Like the best Hollywood movies, it is perhaps the most exciting moment of the film when the building finally explodes, with all the props effects making the best shot, not letting any detail go, the explosive wave, the destruction, the heat of the fire and a huge flame that rises beyond what was the building leave us stunned, with the adrenaline of what will follow. The truth is that fantasy is not very far from reality, in this case, the subject is not so exciting when we learn that a loved one died or that a company was destroyed, product of the flames caused by the explosion in a production plant.
What is a dust collector and how does it work?
Basically, a dust collector, also called baghouse or baghouse, is a cylindrical or rectangular container capable of collecting impurities from a production area thanks to the suction caused by a fan. Inside the container there are filters, also called sleeves or bags, made of flexible material, which capture the impurities and, after a certain time, through a programmed air injection, the impurities fall by gravity towards the lower part of the collector. The product is removed through a lock or rotary valve, hopefully in more compact and larger particles that can be reused.
Can there be explosion risks in dust collectors?
According to Dust Safety Science’s annual report on incidents involving combustible dusts, dust collectors have been involved in explosive incidents that have caused substantial losses in terms of both personnel and facilities. When asked if there are risks with this equipment, the answer is a resounding yes. The graph below illustrates a little of what has been recorded with this type of equipment between 2018 and 2021 in the United States:
In short, dust collectors have a “magnet” for problems.
In the last few weeks alone, there have been reported incidents involving dust collectors, for example:
Case 1: Just on October 5, 2022 in New York City (USA) there was an incident associated with an aluminum dust collector at Briggs Stratton, a company dedicated to the manufacture of engines; the Daily Sentinel newspaper records the news. Although there were no injuries, there was considerable destruction around the dust collector. The matter is still under investigation. See full story at the following link: https://romesentinel.com/stories/fire-outdoors-at-briggs-amp-stratton-in-sherrill,145268
Case 2: On October 6, 2022 in Illinois USA) there was an associated fire at the powder collector of the Quad printing services company, fortunately there were no injuries, the Daily News reports the full story at the following link: https://www.effinghamdailynews.com/news/local_news/fire-at-quad-graphics-quickly-contained/article_c3b75212-45af-11ed-b090-2b7c5189c096.html
Case 3: On August 22, 2022 in Illinois (USA) there was another case around the metal dust collector in Westermeyer Industries, a family business dedicated to the manufacture of parts for industrial refrigeration, an employee who did not want to give the name indicated “that it all started in the dust collector and within minutes the building was engulfed in flames”, the plant was completely destroyed, the local newspaper WLDS reported what happened: https://wlds.com/multiple-alarm-fire-destroys-majority-of-westermeyer-industries-complex-near-bluffs/
These are some recent cases from the last few weeks, but what causes this equipment to become a “ticking time bomb”?
While it is an industrially used piece of equipment, it is one of the most neglected and overlooked.
How does a dust collector become neglected?
Neglected because people do not do periodic maintenance that involves constant cleaning to avoid prolonged accumulation of dust inside, add to that the bags or filters are broken, with this equipment work inefficiently, the dust accumulates in parts that do not have to be, For example, contact with mechanical parts such as the suction fan, that really NEVER has to happen, and although most equipment has differential pressure sensors to know if a filter is broken the “it is not yet time to stop” leads to a real oversight. Another factor is the failure of the lock, which in theory is responsible for removing the material collected in the hopper, leading the operator to intervene manually with tools (hammers, sticks, sticking his hand etc.) generating static, leaving inadequate air inlets and what to say about the solenoid valves, responsible for cleaning the bags in a controlled manner, if they fail there is no automatic cleaning and dust accumulation is a fact. If the suction fan does not operate with the standards with which it was established, the suction or vacuum inside the collector is a fiasco.
By operating inefficiently the accumulation in the plant is a reality, exhaust hoods do not pick up as they should, leading to dust in electrical chutes, light fixtures, hallways, offices, whether the dust collector is inside a building or not.
How can a dust collector be unknown?
Ignorance of what happens inside a dust collector is the most serious factor. NFPA 652 in appendix A mentions the 5 basic components for a product to generate problems inside a dust collector.
a) A dust small enough to burn rapidly and spread the flame.
b) A suspended cloud in a concentration greater than the minimum explosion concentration (MEC).
c) Confinement of the dust cloud by an enclosure or partial enclosure.
d) The atmosphere to support combustion.
e) An ignition source of adequate energy or temperature to ignite the dust cloud.
Next NFPA 652 -2019 reflects the fire pentagon as a factor of what will happen if they find the necessary components for fire, flash fire or an explosion to exist.
Why is a powder explosive?
Because it is in its nature, because it is a combustible powder, that is, perhaps we study the behavior of the raw material of our production processes from the operational point of view, for example, that an agricultural product has a certain component of humidity, temperature, granulometry, etc. compared to another, variables that affect the finished product to come out to what the customer wants. For years the different materials that are part of a production process have been the subject of study, perhaps we never imagined that powdered milk, malt, rice, aluminum and many more, given certain characteristics of granulometry, suspension and confinement are explosive, i.e. generate uncontrolled explosions raising the pressure and temperature of the container in which they are, causing inadequate combustion causing flames that can cause significant damage even DEATH.
Product intrinsic variables such as MEC, MIE, MIT, Pmax, KST are key to determine the damage that a dust can cause inside a collector.
MEC (Minimum Explosive Concentration) is the minimum amount of dust concentration required in size per unit volume (kg/m3) necessary to produce an explosive incident, any concentration less or even greater than this measure does not result in an explosion.
MIE (Minimum Ignition Energy) The minimum amount of energy in joules required to ignite a powder, in other words how sensitive a powder is to ignition by electric or static spark.
MIT (Minimum Ignition Temperature) The minimum amount of temperature that can cause ignition due to overheating of engines, bearings and any hot surface.
Pmax: It is the maximum pressure (barg) that can cause an explosion, it is what makes it more dangerous, equipment such as dust collectors are not designed to reach that pressure, so they break violently causing great damage.
KST: Dust deflagration rate (barg * m / second) is how fast the dust housed in the collector comes to generate the maximum pressure (Pmax), that speed is key to take control action. If the action is not fast enough, the flame travels uncontrolled through ducts causing more dangerous explosions (secondary explosions) that can even double the Pmax pressure and generate devastating effects.
There are specialized firms such as FIKE CORPORATION that offer the service of dust analysis to determine how dangerous it is. Perhaps the variables that make the difference in a dust are its Pmax and the KST. The NFPA has determined in a category of four groups which product is more explosive than another.
The graph below shows two examples of two different products to understand the difference in Pmax and KST.
From the above graph it can be seen that product one with Pmax1 and Kst1 is more explosive and devastating than product two with Pmax2 and Kst2 in the same container in cubic meters (m3), such as the dust collector, therefore, the response speed to mitigate or control the problem should be greater with product one than with product two.
With the arguments raised, it is necessary to know what dust we are dealing with, perform periodic and adequate maintenance, clean the filters properly, guarantee the suitability of the filters or bags, make the proper corrective actions and prevent sparks from being generated in fans, which help to counteract the inconvenience present in the collectors and that it becomes a time bomb. In fact, the collector does NOT explode, we are the ones who let it explode. But is it enough?
Explosion mitigation, a viable and economical solution…
No matter how hard we try to avoid a dust collector collapse, the conditions for an explosion will always be present, no matter how clean the plant is, no matter how many sensors the collector has, no matter how efficient the suction fan is, even if the probabilities are remote, the possibility of an explosion cannot be left to chance.
There are mechanical components called vent panels, which are designed to break once the explosive phenomenon occurs and relieve the energy of the explosion (pressure and heat) in a controlled manner from inside the collector, the graph below reflects what actually happens:
If there were an explosion the panels would not let the collector reach the Pmax but the Pred, i.e. a “reduced pressure” below the design pressure of the collector, so the collector is never going to break apart and much less hurt someone. What is sought is that the energy relieves in a controlled manner where I want and not where the collector “wants”.
As can be seen in the following graphs, an uncontrolled explosion produces a fireball in a sphere or ellipse at 360 ° taking everything that is around and a controlled explosion through one or more vent panels in a dust collector would look like this.
NFPA 68 (2018) in section 8.9 describes the associated variables and resulting geometry of a controlled explosion, in summary we can say that the resulting distance depends on:
With this geometry I could tell how much damage can be caused around the collector, its dependence is on how big the dust collector is, what product we are handling and the number of vents or vent panels. With this result we can see how big a “fireball” is and how we can be aware of it if it occurs.
FIKE CV Vent Panels – a “bomb shutdown” solution
FIKE’s vent panels offer a useful and effective solution to mitigate explosions inside dust collectors as they are designed to withstand vacuum pressure, the environment in which this equipment operates.
The panels are made of 316L stainless steel sheet and can be round or rectangular. One or several of them are placed around the dust collector; they can come with breakage indicators, capable of giving an electrical signal that shows the status if the panel is broken or not.
Among its features are:
- 100% venting efficiency.
- Composite design without fragmentation.
- High mechanical integrity.
- NFPA 68 compliant.
As if it were the best solution for a “time bomb”, the vent panels are undoubtedly a real “problem extinguisher”. Although the explosion may occur, with FIKE’s CV and CV-s reference vent panels, the explosion is controlled without affecting the company’s assets and much less the lives of its employees. Remember that the collector “DOES NOT EXPLODE” it is us who “LET IT EXPLODE” in an uncontrolled way and that is what takes its toll on us sooner or later.