Dust Gets Its Due

Dec. 15, 2010
Industry is placing greater emphasis on addressing combustion hazards.

The dangers posed by combustible dusts no longer are being swept under the rug. Tougher regulations and greater corporate resolve are making dust hazard management an increasingly important topic for every manufacturing sector including the chemical industry.

Testifying to the topic's popularity, a recent CP webinar on dust control (now available on demand at http://video.webcasts.com/events/putm001/33721/) attracted the second largest attendance of any such event. In it, speakers from the National Fire Protection Association (NFPA), Quincy, Mass., Fike Corporation, Blue Springs, Mo., Chilworth Technology, Plainsboro, N.J., and Camfil Farr APC, Jonesboro, Ark., outlined the key challenges facing the industry today.

The importance of dust management really started to come under regulatory scrutiny in the U.S. in 2003 when the U.S. Chemical Safety Board (CSB), Washington, D.C., determined that lack of attention to dust had resulted in explosions at three U.S. manufacturing plants that year. A January fire and explosion at West Pharmaceutical Services, Kinston, N.C., was caused by a fine plastic powder that gathered above a suspended ceiling over a manufacturing area; six workers died and many more were injured. Three weeks later, an accumulation of resin dust from a phenolic binder used in a production area led to a blast that killed seven workers at fiberglass insulation manufacturer CTA Acoustics, Corbin, Ky. Then, aluminum dust was found to be the culprit for an October explosion that killed one person and injured many others at Hayes Lemmerz's aluminum wheel plant in Huntington, Ind.

One recommendation of the subsequent CSB studies was that the U.S. Occupational Safety & Health Administration (OSHA), Washington, D.C., should get more actively involved in combustible dust hazard management. In late 2007, OSHA issued its National Emphasis Program (NEP) on combustible dust.

Flameless Explosion Vent
Figure 1. Layers of stainless steel absorb heat generated by combustion, allowing venting without flame release. Source: Fike.

However, in February 2008, a huge explosion and fire at Imperial Sugar's Port Wentworth refinery, Savannah, Ga., left 14 dead and 38 others seriously injured. The explosion was fueled by massive accumulations of combustible sugar dust throughout the packaging building.

Guy Colonna, division manager of the industrial and chemical engineering department of the NFPA, says he has attended an increasing number of dust-management events like CP's webinar over the last two years. He attributes their popularity to a much greater focus on hazard awareness across all industries and the public following the explosion at Imperial Sugar.

"Earlier incidents during the decade noted and investigated by OSHA and CSB were no less significant in terms of their losses to people and property, but didn't stir everyone the way the Imperial Sugar incident seems to have established a resolve across all affected interests."

He cites data reported by CSB in its November 2006 dust report and similar incident data published by the insurance industry that show explosions and fires involving combustible dusts aren't that rare — about ten incidents per year took place on average from the early 1980s until 2005 in the U.S.

"Those numbers suggest the various industries have 'coped' with the hazard and operated around the hazard. At this stage, it would appear that industry is no longer looking to cope with the problem and is resolute in finding common understanding about the phenomenon that can lead to more effective hazard assessment and control solutions."

In Colonna's experience, two issues commonly crop up in question-and-answer sessions these days — and both are core to coming to grips with combustible dust hazards.

The most common relates to the hazard identification process for examining a facility and any combustible particulates that might be involved as process input, intermediate or final product. This generally requires testing of a sample and then interpreting the results.

One property commonly determined is the dust's deflagration index, KSt , which indicates how rapidly a pressure front forms and moves through the combustion zone.

"Often, those interpreting the data are seeking guidance on the interpretation for the value reported. Their question asks whether a 'low value' for the KSt means there is no problem from the dust in terms of explosibility. The answer to that question is NO. There is no target or threshold value below which no hazard is assumed. Lower values of the index just mean the rate of rise of the pressure with respect to time will be 'slower' (in a relative sense) than other materials that have higher values for the index. The damage pattern from a low KSt value dust is different than the damage experienced when the index is greater, but the overpressure is still present and capable of destroying structures — and the fireball also poses a hazard to both personnel and the facility."

Blowout Panels
Figure 2. Rebuilt Port Wentworth, Ga., sugar refinery now contains such wall panels for fire and dust safety. Source: Imperial Sugar.

The second issue involves the characterization of the dust hazard condition or area. In many cases this currently is based on the thickness of the layer of accumulated dust. Colonna explains that NFPA 654 presents an equation that enables adjusting the permitted or target layer thickness based on the specific material's bulk density — how packed or non-packed a settled accumulation could be — while still yielding an equivalent mass per area. The layer thickness as used today serves as a trigger for various protective measures as well as housekeeping. The prorating equation in the standard permits users to adjust their layer thickness (ultimately that accumulated mass) based on the specific bulk density.

"So, materials like tissue paper, which have a very low bulk density compared to wood and other materials, when evaluated for layer thickness on the basis of the bulk density prorating equation are recognized as accumulating in a less packed or thicker layer while still yielding the same amount of accumulated dust mass across a given area. The equation method for establishing the layer thickness doesn't penalize those materials whose bulk density is low relative to other materials by requiring them to comply with the same layer thickness as more densely layered dusts," he says.

Lately, the chemical industry also has become concerned about various regulatory standards and consolidations currently afoot. Much of this revolves around OSHA's notice of its intention to develop a federal standard to address workplace safety requirements to protect workers from combustible dust fire and explosion hazard conditions.

One solution could be to adopt the five existing NFPA dust standards — there are arguments both for and against this strategy. So the NFPA has challenged its four combustible dust technical committees to determine a path that would consolidate the various requirements of its dust standards into a single comprehensive standard.

Dust Collection
Figure 3. Sugar-refinery rebuild also included this dust-collection system. Source: Imperial Sugar.

"Those supporting the concept see that many of the steps in the hazard assessment process are the same regardless of dust type and, once the explosibility properties are determined, the protective measures are the same in general and only become unique in their design due to those properties which become part of the design process. A single comprehensive standard makes enforcement easier and, thus, stronger," notes Colonna.

This changing regulatory landscape is driving increased interest in combustible dust hazards by all industry groups, says Bob Korn, director of sales for explosion protection products for Fike: "OSHA's NEP has pushed a new emphasis on OSHA inspections of U.S. manufacturing facilities. In October, OSHA chief Dr. David Michaels told a safety group that in the three-year-old combustible dust NEP, nearly 9,100 violations have been found, although not all of them have been for dust violations, with initial penalties totalling more than $19.5 million.

At a symposium, also in October, OSHA assistant secretary Jordan Barab said OSHA's inspections of refineries under the process safety NEP has resulted in an average of 17 violations per inspection with penalties averaging $166,000. He said 53 of 58 refineries have been inspected. At hazardous chemical manufacturing facilities, inspectors have averaged nine violations during each of 136 inspections."

This increase in inspection rates, coupled with OSHA's commitment to the creation of a combustible dust standard, has the U.S. chemical industry scrambling to understand the hazards and develop strategies to protect their facilities, Korn believes.

This scramble is reflected in the sort of queries he faced in the webinar question-and-answer session. These typically concerned equipment applications, code compliance issues and hazard assessment. For example:
• What is the anticipated timeline from OSHA on the implementation of its widely discussed combustible dust standard?
• Does your suppression equipment need to be inspected on a regular basis and, if so, how often and who can do the inspection?
• How do the operating costs of inert atmospheres compare with capital costs of suppression or flame-arrest systems?
 • What type of explosion suppression/relief systems have you seen installed on direct-contact rotary dryer applications?
• What methods do you use to determine particle size and shape?

Explosion Supression
Figure 4. Multiple units now are installed at Port Wentworth sugar refinery. Source: Imperial Sugar.

Overall, the best advice he can give to a chemical maker today is to perform a hazard analysis or risk assessment of its facilities and understand where it needs to improve housekeeping, add dust collection and provide protection for processes at risk.

"Organizations that are actively engaged in understanding the hazard and taking the necessary actions to provide a safe work environment will dramatically lessen the effect of an OSHA inspection and any resultant fines. The chemical industry is in the OSHA spotlight due to other recent serious accidents; consequently they are in the top three of industries inspected by OSHA under the combustible dust NEP."

For Fike, whose business centers on supply of explosion-protection and fire-suppression systems, all this regulatory activity is spurring product development. The company relatively recently introduced rectangular flameless explosion vents (Figure 1). It's working on faster-responding suppression devices because speed is key when dealing with a deflagration. "Our systems must detect the combustion event and inject a chemical suppressant in a matter of milliseconds. From time of detection to full release of the suppressant happens typically in less than 50 milliseconds. It takes approximately 250 milliseconds to blink your eyes, for comparison."

The company also is looking at impulse valve technology for opening its chemical suppressant containers for explosion suppression and chemical isolation of interconnections between vessels. "The impulse valve will provide the quick activation we need without the use of a pyrotechnic GCA [gas charged activation] device that is currently used. This will be a great benefit to future systems as we can eliminate the shipping and handling issues associated with these GCA devices," he explains.

This is a key issue, stresses Dr. Vahid Ebadat, CEO of Chilworth Technology: "The severity of an explosion is often directly related to the size/quantity/spread of the available fuel, in this case, the dust. When one studies the large dust explosions that industry has been experiencing, one realizes that almost always the fatalities and most of the damage has not been the result of the initial event but the so-called secondary dust cloud explosion. Therefore, ensuring that the dust is contained within the protected processing equipment and an effective housekeeping measure is in place would go a long way towards ensuring the safety of people and facilities."

The huge accumulations of sugar dust at the Imperial Sugar refinery clearly resulted from a failed housekeeping strategy, something that Chilworth has been working with the company to rectify.

The last stage in the rebuild at the refinery involved the three sugar silos, each with a storage capacity of 6.5 million pounds of sugar, or 19.5 million pounds total — three million pounds more than the former silos held (Figure 2). All sugar begins its curing journey in a primary conditioning silo, where dehumidified air is percolated through the silo for 24 hours and where a dust-collection system removes dust. The sugar then moves to one of the other two silos, where it sits in storage before moving to packaging or to a bulk station for distribution.

Following consultations with Chilworth, Imperial Sugar opted for 56 pressure-relief vents on each of its silos at the Port Wentworth refinery. More unusual, however, is the conveying sugar through the silos via a dense-phase system instead of belt or screw conveyors or bucket elevators. The advanced system uses high-pressure air to pump sugar within pipes at a rate of 225 tons/hr.

"Dense-phase conveying is a form of pneumatic conveying that is used in the grain, flour and chemical industries, and for wood chips and sawdust, and infrequently in the sugar industry. However, use of this method for conveying sugar to the top of a 175-foot-high silo — as at the Port Wentworth refinery — is unusual. Among the safety features of dense-phase pneumatic conveying in conductive piping are: complete containment of the product; minimized generation of static electricity; and no moving parts that could cause frictional heating or impact sparks," notes Ebadat.

The rebuilt refinery also features wall blowout panels (Figure 1), dust collection systems (Figure 2) and explosion suppression devices (Figure 3).

The OSHA NEP has led to an increasing degree of awareness in the issue, he believes. "And rightly so. Many companies still don't realize that combustible dusts can be just as dangerous as flammable liquids and gases." However, in preparing for an OSHA inspection, companies handling and processing combustible powders and dusts have become increasingly more proactive with their combustible dust management activities, he adds.

Ebadat offers four pointers on how to succeed in such activities:
1. Properly assess your dust's fire and explosion characteristics so adequate measures can be taken for the prevention and mitigation of hazards in your own facilities and, if you are shipping the dust to some other facilities, at those locations.
2. Understand your own powder handling and processing operations. You should be able to identify likely ignition sources during both normal and abnormal operating conditions. Also pinpoint location(s) where combustible dust clouds could exist during normal and abnormal operating conditions.
3. Take effective measures to avoid or control ignition sources and formation of combustible dust clouds. Also consider explosion protection (such as venting and suppression) and isolation to lower the risk to a tolerable level.
4. Maintain dust explosion prevention and mitigation measures.

Seán Ottewell is Chemical Proessing's Editor at Large. You can e-mail him at [email protected].

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