Safety and Prevention at the Heart of Powder and Bulk Materials Transfer

Explosion prevention and suppression in powder and bulk transfer is a top-of-mind concern for regulators and inspectors. Fortunately, suppliers with broad material-handling expertise help deal with it.

By Kevin T. Higgins, Managing Editor

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Application-specific solutions are proliferating, agrees Wylie Musel, regional sales manager with BS&B Safety Systems Ltd. in Minneapolis. “It used to be either vent or suppress; now there are multiple options,” he says. Insurance companies are ratcheting up the pressure on manufacturers to either address the issue or face sharply higher premiums, lending immediacy to the selection of appropriate safety solutions.

NFPA 654 only applies to vessels and enclosures that are 8 cubic ft. or larger, points out Nick Hayes, president of Volkmann Inc., a Bristol, Pa., supplier that focuses primarily on dense-phase vacuum conveying. With U.S. standards still evolving, the gold standard for equipment operating in environments with explosive atmospheres is the EU’s ATEX directive, according to some safety experts. Sugar and flour dust have low ignition points, Hayes points out, and his equipment often is used in those applications. Volkmann’s equipment is certified under ATEX by TUV, the German safety-validation equivalent of UL.

Volkmann supplies some positive-pressure systems, but most of its pneumatic installations are vacuum conveyors. Dust-free material transfer is easier with vacuum, since a leak in the line will draw in air rather than expel dust, and specialized filters within the receiver ensure dust-tight operation.

Scientific understanding

Conveying food powders and bulk solids is an exercise in applied physics, with particle size, density, water content and other variables affecting performance. “No two powders are ever the same,” notes Hayes. “My advice: test, test, test.

“As an engineer, I hate to say it, but powder conveying is more art and experience than science.”

“Bulk-solids handling traditionally has been based on heuristics and trial and error,” suggests Mark Jackson, head of Kansas State University’s engineering technology department, which hopes to advance material-science research and develop general equipment requirements and resolve common problems at a new technology center.

Until then, the specific needs of food processors are driving vendor innovations. One example is reducing the time required to clean filters and prevent cross-contamination between production runs.

Cleaning the filter required to serve an 8 cubic ft. vacuum receiver can take a couple of hours, according to Doan Pendleton, vice president of Vac-U-Max, Belleville, N.J. His firm’s solution is the “swing way” cover for large vessels. A hydraulic jack lifts the filter’s top, which then pivots to the side to expose the bank of filters, which can then be accessed and changed in about 20 minutes. The company still offers filters with side-door access, but “this is the direction we want to move in,” Pendleton explains.

Faster filter cleaning also was goal for a sanitary filter receiver (SFR) introduced by Coperion K-Tron. Fabricated in 304 stainless with polished surfaces and available in 36- and 48-in. diameters, SFR employs spunbond-polyester tube filters supported on a single hinge for quick removal and replacement, eliminating the horizontal ledges needed to support pleated filters in a typical design. If allergen cross-contamination isn’t an issue, sequential pulses of compressed air from a mounted accumulator automates the cleaning process.

Pneumatic conveyors typically are stationary, though mobility is a desirable attribute in some applications. That’s the domain of flexible screw conveyors. Simplicity of design is one of their strengths, and suppliers like Hapman Inc. are reluctant to introduce changes unless they address end-user needs, such as faster cleaning.

For example, a quick-release discharge box recently was added to Hapman’s Helix flexible screw conveyor, says product manager Mike Zeluff. Tri-clover clamps on the Kalamazoo, Mich., firm’s “dairy style” units were added a few years ago, he adds.

Customized changes are often made by end users. Zeluff cites the case of Guixens Food Group Inc., a Miami-based processor that opened a second plant in Tampa two years ago. Inspired by the trolley lines used in meatpacking plants, Guixens installed a ceiling trolley that suspends the motor end of the conveyor. Operators roll a floor-level hopper between filling machines, dragging the motor at the other end of the tube to the next location.

“It’s a little unusual: most of the time you see a dedicated conveyor going to a dedicated line,” says Zeluff, but Guixens’ solution reduced capital costs and takes full advantage of flexible screw conveying’s mobility.

A close cousin of the flexible screw conveyor is the tubular drag conveyor with a cable or chain to drive flights. Powders and sticky products are inappropriate for these enclosed systems, allows Bert Bertolo, regional sales manager with Spiroflow Systems Inc., Monroe, N.C. On the other hand, large particles and products requiring gentle handling are a good fit for Spiroflow’s tubular drag conveyor, which features a stainless steel cable and plastic flights impregnated with metal-detectable material. Bertolo says a constant-tension system reduces the likelihood of cable stretch.

An abundance of bulk bag filling and discharge systems were on display in Rosemont. According to Bertolo, the point of distinction of Spiroflow’s CTE bulk-bag filler is the vibrating cone table that breaks up material being fed in and helps pack more of it into the bag at a high fill rate. For high-volume applications of 5,000 totes a month, he estimates the system delivers an ROI in about a year by packing 10 percent more material into each tote.

Incremental improvement is the general rule in process industries. As suppliers expand their capabilities and the understanding of bulk material and powder flow advances, the pace of change in dry material handling is likely to accelerate.

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