Dust Gets Its Due

Industry is placing greater emphasis on addressing combustion hazards.

By Seán Ottewell, Editor at Large

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"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 sottewell@putman.net.


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