Example of standard elements in a bulk solids processing facility.
Photos courtesy of Rembe.
CHARLOTTE, NORTH CAROLINA, U.S. — Combustible dust is a risk when conveying materials, but reviewing where some of the pitfalls may lie and taking appropriate corrective measures can help eliminate the risks.

Most production processes in mills, mixed feed plants, breweries, power stations and the like start with the unloading of “raw material.” After its delivery by road or rail, the material is often poured into conveying equipment. Organizational precautions can be taken such as allowing the truck to cool down for at least 15 minutes before starting the unloading process, said Charlotte, North Carolina, U.S.-based REMBE, Inc. This substantially reduces the potential for ignition hazards from hot brakes, hot exhaust pipes or catalytic converters.

As an additional preventative measure, grounding can also offer protection against spark discharge. If movable vehicles are involved, such as trucks and rail cars, it is vital to proceed with great caution and regular staff training should be provided, REMBE said. To continue the typical scenario, the raw material is taken downstream via elevator or conveying line.

Example of REMBE Q-Box and REMBE explosion panels on a conveyor.
Conveyors differ in their design so they require different protection methods. All of these methods are primarily designed to reduce/prevent ignition hazards that might arise from the conveyors themselves – by limiting the speed, ensuring appropriate combinations of materials and using a safety-compliant design.

Open conveyor belts are considered to be the least hazard-prone, since the conveyed material does not typically create a dust cloud and is not in direct contact with hot surfaces – unlike chain conveyors and screw conveyors, which operate differently.

Explosion venting devices such as REMBE explosion vents or REMBE flameless venting systems will likely be necessary, depending on a number of variables, including:

• Material characteristics: fineness, moisture level, dusting propensity

• Conveying type and speed

• Interconnected plant equipment

• Zoning

• Ignition risk assessment

REMBE explosion vents will rupture at a defined pressure, protecting the conveyor by reducing the overpressure within it and releasing the explosion into the surrounding area. For inside installations, REMBE flameless venting systems incorporate this same explosion panel or disc, but the pressure and flame will be contained within the metal mesh body of the flameless vent. The metal mesh extinguishes flames through heat dissipation and reduces pressure/noise to negligible levels.

In addition to venting, isolation systems are mandatory to prevent explosion propagation through interconnected plant processes/equipment. The general standard is to install ATEX-approved quench valves or quick-closing valves such as the REMBE EXKOP System, passive isolation devices such as the REMBE Q-Flap, chemical barriers or rotary valves.

REMBE EXKOP Isolation System.
With appropriate expertise and design, a screw conveyor itself can be converted into a protective system with an isolating effect by removing one or two flights of the screw conveyor, REMBE said. Whether or not this is feasible depends, in particular, on the flow quality of the material and the installation position of the screw. Ideally, the conveyed product will fill the entire cross section of the pipe in this area, thus plugging the conveyor and preventing propagation.

The situation is different with a trough auger, where the upper cross section is not filled with product. In these applications, REMBE flameless vents or chemical barriers are necessary. Chemical barriers are controlled by pressure detectors or infrared sensors, which identify flames and explosions. This triggers the release of an extinguishing agent within a few milliseconds, thus extinguishing the explosion.

Outdoors, on the other hand, it may be wiser and more cost-effective to use approved pressure venting devices such as REMBE explosion vent panels or REMBE flameless venting devices.