Explosion hazards during the blasting of metals

Status:

completed 12/2009

Aims:

During the blasting of light and other metals in blasting cabinets, dusts are generated by the abrasion of blasting agent and workpiece. The dusts frequently contain further substances from the surfaces of the workpieces, such as paint or anticorrosive oils and agents. These blasting dusts are exhausted from the blasting cabinet, and accumulate in the dust filter system. Studies conducted to date at the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) into the combustion and explosion behaviour of blasting dusts from dust collectors produce widely varying results. These range from "no dust explosibility" and burning index BZ 1 (no ignition) to "explosible" (dust explosion class St2) and burning index BZ 4 (spread of a glowing fire). In some cases, comparatively low values are also determined for the minimum ignition energy.
Based upon these study results, it may be assumed that in many cases, explosive dust/air mixtures may arise in the dust filter systems of blasting installations, particularly during cleaning of the filter elements. A large number of fires and explosions in such filter systems (cf. for example BIA Report 11/97) confirm this assumption. The systems frequently use filters which are not intended for the exhaust of combustible dusts.
The purpose of the project was to study the combustion and explosion behaviour of blasting dusts from installations with known boundary conditions in order to extend the existing body of data. In addition, the project was to examine the extent to which paints, preservation oils and other organic constituents and inert constituents or metal oxides present in the blasting dusts influence their behaviour. Based upon the results of the studies, the processes, types of blasting installations and areas within the installations and also certain combinations of workpiece and blasting agent were to be identified which present a hazard or are associated with particularly high hazards.

Activities/Methods:

Comprehensive studies were planned of the combustion and explosion behaviour of blasting dusts from installations with known boundary conditions. The studies were to determine the combustion characteristics, lower explosion limit, maximum explosion pressure, dust explosion constant, glow temperature, ignition temperature and minimum ignition energy, and also the spontaneous combustion properties. In the past, many studies of blasting dusts determined only the burning index and the facility for explosion, or failed to provide information on the installations from which the samples originated.
Where possible, dust samples should also be examined for flammable organic constituents and for inert or fully reacted constituents.

Results:

Fire and explosion hazards may be caused in blasting installations not only during the processing of light metals, but also by materials made of steel. This particularly applies to the filter systems in these installations. Evaluation of the laboratory analyses however shows the explosion hazards presented by blasting dusts to be considerably lower; frequently, only fires or low-pressure explosions occur. Stricter safety measures are to be recommended for blasting dusts of burning index 4 and dust mixtures comprising steel/cast steel and aluminium.

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