Most people are aware that organic materials such as flour and sugar dust can cause large explosions. However, many types of metal dust are also explosive. Aluminum, bronze, magnesium, silicon and zinc can all have explosive characteristics. In fact, both magnesium and aluminum can be more explosive than flour or sugar depending on the circumstances.
In May of 2018 there was an explosion at the Meridian Magnesium Products plant in Eaton Rapids Michigan. It is believed that the explosion was associated with Magnesium dust in a conveyor tunnel. The plant suffered millions of dollars in damage and was shut down for several months. According to a report by the US Chemical Safety Board, 20% of all dust explosions from 1980-2005 were associated with metal dust (“Combustible Dust Hazard Study” Report No. 2006-H-1 – 2006). Various newer studies suggest that the relative frequency continues to be about the same.
To compare metal dusts with their organic counterparts, it is helpful to look at some of the characteristics that are used to calculate and design explosion relief systems and explosion suppression systems. Kst and Pmax are defined in NFPA 86 Standard on Explosion Protection by Deflagration Venting and are measured per ASTM E 1226, Standard Test Method for Explosibility of Dust Clouds. Kst is “the deflagration index of a dust.” Pmax is “the maximum pressure developed in a contained deflagration of an optimum mixture.” Together, they provide an idea how hazardous one type of dust is compared to other types of dust.
The numbers in the following table are taken from Appendix F of NFPA 86. Notice the relatively high numbers for Pmax and Kst for Aluminum and Magnesium.
|Material||Mass MedianDiameter(μm)||Minimum FlammableConcentration(g/m3)||Pmax(bar)||Kst(bar-m/s)||Dust HazardClass|
Metal dusts can behave much differently in comparison to organic dusts if a cloud is ignited. The flame temperature of some metal dusts can exceed 3,500°C, which is more than a 1,000°C higher than an organic flame. This can result in high explosion pressures, Pmax. Also, the pressure rate of rise, (dP/dt)max, as well as the flame speed, can greatly exceed that found with organic fuels. Taken all together, these factors can make it much more challenging to control an explosion involving metal dust.
When considering the loss control measures that should be taken for a process that generates metal dust, the basic approach is the same as it would be for any combustible dust:
- Always have the dust you are working with tested to determine the specific characteristics of explosibility. Variations in particle size, moisture and contaminants can all have an influence on the level of hazard for a given dust.
- Provide appropriate protection for all dust handling systems and collection systems. This would include a combination of explosion relief, explosion suppression and explosion containment. Explosion relief should always be ducted to the exterior of the building.
- Systems that handle or generate dust should be designed to minimize fugitive dust. Fugitive dust is the dust that escapes from the process equipment and settles on floors, equipment and building structures. In many situations where there has been a severe loss due to a dust explosion, it was the fugitive dust that caused the most damage. After an initial explosion inside a piece of equipment, fugitive dust is shaken loose from is resting places throughout a building and can then ignite large secondary explosions.
- Implement good housekeeping procedures to deal with any fugitive dust that cannot be contained inside equipment. Schedules should be designed to keep dust accumulations to an absolute minimum, and procedures should cover dust found on floors, equipment, and building structures (including roof beams and bar joists).
- Never use compressed air to blow down fugitive dust. Sweep and vacuum only.
There are some important and critical differences when it comes to metal dusts:
- Explosion relief systems that were designed with organic materials in mind may not be adequate for metal dusts. Explosion relief should always be designed and evaluated considering the specific type of dust involved.
- Explosion suppression systems designed for organic materials may not be suitable for metal dusts. The detection systems may not be able to react fast enough. Some types of suppression agents may not work effectively with metal dusts and could even react to make the explosion worse. Suppression systems should always be engineered and tested for the specific dust involved.
Any process that generate metal dust should be given careful consideration. These processes might include sawing, grinding, buffing and polishing operations conducted on metal or metal alloys – especially aluminum and magnesium. Contact Risk Logic with any questions you may have regarding the hazards of metal dusts.