Dust / Conveying Equipment

Attention Turning To Dust Explosion Prevention

Food ingredients are the leading cause of explosions, and containing those events no longer is enough.

By Kevin T. Higgins, Managing Editor

Awareness is the first step to preventing dust explosions in food and beverage facilities. Too often, awareness is part of the residue of a catastrophic event.

A feed mill explosion in February 2016 is a recent reminder of the dust explosion risk. A 25-year-old man was killed and five others were sent to hospitals when grain dust ignited in the hammer mill area in a Koch Foods Inc. chicken feed mill in Rockmart, Ga. The estimated $3 million in damage resulted in the facility’s demolition.

“This incident and this man’s death were preventable,” Christi Griffin, area director of the U.S. Dept. of Labor’s Occupational Safety and Health Administration said in a statement announcing $112,000 in proposed fines. Ownership “needs to take a proactive approach in their safety and health program to assess the workplace for hazards and correct them to ensure worker safety,” she added, citing the 23 safety and health violations found by inspectors.

In a report on combustible dust incidents in North America last year, Chris Cloney (www.mydustexplosionresearch.com) found that food products and food production environments accounted for one-third of all incidents, exceeding wood products and lumber facilities. Grain milling was the most common fuel source, but protein, starch, oregano and fishmeal were among leading sources.

Events like the Rockmart explosion give impetus to tighter National Fire Protection Assn. (NFPA) standards for explosive environments. Stricter standards usually provoke grousing from those who perceive dust explosions as freakish or isolated events. Whether they are in fact rare is open to debate: Small explosions in confined spaces seldom are reported if no one is injured or emergency crews don’t respond to a fire. But fine particles can ignite and explode, as widows and widowers of the Rockmart and 2008’s Imperial Sugar explosions can attest.

The good news is that awareness is increasing. While some of the guidance available prescribes ways to contain an explosion, new and revised standards aim at preventing them in the first place. A prime example is NFPA 652.

Under NFPA 652, which became effective Sept. 7, 2015, manufacturers are required to conduct a comprehensive dust-hazard analysis. Existing facilities have until 2020 to comply, and insurance underwriters, not OSHA, are wielding the big enforcement stick. Food & beverage companies face sharply higher insurance premiums or denial of coverage if they are not already working toward NFPA 652 compliance.

Creation of dust

Oxygen, fine particles and ignition are the ingredients of a dust explosion. OSHA defaults to NFPA on defining the conditions needed to pose a risk. Those levels are in flux, however, as experts and regulators adopt a less prescriptive approach to safety.

For example, particles smaller than 450 microns were considered the danger threshold until it moved to 500 microns. But wood particles up to 1,000 microns will explode, points out Jack Osborn, senior engineer at Airdusco Inc. (www.airdusco.com), a Memphis application engineering firm and manufacturers' representative.

While NFPA 652 requires an assessment of the explosive potential of the particles handled in a plant, Osborn points out that minute dust particles can be created when materials are transferred. A grain of rice isn’t explosive, but the dust created in transit can be.

Osborn speaks with authority: He is the only individual who serves on all NFPA combustible-dust committees. By his own estimate, he has evaluated 2,000-3,000 manufacturing systems, many of them in food & beverage plants, giving him a clear understanding of where the greatest risks reside.

Poor housekeeping is the biggest issue, Osborn observes. That was the root cause of the secondary explosion that devastated the Imperial Sugar facility. Storage silos, dust collection systems, vacuum conveying systems, weighing and batching areas and hammer mills and other size-reduction equipment are other potential danger points. The bearings in bucket elevators are particular danger points.

“Common sense things like managing change and contractors who are not made aware of the risks” are mistakes he finds particularly galling. Osborn recalls a case where a contractor was welding a support on the side of a dust collector. Even though the unit was not in operation, the sparks were enough to trigger an explosion.

Minimum ignition energy is a point where potential danger and reasonable caution must be balanced. Volkmann Inc. (www.volkmannusa.com), Bristol, Pa., takes its cue from ATEX, the European counterpart of NFPA, when engineering pneumatic vacuum conveyors. As a result, it redesigned equipment to be explosion-resistant when energy inputs of as little as 1 millijoule are present, down from 3 millijoules. That’s a trace amount, concedes president Nick Hayes, well below the energy in an electric spark, but static electricity is among the 13 potential ignition sources identified by ATEX.

“An operator walking on nylon carpeting that’s positively charged could put his hand into a reactor with a negative charge and cause an explosion,” points out Hayes. True, allows Osborn, though “there’s not much energy” in a millijoule. Even 100 millijoules contain as little energy as 0.0001 Btus. Given the low ignition risk, he prefers to focus resources on other areas.

Explosibility characteristics of the material in question are the key to hazard identification, and NFPA 652 provides those characteristics for a number of materials. For true peace of mind, Hayes advocates taking the powder in question, placing it in a pressure vessel, and hitting it with compressed air and a spark. If the results are satisfactory, priorities should shift, in particular to the upstream and downstream equipment that could be impacted by the conveyor.

Isolation and containment devices are important, though Hayes likens them to “closing the barn door after the event.” A better strategy is preventing explosions in the first place. “There’s been progress in that area and more awareness, but education is the key,” he summarizes.

Addressing needs

Vac-U-Max is another supplier gearing up to assist food companies in meeting the new combustible dust commodity standards, in particular the required process hazard analysis.

“Pneumatic conveying is the preferred technology for powders, and vacuum is the best approach to explosion protection,” observes David Kennedy, business development manager at the Belleville, N.J., firm (www.vac-u-max.com). “If the system has a leak, it leaks inward, pulling dust in.”

“We need to assess each product and process for compliance with the combustible dust standards,” adds his colleague, Tom Schneider. “Just about anything can combust, so you have to look at the application before determining what we can do to mitigate risk.”

Safety is top of mind but not the only improvement focus of powder and bulk handling. Flexibility, reliability, precision and waste reduction also are priorities. Flexibility and mobility guided development of a mobile vacuum conveying system that recently was commissioned.

The manufacturer had been feeding a blender from super-sacks that were positioned 25 ft. above on a mezzanine. Workers had to scale ladders to access the bulk materials, creating a dust issue and some product damage when gravity carried the potato flakes to the blender below.

A mobile system developed by Vac-U-Max had to be modified to attain the 0.5 percent plus or minus weight accuracy that the process demanded. “Beefy was the main consideration,” says Schneider. Five-inch heavy gauge steel was used to build the machine’s frame, and additional modifications in the field were needed. “We actually exceeded the rate they were looking for, and the product is intact when it reaches the blender,” he reports.

Another piece of equipment shares space on the mezzanine. When that machine is in use, the mobile conveyor is wheeled out of the way.

That kind of customization is standard procedure in material transfer. When Flexicon installed a tubular cable conveyor at Bakery on Main’s new plant in Glastonberry, Conn., it incorporated transparent sections to allow operators to eyeball system performance and fill levels of the conveyor pockets.

Flexibility was a top consideration when Doumak Inc. commissioned a Vac-U-Max system in its Bensenville, Ill., marshmallow plant. Bucket elevators and open-air conveyors were contributing to product damage, and starch dust created a potentially explosive environment that necessitated masks and respirators for workers.

“There’s a need for flexibility and integration that’s often unmet by bucket elevators or similar conventional equipment,” noted Mike Morgan, the facility’s maintenance supervisor. “When a packaging machine malfunctions with a bucket elevator, production shuts down for at least 30 minutes."

Vacuum conveying resolved the downtime issue and, not incidentally, addressed the combustible dust issue, an outcome sure to please Doumak’s insurance underwriter.