We Are Stonehouse Process Safety

February, 2008

Coal dust silo explosion injures 6 workers

Stonehouse Process Safety commentary: Silo explosions are not uncommon. Over the years there have been many silo explosions from combustible powders ranging from agricultural products to various chemicals, and in this particular case, coal. Often the finer particles (dust) remain in suspension in the silo headspace, with a concentration that exceeds the minimum required to form an explosible dust cloud. An explosion will occur if a sufficiently energetic ignition source is present. Common ignition sources include burning embers entering the silo from upstream processes, static electricity, friction/impact sparks, and self-heating. Coal powder/dust in bulk is particularly susceptible to self-heating. Self-heating has been identified as the sources of many reported coal dust fires and explosions in silos, railcars, and dust collection systems. There are practical techniques available for ensuring safety including control of the likely ignition sources, but explosion protection is generally also required.

The following text by TOM KERTSCHE and DON BEHM, Journal Sentinal has been redacted to remove company names.

“Oak Creek – An explosion Tuesday morning inside a coal dust silo rained flames down on a group of contract employees who were making preparations for repair work to begin.

Four employees were inside the 65-foot-tall structure and two outside when the explosion occurred, said a We Energies spokesman. A doctor said a 43-year-old man pulled his son, 22, and at least one other co-worker to safety.

The 22-year-old was the most severely injured, suffering burns to more than half his body, according to Tom Schneider, medical director of the Columbia St. Mary’s Regional Burn Center in Milwaukee.

The cause of the blast, reported at 10:53 a.m., has not been determined. Federal and local authorities will be investigating, officials said.

The six workers are employees of the Milwaukee branch of a Waukesha-based company that provides scaffolding services, said Michelle Dalton, a company spokeswoman. She would not identify the workers.

After the blast, firetrucks and rescue squads from as far away as Wauwatosa and the North Shore responded.

Two of the victims were transported to hospitals by helicopter, said Oak Creek Assistant Fire Chief Tom Rosandich.

The 43-year-old and other workers described a bit of their ordeal while the emergency room doctors and burn unit surgeons tended to them in the early afternoon.

They described the fire rolling down at them from the top of the silo, and a fast scramble to escape through a door. One worker jumped from a scaffold to escape, according to Schneider.

The 43-year-old and a 23-year-old co-worker were treated at Columbia St. Mary’s and released Tuesday afternoon. They had minor burns to their hands and faces.

Three other workers, ages 27, 29 and 34, suffered second- and third-degree burns, also predominantly to their hands and faces. All were in fair condition, Schneider said.

He said one of them will need skin grafts on his hands, likely requiring a hospital stay of 10 to 15 days, but the other two should be discharged within a day or two.

The 22-year-old worker was taken to Froedtert Hospital in Wauwatosa because of challenges in establishing a clear airway, Schneider said.

He was later transported to Columbia St. Mary’s, where he was in critical condition Tuesday evening, said hospital spokeswoman Kathy Schmitz. No further information on his condition would be released, Schmitz said.

The silo, one of nine at the plant, is used to collect coal dust that accumulates from coal that is brought by train to the plant, said We Energies spokesman Barry McNulty.

He said the dust is compacted and, like coal itself, is burned for fuel.

Much like gas vapors, coal dust becomes explosive when it reaches certain concentrations in an enclosed area.

An explosive concentration would obscure objects viewed from about 6 feet away, according to Guy Colonna, a combustible dust expert with the National Fire Protection Association.

The lightest dust particles become the most hazardous, rising unnoticed to the upper reaches of a workspace, he said.

Colonna said heat or sparks from operating machinery, static electricity or some type of cutting or welding are common ignition sources in industrial settings.

The coal-handling facility that includes the silo that exploded is part of a $2.3 billion construction project.

Thomas Content of the Journal Sentinel staff contributed to this report.

Rail car facility at silo had undergone repairs

By Don Behm of the Journal Sentinel

Posted: Feb. 3, 2009

“The coal dust collection silo where an explosion occurred Tuesday at an Oak Creek power plant had not been used since at least Jan. 25, said Tom Zelinski, an air management engineer with the state Department of Natural Resources in Milwaukee.

Zelinski said that on Jan. 25, the utility reported mechanical problems there.

Prior to Tuesday’s blast, the silo was to resume operating today. Zelinski said he had planned to inspect the dust collection system Thursday to check whether it was operating properly.

Heavy snow that accumulated atop coal in rail cars this winter caused intermittent problems with the collection system, Zelinski said.

The Oak Creek plant burns sub-bituminous coal shipped by rail from the Powder River Basin in Wyoming, said Timothy Le Monds, a spokesman for the state Public Service Commission in Madison.

Powder River Basin coal comes with a lot more dust particles than bituminous coal, Le Monds said. The dust is collected because it is a fuel that can be burned in the plant. Collection also prevents its release to the air.

Ryan Haggerty of the Journal Sentinel staff contributed to this report.

March 2011

Workplace safety practices reviewed

Stonehouse Process Safety commentary: Highly flammable chemicals are in widespread use in many process industries. For as long as these materials have been in use they have been the fuel for numerous fires and explosions with a devastating loss of life, injuries, and environmental and property damage. A low energy ignition source is often more than capable of igniting flammable vapor and gas atmospheres. Common ignition sources include electrostatic discharges, frictional sparks, and electrical arcs and sparks. Safety from fires and explosions could be based on avoidance of flammable atmospheres, controlling ignition sources, or explosion protection. It is often possible to achieve an effective and practical method for ensuring safety based on the specifics of the process and pertinent properties of the chemicals.

The following text by MARTIN FINUCANE, Boston Globe [mfinucane@globe.com] has been redacted to remove company names.

“Federal investigators are looking at workplace safety issues at a manufacturing plant in Middleton where a chemical explosion Sunday injured four workers.

State Fire Marshal Stephen D. Coan said his office and local fire officials were investigating the cause of the fire, while investigators from the US Occupational Safety and Health Administration officials are reviewing workplace safety.

He said investigators continued to examine the scene, interviewing witnesses, and learning more about the chemical processes that were used in the building.

“Investigators continued to conduct interviews and their forensic scene examination today, both of which were very helpful,’’ said Jennifer Mieth, a spokeswoman for Coan’s office. “They are still working to determine the ignition source and which of the many possible flammable materials were involved in the explosion.’’

In an interview, Coan said yesterday that the chemical nature of the fire demands a more complicated and delicate investigation.

A chemical engineer and explosives expert may be called in to consult.

“It’s not like a regular fire, where you look for the burn pattern and determine the cause,’’ Coan said. “We have a good-sized team out there, and it could take several days.’’

Officials said Monday that they had found no air quality concerns following the explosion, and no evidence of pollution from chemical runoff in the nearby Ipswich River.

“Air quality sampling and monitoring processes were immediately deployed, and they indicated there were no imminent air quality concerns,’’ officials for the company, which operates the plant, said in a statement. “No evacuation of residents in the area was required nor is any future evacuation action anticipated. Monitoring of both the Ipswich River and air quality around the plant is ongoing, as a precautionary measure.’’

The three men and a woman who were hospitalized after the explosion shortly after 7:30 p.m. Sunday were released on Monday.

Coan has said the blast was particularly worrisome in light of other chemical explosions that have occurred in the past six years, including those in Leominster in 2005 and Danvers in 2006.

Investigators focus on causes of fire at Taiwan theme park that burned 498

June 2015

Stonehouse Process Safety commentary: Holi is an ancient Hindu religious festival which has become popular with non-Hindus in many parts of South Asia, as well as people of other communities outside Asia. In recent years the festival has spread to parts of Europe and North America as a spring celebration of love, frolic, and colors. Dust cloud flash fires and explosions have been recognized as real and present hazards in many industries. However, it may come as a surprise to many party goers that the magnificently colored powders that they throw into the air or at each other are mostly combustible and under the right conditions could present a flash fire or explosion hazard. It is, therefore, necessary that precautions be taken to only use powders that are inherently non-combustible or prevent their inadvertent ignition.

The following text by RALPH JENNINGS, Associated Press.

“Investigators in Taiwan were focusing Monday on the possibility that a cigarette butt or spark caused the blaze that burned 498 people, killing one, at a weekend water park party when colored powder sprayed from the stage caught fire.

More than 400 people remained hospitalized, including 202 in serious condition, city officials said. Taiwan’s Central News Agency reported one death, a 20-year-old woman with burns to 90 percent of her body who was taken off life support with her family’s consent. It said her 12-year-old brother also had burns on 90 percent of his body.

Police recommended criminal charges against the organizer of Saturday’s party, as well as two technicians, at the Formosa Fun Coast park in suburban Taipei.

“It’s still not clear what happened, but there were a number of people smoking and the weather was warm, New Taipei City news department head Lin Chieh-yu said. Temperatures around greater Taipei topped 36 degrees Celsius (96.8 F) before the party.

The three tons of colored starch-based powder bought by the organizers from Tai Won, a seller in the island’s southern county of Yunlin, were flammable, said Chou Hui-fang, a representative of the seller. She said the buyer was informed about the risk of fire.

“Whether it’s corn starch or flour starch, this kind of stuff, no matter how long it’s been around, if it’s in dense quantities and if it’s hot, it can catch fire, Chou said. She said her 4-year-old company has been questioned by police and health officials but was not considered at fault.

“We didn’t know what the buyers were going to do with it or how much they would use, she said. “It might have been supplies for a whole year.

Taiwan Premier Mao Chi-kuo banned use of the powder at future private events. Colored powder is often thrown on revelers during the annual Holi celebrations in India and Nepal, a Hindu festival. The powder at Saturday’s party was made in Taiwan, Chou said.

The water park was ordered to close after the fire.

Taiwan police recommended charges of professional neglect and public endangerment for party organizer Lu Chung-chi, who was arrested but released on bail of 1 million Taiwan dollars (US$32,000) and restricted from leaving the island, a New Taipei City police spokesman said.

Local media photos showed Lu kneeling on the ground to apologize, pledging to take full responsibility.

Police also recommended charges for the stage hardware technician and the person responsible for shooting off the powder. Each was given bail of 300,000 Taiwan dollars. Officers questioned two others involved in the event but did not recommend charges, said Yan Bo-jen, news liaison for the local police precinct.

Taiwan university student Liang Sheng-kai said flaming powder hit his legs, apparently catching fire after it was sprayed from a concert stage into the front row where he was standing.

With the park’s water features several hundred meters (yards) away, too far to douse the fire or ease burns, people screamed and panicked to find exits as balls of fire surged from the ground, he said. He said the right and left sides of the stage were blocked.

“It was very messy and a lot of people fell over or got knocked down, said Liang, 20, who is being treated for his burns in a Taipei hospital.

Five victims were from Hong Kong, two from mainland China and one each from Japan, Malaysia and Singapore. Authorities were still checking on the nationalities of three other foreigners. More than 200 were students, Central News Agency said.

Hong Kong is reviewing venues and other factors before it gives licenses to two local events similar to the water park party, Secretary Ko Wing-man said.

A total of 498 people were injured by the fire, according to the latest statement from the city government’s health department. It said 202 were badly injured, 236 lightly or moderately injured, and 60 others were unclassified.

July 2017

Chemical Fire Fought with Sand, Rock Salt on Detroit’s East Side

Stonehouse Process Safety commentary: There are a number of reasons why metal dust explosions are particularly hazardous. Firstly, dust cloud explosion temperatures and pressures for most organic materials are limited to about 2,300oC and 9 barg respectively. In contrast, explosion temperatures and pressures for most metals can reach around 3,500oC and 12 barg, and some metals such as magnesium can react with the nitrogen in the air, generating even higher values. Secondly, metal powders, particularly Aluminum, Magnesium, Chromium, Titanium, Thorium, Iron, Tantalum, Zinc react with water and generate hydrogen gas. Hydrogen gas is very easily ignitable over a very wide concentration range. And thirdly, “fresh metal surfaces could even be pyrophoric.

Explosion prevention and protection measures are important when processing/ handling mental powders. Fires are generally smothered to extinguish since water cannot be used.

The following text is an extract from The Detroit News of July 25^th, 2017 by JAME DAVID DICKSON It has been redacted to remove company names.

“It started with a “minor explosion,” and ended with firefighters and plant employees working “hand-in-hand”

On Tuesday morning, the Detroit Fire Department fought fire with dry sand and rock salt at a plant on the city’s east side.

It all started with a “minor explosion in a machine that processes magnesium at a metal processing plant on the 19300 blocks of Filer. This was at about 5:10 a.m., said Dave Fornell, deputy commissioner of the Detroit Fire Department. That’s north of East Seven Mile, and just east of Mount Elliott.

Of the three buildings on site, the explosion took place in a building where the company processes magnesium.

When firefighters arrived, the fire had spread to two storage bins, Fornell said.

“Magnesium, when it ignites, it burns with a white-hot heat, and you can’t use water to put it out, Fornell said. “The water turns to steam and causes minor explosions, which spreads the material.

There was also titanium involved.

Such a fire can’t be extinguished with water, Fornell said; it must be smothered with dry sand. In this case, dry sand and rock salt. And unlike at most of the blazes the department fights, this time workers at the site “worked hand-in-hand with firefighters over the next hour. The sand and rock salt were already on site.

“They knew what they were doing, Fornell said of the employees. “These people deal with this every day. They’re trained; it’s their job. They’ve got the equipment, they’ve got the expertise, they know what valves to turn on and off.

As for the building itself, it’s “pretty well-intact after the explosion, Fornell said. “There’s no damage to the structure. No one was hurt.

October 28, 2018

No injuries at powdered-milk plant in industrial park; friction from powder may have sparked blaze

Stonehouse Process Safety commentary: Spray driers, in general, have been the source of many dust fires and explosions over the years, yet it is a surprise to many people that many materials like milk powder will give rise to an explosion hazard when dispersed in the air. A sufficiently energetic ignition source is required to start a fire or an explosion, and a friction spark is an example of one such ignition source, but milk powder can also create its own ignition source through self-heating. There are practical techniques available to reduce the likelihood of friction sparking and self-heating taking place, but explosion protection is generally also required.

The following text by VALERIE GIBBONS [vgibbons@visalia.gannett.com] has been redacted to remove company names.

“An explosion in an eight-story milk-evaporation tower blew out a wall, sparked a blaze and forced the evacuation of the dairy plant in the Visalia Industrial Park Monday morning. There were no injuries.

The plant, which opened in January, produces powdered milk. It employs about 75 people and boasts the largest evaporative “dry unit” in North America.

Evaporative milk is made by spraying tiny droplets of condensed milk into a heated tower. As the water is baked out of the milk, it turns into a fine powder.

‘A significant explosion’

Officials with the Visalia Fire Department believe friction from the powder as it moved throughout the tower sparked the explosion and fire. The incident on the tower’s fourth floor occurred just before 10 a.m.

“It was a significant explosion,” Visalia Fire Chief Mark Nelson said. “It blew out a panel of the wall, which is designed to blow out in case there is an explosion. It did what it was supposed to do.”  The structural integrity of the tower was not compromised,” Nelson said.

The plant was evacuated and safety officials counted the workers as they gathered in the facility’s west parking lot. Smoke could be seen billowing from of the tower’s ventilation shafts.

“Sprinklers in the unit contained the fire for nearly an hour while firefighters ventured deep into the tower to find its source,” Nelson said.

March 2019

In late May last year, in South Sioux City, engineers were pondering the best way to demolish a grain elevator. The reason; a series of explosions in the 200-foot elevator tower had rendered the structure unsafe; and families from a dozen homes nearby, around fifty people, were waiting to hear when they would be allowed back home. Of course, their plight belies the grief of workers injured and their families, in one case fatally, who were caught in the explosion.  Source: Dust Safety Science

Stonehouse Process Safety Commentary

We do not comment on this serious event. We do ask why such events happen with worrying regularity and why, when much is known about safe elevator and silo explosion design, we continue to witness these shocking events time and time again. In Sioux City in the ‘70s, just a few miles away from last year’s elevator explosion, there was an elevator/ silo grain explosion that struck a bunch of grain silos – that time resulting in the death of 4 men.

People are injured, businesses are destroyed, lawsuits are filed, and communities devastated.

A way to view risk and focus the mind

When we consider dust explosion RISK we think of its components – likelihood and consequence. In our work as process safety consultants we are constantly reviewing how our clients can reduce the likelihood of explosion at their facilities. We are also looking at what would be the effects (consequences) of explosion and how they can be safely managed. It is through this twin-track approach that we can establish a basis of safe operation and can help our clients enhance safety and protect their staff and their businesses. But actually, this twin track approach is not always symmetrical. To have established through a Dust Hazard Analysis that an explosion/ flashfire is unlikely is good news; but measures to protect against the effects of an explosion could also still be required. If the consequences of explosion are potentially huge and devastating, then we would argue that protection measures become so much more important and sometimes critical.

So now we address the BIG ONE – the large silo explosion, the tall elevator explosion and so on. Well, in plant and equipment of very large volume, any explosion has the potential of being BIG – which can imply destructive and life-threatening potential. For this reason, silos and elevators will most likely be designed and operated to be able to cope with the consequence of a dust explosion, even if it’s unlikely to happen. Explosion relief venting is the usual way of achieving this, but the area and activation pressure of those explosion vents must be designed taking into account the explosibility of the contained dust (Kst value and explosion pressure), the volume, strength, and shape of the silo/ elevator. In our experience, there are too many silos out there that have poorly-designed, inadequate or even no explosion protection (e.g. venting). Of course, this does not mean that taking preventative measures does not matter. Regular maintenance, temperature monitoring, correct choice of electrical equipment, control of static electricity, hot-work permit systems, and so on must always be reviewed and corrective actions taken where deficiencies are identified. But although explosion prevention measures are clearly important, it would need specialist study and meticulous implementation of prevention measures in practice to bring the ignition risk to an acceptable/tolerable level. So, in our view, with very low probability events in BIG equipment, given the consequence, the importance of protection to safely deal with an explosion would usually be prioritized. With the BIG event, consequence is often king and can focus the mind very well when it comes to deciding on where to prioritize spend from that limited safety budget.

And finally; food for thought

Beware when you hear the statement “this hopper, silo, elevator, dryer, conveyor… has been operating safely for years. Please feel free to add “so have most facilities that have just blown-up.

February 2020

When something serious goes wrong on powder handling plant, how long do I have to get out before a dust explosion develops?

I think we all know that individual dust explosions last for a matter of seconds. Once ignition has started, a flame front grows from the ignition source with the generated heat developing pressure waves which travel ahead of the flame front – at the speed of sound. Devastation will follow unless proper explosion protection and isolation measures are in place. Of course, combustible dust explosions can be multiple explosion affairs, where a primary explosion raises dust that leads to a secondary explosion and so on; and in extreme conditions (for example in a pipe system) it is even possible for the deflagration to develop into a detonating dust explosion, usually with more extreme consequences. In most cases it is clear that there is often very little time available to evacuate a facility.

So, you will be surprised to hear that in Belgium last year, a combustible dust explosion occurred that led to one fatality and 3 serious injuries; the explosion took place one day after the incident began. How can this be – and surely a day is enough time to evacuate a factory…….

The problem began on a Thursday back in January last year when a fire was reported in a silo at a factory making wooden windows, gates and doors. (ref. de Standard) https://www.standaard.be/cnt/dmf20190125_04130842 (ref belgischeradiounie.net)

https://www.belgischeradiounie.net/2019/01/stofexplosie-bij-firma-pouleyn-in-anzegem-eist-zware-tol/.

The fire service was called to the facility. The next day, at around 8:00am employees of a contract cleaning company opened a hatch in the silo. According to reports, an explosion occurred at this point and the four contractors sustained their injuries.

The account of events is consistent with known phenomenon associated with combustible wood dust and oxygen depleted atmospheres. Like many organic materials, wood dust can smolder for long periods with the combustion products creating a depleted oxygen gas atmosphere above the smoldering dust. It is only when oxygen is introduced that enhanced fire and even explosion becomes possible. Indeed, a sudden in-rush of air can even help raise partially combusted dust into the air to create an explosible atmosphere of dispersed dust and air, with the still smoldering dust providing an ignition source. The fire triangle of air, fuel, and ignition source is complete.

We have reported previously on material self-heating phenomenon in powders which could also be relevant to this incident ref https://stonehousesafety.com/powder-thermal-instability-self-heating-in-spray-dryers/

The time to evacuate a facility at risk of dust explosion is, of course, generally very short. Smoldering materials, however, have to be watched for. Indeed, some facilities will install carbon monoxide detectors to check for the presence of smolding in process plant – the dairy industry uses such in spray drying of milk. However, once smoldering is detected the courses of action taken, be they water flooding or simply waiting for the combustion to run its course must never introduce air or encourage powder dispersion. As we have seen, if this happens the consequences can be very serious indeed.

For more information on dust explosions, combustible dust, or for a complimentary discussion with one of our dust explosion experts, please visit the dust explosion prevention and mitigation pages of our website, emailed us at info@stonehousesafety.com or give us a call on 609-455-0001.

Somewhere in America in the late 1880’s a man stood with a camera facing his subject. In one hand he had a line of magnesium powder a-top a pole. A smile, a spark and a brilliant flash later he had captured a photograph (ref. 1). Fast-forward a century…

Explosion: In the early hours of the morning of May 2^nd last year, a white glow was seen at the bottom of a magnesium scrap conveyor at an automotive MAGNESIUM parts manufacturer in Michigan (ref. 2). Then there were explosions in a tunnel at the bottom of the conveyor. There were also reports of one operator being injured by debris blasted from the plant and another of an employee being “thrown through the air on the way out of the building.

And another one…

Explosion: On October 22^nd last year there was an explosion at a MAGNESIUM diecast manufacturing plant in Ontario, Canada (ref 3). Two people were struck by falling debris and suffered minor injuries, according to Global News, and structural damage was sustained to the facility.

Stonehouse Process Safety Commentary:

MAGNESIUM powder is incredibly useful today. Everything from aircraft parts to automotive components are made from magnesium, benefiting from the metal’s light weight and high strength giving a favorable strength-to-weight ratio. But the metal must be treated with great respect and thoughts of the photographer of yesteryear is the key to why.

If exposed to fire, thin sheets of magnesium can ignite in a matter of seconds. Even thicker castings can be ignited with enough heat over a few minutes. But finely divided magnesium, even as a layer, will explosively flash – as we know from our photography history lesson. But if that same magnesium powder is dispersed in the air, explosion will result with devastating rates of pressure rise (St-3 Classification). Many metals behave this way and great care is needed to avoid dust explosion.

There are even more dangers with metal powders that may surprise some. Some metal dusts can react with water to produce hydrogen – a gas that explodes so strongly when mixed with oxygen that it is used to propel spacecraft into orbit. And yet another danger arises when trying to extinguish a fire of burning metals. Many metals burn with such intense heat (perhaps 3000ËšC) that the application of water results in instant explosive phase change from water to steam and decomposition… and another explosion results.  There is clearly a lesson there for first-responders – and also indication of the risks introduced by inappropriately located sprinkler systems. [Metal fires are frequently left to burn or else smothered with, for example sand, where the fire is small.]

NFPA 652 has lots of general information about dust explosions, but metal dusts have their own standard, NFPA 484. At Stonehouse we have many years of experience advising companies control their fire and explosion risk. We have also looked at our fair share of incidents involving metal powders. Consult our metal industry web page https://stonehousesafety.com/markets-we-serve/metals-machining/#main for more information or speak to one of our process safety specialists.  Call: 609 455 0001, Email: Info@stonehousesafety.com, Web: www.stonehousesafety.com.

References

1. http://hspvaragtime.weebly.com/home/jacob-riis
2. https://www.lansingstatejournal.com/story/news/local/2018/05/21/meridian-magnesium-report/628633002/
3. https://www.autonews.com/article/20181023/OEM10/181029886/factory-explosion-fire-idle-supplier-meridian-in-canada

July 2019

In this issue we dig into our own case files for an explosion/ flash fire caused by STATIC ELECTRICITY. As they say, all names, places and facilities have been changed to protect the innocent…. but the story contains the essentials of a real incident and is relayed here to help us all learn from past mistakes. Look out for the EARLY WARNING SIGNS!

On a bright, sunny winter’s morning somewhere not too far away, an operator in a pharmaceutical secondary processing facility was discharging a stainless-steel blender into a stainless-steel drum below; something he and his colleagues had done many times for some years. The operators noticed yet again that with this particular batch, as with some others before, they were getting small electric shocks from the lip of the stainless-steel drum. Nothing unusual here, then. In any case, they had a solution to this discomfort that worked. Thick rubber gloves usually did the trick when it came to grabbing the drum to move it. Or else contact with the drum to an arm through clothing before using hands worked sometimes too.

On that very cold, bright and sunny day, there was a flash from the drum – and an expanding flame. The operators fled their station, to raise the alarm. A fire developed which was extinguished by fire crew before the room was fully destroyed.

Our incident investigation team was called in and quickly got to work, reviewing plant and the process, testing similar equipment, performing some onsite electrostatic measurements, and interviewing operators. It became apparent that the stainless-steel drums were mounted on nylon wheels, and the floor in the production area was finished with epoxy – a very practical and easy-to-clean finish. Unfortunately, both the drum’s nylon wheels and the floor surface introduced a very high electrical resistance between the drum and electrical ground.

We moved on to the powder. We measured the electrical volume resistivity of the power which was found to be well in excess of 10⁹ ohm.meter. We also measured the minimum ignition energy of the powder using our dust explosion laboratories and found powder MIE to be less than 3mJ. We also performed a powder charging experiment using our tribochargeability rig and discovered high levels of charge generation when the powder contentiously made and then broke contact (colliding/rubbing) against stainless-steel.

Thinking of the basics of static electricity, we had a charge generation mechanism: the tumbling blender produced many opportunities for powder to charge by contact and separation with the blender walls. The electrostatic charges could accumulate with the powder as it bulked into the drum below (no earth path) and the opportunity to ‘spark’ was present as operators tended/approached the filling drum. As was mentioned earlier the powder minimum ignition energy was measured and found to be very low, making it easy to be ignited by a static spark from the drum. With hindsight, we had ‘the perfect storm’ brewing! We even had EARLY WARNING SIGNS (operator experience sparks) to point to the danger.

No one was injured in this incident. Of course, they could have been. The production facility was damaged and out of action for a few months, but it could have been much worse.

Our investigation and diagnosis allowed us to work with our client to come up with practical solutions to manage the risk. Static control measures include grounding provision – but there are better solutions than providing a grounding cable and clamp – and hoping someone remembers to use it. They also included better dust control. Sometimes there are means by which static charge generation can be reduced but these are not always practical.  Training for operators on the dangers and control of static electricity is also important. Had operators been trained to realise and understand the early warning signs, they could have reported them and control measures could have been taken. For information on in-house training courses by Stonehouse Process Safety, please visit our training and competency pages on our website.

So, what about the cold bright sunny day. Well, in winter, cold, dry days are often associated with low atmospheric humidity – conditions in which electrostatic phenomena tend to come onto their own. But I hear you say ‘our facility is humidity controlled – it has to be to keep moisture absorption to our product under control’. But that de-humification can equally well encourage static electricity.

For more information on static electricity and its practical control or for a complementary discussion with one of our electrostatic specialists, please visit the static pages of website or give us a call on 609-455-0001.

November 15, 2018

Source: East Anglian Daily Times

Stonehouse Process Safety commentary: The humble screw conveyor is staple equipment for moving bulk product from A to B. Use it well and wisely and you have plant that not only will move grain or powdered materials through your facility, but it can also be designed to act as a choke in your dust explosion protection and isolation system – to stop a dust explosion propagating from one part of plant to another. But get it wrong and the screw conveyor can become a source of ignition in itself, creating heat or mechanical or electrical sparks from overheated bearings or equipment. If not shut down, the screw conveyor can also facilitate the transfer of smoldering material to downstream equipment such as hoppers and silos, with serious risk of dust explosion. Bearing design, maintenance and monitoring as well as proper bonding and grounding all affect the risk associated with screw conveyors.

The incident described above had limited impact due to a vigilant employee. Others have been less fortunate with smoldering material being conveyed downstream, only to initiate serious explosions, destroy plant and equipment and cause loss of life. Dust Hazard Analysis (DHA) provides a route to identifying and assessing hazards – and of mitigating the hazards.

“In October 2018 in the small market town of Stowmarket in the United Kingdom, nine fire engines and crew were called to a malting plant when a vigilant worker noticed that a grain intake screw conveyor appeared warmer than usual; he also thought he might be smelling burning. The source of the burning was traced to a screw conveyor, and fire crew worked to extinguish the fire.

A spokesperson for the company, which exports malt and malt extract around the world, said “We are thankful to the fire service for their timely response. The exemplary vigilance of our operator and assistance from the fire service ensured this was a minor incident.

September 2019

Three weeks ago, at 7am on August 23rd, the life of a 38-year-old woman in Lower Saxony, Germany, was about to take an unpredictable turn for the worse (ref 1&2).  Within the hour, she found herself being rushed to the hospital in Braunschweig by helicopter, having been caught as a bystander in a significant dust explosion. She was seriously injured while on shift as a worker at a particle board manufacturing plant; she was outside a dry cutting area building at the time of the blast. Unfortunately, she was in the wrong place at the wrong time and was caught in the path of a roller-door that had become airborne and detached from the building by the explosion forces.  The ignition source was reported to be a ‘spark’ from a mill in the dry cutting area.  Source: Volkstimme

‘Particle board’ refers to low-density fiberboard (LDF), and chipboard. It is a product manufactured from wood chips, sawmill shavings, and sometimes sawdust, which are mixed with a suitable binder – usually a synthetic resin. The product is pressed and extruded to construct the boards we routinely see in our favorite home improvement stores.

It is not completely clear from reports of the incident exactly what happened, but the consequences of the explosion, which turned building components (including windows and gates) into missiles, included injury and damage to plant. A number of fires followed the blast which were extinguished by the attending fire service. The incident was reported to have caused over $300,000 of damage and, of course, there would have been significant disruption to operations.

Stonehouse Commentary: Impact Sparks

At the time of writing this article, the police had reported that there had been a dust deflagration in the so-called mill building and that it was caused by a ‘spark’. Regular readers of our Safety Dispatch articles will be aware that a number of our previous stories have concerned ‘sparks’ of the electrostatic type but given this incident, we thought it may be of interest to readers to take a quick look at ignitions caused by friction or impact sparks. It’s worth pointing out that we are not party to private or privileged information on the incident reported here, but many readers will be operating mills and cutting equipment that are quite capable of producing incendive impact and friction sparks, and so we take the opportunity to look at these in a little more detail.

Impact ‘sparks’ have very little in common with electrical or electrostatic ‘sparks’, despite sharing the same name. Impact sparks generally involve hot or glowing material that flies off at impacting surfaces when they come together with force – such as inside a mill or during mechanical cutting. Electrical or electrostatic sparks, on the other hand, involve the breakdown of gas – usually air, due to voltage differences – and a flow of ions (current) through the air.

Perhaps the best-known type of impact spark involves alloys of light metal such as aluminum, magnesium or titanium with oxygen-rich materials such as rusty mild steel; These sparks are known as thermite sparks. The impact of rusty metals on certain surfaces (e.g. a rusty tool impacting a painted surface containing aluminum) can readily generate sparks, or spark showers.

The big question, of course, is that although impact sparks can look quite spectacular, what is their capability to ignite flammable dust or flammable vapor atmospheres? Some would claim that impact sparks are not as ‘incendive’ as they look. In fact, an electrical discharge channel is arguably more able to transfer its heat energy to a flammable atmosphere than can a glowing piece of metal, for example. However, it is known that impact sparks can indeed ignite flammable gas/vapor atmospheres and some dust clouds.

The good news is, though, that dust explosion testing is available to establish the susceptibility to ignition of flammable dust atmospheres by impact sparks. These tests generate information on the sensitivity to ignition of the powders to both heat (auto-ignition) and also to electrostatic sparks which, it turns out, can be combined to ascertain and overall sensitivity to impact sparks of various types. Read about Laboratory Testing.

The sensitivity of flammable mixtures to impact ‘sparks’ is a big topic and one to which we intend to return in future Safety Dispatch articles. In the meantime, if you have any concerns about the sensitivity to impact spark ignition of materials you process, or any questions on electrostatic safety, static electricity warning signs, or dust explosion safety, please feel free to contact us.

References:

1. https://www.euwid-holz.de/news/holzwerkstoffe/einzelansicht/Artikel/verpuffungsexplosion-im-glunz-werk-nettgau.html

2. https://www.volksstimme.de/lokal/salzwedel/verpuffung-arbeiter-schwer-verletzt

November 2018

On November 8, 2018, fire crews from four local fire departments were called to a reported dust explosion and fire at a woodworking facility in the Midwest. When the crew arrived, they successfully tacked a fire raging in a dust collector and trailer used to collect sawdust from the facility. It was reported that a saw had shut down, sending a spark into the dust handling system, which is presumed to have been the ignition source. The event required the closure of a highway during morning hours while the fire crews were on the scene. Fortunately, no one was reported as hurt. Source: The Crescent-News

Stonehouse Process Safety commentary: Most of us have tried to start a camp fire – and usually struggled. We know if we first prepare kindling, we might light that first and hope that the logs will eventually flame. What we may not realize is that if we take sawdust from that same wood and disperse it in the air, we could have a flashfire or dust explosion on our hands. Wood needs a lot of surface area to combust, and the more you have, the faster will be the combustion.

Sawmills and wood workshops throughout the country create wood dust in their operations, and special precautions are required to prevent that dust from inadvertently catching fire or even creating an explosion. Electrically operated saws can overheat and cause dust to smoulder, and they can also create sparks, especially if they encounter a hard object such as a piece of metal or stone – and they can sometimes generate burning or smouldering dust that gets transferred along dust extraction systems. When this burning or smouldering dust reaches a dust collector or collection bin it can cause fire or, if the dust concentrations are sufficient, an explosion.

The risk of injuries to employees or others from events like the one described here is real. However, dust explosions and fires like these are preventable. There are systematic techniques available to conduct Dust Hazard Analysis (DHA) in accordance with NFPA 652 that can significantly reduce both the likelihood of events like this happening, as well as ensure that if a problem does arise, that the consequences are minimized and controlled.

October 2019

Last month (September 2019) in the sleepy Dutch town of Wijhe, an explosion occurred at the Arva Spice factory. The explosion, which left 3 people injured and taken to hospital, was reportedly caused by a blockage in an herb grinder (ref 1). It appears as though the blockage led to heat accumulation and product fire and it seems likely that it was the fire that initiated the explosion of the herb dust/powder. The combustible dust explosion caused a pressure rise inside the building which dislodged sections of the building façade. A roller door was also removed from its mounts. The fire was extinguished by the local fire service.

Arva undertake operations that would be recognized by many food ingredient manufacturers. They process raw materials such as herbs and spices by grinding, mixing, packaging and distributing them. Indeed, back in September 2016 at High Quality Organics at Stead near Reno, Nevada, an incident (ref 2) with significant similarities took place. There, the herb oregano was being milled when a fire and explosion occurred in a grinding hopper. Again, the fire service was in attendance to extinguish the fire. Fortunately, in Reno, no one was reported as being injured.

The damage at the Reno facility seems to have been less than that in the Netherlands but the source and effects of the ignition were similar; so, it’s worth a look at what could be going on when herbs and spices (and other food ingredients) are milled.

The Dust Hazard Analysis (DHA)

Milling in general is a high-energy process. Electrical energy is converted to mechanical energy in the mill to effect size reduction of the ingredients. But if anything goes wrong in the mill and if no action is taken, the mechanical energy can be converted to heat energy. A blocked or overloaded mill, for example, could heat the product to its self-heating temperature, at which point thermal runaway and fire can result. Alternatively, mill bearings could overheat, and even foreign objects can enter the mill and result in friction or impact sparking inside the mill itself; all potent sources of ignition.

Creating an ignition source is always to be avoided, but some food ingredients are particularly sensitive to ignition through self-heating or to impact and friction sparks. Fortunately, dust combustibility tests can be undertaken that will search out powders and dusts that are prone to such ignition sources so that appropriate precautions can be taken when processing. These tests include thermal stability tests, ignition temperature and ignition energy tests. Armed with such stability and combustibility data, a DHA (Dust Hazard Analysis) can be performed on powder handling plant with a view to establishing a clear basis for safe operation; and also ensuring that if something does go wrong, both personnel and plant are well protected. All this in accordance with NFPA 652 Standard on the Fundamentals of Combustible Dust.

Precautions that can be taken on mills include regular maintenance, temperature sensors capable of initiating shutdown, and screening for foreign objects in the feed. Explosion protection measures for the downstream receiving/storage vessels and equipment include explosion relief venting and explosion suppression. In extreme cases, even milling under an inert gas can be used. The best decisions on explosion prevention and protection can be made once a Dust Hazard Analysis (DHA) is complete – and that Dust Hazard Analysis (DHA) should best be guided by the fire and explosion properties of the ingredients being milled.

For more information on Dust Hazard Analysis (DHA) according to NFPA 652 click here. For more information on laboratory testing for dust combustibility/ explosion properties click here.

References:

1. Netherlands explosion: https://www.rtvoost.nl/nieuws/318209/Drie-gewonden-bij-explosie-in-Wijhe
2. Nevada explosion: https://mynews4.com/news/local/reno-fire-responds-to-dust-explosion-at-spice-manufacturing-building-in-stead