in numbers.

The industry provides also effective and reliable protective systems (when properly designed and assembled), which ensure limitation of the negative effect of explosion (C). Applying such systems requires significant experience and responsibility. The solutions include explosion venting (gases: PN-EN 14994, dusts PN-EN 14491), explosion suppression (PN-EN 14373) and explosion isolation systems (PN-EN 15089). It must be emphasised that explosion venting is to direct and (safely) discharge the effects of an explosion to atmosphere, whereas explosion suppression prevents explosion origination in a device. Both methods (venting and suppression of an explosion) serve the protection of process devices. For the purpose of protecting process systems the explosion suppression/venting and isolation methods are merged. Designing such complicated protective systems is difficult in many cases.

Paragraph 4.2.

The protective measures referred to in paragraph 4.1 may be supplemented by the employer or merged with measures preventing explosion propagation.

This paragraph reflecting the reason for explosion isolation (“as a measure of preventing explosion propagation”) is important but still imprecise. The statement that the employer “may merge or supplement” the solutions leaves the employer a huge space for decision. This is not a good solution while it is known that in order to protect process systems against the consequences of explosion propagation, it is of key importance to ensure effective explosion isolation. Isolation means cutting off the respective device (devices) threatened with explosion from the rest of the system. Moreover, we still have paragraph 12.2. Therefore, in our opinion paragraph 4.2 should include the words “must be supplemented”, for example.

Paragraph 12.2.

The measures referred to in paragraph 12.1 should prevent explosion, or in case of explosion should control it and minimise its propagation in the place of work or in equipment, such as to minimise the consequences for the workers.

This important paragraph states that the applied measures “should minimise explosion propagation”. Such wording is better than that of paragraph 4.2, but it is not sufficiently precise. Minimisation of explosion propagation may be provided by a properly designed explosion isolation, as mentioned earlier in paragraph 4.2.

Paragraph 4.4.

The employer carries out a comprehensive risk assessment related to the potentially explosive atmosphere in the places of work, taking into account at least:

  • the similarity and time of explosive atmosphere origination (R1),
  • probability of presence and activation of ignition sources, including electrostatic discharge (R2),
  • systems, substances and mixtures used by the employer, as well as the processes performed and their interactions (R3),
  • size of the expected explosion effects (R4).

Proper explosion risk assessment is not a simple task, although a very responsible one. The document may be prepared by a person authorised by the owner (a plant employee or an external person) or by an external company. This is not regulated by any legal provisions. However, the risk and liability related to the explosion risk assessment made are vested in the owner/user (employer). Therefore, the basic condition is that the person/company has sufficient practical experience, knows the process, knows the legal regulations (in that scope) as well as design standards, and reflects high professional ethics.

Explosion risk assessment takes into account the probability and time of explosion atmosphere origination (R1), but also the probability of presence and activation of ignition sources (R2). This is clear. Yet, it is not clear how to determine the probability in numbers. How to define practically, whether the risk is high, low or negligible? What does the probability depend on? How to determine, at least in some process cases and situations, the time of explosive atmosphere origination? Whether 1 hour, for example, is a lot or too little? The typical solution in that scope described in [1] does not provide any quantitative recommendations. The terms “high”, “low”, etc. applied there must be defined in numbers. What is high probability in a given process? The confirmation of such quantitative interpretation problem is found in the cases described in [2]. In the explosion risk assessment process it is also necessary to determine/evaluate the expected explosion effects (R4). The Regulation provides no recommendations in that regard. So what remains is the experience of the person/company preparing the Explosion Risk Assessment Document. Risk assessment based, for example, “on other similar situations” is a difficult and responsible task.

Paragraph 9.1.

The employer should also provide staff who work in the possibly explosive atmosphere with adequate training in explosion protection, within the mandatory occupational health and safety training.

The provision means the “obligation” of periodical repeating of training, which is good, as it takes into account the fact of employing new staff, among other things. However, is training systematically provided at plants? What is the scope of knowledge passed? Who should participate in the training?

Paragraph 10.2.

If explosive atmosphere consists of various types of flammable gases, vapours, mists or dusts, protective measures adequate to the highest potential hazard must be applied.

This is an important provision, as it is known from experience that an explosive atmosphere originating as a result of the applied technology may consist of various components. However, quantitative assessment in the simultaneous presence of various components is very difficult, not only because of the changing content of the explosive atmosphere mixture, but also (very often) lack of knowledge of the flammability and explosive characteristics of the particular components.

It is also a problem to determine the highest potential hazard, while the hazard resulting from the presence of a given component in an explosive mixture does not refer only to high values of its flammability or explosive characteristics, but also (or mainly) from result from its quantitative share in the mixture. Therefore, how should we define the “highest potential hazard” in numbers? The technology and scientific knowledge provide only a few proven solutions in that regard. An additional problem is that the databases regarding flammability and explosive characteristics of individual (particularly atypical) substances are incomplete.

How to handle situations in which we deal with hybrid mixtures (mixture of flammable and explosive dusts and gases/flammable vapours of liquids)? The scientific knowledge in that regard is still very poor…

Paragraph 12.1.

The employer should apply any necessary measures in order to ensure that the place of work, equipment and connecting elements available to the staff have been designed, assembled, connected and installed, as well as maintained and operated in a manner that minimises the explosion hazard.

Indeed, this should be followed because only ensuring proper technical and organisational solutions will enable fulfilling such condition. But how to determine the “necessary measures” quantitatively and qualitatively? Who is to propose the proper solutions in each engineering situation: the designer (design bureau), operation maintenance engineer, technical department manager, director, an external company dealing with explosion hazards? Another problem is high responsibility and adequate knowledge.

Paragraph 7.1.

Before rendering a place of work the employer should prepare an Explosion Protection Document (EDP) based on the risk assessment referred to in paragraphs 4.4 and 4.5.

Another interpretative problem – “before rendering a place of work”. Does this mean a new system, new production unit or modernisation deemed to be significant by the employer? Does this mean that EDP must be prepared before test run or at the moment of the system/process unit commissioning?

From the above analysis it appears that the Regulation in question imposes on the employer (owner of a plant or system intended for operation in an explosion hazard zone) a series of duties, which are rather hard to be fulfilled.

The Regulation does not specify what an employer should do when building a new system or modernising an existing one through an external contractor. How to ensure that the resulting design solutions (concept, technical design), machines and equipment, protective system or electric devices fulfil the requirements of the Regulation at the adequate level (for the employer)?

In our opinion the basis is to include in the ToR within a contract award procedure such provisions that impose on the contractor the duty to fulfil the requirements of the Regulation regarding minimum requirements for occupational safety and health (ATEX 137) and the Regulation regarding basic requirements for equipment and protective systems intended for use in potentially explosive atmosphere (ATEX 95) at sufficiently high level. The manner in which the contractor will ensure the minimum requirements fulfilment must be accepted by the employer before signing the contract. The manner should result from the standards applied by the contractor in ensuring production safety and the assumed explosion safety level. Later discussions in that regard are very difficult and not always fruitful.

Further questions appear: what level of process and explosion safety (translating into the risk level) should be assumed. What should that stem from, who shall define the risk, and who should be responsible for the risk?

Replies to the questions are very difficult. The Regulation, as it has been said herein above, does not comprise any good quantitative conduct criteria. It is not clear, however, whether such criteria may be included in a regulation. They should depend, to some extent, on the type of process, technology, unit operations, machines, equipment and other organisational and technical factors. The number of possible solutions is large, and specifically the number of solutions offered in various industry sectors or production technologies. In our opinion the below specification (Table 1) may be helpful.

Device/process

Share [%]

Silos (storage)

20

Filters/cyclones (dedusting)

17

Mechanical transport

10

Total:

47

Milling

13

Drying

8

Dedusting systems

5

Mixing systems

5

Grinding and polishing

5

Screening

3

Other

14

Total:

100

Table 1. Specification of devices and unit processes as the potential sources of explosion hazards in industry (dusts)

The specification reflects that typical devices and unit operations (broadly applied in various industries) are statistically responsible for as much as 50% of explosive hazard reasons. So they should be particularly taken into account when assessing explosion hazard related to the presence of flammable and explosive dusts. This is nothing new. From the industrial practice it appears that explosions happen most often in silos (storage), filters (dedusting) and during mechanical transport (e.g. bucket conveyors).

Do the staff responsible for commissioning production systems sufficiently consider the aforesaid hazards?

This is the topic of the upcoming INDEX Conference – learn more http://www.strefaex.eu/en/atex-conferences/