This question repeatedly arises in the context of explosion safety during abrasive blasting of metals. This article encourages contributions from practitioners and specialists in risk assessment and risk management, including those from the insurance sector, to enrich the analysis with their knowledge and experience.
To begin, it is worth clarifying what metal abrasive blasting is. It is a technological process in which the surfaces of metal components are cleaned using a stream of compressed air and an appropriately selected abrasive medium, such as glass beads, corundum, sodium bicarbonate, steel shot, slag, or sand.
During abrasive blasting, particularly when conducted in enclosed chambers, a variety of dusts are generated. These contain particles originating from the processed material (e.g., metal oxides) as well as secondary contaminants created during the removal of coatings, degradation of surfaces, and chemical deposits. Such contaminants may include combustible components such as solvents, metallic pigments, organic compounds, oils, greases, corrosion inhibitors, welding auxiliaries, silicones, waxes, and other hydrocarbon-based substances.
The case analysed here concerns abrasive blasting processes using media such as cast steel shot, glass beads, or corundum. In such installations, the abrasive is recirculated and separated, for example in cyclones, which separate dust generated in the blasting chamber from reusable abrasive media. The separated dust is subsequently transported to a dust collection unit, while reusable abrasive is returned to the process.
Numerous documented cases of fires and explosions occurring in cyclones and baghouse filters involving metal dusts, including those associated with shot-blasting operations, indicate that this hazard should not be underestimated. Therefore, it is essential to identify the processes, technological operations, and critical areas within the installation, and subsequently perform an appropriate risk assessment. The analysis should determine the extent to which contaminants such as paints, oils, other organic constituents, and metal oxides present in collected dust contribute to increased fire and explosion hazards.
In some cases, the dust generated during the process may not exhibit explosive properties under test conditions, nor show a tendency to ignite or propagate combustion along deposited dust layers (flammability class according to VDI 2263: BZ1 – no ignition and no flame propagation). In other cases, however, dust may exhibit combustible properties and be capable of forming an explosive atmosphere while deposited dust layers may be classified as highly susceptible to flame spread (flammability index BZ4 on a six-point scale – dust readily ignites and combustion spreads rapidly along the surface of the layer). Depending on the case, relatively low minimum ignition energy (MIE) values may also be determined, which is highly relevant when assessing elevated explosion risk.
A particularly hazardous situation arises in shot-blasting installations where dust collection units are located adjacent to the blasting chamber inside the production hall and are not designed for handling combustible dust. Due to the non-combustible nature of the abrasive medium, preventive measures or explosion protection are often not implemented. This hazard is particularly relevant in dust collection systems where fine fractions generated during the process exhibit altered physicochemical properties compared to clean abrasive material.
The risk of severe consequences also increases when secondary contaminants accumulate in dust collection units, dust-laden air ducts, and on other surfaces. A typical example is the automatic regeneration of filter elements using compressed air, which, during normal filter operation, may cause accumulated dust to become dispersed. Dust dispersed in this manner may form an explosive atmosphere, which, in the presence of an ignition source, may lead to an explosion. Therefore, despite low concentrations of dust in the process stream, accumulated dust layers inside the filter housing and on filter elements represent a constant potential for forming a dust cloud at concentrations within explosive limits. It should also be noted that dust filtration inherently leads to the separation and accumulation of fine fractions. Fine, combustible particles originating from residual paint coatings and other organic materials removed during processing remain suspended for longer periods and tend to deposit on the surfaces of filter housings and filter elements. Conversely, the coarse fraction, originating from generally non-combustible abrasive material, is more rapidly separated from the fine fraction due to differences in settling velocity. Consequently, an increased proportion of fine, combustible dust accumulates in the filter compared with the incoming material.
End-users often assess the risk based on the small amount of dust discharged by the collection system into drums or containers, which leads to the erroneous assumption of a low concentration of combustible material in the filter. In addition, the material leaving the filtration system often appears oxidised, which may suggest reduced reactivity of the dust.
However, this might not be the case for the internals of the filter, as proven by several incidents investigated by ISMA.
Summary
Numerous studies (DGUV IFA, IChemE, DSS) demonstrate that dust generated during metal processing, including shot-blasting, may exhibit combustible and explosible properties, particularly in enclosed systems such as dust collectors. Such dust may have low minimum ignition energy (<<10 mJ) and be classified as St1–St2, and may undergo rapid deflagration resulting in maximum explosion pressures of approximately 6–10 bar. If metal dust is present, even St3 and higher explosion pressures can be reached, depending on the type of metal.
Explosions in abrasive blasting processes are typically characterised by low explosion severity (low maximum explosion pressure and low maximum rate of pressure rise). Nevertheless, despite their generally low intensity, they remain hazardous to surrounding equipment and personnel, and secondary fires resulting from such events represent an additional risk factor that should be included in risk assessments.
Unfortunately, there is no comprehensive database of documented fires and explosions in shot-blasting operations, as such events are generally not reported. There is also no governmental or private institution that systematically collects and analyses data in this area. Nevertheless, based on the experience of firms conducting post-incident investigations (e.g., ISMA NV from Belgium), it can be concluded that the hazard is real and requires the implementation of appropriate preventive measures and, in many cases, technical safeguards.
Personnel responsible for safety at a facility should first commission tests to determine dust combustibility and explosibility parameters. However, representative sampling is a key challenge. Dust at the outlet of a dust collection system often contains predominantly non-combustible abrasive fractions, while samples collected from filter elements may not reflect actual process conditions, especially since dust composition changes over time, with a risk of underestimating important parameters like the explosion vehemence. (Kst-value). This variability is a critical factor when assessing whether a system transitions from safe conditions to conditions enabling the formation of an explosive atmosphere. The comparison of test results with worst-case literature values is therefore strongly advised.
Subsequently, based on test results, operators should consult appropriate specialists to guide further actions, such as the Belgian company ISMA, a recognised and highly experienced consultancy with over 100 explosion-related incident investigations and more than 5,000 explosion risk assessments conducted.
If dust is confirmed to exhibit combustible and explosible properties, more rigorous technical and organisational measures should be implemented in abrasive blasting processes, particularly where high-flammability and high-explosibility dusts are present.
Authors: Sebastian Słaboszewski and Michel Vandeweyer ISMA NV
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