Living with the Threat of Combustible Dusts
When you are young, cleaning up your bedroom was a chore. How many times can you remember hearing the familiar refrain, “Go clean your room”, and having to trudge off to fix up a messy bedroom? The consequences for procrastinating could have been being scolded or grounded. We never worried about our personal space igniting into a destructive explosion.
Dust collecting on your bedroom dresser was not going to deflagrate or explode.
Housekeeping at work
Today, we work in dusty environments where many dusts are combustible. How many of us make the connection between good housekeeping habits at home and good housekeeping habits at work?
Look around your work environment. Do you see dust on overhead/horizontal surfaces in your facility? If so, you may have a potential problem. It is up to every individual that reads this to know or investigate the nature of the dust in your manufacturing environment.
Many dusts that result from manufacturing are combustible. Over the last twenty-five years while working in the field of industrial ventilation I have personally missed TWO Explosion events. Fortunately, I was in the wrong place at the right time!
The first event was an aluminum polishing operation where I toured the facility so I could provide a combustible dust collector proposal. I thought it odd that I could taste aluminum when I left the plant. Hint: if you can taste the particulate that is combustible, the concentrations of dust may be of sufficient quantities to foster a combustible dust explosion.
The enterprise I visited had a proposal in four or five days. Thirty days later, BOOM! An aluminum dust cloud ignited where two employees were grinding aluminum. Aluminum burns fast, about 24,000 feet per second. It also burns hot. The clothing worn by the employees melted to their bodies, resulting in third degree burns.
The second incident caused an explosion in a PVC plastics plant I was working in. I recall returning a ladder to an area of the plant where plastic dust was piled up like snow drifts about six inches deep along walls and equipment. A year later, an employee lost their life in that explosion.
Even today, we as a company are invited to enter facilities where there are more than sufficient amounts of combustible wood dust, coffee dust, P.E.T. plastic dust and many others, yet some institutions continue to operate without safeguards in place.
My odds of being in the wrong place at the wrong time increase over time. A plant explosion is a result of an alignment of at least five factors. See the pentagram of conditions below. So, you look at it and say to yourself, we don’t have that possibility here at our plant. Let’s examine the forensic findings of a large plastics facility that was leveled by a single explosion and resulting fire.
What does it mean?
OSHA has identified combustible dust as, “. . . defined as a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations” Critically, the dust must be fine to cause an explosion
Are we getting it?
OSHA recently re-issued the combustible dust initiative – CPL-008-003.
WHY did OSHA have to re-issue a safety directive?
The reason for the re-issuance was due to the Imperial Sugar plant explosion in
Georgia. One of the plant’s managers predicted the impending explosion. Yet, employees still went to work and managers continued to operate under the less than favorable conditions and the danger was plainly visible to the naked eye.
In Plain Sight
Are you cognizant of the visible signs of danger in your plant or are you oblivious to the potential danger of dust accumulation? Does your plant take a pro-active approach to safety or a reactive approach to safety?
The answer(s) surround you.
Ask or determine the composition of the products your plant manufactures or works with. OSHA publishes a Combustible Dust Poster that lists types of dust and their propensity to combust, explode or deflagrate. See it here -
There is a relatively inexpensive go/no-go test for determining if a suspected dust is combustible. Have your dust tested.If you have determined the dust is combustible, walk to the area of the plant where you know there is a buildup of dust. Look on overhead rafters, bar joists, tops of equipment, crane rails, conduits, pipes, etc. If the dust layer is thicker than a paper clip, it’s time to decide. You are now no-longer oblivious to the possibility of a future event that may change the course of your life and others around you as well as the potential for a future event to damage or destroy the facility you work in.
Institute a housekeeping plan per OSHA’s recommendation.
Verify that all processes that create dust are addressed per the ACGIH’s Industrial Ventilation, Manual of Recommended Practice (www.acgih.org) and NFPA’s Guideline publications for the handling of combustible dusts. See the NFPA website for details - http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=654
It is a well understood fact that dust explosions are a serious matter. Dust explosions are a relatively frequent, highly preventable occurrence. OSHA has developed an extensive list of materials that can deflagrate or cause a catastrophic explosion.
A Formula for Disaster
Dust explosions are not based solely off the fine granulation of the dust. There are five key components that must be met for each explosion to occur:
1. Combustible Dust
2. Confinement of Dust Cloud
3. Dispersion of the Dust Cloud within the Confined Space
4. Oxygen, as with any fire, and
5. The Ignition Source.
The combined effect of these five ingredients can produce a disaster.
Additionally, a key component to consider besides the main five conditions is the material being handled. Each particulate has what is called Kst value (speed of pressure rise) ranging between 0 - >300, and a P-max value (maximum pressure of a dust cloud explosion) determined by the dust in question. These two figures are used to determine the Explosion Severity Test.
Each kind of dust also has varied levels of explosion points determined by the ‘ignition sensitivity’ and the ‘explosion severity’. The explosive index is a set range between 0 - 10. Materials low on the scale are at low risk, and those higher far more volatile. This scale is similar to the Kst and P-Max determined Explosion Severity Test.
An occasionally overlooked combustible fuel besides many of the commonly considered combustible dusts like wood by-products, fugitive grain dust, and combustible metals is the plastic industry. No matter the fuel, the devastating results remain the same.
The plastic industry has experienced many of the same challenges faced by others handling combustible materials. OSHA has listed many guidelines on how to handle combustible material, but much of the implementation is left to the employer’s discretion. This leaves many tough questions for manufactures who need to meet shareholders expectations via the bottom line and at the same time, ensure a safe work environment for their employees and the future earnings of the company. This article will provide a general example of factors to consider when modifying, shaping, processing, blending, mixing, transporting, collecting, filling or handling combustible materials.
Out of sight – out of mind
It’s true the example plants above did not have all five conditions present during any one of the plants normal processes. However, over time a mixture of plastic dust built up layers above the ceiling forming a chemical mixture of combustible dust.
Then, one day the plant was destroyed when a small “event” occurred in an enclosure. The event sent a shock wave through the plant, which dislodged the “mix” of plastics in the rafters. As the combustible mix rained down towards the small fire created during the first event the second event destroyed the entire plant.
To clean or not to clean
When developing a plan to protect your business from a negative event involving combustible dust, it is critical to consider how each element of the five conditions necessary for a dust explosion are represented in your facility. For this article, let us create a mock company, Plastic Co., to represent the do’s and don’ts of the combustible checklist.
A common plastic used for varied applications is polyethylene terephthalate (PET) – more commonly known by its family name - polyester. PET has a relatively high Kst value of 156. Additionally, per studies conducted by the Department of the Interior PET ranks 7.5 out of 10 on their explosivity scale. In this article, we will use PET as an example of the product Plastic Co. produces to demonstrate specific ways in which combustible plastic dust can cause challenges, and explore possible solutions to them.
Plastic dust is a by-product of the manufacturing and recycling process. When the plastic composite is being machined, heated, and formed, bits are shaved off and distributed on the floor and in the air. The size of the dust is determined by the application or specific operation. This is critical to understanding how volatile your dust may be. Dust that is coarse has a less likely chance of explosion than dust that is very fine. OSHA stipulates that dust that can pass through a U.S. No. 40 standard sieve (420 microns) has the capacity to explode, combust or deflagrate.
Below: Dust collecting on pipes & rafters
Let’s look at our example of PET to see a specific application. PET dust in
certain applications can easily pass through a U.S No. 40 standard sieve. At Plastics Co. the manufacturing facility has high ceilings with cross beams for support, and ledges along the walls. These spaces provide surfaces that permit PET dust to accumulate quickly. The standard house-cleaning process employs compressed air blow down of the accumulated dust followed by a manual sweep of the piles. Although this process is inefficient (herding dust) it is also a high-risk behavior. Dust blown off ledges etc. easily recirculates in the plant and nearby HVAC returns where it is re-distributed into other areas of the facility! Eventually, these areas can accumulate to dangerous levels.
The standard rule by OSHA is any plastics over 1/8th of an inch, and in some cases 1/32” have the capacity to explode. 1/32” is the thickness of a single paper clip.
Now the layered dust shouldn’t be cause for too much alarm. Remember, it is the combined effects of the five conditions that cause the explosion. But it is important to keep in mind that most facilities are already “contained” by the walls of the building. Furthermore, the PET dust located on the rafters and ledges is a breath of air away from forming a dust cloud. This coupled with the compressed air cleaning having spread dust around to far corners of the facility has produced dispersion. And finally, the air itself within the facility produces the fourth piece to our explosive pentagon - oxidation. All that is left is the spark from a faulty wire, machine, or careless mistake.
When placed in comparative perspective, it becomes quite clear the virulent danger implicit in the manufacturing process of plastics, and all combustible material. These hazards increase the longer no action is taken. Don’t wait for disaster.
Allow your eyes to be the conduit for awareness and your knowledge to seek out a solution for yourself and others around you.
 See OSHA’s “Hazard Communications Guidance for Combustible Dust”