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Thursday, September 27, 2012

Ask The Experts - Spark Detection vs black body ember detection

August @ 2011 @ Ask The Experts

Near Infrared Spark Detection vs. black body ember detection. 

Definition of spark detection as defined by the experts.


Q. We have differing recommendations on spark detection systems for dry wood dust collection. One recommends only detecting and extinguishing black body radiation for specific ignition temperature. Another specifies to detect and extinguish any and all sparks prior to the collector. Given limited information available, which method is preferable?

NFPA 664, Paragraphs A.8.6.2.2 states: “Provide a spark detection and extinguishing system on the main airflow duct between the dryer drum and cyclone. The spark extinguishing system should activate every time a single spark is detected [emphasis added]. It will reset after a few seconds (if no additional sparks have been detected), and the dryer can continue to operate. The spark counting features available in some approved spark extinguishing systems can be used to shut down dryers when an excessive number of sparks are encountered, but they should never be used as a measure of when to actuate the extinguishing spray.”


There is no discussion concerning “black-body radiation” in NFPA 664, and the only discussion of “black body” is in NFPA 1971 and concerns flammability of clothing. However, there is good discussion of detection methods – including a mention of “Planck’s Law” [regarding black-body radiation] – in NFPA 72, in Sections 3.3, 5.8, A.5.8, and B.5.1.4.

If small, low-energy sparks occur so frequently that spark extinguishment interferes with production AND a prolonged historical record indicates no significant explosion hazard from such sparks, then a “cut-off” based on thermal [black-body] radiation might be appropriate. However, for infrequent sparking, a large low-temperature spark or firebrand might have a temperature below an established black-body temperature criterion but might have more energy – to ignite a dust cloud – than a small particle having a temperature far above the black-body temperature criterion.

Thus, the “bottom-line” response to this question would be a suggestion to consider the above-quoted guidance from NFPA 664, and detect and extinguish every single spark.

Based on the results of a recent explosion investigation by Chilworth, it also would be prudent to count all sparks and document the counts, rather than document the number of extinguishment actions. An increasing number or frequency of sparks could indicate serious problems with upstream equipment.

Wednesday, September 26, 2012

NFPA Offering Free Combustible Dust Webinar

NFPA Offering Free Combustible Dust Webinar Oct. 11 -- Occupational Health & Safety


Guy Colonna, division manager for the association's Industrial & Chemical Engineering -- Hazardous Chemical & Materials Department, will discuss the 2013 edition of NFPA 654 standard.
The National Fire Protection Association is now registering attendees for a free webinar about 2013 NFPA 654 and combustible dust hazard assessment that will be presented from 12:30 to 2 p.m. EDT on Oct. 11 by a well-known expert on the topic: Guy Colonna, division manager for the Industrial & Chemical Engineering -- Hazardous Chemical & Materials Department at NFPA.
He'll be discussing the 2013 edition of NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, as well as related standards and safety best practices and controls.

Colonna has engineering degrees from the US Coast Guard Academy and Stanford University and is a registered professional engineer in chemical engineering in Massachusetts. He is Staff Liaison to NFPA committees concerned with safeguards at dust hazard process locations, explosives, industrial fire brigades, explosion protection systems including venting of deflagrations, and pyrotechnics and special effects, according to the webinar's registration page.
The webinar is sponsored by 3D Instruments LLC of Anaheim, Calif.

Friday, September 21, 2012

Dust to Dust

Dust to Dust | OHS Canada


Dust to Dust

OHS Canada

By: Jean LianJuly/August 2012
2012-07-16


For a dust explosion to occur, everything has to be lined up just right. Unlike a fire, which can be started by a trio of elements — fuel, an ignition source and oxygen — a dust explosion can occur only if a pentagon comprising two additional elements, dispersion and confinement, come together.

That destructive constellation is now being investigated as a possible cause behind the explosion of two sawmills in British Columbia that blew up months apart from each other. On January 20, 2012, an explosion and fire destroyed Babine Forest Products mill in Burns Lake, killing two workers and seriously injuring more than a dozen. On April 23, an explosion and the resulting fire razed Lakeland Mills in Prince George, claiming two employees and injuring 22.

The similarities between the two explosions are striking — both are sawmills, dust was present in both facilities and both are working with beetle-infested wood. A similarity so uncanny that it prompted a review, an industry-wide directive order and several investigations to find out just what went wrong, and how it could go so wrong.

The destruction of the two sawmills has triggered a search for answers that will be close to the heart of many industries that generate fine particles — from mills that process wood, grain and sugar; chemical manufacturing plants and facilities that fabricate rubber and plastic products; to coal-fired power plants.
On April 25, provincial labour minister Margaret MacDiarmid convened an urgent meeting involving representatives from the industry, United Steelworkers and WorkSafeBC to review mill safety and compile a document on industry best practices relating to combustible dust control in sawmills.

 The following day, the provincial safety regulator issued a directive order to all sawmill operators to conduct a full hazard identification, risk assessment and safety review with particular focus on combustible dust, dust accumulation and potential ignition sources. “WorkSafeBC officers will be following up on these orders to confirm the ordered actions have been taken and sawmills are in compliance with the Workers Compensation Act and Occupational Health and Safety Regulation in regard to combustible dust and potential safety hazards,” a WorkSafeBC statement noted in April.

Until the investigation has concluded, Roberta Ellis, vice-president of corporate services with WorkSafeBC in Richmond, British Columbia, says it “cannot speculate, based on the similarities, as to the cause of these events.”

It did, however, reveal in a statement in May the ignition sources in both investigations appear to have been located at the conveyor belt level where electrical and/or mechanical equipment was in operation in areas contained by walls and equipment. These areas are located at the basement or lower level of both mills.

Officers inspecting all sawmills and associated site facilities under the directive order “will be paying particular, but not exclusive, attention to areas similar to those identified in the Babine Forest Products and Lakeland Mills investigations,” the statement notes.
The Council of Forest Industries in Vancouver has also established a task force comprising representatives and stakeholders from the wood products manufacturing industry to investigate combustion risks in mills. The task force, which reports to a CEO Action Committee, “is a significant collaborative effort to ensure that British Columbia’s wood products manufacturers are operating to the absolute highest standards,” Hank Ketcham, chief executive officer of West Fraser, says in a statement.

Don Kayne, chief executive officer of Canfor, adds that “we have taken every opportunity to increase our efforts in safety since the serious incidents at Lakeland and Babine, and this joint initiative is a next and very important step.”

Stephen Hunt, director of western Canada (district 3) with the United Steelworkers, went to Babine Forest Products mill with several union representatives following the incident. “Nobody from anywhere could recall a catastrophic explosion in a sawmill that literally blew the mill right off the face of the earth,” he says. When the mill in Burns Lake exploded, “I think it is safe to say that everybody is convinced that it was a one-off; something happened in that mill that was so unique that they could not be replicated,” Hunt continues. “Then three months later, we blow another one up.”

FACT CHECK
The two incidents have raised many questions on why dust particles are hazardous and what makes them combustible.
Combustible dust — also known as dry, deflagarable dust — is a wood particulate with an average diameter of 420 micrometers or smaller and has a moisture content of less than 25 percent, notes information from WorkSafeBC. “The drier the dust, the finer it can become and it is more of a fire explosion hazard,” says John Astad, director of the Combustible Dust Policy Institute in Santa Fe, Texas.
Combustible dusts are often organic or metal dusts that are finely ground into very small particles. Types of dusts include, but are not limited to metal dust (such as aluminum and magnesium); wood dust; plastic dust; biosolids; organic dust (like sugar, paper, soap and dried blood); and dusts from certain textiles, notes information from EMSL Canada, Inc. in Mississauga, Ontario. The United States-based firm is one of two companies identified by WorkSafeBC as being able to perform combustible dust testing.
Suspended dust burns more rapidly while confinement allows for pressure to build up. The initial explosion can cause dust that has settled over the years to become airborne, resulting in a secondary explosion that propagates throughout the plant, often with disastrous results.

Apart from primary and secondary explosions, there is also a “multitude of non-consequential combustible-related fires,” Astad notes. While the majority of combustible dust-related incidents do not result in injuries, fatalities or property loss, “these can be precursors to catastrophic events.”

WEAKEST LINK
No amount of dust, it seems, is too little to be ignored. Any surface that has more than five per cent of dust accumulation on it has to be cleaned off. In fact, a thickness that exceeds an eighth of an inch is too much, says Craig Kennedy, owner of Kennedy Forest and Safety Consultants in Williams Lake, British Columbia. “That is the thickness of two nickels.”

Potentially explosive particles can accumulate wherever dust is generated, says Dr. Graeme Norval, PhD, associate chair and undergraduate coordinator with the department of chemical engineering and applied chemistry at the University of Toronto. For sawmills, this means places where the blades are located and where lumber gets transported. In sugar mills, vulnerable spaces include those where grinding operations, transportation of materials on conveyor belts and bagging of sugar and flour take place.

In other words, “anytime you are changing a particle size, that is when you have a vulnerability,” Astad notes.

Many new, automated mills are using more efficient, computerized technology to process wood, generating finer dust in greater amounts which, when suspended, presents an explosion risk, Kennedy explains. “They are pounding wood through really fast,” he adds, citing bandsaws, fast-moving table saws and circular saws as examples.

Hunt agrees. “We are finding that, like in many workplaces, the size and speed of the saw blades have changed; the nature of the dust coming off has changed.” These changes demands the accompaniment of safety controls, although “it appears that that may have been left behind,” Hunt suggests.

Transfer points like conveyor belts, bucket elevators, mixers and dryers are vulnerable to the risk of dust explosion. Friction from moving equipment — like a bucket elevator with its metal rubbing against a casing — and industrial-powered equipment, such as forklifts, can also cause fires or explosion when hot surfaces from motors, brakes and engine exhaust system meet suspended dust in sufficient amounts. “All that is missing is an ignition source,” Astad says, noting that explosion-proof forklifts should be used to ensure safe operation in environments where dust is being generated.

Any activity that involves heat, electricity and an open flame can serve as an ignition source. A worker doing hot work on the other side of a process equipment, Astad cites by way of example, can accumulate sufficient electrostatic charge to potentially ignite dust accumulation in that area. The possibility of an electric rotor or electrical panel arcing or sparking presents an additional risk.
“A lot of these ignition sources are inherent with the process equipment,” he notes. “It is not just one area we have to look; it is a holistic approach.” 

Determining the minimum ignition temperature at which dust clouds can ignite is key to preventing dust explosions. There are two types of minimum ignition temperature: layered (for undisturbed dust) and suspension temperature (for suspended dust). The American Society for Testing and Materials provides guidelines for performing laboratory tests to evaluate the deflagration parameters of dusts in ASTM E1226: Standard Test Method for Explosibility of Dust Clouds.

These tests provide crucial information that forms the basis for the development of a dust accumulation control program. Once the minimum ignition temperature has been determined, “we can evaluate our facility and see if we have temperatures that are approaching that,” Astad says. From that knowledge springs the need to communicate the hazard to rank and file, implement engineering and administrative controls and evaluate the training required, he adds.
Characterizing Dust Samples
A combustible dust analysis comprises a series of tests to determine whether or not a dust is combustible and potentially explosive. At least one litre of dust must be collected and submitted for analysis, notes information from WorkSafeBC. Combustible dust is typically analyzed in two ways:
- Particle characterization determines particle size and moisture content. The most important part of this test is determining the percentage of the dust sample that is both combustible and small enough to pass through a 40-mesh sieve less than 420 micrometer in size.
- Class II test involves a number of parameters that determine how explosive the dust is and if the sampled dust is considered a Class II hazardous material.
In view that the entire Class II test is expensive, costing more than $4,000, the most practical way is to perform an explosive severity test, which is an initial screening to find out if the dust has explosive properties before continuing with the remaining tests. ‘If the dust is not found to be an explosive threat, the analysis can be aborted to avoid unnecessary fees,” WorkSafeBC information notes.
For combustible dust sampling and analysis, the safety regulator recommends following the Occupational Safety and Health Administration (OSHA) ID-201SG sampling method guideline. The OSHA Combustible Dust Emphasis program (CPL 03-00-008) also provides information on sample collection.
IN SYNC
Undertaking a risk assessment to identify high-hazard areas and developing a preventive maintenance program is a good starting point. However, housekeeping can only mitigate the hazard so much if the interaction between the operational environment and engineering controls do not work in sync to keep the dust out.

Kennedy cites a wood processing facility he inspected that has two large openings in one part of the mill. “The wind is blowing through one side and out through the other. It is like a big wind tunnel out there,” he describes. “The dusts that were cleaned off were blown back onto the girders and the beam by the wind.”

Seasonal change also needs to be factored into consideration when it comes to designing engineering controls. Water misters, used to dampen down airborne dust, are not suited for use during winter when the temperature drops below freezing. Roof fans, which keep small amounts of fine dust out of a building, introduce hot air into a facility in summer “and in winter, it sucks the cold air in,” Kennedy says.
“Many facilities think housekeeping is just blowing it down with compressed air,” Astad says. On the contrary, that  can put dust in suspension and heighten the risk of an ignition, he cautions. 

In fact, equipment designed to collect dust can sometimes pose a hazard itself. About 40 per cent of combustible dust explosions reported in the United States and Europe over the last 25 years have involved dust collectors, notes information from Designing Your Dust Collection System to Meet NFPA Standards, published in 2008. Down south, dust collection systems have now become a primary focus of inspection required by the Occupational Safety and Health Administration’s National Emphasis Program on the safe handling of combustible dust since 2007.

One of the key factors in preventing explosions in the dust collection system is to maintain a reasonable conveying air velocity in every part of the duct. Low velocity can cause dust to drop out of the air and build up inside the duct. Too high a velocity, on the other hand, results in energy wastage. The characteristic of the dust is also an influencing factor. Coarse dust can cause erosion to the duct while moist and sticky dust can smear the duct wall, the paper notes.

“The most important thing that should be considered is the efficiency of the collector,” says Bes Blentic, sales manager with Camfil Farr Air Pollution Control in Toronto. Finer dust requires a lower velocity while heavier dust needs air to be churned at a higher speed to prevent them from settling in the duct, Blentic adds.

Personnel who maintain dust collector systems should be provided with training to ensure that they understand the fire and explosion hazards associated with combustible dust, Astad notes. He is, however, quick to point out that training on preventing dust explosion hazards should involve not only workers and stakeholders, but also all relevant regulatory agencies, such as the Fire Commissioner. “This is not solely a WorkSafeBC issue,” Astad says, noting the provincial safety regulator primarily looks at the operational environment of a plant from an occupational safety perspective.

On the other hand, the built environment involves looking at the physical structure of the plant and its fire safety and prevention, which falls under the purview of the Office of the Fire Commissioner. This includes administering the Fire Services Act, enforcing fire safety legislation, inspecting and investigating fire incidents, and responding to major fire emergencies.

“We need to distinguish what is an operational environment and what is a built environment,” Astad notes. “Workplace fire and explosion hazard is an issue where guidance can be acquired from both local assistants to the Fire Commission and WorkSafeBC fire prevention officers,” he adds. “Solely relying on one agency and ignoring the other does not comprehensively address the issue.”

ON ALL FRONTS
A multi-faceted dust accumulation control program involves conducting regular risk assessments; dust control through containment, engineering systems and housekeeping; ignition control and emergency procedures in the event of a dust explosion, notes information from the Forest Industry Task Force on Mill Safety. 

For risk assessments, workspaces to consider include sawing and debarking operations, planers, chipper enclosures and chip screening areas. Particular attention needs to be paid to less conspicuous areas, such as conduit, pipe racks, cable trays and rafters. Areas that produce fugitive dust should be identified and ways to enclose or contain it in that location need to be developed. All dust control systems must be inspected, cleaned and maintained in good working condition, the information adds.

Clean-up should also be scheduled in relation to the extent of possible dust accumulation. Apart from horizontal surfaces, overhead and vertical areas — such as beams and walls — need to be covered. “You have to be conditioned to notice that there is dust collecting and deal with it,” Dr. Norval says.

One common oversight is dust accumulation at heights, such as on top of girders or overhead pipes. “One day, the pipe shakes and now you have a large accumulation of dust in the air,” Dr. Norval says. “Just because you don’t see dust on the floor doesn’t mean there’s no dust problem. People look at the floor, but they don’t look at on top of things.”

When performing clean-up operations — methods of which include vacuuming, water wash, brooms and compressed air — personal protective equipment and proper safe work procedures should be provided.

A comprehensive combustible dust control program, however, is not complete until that hazard has been identified and communicated to all workers. Until then, “we can talk about engineering and administrative controls all day, but that’s putting the cart before the horse,” Astad suggests.

In the meantime, the search for answers to the two devastating explosions continues. The United Steelworkers is engaged in the investigation conducted by WorkSafeBC, as well as conducting its own investigation, which involves interviewing workers at Lakeland Mills, Hunt says. “The key for us is to ensure the timing is right for the investigation,” he notes. “If you go to people too quickly, there is still quite a bit of anger and frustration.”

The Council of Forest Industries task force is looking at establishing an auditable dust control standard that can be implemented through a third party to serve as a “powerful means of ensuring a consistently high standard across the industry,” says Corinne Stavness, manager of public affairs with the council.

“We are making good progress,” she adds, noting that the two incidents have brought together the forest products industry in British Columbia in an unprecedented fashion. “It is our aim that this will be a positive legacy of these horrible tragedies — that we have really set a new standard in coordination and openness between companies, sharing more information on dust and all other safety hazards and increasing the pace at which we learn from each other and implement safety improvements.”

Since May 28, Lakeland Mills had restarted its planer mill for about six weeks to process inventory that remained following the explosion and fire. A company statement says there is enough lumber to keep the mill running for approximately 30 shifts, or one shift per day for six weeks. No decision has yet been made on the longer-term future of the mill’s operations.

“What we learned from this is don’t take any dust for granted,” Hunt says. “It can all explode if they have enough quantity.”  
Jean Lian is editor of ohs canada.
Under Scrutiny
On May 2, WorkSafeBC released an investigation update on the explosion at Babine Forest Products that leveled the mill on January 20, 2012. More than 80 witness interviews have been conducted, with as many as 20 WorkSafeBC personnel working on the investigation. The footprint of the incident site required in excess of 3,000 lineal feet fencing, placing it amongst the largest scene investigations in organizational history.
Possible factors contributing to the risk of explosion that continue to be examined include production level records (both recent and historic); type of wood being milled prior to the incident; exhaust and ventilation systems and schedules; effect of cold weather in the days preceding the incident date (-41 degrees Celsius); effect of cold on water pipes and misters; and sawdust accumulations.
There is no evidence to suggest the explosion was caused by arson, lightning strike, hot oil, hydraulic oil, gear oil, oxygen and acetylene.
Natural gas, propane and sawdust continue to be examined as possible fuel sources, while hot surfaces or friction, electric arcs from motors and switches, and several other electrical components are being looked at as possible ignition sources.

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Reader Comments

Most recent firstOldest first
Eyal Zadok
Hello, This post is given to OSH Canada in order to assist the scrutiny to focus on a possible ignition source that could be generated from the described physical evidences in the two explosion events. I am a physicist and electrical engineer. For more than 30 years I am working in the field of static electricity risk analysis in industry, including investigation of dust materials fire & explosion. The very cold and dry weather has a strong effect on the build up of electrostatic charge due to any kind of friction between materials/parts in the mill. The charge can accumulate on electrically conductive parts as well as isolative materials. The electrostatic mechanism of dust ignition involves some parameters such as discharge type (spark, brush, PBD, cone) and charge density/energy, dust particle (distribution size, concentration in space, moisture content). Dust explosions created by such ignition source are generally characterized by two explosions following each other, the first is weak (sometime even not seen or heard) and the second is much stronger that is yielding mass destruction. The investigation already eliminated some common ignition source and now some electrical components are being looked. As static electricity is known to be a hidden ignition source with very unique characteristics, I strongly recommend OSH Canada to include in the investigation and to look for the possibility for electrostatic ignition. Feel free to contact the undersigned for any further assistance if needed. Sincerely, Eyal Zadok E-mail: eyalzad@netvision.net.il
Posted July 18, 2012 04:26 AM

Tuesday, September 11, 2012

Combustible dust: Identifying, addressing explosion risks can save lives

From Plant Engineering and Camfil Farr, an excellent commentary on the state of the art in current combustible dust explosion prevention/protection standards and design.


Combustible dust: Identifying, addressing explosion risks can save lives  | Plant Engineering


Combustible dust: Identifying, addressing explosion risks can save lives


Review of the current status of the OSHA National Emphasis Program for combustible dust, the NFPA standards that address how to prevent or limit explosion hazards, how to identify these hazards, and the types of equipment used to eliminate or control explosion hazards.

Tony Supine and Mike Walters, Camfil Farr APC

06/09/2012

The National Fire Protection Association (NFPA) sets standards and codes to protect buildings against fire and explosion risks, and OSHA is enforcing these standards with increasing vigilance. When it comes to combustible dust, several standards must be considered. Combustible dust explosions are a risk in many areas of a plant, but one of the most common locations is the dust collection system. How do you know if your dust collection system complies? What do you do if it doesn’t? Are your employees at risk? What are the hazards and how do you identify them?
There is more information on the topic of combustible dust collection and safety available at www.plantengineering.com. Among the items available online:

A detailed list of the agencies involved in monitoring the safety of dust collector systems. Among those agencies are OSHA, NFPA and the U.S. Chemical Safety Board. Learn more here.

A list of relevant NFPA standards that cover combustible dust issues.
This article reviews the current status of the OSHA National Emphasis Program for combustible dust, the NFPA standards that address how to prevent or limit explosion hazards, how to identify these hazards, and the types of equipment used to eliminate or control explosion hazards. We will also examine the most common shortfalls to compliance and how to avoid them.
At a the start of a dust explosion and a passive system of protection, explosive dust is injected into the dust collector to create a flammable cloud. Courtesy: Camfil FarrAPC 
Ten-year retrospective
An explosion at the West Pharmaceutical facility in Kingston, N.C., in January 2003 killed six workers and injured 38 others, including two firefighters. The culprit was inadequate control of dust hazards at the plant. Just a month later, in February 2003, another explosion and fire damaged the CTA Acoustics manufacturing plant in Corbin, Ky., fatally injuring seven workers. Investigators found that resin dust, accumulated in a production area, was likely ignited by flames from a malfunctioning oven, triggering the explosion.

Probably the most famous combustible dust explosion in the past decade—and the one that re-focused the national spotlight on this issue—was the February 2008 accident at the Imperial Sugar Company’s Wentworth, Ga., refinery. A dust cloud explosion triggered a fatal blast and fire that killed 13 workers and injured 42 others, generating a storm of media attention and government scrutiny.

These are not the only fatal explosions to occur in U.S. manufacturing plants, though they are the three deadliest to be investigated. More recently, in December 2010, two brothers lost their lives in a chemical explosion at the New Cumberland, W.Va., plant of AL Solutions. And during 2011, three deadly fires and explosions occurred at a Hoeganaes Corp. plant in Gallatin, Tenn. Investigators found that accumulations of fine iron powder in the facility led to the explosions.

About 5 ms later, the dust ignites and the vent is opened. Courtesy: Camfil Farr APCIn the U.S. alone between 1980 and 2005, the Chemical Safety Board reported 281 explosions caused by ignited combustible dust. These explosions resulted in 199 fatalities and 718 injuries. Combustible dust explosions over the past 10 years in U.S. plants are blamed for well over 100 fatalities and hundreds more injuries. Sadly, experts believe that all of these accidents were preventable if the companies involved had followed best practices for fire and explosion protection such as the methodologies described in this article.

Explosion protection solutions
There are two general categories of equipment used to comply with NFPA standards for the explosion protection of dust collection systems: passive and active. Passive systems react to the event, while active systems detect and react prior to or during the event. The goal of a passive system is to control an explosion so as to keep employees safe and minimize equipment damage in the plant. An active system, by contrast, can prevent an explosion from occurring. An active system involves much more costly technology and typically requires recertification every three months.

Passive devices
Almost immediately the flame is extended away from the dust collector and into a safe area. Courtesy: Camfil Farr APC

Explosion venting: Designed to be the “weak” link of the dust collector vessel, an explosion vent opens when predetermined pressures are reached inside the collector, allowing the excess pressure and flame front to exit to a safe area. It is designed to minimize damage to the collector and prevent it from blowing up in the event of a deflagration, thereby reducing the safety hazard.

Flameless venting: Designed to install over a standard explosion vent, a flameless vent extinguishes the flame front exiting the vented area, not allowing it to exit the device. This allows conventional venting to be accomplished indoors where it could otherwise endanger personnel and/or ignite secondary explosions.

Passive float valve: Designed to be installed in the outlet ducting of a dust collection system, this valve utilizes a mechanical barrier to isolate pressure and flame fronts caused by the explosion from propagating further through the ducting. The mechanical barrier reacts within milliseconds and is closed by the pressure of the explosion.

Back draft damper: A mechanical back draft damper is positioned in the inlet ducting. It utilizes a mechanical barrier that is held open by the process air and is slammed shut by the pressure forces of the explosion. When closed, this barrier isolates pressure and flame fronts from being able to propagate further up the process stream.
The smoke and dust quickly clear. the entire event took just 150 ms. Courtesy: Camfil Farr APC 
Flame front diverters: These devices divert the flame front to the atmosphere and away from the downstream piping. Typically, these devices are used between two different vessels equipped with their own explosion protection systems. The flame front diverter is used to eliminate “flame jet ignition” between the two vessels that could overpower the protection systems installed.

Active devices

Chemical isolation: Designed to react within milliseconds of detecting an explosion, a chemical suppression system can be installed in either inlet or outlet ducting. Typical components include explosion pressure detector(s), flame detector, and a control panel. This system creates a chemical barrier that suppresses the explosion within the ducting, reduces the propagation of flame through the ducting, and minimizes pressure increase within connected process equipment.

Chemical suppression: Whereas chemical isolation is used to detect and suppress explosions within the ducting, chemical suppression protects the dust collector itself. It is often used, together with isolation, when it is not possible to safely vent an explosion or where the dust is harmful or toxic. The system detects an explosion hazard within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.

Fast acting valve: Designed to close within milliseconds of detecting an explosion, the valve installs in either inlet or outlet ducting. It creates a mechanical barrier within the ducting that effectively isolates pressure and flame fronts from either direction, preventing them from propagating further through the process.

High-speed abort gate: The gate is installed in the inlet and /or outlet ducting of a dust collection system and is used to divert possible ignition hazards from entering the collector, preventing a possible explosion from occurring and preventing flame and burning debris from entering the facility through the return air system. A mechanical barrier diverts process air to a safe location. Abort gates are activated by a spark detection system located far enough upstream to allow time for the gate to activate.

When planning explosion protection, don’t overlook additional devices and materials that can help reduce fire risk within the dust collection system. For spark-generating applications, a range of features and technologies is available, from flame-retardant and carbon anti-conductive filter media to spark arrestors in the form of drop-out boxes, perforated screens, or cyclone devices installed at the collector inlet. Fire sprinkler systems may also be required with some installations.

A dust collector that uses vertically mounted filter cartridges can also reduce fire and explosion risks. With horizontally mounted cartridges, dust becomes trapped in the pleats in the upper third of the filters. This trapped dust can burn even if the filter media is fire retardant. Horizontal cartridges are also exposed to all of the dust entering the collector, coarse and fine. This leads to premature failure from abrasion and leaks. These leaks can go unnoticed for quite some time while fine combustible dust is blown into your facility. Vertically mounted filters use baffle systems to segregate much of the dust into the hopper, which reduces the load on the filters and helps eliminate these problems.

Avoid pitfalls
A variety of situations can place a facility at risk, but there are some common denominators. The ones we have most commonly encountered in our field experience are:

Complacency about maintaining the status quo: “I’ve been here for years and we’ve never had a problem” is an all-too common refrain. This mind-set stems in part from a common misconception that the facility’s dust is not explosive because it has not had an event, when in fact the opposite may be true. In some cases, it may take years for dust to accumulate to explosive levels as seen in the CTA Acoustics event.

To understand the risks, let’s review the five elements that comprise what is known as the “dust explosion pentagon”:
  • Combustible dust
  • An ignition source
  • Oxygen in the air
  • Dispersion of the dust in sufficient concentration to be explosive
  • Containment of the dust cloud within a confined or semi-confined vessel or area.
All five of these elements may exist in an industrial facility, but all must be present at the same time for an explosion to occur. If there is no containment, it is still possible for a flash fire to erupt if elements 1-4 are present simultaneously.
In a closed vessel such as a cartridge dust collection system, an explosion typically begins when an ignition source enters the dust collector. This ignition source can come from many things and in most cases is never identified. When a pulse cleaning event occurs, a suspended cloud of combustible dust is present in high concentration within the collector.

Though some incidents involve a single explosion, it is more common for a series of deflagrations to occur. The initial explosion can dislodge ignitable dust hidden on overhead surfaces or other areas over a large area and trigger secondary explosions that can be ignited from the initial explosion or from other ignition sources. It is these secondary explosions that have historically caused the majority of injuries and damage to property.

How do you know if your facility is at risk? Even if there has never been a problem before, this is no guarantee of future safety. The level of hazard can change from day to day and even from moment to moment—whether due to the introduction of a new process, a temporary lapse in housekeeping, or a static electricity discharge caused by improper grounding. It takes ongoing vigilance and management of change to identify conditions in your plant that might cause a potential safety problem.

Failure to conduct a hazard analysis (also called a risk evaluation): This is probably the biggest oversight that we see. The NFPA states that a hazard analysis is needed to assess risk and determine the required level of fire and explosion protection. The analysis can be conducted internally or by an independent consultant, but either way the authority having jurisdiction will ultimately review and approve the findings.

When it comes to explosion protection, the first step in a hazard analysis is to determine if your dust is explosive. Most commercial test laboratories offer a low-cost test to establish whether a dust sample is combustible. If the test is positive, then the explosive index (Kst) and the maximum pressure rise (Pmax) of the dust should be determined by ASTM E 1226-10, Standard Test Method for Explosibility of Dust Clouds.

Your dust collection equipment supplier needs the Kst and Pmax values in order to correctly size explosion venting or suppression systems. Failure to provide this information will increase your costs, since the supplier will use worst-case estimates of the Kst and Pmax values or may even refuse to provide the equipment. The liability to the manufacturer and to the equipment purchaser is too high to ignore the life safety objectives.

The fact is, any dust above 0 Kst is now considered to be explosive, and the majority of dusts fall into this category. If OSHA determines that even a very low Kst dust is present in a facility with no explosion protection in place, a citation will result. This is one of the biggest changes to occur with the reintroduction of the OSHA NEP in 2008.

Shopping for price over quality: Every plant engineer is acquainted with the benefits of basing purchasing decisions on life-cycle cost—sometimes called “total cost of ownership”—over choosing equipment with the lowest price tag. A dust collector is no exception. A well-designed dust collection system can pay for itself rapidly in energy and maintenance savings, costing far less to operate than a unit with a low initial price. A high-quality, heavy-duty collector can also offer a less obvious advantage in the event of a combustible dust problem.

As documented both in full-scale testing and field experience, in the event that a dust explosion occurs in the collector, a low-end model will more than likely require total replacement. A collector made of thicker-gauge metal with higher vessel strength, however, will survive an explosion and can often continue in service with only the explosion vent and filter cartridges needing to be replaced.

Use of noncompliant devices: A close cousin to the “price versus quality” issue involves the use of noncompliant or uncertified explosion protection devices. As an example, sometimes products, such as back flap dampers, may be reverse-engineered by suppliers that do not have any expertise in explosion protection or have chosen not to perform the required testing to satisfy the standards and/or the performance-based provisions. No testing exists to prove that the device will comply with current standards. If an OSHA inspector finds this situation in the field, the plant will have to replace the device and may be subject to a fine. Worse yet, if a combustible dust problem should occur, there is no guarantee that the device will perform as expected.
It is also worth noting that there is no such thing as an “NFPA-approved” device. A supplier may correctly state that a device “carries CE and ATEX certifications” or is “manufactured in accordance with NFPA standards,” but test data must be available to support these claims. Such a device might cost more than its noncompliant counterpart, but in the long run it can save money, headaches, and even lives.

Housekeeping deficiencies: In its October 2011 update on the Combustible Dust NEP, OSHA reported that one common violation encountered during inspections involved “hazardous levels of dust accumulation in the workplaces due to poor housekeeping practices.” In the authors’ experience, as a rule of thumb, if an OSHA inspector can run his finger across a dusty surface or see a footprint, that is considered a citable condition. Even diligent cleanup of floors and work surfaces is not sufficient if more elevated areas are neglected: Dust accumulation on rafters and other horizontal overhead surfaces or on top of machinery is a frequent culprit. Also, the NFPA recently tightened its definition of hazardous surface dust (in NFPA 654), defining it as any dust layer of 1/64 in. (0.4 mm) or greater, compared to a previous limit of 1/32 in. (0.8 mm) of dust.

When it comes to the dust collector, a simple but important housekeeping requirement is to change filters when airflow through the system reaches a differential pressure limit as prescribed by the manufacturer or when the pressure drop across the collector is negatively affecting the ability of the dust collection system to capture the dust, thus allowing it to escape into the facility.
Some long-life cartridge filters available today can operate for two years or even longer between change-outs; but for heavy dust-loading applications, filter replacement might be considerably more frequent. Also, use of a listed portable vacuum helps keep the surrounding area free of spilled dust and surface dust. Use of compressed air to control dust is permitted only under certain conditions because it can increase the hazard by creating a combustible dust cloud.

Another housekeeping misstep is storing dust in the dust collector’s hopper. The hopper should be equipped with a device that discharges the dust into a separate drum or storage container after it is pulsed off the filters during the cleaning process. Equally important, this storage container must be emptied regularly, or dust can back up into the hopper. Dust sitting in a hopper creates a potential fire or explosion risk, and may also affect performance of the dust collection system. This will lead to loss of airflow, which will reduce conveying velocities, allowing build-up of dust in the ducting and emissions of dust at the process hoods.

Misconceptions about “open” style dust collectors: There is a fairly widespread misconception that open-type dust collection systems, such as those incorporated into downdraft tables and booths, are not a hazard. While these collectors admittedly differ from traditional dust collectors in that they do not take the form of a tightly contained vessel, at least four of the five ingredients of the explosion pentagon may still be present: combustible dust, an ignition source, oxygen, and dispersion of the dust in sufficient concentration to pose a hazard. Thus, there is still a risk of flash fire directed by a pressure front—a potentially fatal risk, given that workers are in close proximity in these environments. If you are using an open-type dust collector, you must still test and evaluate the combustibility of the dust and equip the area with fire and/or explosion protection equipment as required.

The mistake of over-engineering: The problems and pitfalls described thus far involve not doing enough in one way or another. But sometimes plant engineers err on the side of doing too much—the error of over-specification, which results in explosion protection solutions that may be needlessly expensive and time-consuming to maintain and monitor.

The NFPA uses relatively conservative textbook calculations in its standards for explosion protection equipment, and justifiably so. However, the NFPA also allows real-world destructive test data to be used in place of its own standard calculations, provided the dust collection supplier can provide adequate data to prove that the collection system is designed to meet a specific set of criteria for a given situation. The use of real-world destructive test data is thus a permissible and sometimes overlooked strategy.
An example is actual explosion testing of a dust collector to show that it will stand up to anticipated pressure conditions, instead of using the reduced pressure calculations in NFPA 68. By combining field testing and full-scale dust collection laboratory test apparatus to prove certain assumptions, this approach might allow you to install longer duct lengths in a given application; to use a single explosion vent where multiple vents might otherwise have been needed; or even to use explosion venting in place of a more costly chemical suppression system. Find out if your dust collection supplier can provide real-world test data to assist in a strategy that may help you to avoid over-engineering and save on equipment costs without compromising safety.

Conclusion
There is no universal agreement about the best way to tackle the combustible dust problem. Some concur with the CSB position that OSHA needs to accelerate efforts to produce and enforce its own standard, citing a long-standing precedent with the grain industry.
Explosions in grain bins used to be one of the biggest safety problems in the U.S. In 1987, following a series of deadly explosions, OSHA promulgated a Grain Handling Facilities Standard that remains in effect today. This standard has yielded major improvements in combustible dust safety in these facilities.

According to OSHA, “The lessons learned in the grain industry can be applied to other industries producing, generating, or using combustible dust.”

Others argue that more stringent and perhaps consolidated dust standards from the NPFA, diligently enforced by OSHA and local authorities, would be preferable to a separate OSHA standard. What everyone does seem to acknowledge is that more drastic action is necessary to prevent combustible dust tragedies from continuing to occur.

Until such action is mandated, a certain degree of self-regulation is called for. Every plant engineer can choose to be part of the problem or part of the solution. By following the guidelines in this article, and securing the help of engineering consultants and equipment suppliers with a proven track record in combustible dust applications and performance-based solutions, you can minimize risk factors and maximize combustible dust safety in your facility.

Supine has held numerous positions with Camfil Farr APC including research and development manager, technical director, and currently plant manager. Walters, a registered professional engineer with 30 years’ experience in air pollution control and dust collection systems, is a senior engineer with the company. The authors can be reached at 800-479-6801 or 870-933-8048; e-mail filterman(at)farrapc.com website www.farrapc.com.

Monday, September 10, 2012

Lakeland Mills sawmill warned multiple times before fatal blast

Inspection photos show dust buildup at Prince George facility

Prince George Fire and Rescue inspection photos show buildup of wood dust at Lakeland Mills in Prince George on Nov. 29, 2011, five months before an explosion and fire killed two workers and seriously injured others.

Five months before a deadly explosion at Lakeland Mills sawmill, photos show combustible wood dust built up on ledges, under a machine, and on hand railings, light fixtures and pipes for the water-sprinkler system.

In several photos of the Prince George mill, obtained by The Vancouver Sun under a freedom of information request, the dust is so thick it is visible in the air as hazy, luminescent dots.

A five-year span of fire inspection reports, as well as the Nov. 29, 2011 photos, show Lakeland Mills was warned several times about combustible dust hazards before the April 23 explosion that killed two workers.

Combustible dust is defined in the B.C. Fire Code as “dusts and particles ignitable and liable to produce an explosion.”
WorkSafeBC inspections of the mill, which have been previously reported, noted high levels of dust but keyed on the harm that wood dust could do to workers’ lungs.

Inspection reports by Prince George Fire and Rescue between 2007 and 2012 warned Lakeland about the accumulation of wood dust three times.

View photos of dust-covered sawmill

In November 2011, fire officials cited Lakeland Mills for a “deficiency” under the B.C. Fire Code for not keeping building and machinery surfaces clean of accumulation of combustible dusts.

A followup letter dated a day after the inspection warned Lakeland Mills that parts of the mill had “excessive amounts of accumulated fine wood dust.”

Prince George Fire and Rescue — which has a duty to inspect public buildings, including factories, under B.C.’s Fire Services Act — requested the mill develop and adopt a policy that describes “the procedure, frequency and documentation for the cleanup and removal of this combustible hazard.”

On March 19, 2012, months after a fatal explosion at a sawmill in Burns Lake, Lakeland Mills received yet another warning.
Prince George Fire and Rescue Lieut. Steve Feeney noted in his inspection report the “unacceptable” amount of dust present during the 2011 inspection had been “significantly reduced.”

Nonetheless, the mill was again cited for being “deficient” in not keeping the building and machinery clean of combustible dust. The fire department repeated its request that Lakeland create a cleanup policy.
An explosion and ensuing fire burnt Lakeland Mills to the ground April 23.

Prince George Fire and Rescue chief John Lane declined to answer questions about the inspection reports because the investigation into the explosion is still underway.

“It really would be inappropriate for us to offer any specific comments,” said Lane.
However, in general, deficiencies identified during inspections are “certainly” orders.

“They reflect the measures that are required for the business or the building owner to be in compliance with the fire code, and those orders are followed up until they are complied with,” he said.

Sinclair cct Group president Greg Stewart, which owns the majority of the mill, cautioned against reading too much into the inspection reports.

Dust is a fact of life in mills, Stewart said. Therefore, an inspection immediately after a cleanup would find little dust while an inspection at the end of a shift could find more dust.

He said the mill had increased its cleaning crews to three workers from two in April.
And Stewart pointed to the March fire code inspection report, saying it showed the mill was making improvements.

The Sun had to provide a copy of the inspection reports and photos to the lumber company because it no longer had them. The reports had burned up in the Lakeland Mills explosion and fire, said Stewart.

Lakeland Mills workers told The Sun they believe dust buildup was still a problem in the weeks before the explosion.

Stewart said he couldn’t comment on the workers’ observations because he had no way of knowing what level of dust they were using as a comparison. “In addition to that, I don’t want to speculate that the dust was the cause of this incident,” he said.

Lakeland Mills workers said the wood dust problem had not improved in the weeks leading up to the deadly explosion.

“It was still pretty bad,” said Allan Morin, a 37-year veteran worker whose hands and face were severely burned in explosion.
“The sun would shine into the mill once in a while, and you could see the dust in the air,” he said.

Morin said he was only protected from receiving worse burns because he was sitting in the cab of his machine when the blast tore through the mill.

He described hearing something like an electric zap just before the explosion. “It was like a loud ‘zzzzz’ and all of a sudden the power went out, and then the fireball hit me.”

Morin said an incident that took place in the winter, several months before the explosion, now seems like a warning. A spark from a saw ignited wood dust, creating a small fireball, he said.

Lakeland Mills worker Lorne Hartford also said there was dust buildup in the weeks before the explosion. Hartford said the mill was short-handed, so cleanup was neglected. “It got really bad,” he said.

The Prince George fire department called on Lakeland Mills to create a fire safety plan in September 2008, a request repeated in September 2010, November 2011 and March 2012.

The inspections record noted numerous other deficiencies.

They included that exit lights need to be illuminated at all times when the building is occupied; fire extinguishers must be mounted on wall hangers and protected from dust; fire hoses and nozzles need to be inspected annually, and rated fire doors must be kept closed at all times.

WorkSafeBC has not directly linked the deadly explosions with wood dust, but after the explosion at Lakeland Mills ordered all sawmills in B.C. to clean up wood dust.

An explosion and fire at Babine Forest Products in Burns Lake on Jan. 20 also killed two workers and seriously injured others.
Dust samples have been collected at both mills by WorkSafeBC for explosive testing.

ghoekstra@vancouversun.com
Combined Pdfs

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