OHS Canada Magazine

Combustible dust, which is a mixture of fine solid particles that are liable to catch fire or explode upon ignition when dispersed in the air, is a hazard com­mon to many industries. Woodworking facilities, chemical manufacturing, food production, grain elevators and any fa­cility that manufactures or processes sugar, flour and metal are workplaces where combustible dusts are present.

While well-intentioned companies try their best to man­age explosive dusts in their facilities, many make mistakes that render their hazard-mitigation ineffective — even costly.

In 2018, there were 194 dust fires and explosions result­ing in one fatality and 39 injuries in North America, com­pared to 145 dust fires and explosions resulting in six fatali­ties and 52 injuries in the previous year, according to Jason Reason, director of combustible dust services with SEAM Group in Indianapolis.

While conducting a dust-hazard analysis (DHA), imple­menting controls and documenting the effectiveness of the preventive measures taken is a good process to follow, “there are a lot of mistakes that occur,” says Reason, who spoke at Safety 2019 in New Orleans on June 10. “Most of the time, it is possible to avoid these mistakes.”

Some common combustible-dust mistakes are as follows:


Ignorance about dust hazards

The most common mistake is not knowing the hazards of the dust present in a workplace. “Wood dust is not wood dust; corn is not corn,” says Reason, adding that there are differ­ences that affect their explosive properties. He showed a vid­eo of sparks emitted each time rain hit iron dust on a filter. “Dust is so small that when water hits it, it actually sparks. And this is just plain iron dust coming out of a steel shop.”

Testing is necessary to know if combustible dust is pres­ent and what type of dust it is, so that appropriate safety measures can be taken. “This is why knowing your dust becomes invaluable,” Reason says. “Whatever test data you have is going to affect the next step.”

Not understanding a dust-hazard analysis

According to Reason, many people do not understand what a DHA is. So what exactly is a DHA? “Let me tell you what it is not,” he says. “It is not dust testing,” which indicates whether combustible dust is present in a workplace, but does not tell you whether or not you have a hazard. “The DHA is going to tell you if you have a hazard.”

A DHA is a systematic review to identify and evaluate the potential fire, flash fire and explosion hazards associated with the presence of one or more combustible particulate solids in a process or facility. Reason describes a DHA as a road map to hazard mitigation and shows a company what it needs to do to mitigate the risk. An effective DHA will gather and review information, conduct onsite visits to ob­serve processes, evaluate risks and safeguards, issue a report with detailed recommendations and provide a finalized re­port based on feedback and comments.

Elements of an effective DHA include looking at the following: sources of ignition and industrial ventilation sys­tems; dust testing and dust reactivity; environmental health and safety programs and policies; housekeeping and clean­ing methods; fire-protection, explosion-protection and pre­vention systems; preventive maintenance programs; emer­gency egress; and personal protective equipment.

A DHA that is done right can yield five to 10 times re­turn on investment in the form of savings from unnecessary controls and sometimes increasing productivity, he adds.

Under- or over-testing

The other common mistake is not doing enough testing or over-testing. Testing may not be required where reliable in-house, commodity-specific testing data or published data of well-characterized samples are available. But the absence of previous combustible-dust incidents cannot be used as a ba­sis for deeming a dust to be non-combustible.

As there are easily a dozen types of dust tests out there, Reason advises companies that may have combustible-dust hazards on their premises to conduct a DHA from scratch.

“What you are trying to do is to figure what is the mini­mum number of tests you need to do and the minimum number of samples you need to take to do whatever it is you are trying to do,” he says.

The importance of a qualified person

A DHA must be performed or led by a qualified person who possesses a recognized degree, certificate, pro­fessional standing or skill and who by knowledge, training and experience has demonstrated the ability to deal with problems related to the subject matter, work or project.

Reason lists safety professionals, engineers, engineering firms and insurance carriers in his list of internal and ex­ternal personnel who are not qualified to conduct a DHA.

An unqualified individual will downgrade the existence or seriousness of a combustible-dust hazard. Either they may find it and not say much about it, or just flat out ignore it,” Reason says. An unqualified person may also recommend the implementation of unnecessary hazard-mitigation con­trols that drive up operating costs.

The following are good questions to ask to determine if an individual is qualified to perform a DHA: the experi­ence the individual has that qualifies him or her to perform a DHA; how many DHAs he or she has performed; has the person performed a DHA for similar processes; can the per­son provide references from previous companies they have conducted DHAs for and provide a sample DHA report; and who will be performing the DHA, or will it be outsourced to a third party.

Asking the individual to describe the DHA process is also a good indicator of their knowledge and experience in this area. “Unfortunately, there is not a prescriptive way to do it. There is a lot of wiggle room in this,” Reason says.

It is also important to read the report that follows a DHA and determine whether it will address existing problems and if the recommendations can practicably be acted upon

Testing raw materials

For facilities that mix raw materials, Reason offers a word of caution with regard to testing raw materials to determine the presence of combustible dusts and their explosive prop­erties, because the dust that comes out of a mixer is very dif­ferent from the individual properties of the raw materials that go into the mixer.

“I have had facilities that spend $50,000 or a $100,000 testing raw materials and still did not get all the test data they need, because all those raw materials don’t end up in the dust collector — your mixer does. So you got to test those mixers, because there are no reliable data for those.”

When it comes to mitigating combustible-dust hazards, administrative controls (like housekeeping, training work­ers and preventive maintenance programs) rank above engineering controls as the former tends to be more cost-effective. “They require a lot of babysitting to make sure they are implemented right, but from a cost perspective, they are a lot less,” Reason says of administrative controls.

“If those don’t work, that is when you want to go to engi­neering controls.”

Measuring combustible dusts

Combustible-dust explosions often involve two explosions: primary and secondary. The primary dust explosion is the first explosion that occurs when dust suspension in a confined space is ignited and explodes. This first explosion will shake other dust that has accumulated which, when airborne, also ignites. A secondary dust explosion is often more destructive than the primary one. The following are measurements to determine the explosive properties of combustible dusts:

  • Kst is a dust deflagration index that gives an indica­tion of the severity of a dust explosion. The larger the Kst value, the more severe is the explosion.
  • Maximal Pressure, or Pmax, defines the maximum ex­plosion pressure of a cloud of dust under standardized conditions.
  • Minimum Ignition Energy, measured in millijoule, refers to the minimum amount of energy required to initi­ate the combustion of a cloud of dust, vapour or gas. The lower the number, the more hazardous it is.
  • Minimum Explosive Concentration measures how much dust need to be present to cause an explosion. The lower the number, the more hazardous it is.

Jean Lian is the former editor of OHS Canada.


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