Tuesday 19th Nov, 2019

Should I be concerned about the explosive potential of my process dusts?

In 2017, a corn dust explosion killed five workers, injured 14 others and reduced a facility to rubble.

In this regular column, experts from specialist bulk materials engineering firm Jenike & Johanson answer readers’ queries around problems at their sites. In this edition, the company’s general manager, Grant Wellwood, explains the safety risks and explosive potential of dust.

If the bulk solids you are handling are oxidisable, you should definitely determine the explosive potential of the dusts inevitably generated by your processes.

When we think of process risk, we are often blinded by a cognitive bias known as the salience effect, more commonly referred to as the “squeaky wheels get the most oil”. This is often the case in many process developments when it comes to addressing risk elements, with the “wheels” being risk elements in the project’s master risk register and the “squeak” proportional to the rating it is assigned.

Now, this is not intrinsically negative as it makes sure the focus is on the elements that matter the most, as opposed to the ones with a higher level of professional interest associated with them.

But what if an important risk element is not on the list to start with, or has been assigned unrealistic likelihood or consequence score? According to this common model, it might not get the attention it demands until it is too late.

Explosive dust is a typical risk element that fits into this category. Many materials which are inert when they are in lump form are still chemically oxidisable, explosively so when they are finely divided, and whenever you handle materials, you will generate dust.

Therefore, the first step in an assessment of your process dusts is to determine whether they are actually combustible. Any material that will burn in air in a solid form can be explosive when in a finely divided form. Combustible dust is defined by National Fire Protection Association (NFPA) 654 as “Any finely divided solid material that is 420 microns or smaller in diameter (material passing a US No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed and ignited in air.”

Common examples include coal, sawdust, plastics and certain chemicals, and many otherwise mundane organic materials such as grain, flour, starch, sugar, powdered milk, cocoa, coffee, lime, and pollen. Powdered metals (such as aluminium, magnesium, and titanium) can also form explosive suspensions in air if finely divided.

While this is a broad classification, how many times have you had the explosive potential of dust from your process materials quantified? If any of the bulk solids you handle as part of your process (feeds, products, intermediates, additives, etc) are combustible, it is prudent to have them properly tested and assessed. Even products like mineral concentrates (Department of Mines Western Australia, 2019) are oxidisable and prone to combustion if and when they dry out. It’s also important to also test systems and peripherals such as dosing plants and dust controls for their risk potential.

If materials and wastes processed have an explosive potential, then there is a risk likelihood factor. This will need to be itemised on the register and an appropriate consequence score should be considered.

There is a sad history of major loss associated with dust explosions, many of which appear in the Combustible Dust Database (Cloney, 2019). Every dust related event has potential for significant loss and the number of organisations that think it could never happen to them seems to be increasing year on year.

Table 1: Dust Explosion statistics 2017
Table 1: Dust Explosion statistics 2017

In terms of the industries, materials and devices involved, the year on year trends are relatively consistent, indicating the lessons are not being learnt. Statistics from 2017 show that wood and wood processing was the industry that reported the most incidents (figure 1), with wood as found to be the most frequent material involved in a loss event (figure 2). Bagfilters topped the list of the equipment cited to be the root cause of loss events, followed by storage silos (figure 3).

Figure 1: Breakdown of industries reporting dust related loss events-2017

One of these incidents in 2017 statistics was the Didion Milling dust explosion, which killed five people and injured thirteen more. The investigation report (Chemical Hazard and Safety Board, 2019) should be compulsory reading for anyone with a process involving oxidisable dusts.

Figure 3: Breakdown of materials reported as being involved in dust related loss events -2017

So, if you think your project could be at risk, you should get proactive and undertake a dust hazard risk analysis (DHA). While there are many good general reference documents like the US Occupational Health and Safety Administration (OSHA) bulletin Combustible Dust in Industry, they are often not well suited to the incidental facilitator.

Figure 4: Equipment cited as being the root causes of dust related loss events -2017
Figure 4: Equipment cited as being the root causes of dust related loss events -2017

Conducting a comprehensive DHA is a specialised skill that ties back to bulk solids handling, system thinking and solution development. Fresh and experienced eyes can add a lot of value in this context, so to avoid distracting the core team from their mission, engaging an external resource to handle the issue is usually the best investment.

There are four main factors to look for when selecting a suitable DHA partner:

  • History of testing powders, working with bulk solids and analysing the issue of dust generation,
  • Appreciation of standard test scoping, sample sizing/receipt/processing, testing, and data interpretation/presentation,
  • Ability to see your value chain as a system and provide guidance on engineering solutions for safe dust handling as opposed to a simple factual report and/or impractical solution,
  • Experience

Do you have a bulk solids handling question?  Jenike & Johanson has developed the science of bulk solids flow and specialise in applying it to solve the most challenging bulk solids handling problems.  So why not put us to the test with your question? The harder, the better.

Note: The advice here is of a general nature.  Specific solutions are very sensitive to their circumstances; therefore, you should consult with a specialist in the area before proceeding.

References:

Chemical Hazard and Safety Board (CSB). (2019). https://www.csb.gov/. Retrieved from Factual Investigative Update-Didion Milling: https://www.csb.gov/assets/1/20/final_didion_factual_investigative_update.pdf?16220 Accessed February 6th, 2019

Cloney, C (2018), “2018 Mid-Year Combustible Dust Incident Report – Version #1” DustEx Research Ltd. Retrieved from http://dustsafetyscience.com/2018-Report

Cloney, C. (2019,). Combustible Dust Incident Database. https://dustsafetyscience.com/ Accessed February 6th, 2019

Department of Mines Western Australia. (2019). Department of Mines, Industry Regulation and Safety. Retrieved from http://www.dmp.wa.gov.au/Documents/Safety/MSH_SB_001.pdf Accessed February 6th, 2019

US Department of Labor. (2019). Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions. Retrieved from Occupational Safety and Health Administration: https://www.osha.gov/dts/shib/shib073105.html Accessed February 6th, 2019