Andrzej Wolff, Ph.D., has developed AW-OZ, a novel method for assessing the hazards of typical industrial equipment and installations used in the processing of bulk materials. The AW-OZ approach is based on the design, operating conditions of the equipment and the presence of combustible and explosive dust.
In the realm of industrial safety, the limitations of the current explosion risk assessment methodologies, mandated by the ATEX Directive have become increasingly apparent. Recognizing these shortcomings, Dr. Andrzej Wolff proposes a groundbreaking methodology that shifts the focus from probability-based assessments to a more objective analysis grounded in process data. What are the nuances of this new methodology and what is its potential to revolutionize explosion risk management?
Limitations of the now recommended explosion risk assessment: the legal status
The need to assess the risk of explosion in production conditions threatened by the presence of flammable and explosive gases, dusts, mists, vapors of flammable liquids and fibers results from the provisions of the ATEX Users Directive (99/92/EC). According to the Directive, explosion risk assessment related to explosive atmosphere in workplaces should take into account the following:
- probability and duration of an explosive atmosphere,
- likelihood of both occurrence and activation of ignition sources, including
electrostatic discharges, - installations operated by the employer, substances and mixtures used
- production processes involved, and their interactions
- the magnitude of the anticipated consequences of an explosion
In practice, the explosion risk assessment techniques used mainly emphasize the evaluation of the probability of an explosive atmosphere, the probability of occurrence and activation of ignition sources, and the magnitude of the possible consequences of an explosion.
The typical approach to explosion risk assessment is therefore based on hazard identification, with an estimate of risk R. It belongs to the Preliminary Hazard Analysis PHA assessments. This approach, combined with the knowledge and experience of those performing the assessment, is intended to ensure that hazards are identified and assigned an appropriate level of risk R.
The risk R is derived from the assumed values of P and S and is calculated from the equation: R = P x S,
where: P – the frequency of occurrence of the danger zone / effective ignition source
S – the consequences that may occur when an explosion occurs
Based on the above, the level of risk acceptability is widely assessed. However, Dr. Andrzej Wolff proposes a different approach.
– The primary goal of our work was to reduce the role of subjective assessment based on the probability of the presence of an explosive atmosphere or the occurrence and activation of ignition sources, as well as the magnitude of the anticipated consequences of an explosion – comments Dr. Andrzej Wolff, the originator of AW-OZ.
Approach to explosion risk assessment following the ATEX User Directive
At the implementation stage, the risk assessment methodology of the ATEX Users Directive relates to the analysis of equipment, process and technology in a limited way.
In the typical approach to explosion risk assessment introduces the concepts of probability and of frequency of the danger zone and potential ignition source. These are difficult to quantify, and thus difficult to use in practice. Therefore, hazard identification with R explosion risk estimation is generally based on arbitrary P and S quantities. As a result, explosion risk assessment is largely based on subjective evaluation o the situation, which is not a simple task.
Proposal for a new approach to hazard assessment – AW-OZ methodology
As a result of a number of years of work in the field of explosion and process safety, including hundreds explosion risk assessment documents drafted for process plants, in various industries, we have come to the conclusion that there is a need to develop a new approach to risk/hazard assessment. This new approach is essentially based on process data rather than the concept of probability/frequency of an event. What is taken into account is the typical characteristics of equipment and the relatively easy-to-obtain process data.
The AW-OZ Hazard Assessment takes into account the design and operating conditions of typical equipment used for process processing of bulk materials containing combustible and explosive dusts (except metal dusts). It also considers the possible presence of typical ignition sources.
The goal is to quantitatively estimate the level of process and explosion hazards resulting from the operation of the equipment. The AW-OZ methodology is applicable to equipment in operation, as well as in design. It applies to equipment that has a broad range of industrial applications.
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Depending on the estimated level of hazard/risk, it is proposed to adjust the protection of the equipment against possible hazards leading to explosion/fire. This includes the explosion protection system.
The AW-OZ risk assessment is essentially based on the following provision of the ATEX Users Directive (and follows local regulations, i.e. an ordinance of the Polish Minister of Economy):
“The explosion risk assessment shall take into account: the installations operated by the employer, the substances and mixtures used, the processes taking place and their interactions.”
The AW-OZ approach is based on this provision and is general enough in nature to be applied to typical technological systems designed for processing bulk materials containing combustible and explosive dusts. It also allows a reasonable quantitative estimation of the hazard levels associated with the operation of equipment.
As a result of the assessment, depending on the hazard level estimate, different systems of equipment/process node explosion protection systems are proposed.
Equipment and unit operations considered when assessing process and explosion hazards
The level of risk for equipment used depends on process conditions, including possible ignition sources, equipment design, and the presence and properties of combustible and explosive dust. In each case, the assessment is supplemented with a recommendation of a technical solution for improving process and explosion safety.
The data included in the summary below gives estimated % share of specific equipment apparatus/unit, in which ignition and explosion/fire could potentially occur.
The above compilation shows that potential explosion events are mainly related to storage, dust collection, mechanical transport (bucket feeders), grinding and drying. Based on this summary, it can be estimated that more than 50% of explosions in the industry originate in storage, dust removal and mechanical transport operations. And when grinding and drying operations are included, the risk is more than 70%!
The main advantage of our approach is that we obtain a quantitative assessment of the level of hazard associated with the operation of equipment based on easily obtainable process data. Each piece of equipment under consideration is characterized by an individual list of risk factors. It takes into account its design and operating conditions, as well as the properties of combustible and explosive dusts, including the possible presence of ignition sources.
What process equipment can be evaluated using the AW-OZ risk assessment method?
Nowadays, AW-OZ can be used to assess explosion risk for the following process apparatuses commonly used to process bulk materials:
- storage silos
- intermediate storage tanks
- filters
- bucket elevators
- cyclones
To enable evaluation not only of individual equipment, but also of entire process nodes, the following units are also evaluated:
- screw feeders
- scraper feeders
- pipelines/channels
- conveyors
Equipment assigned to a higher hazard level included: storage silos, intermediate storage tanks, bucket feeders, filters.
Equipment assigned to a lower hazard level included: cyclones, screw feeders, scraper feeders, pipelines and ducts, as well as conveyors.
AW-OZ application available as mobile and desktop
The AW-OZ solution was developed especially for:
– insurers,
– design companies,
– general contractors for turnkey process nodes and industrial installations,
– companies that use process plants processing organic bulk substances in their production (e.g., food, pharmaceutical, wood, chemical or energy industries).
PLEASE NOTE THAT METAL DUSTS ARE NOT INCLUDED IN THE AW-OZ METHOD
How does the AW-OZ explosion risk assessment methodology meet the legal requirements?
Explosion risk assessment is concerned with determining the risks associated with the possibility of explosive atmospheres in workplaces. The AW-OZ methodology complies with the provisions the European ATEX User Directive [1] in terms of meeting the requirements for explosion risk assessment. However, it is based on a different operating procedure.
The foundation for the AW-OZ risk assessment is subsection c of the Directive [1]: “the installations operated by the employer, the substances and mixtures used, the processes taking place and their interactions.”
Based on the obtained knowledge, the level of risk associated with the design and working conditions of the equipment can be quantified. However, the other subsections of the recommended methodology for explosion risk assessment [1]:
- probability and frequency of occurrence of explosive atmospheres,
- probability of occurrence and activation of ignition sources, including electrostatic discharges,
- magnitude of the expected consequences of an explosion,
are estimated in the final stage of the AW-OZ assessment, only after the equipment has been classified to a certain level of risk, based on process knowledge.
