Ignition risk assessment to the non-electrical equipment from hydrogen production, storage, transport and use facilities

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Introduction
Hydrogen has been used for long in a large range of industrial sectors including mainly chemistry (ammonia, fertilizers), oil and gas industry, metallurgy and electronics.In the context of decarbonisation, hydrogen is becoming increasingly important as an energy carrier (thermal utilisation) and a starting material for chemical production lines (molecular utilisation).Hydrogen technologies are undergoing a significant period of growth as their widespread introduction is planned in the energy generation and production industries.Those who knowledge of the properties and flammability of hydrogen will recognise that hazards due to explosive atmospheres must be assessed more and more frequently as the element is https://doi.org/10.1051/matecconf/202438900035SESAM 2023 utilised, and, if necessary, appropriate protective measures must be taken.Hydrogen is a substance that occurs in gaseous form at normal atmospheric temperature and pressure.Hydrogen is a lighter gas than air, which diffuses rapidly with a tendency to rise upwards.The flammability range of hydrogen in air is between 4% and 77% by volume.The flammability range is 5-10 times higher than for conventional hydrocarbons, the minimum ignition energy is 5-10 times lower and the maximum burning rate is also about 5-10 times higher -shown in table 1 and figure 1, comparison of explosion characteristics under atmospheric conditions.In other words, hydrogen explosions lead to much greater consequences.As an intrinsic property, hydrogen is very flammable requiring only a small amount of energy to ignite.As can be seen in Figure 1, hydrogen has a wide flammability range compared to methane and gasoline.In the case of technical installations for industrial processes relating to the production, storage, transport and use of hydrogen, where potentially explosive atmospheres are likely to occur, the risk of explosion must be minimised by taking all necessary technical and organisational measures [1].One of the technical measures required in such spaces classified as potentially explosive atmospheres (also applicable to potentially explosive atmospheres caused by the presence of hydrogen) is the use of technical equipment (electric and nonelectric) intended for use in potentially explosive atmospheres.They must, within the EU, comply with ATEX Directive 2014/34/EU and the applicable harmonised standards.These harmonised standards under the ATEX Directive 2014/34/EU are applied for the purpose of making and assessing explosion-protected equipment [2].
The equipment must be subjected to a formal documented ignition hazard assessment to identify all potential ignition sources that could occur during normal operation, expected malfunction and rare malfunction.Then, depending on the intended EPL (Equipment Protection Level) of the equipment, mitigation can be applied to each of these potential https://doi.org/10.1051/matecconf/202438900035SESAM 2023 ignition sources to minimize the likelihood that they could become effective ignition sources [3].
The importance of protective measures depends on the likelihood of an explosive atmosphere and the consequences of a possible explosion.In this line, the classification of non-electrical equipment is done making a difference between different categories of equipment and protection levels as specified in Directive 2014/34/EU taken into national legislation by GD 245/2016.These categories reflect the measures for different areas and conditions of hazard the non-electrical equipment's for potentially explosive atmospheres are classified in groups and categories -see the table 2 [4].

Methodology for the risk assessment of non-electrical equipment
Ignition risk assessment is a process that consists of a series of logical steps that enable designers and safety engineers to examine in a methodical manner the function of an equipment arising from its use in a potentially explosive atmosphere and to decide whether protective measures and/or types of protection are required -see the Figure 2 [5].Ignition risk assessment includes the following four steps: -product description performance, lifetime and configuration; -identification of ignition hazards; -ignition risk estimation; -ignition risk evaluation.
For equipment and components, identification of the potential ignition sources is the most relevant part of the ignition risk assessment.This paper presents this specific risk assessment procedure and the information required to allow ignition risk assessment to be carried out for the design of equipment or components, as well as recommendations for a decision to be made for the categorization of equipment.In this procedure, the following information is to be taken into account: a) Possible occurrence of an explosive atmosphere inside the equipment or component or penetrating the equipment or component from the outside (in normal operation or during malfunctions) and the amount of explosive atmosphere involved leading to a possible explosion impact inside the equipment or component; b) equipment or components surrounded by an explosive atmosphere (in normal operation or during malfunctions); c) equipment or components wholly or partly surrounded by an explosive atmosphere, including any explosive atmosphere in connection (in normal operation or during malfunctions); d) the presence and likelihood (effectiveness) of ignition sources.

Fig. 2. Ignition risk assessment for design of equipment or component
An important aspect to mention is that the protective measures / type of protection and equipment conformity are not part of methodology for the risk assessment.They are an integral part of the conformity assessment process for equipment intended for use in potentially explosive atmospheres, in accordance with the specific safety and health requirements for the design and manufacture of equipment intended for use in potentially explosive atmospheres.
For identification of all possible ignition hazards it is important to proceed systematically and do it without any assessment aspects to avoid restrictions in thinking.For the analysis of the possible ignition hazards, all utilisable information sources should be used (discussions with experts from test laboratory, universities, users, other manufactures etc.) and all accessible examples should be examined to perceive analogy.

Evaluation of ignition sources specific to non-electrical equipment
Ignition risk evaluation is the comparison of the estimated ignition risk against given criteria to determine the intended level of protection.The level of protection is related to the defined categories and the related requirements.From the ignition risk evaluation emerges whether additional measures shall be required in order to meet the target category.
Ignition of explosive atmospheres by non-electrical equipment occurs when energy supplied by equipment used for processing or conveying of material is converted into heat, usually as the result of a mechanical failure of the equipment or associated systems [6].The most common ignition sources specific to non-electric equipment are hot surfaces, https://doi.org/10.1051/matecconf/202438900035SESAM 2023 mechanical sparks and electrostatic discharges [7].These types of igniting sources develop sufficient energy to initiate the explosive mixtures generated by hydrogen.
Analysis of the physical processes that lead to ignition by mechanical equipment shows that there are at least two key stages: production of heat by conversion from kinetic energy and transfer of heat to the surrounding explosive or flammable atmosphere.The first stage of this sequence is the area that offers the greatest potential for preventing ignition of an explosive or flammable atmosphere by possibly limiting the power/energy fed into the conversion process.In general, the friction processes that need to be considered are rubbing (long duration friction between surfaces producing a hot surface), grinding (long duration friction producing hot surfaces and sparks) and impact (short duration friction producing short duration transient hot surfaces and sparks), or a combination of these.Fundamental studies of friction such as Jaeger [8] and Kragelsky et al [9] show that the power dissipated in the contact region is dependent on material properties, such as hardness, melting point and thermal conductivity, and the conditions of the actual friction event such as rubbing speed, contact force and contact area.
Considering these aspects as well as the explosion characteristics defined for hydrogen, in the specific references of non-electrical equipment intended for use in potentially explosive atmospheres generated by hydrogen, there are a series of requirements for the type of materials used as well as for the energy limits in case of a single impact, depending on the equipment category, EPL (Equipment Protection Level), and explosion subgroup.Group II equipment is subdivided according to the characteristics of the explosive gas atmosphere for which it is intended -see the table 3.For highlighting the specific requirements for specific equipment in sub-group IIC (characteristic gas is hydrogen) compared with the ones in sub-groups IIA and IIB, in the following are presented a series of requirements for limiting the energy of a single impact in case of group II equipment with EPL Ga, Gb and Gc -see tables 4, 5, 6. Analysing the values presented in the above tables, it can be noticed that for non-electrical equipment in sub-group II intended for use in potentially explosive hydrogen atmospheres, the energies are significantly lower compared to those specific to non-electrical equipment in sub-groups IIA, respectively IIB.
In view of all this, the design, manufacture, conformity assessment and therefore the assessment of the ignition risk of non-electrical equipment intended for use in potentially explosive hydrogen atmospheres requires particular attention for both the manufacturer and the staff involved in assessing the conformity of non-electrical equipment with the specific health and safety requirements laid down in Directive 2014/34/EU.
If the manufacturer has not achieved the intended level of protection with this categorization, he has to start a new ignition risk assessment taking into account different design and/or taking further measures until he reaches a level of protection as high as necessary to achieve the intended category taking into account the safety and health requirements.

Table 1 .
Comparison of explosion characteristics under atmospheric conditions

Table 2 .
Relation between Equipment protection levels (EPLs) and zones

Table 3 .
Explosion subgroup for equipment group II:

Table 4 .
Single impact energy limits for EPL Ga

Table 5 .
Single impact energy limits for EPL Gb

Table 6 .
Single impact energy limits for EPL Gc