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Safe and economical processing of bulk materials

Author : Dr.-Ing. Johannes Lottermann, Chief Business Development Officer, REMBE

05 October 2020

During the processing of combustible bulk materials, there is always a risk of explosions. In most cases, each piece of equipment at a plant is considered separately in order to derive suitable measures for explosion safety. However, it is more effective to look at a facility as a whole and to develop integrated explosion safety concepts. This both improves process safety and saves operational costs.

Figure 1 – Standard equipment of a facility for the bulk material processing
Figure 1 – Standard equipment of a facility for the bulk material processing

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During the processing of combustible bulk materials, there is always a risk of explosions. In most cases, each piece of equipment at a plant is considered separately in order to derive suitable measures for explosion safety. However, it is more effective to look at a facility as a whole and to develop integrated explosion safety concepts. This both improves process safety and saves operational costs.

Each facility has its own special features when it comes to bulk material processing. However, there are usually standard machines and equipment installed in most plants, just in different arrangements. This includes equipment for bulk material unloading, conveyors, elevators, silos, screens, mills and filters (Figure 1). Suitable actions and measures for explosion prevention and explosion protection must be provided for each equipment individually but aligned.

Unloading the bulk material

Figure 2 – A flameless explosion vent designed specifically for elevators
Figure 2 – A flameless explosion vent designed specifically for elevators

The respective bulk material is usually transferred into bulk containers from a truck or freight wagon. This is where organisational/preventive actions are mostly sufficient – the risk of combustion from hot brakes or exhaust systems is effectively countered through a wait and cooling time of 15 minutes before the discharge. Grounding systems additionally protect against spark discharges. For moving objects such as trucks or wagons, the staff must work carefully and have received the appropriate training. In addition, agreements should be made with suppliers to ensure that the raw material is delivered without any glowing embers.

Mechanical conveyors

There is no standard concept for explosion safety on conveyors, as the designs and functions are highly diverse. However, there are actions that avoid explosions – without the need to install protective systems. These include limited conveying speeds, suitable material pairings and safety-compliant designs.

Open belt conveyors are relatively uncritical. The material is not swirled up and does not come into contact with any hot surfaces. This is different with chain and screw conveyors. Depending on the degree of fineness, moisture content and dust formation probability of the bulk material, the conveying principle and conveying speed of the connected system parts, there could be a need to implement explosion protective systems such as explosion pressure venting devices. It must be ensured that if an explosion was to occur in a neighbouring part of the system, then it is prevented from propagating through to other connected conveyors. For this purpose, isolation systems such as ATEX-certified rotary valves, chemical barriers, quench valves or quick-acting slide valves are used.

Figure 3 – The “ingredients” for an explosion
Figure 3 – The “ingredients” for an explosion

Tubular screw conveyors have a special advantage. Depending on the fluidity of the bulk material, they can be converted into an isolating protection system in-house with the help of expert guidance. One and a half or two flights are removed from the screw for this purpose. These actions will cause the conveyed product to fill the entire pipe cross-section in this area, which will then form a plug comprised of the product. It refills automatically during normal operation and serves as an isolation element in the event of an explosion.

Chemical barriers are usually required for trough screw conveyors. These are controlled by pressure or infrared detectors, which detect a resulting explosion or flames, and trigger the extinguishing agent container. The extinguishing agent, which is then discharged, will stop the flame. Especially in the outdoor area, however, it can often be more useful and cost-effective to install approved explosion venting devices.

Explosion safety for bucket elevators

Bucket elevators represent a special source of risk because they facilitate the negative explosion prerequisites (explosive mixture and ignition sources) through their function and design. Additionally, pressure waves and flames can propagate across several floors through unprotected elevators and cause severe damages. VDI Guideline 2263, Sheet 8 (8.1 and 8.2), describes explosion safety measures and provides information on the dimensioning of explosion venting devices and the design of explosion suppression systems.

Figure 4 – Protection for screens with a flameless explosion venting
Figure 4 – Protection for screens with a flameless explosion venting

Some manufacturers offer flameless explosion vents specifically developed for the protection of bucket elevators. The particular challenge with elevators is that the protective systems installed must be as light as possible and at the same time achieve the maximum venting efficiency.

Flameless venting devices or conventional explosion panels are installed at critical points of the elevator. These include elevator foot and elevator head. The shafts could also be protected with chemical barriers.

Risk of an ignition source entry into silos

Silos usually do not contain their own ignition sources. This eliminates a decisive parameter that can trigger a dust explosion (Figure 3). However, there is a danger of combustion which is caused by a possible ignition source entry from upstream facility components. Outdoor silos must therefore be protected with explosion vents, and silos inside buildings with flameless venting devices or explosion suppression systems. Depending on the type of bulk material, preventive actions can also be implemented, such as the installation of spark detectors in combination with spark extinguishing systems or quench valves. The omission of explosion protection always remains a special case and requires an assessment from technical experts.

Figure 5 and 6 – An add-on module for explosion vents
Figure 5 and 6 – An add-on module for explosion vents

Dangerous combinations with screens

Especially in combination with upstream dryers, such as spray dryers in the dairy industry, drum dryers in the wood industry or ring dryers in the starch industry, screens can often be extremely endangered equipment. Glowing embers in bulk materials that "survive" in mechanical conveyors and do not ignite there may subsequently be broken open by tumbling or vibrating movements and can then trigger an explosion. Explosion protection is therefore necessary, but not always easy to implement within buildings. The solution: mostly individual, vibration-isolated systems based on a flameless explosion venting device (Figure 4).

Mills and grinder systems

Dust, a closed container, effective ignition sources due to metallic, rapidly interacting components and oxygen – a mill offers all the prerequisites for a dust explosion. Several manufacturers have devices which offer an explosion-pressure resistant design (up to 10 bar). Depending on the size of the facility operator, this represents a cost-intensive investment. Above and below the mill, explosion isolation should always be provided in order to protect other facility components. Some devices are designed for the suction opening of the mill. Through this, the mill can suck in air during normal operation, which is routed through a stainless steel mesh filter without any problems. If an explosion occurs, the stainless steel mesh filter protects the environment from flames and pressure by removing the heat.

Figure 5 and 6 – An add-on module for explosion vents
Figure 5 and 6 – An add-on module for explosion vents

Filter: Increased danger during the pulse-cleaning

The risk of explosion is particularly high in filter systems. Many manufacturers therefore offer products with integrated explosion safety devices. The reason is that in addition to dust extracted from other parts of the system, sparks or glowing embers can also be introduced. It becomes particularly critical when the filter elements are pulse-cleaned. The very fine dust can then spread evenly in the filter, and in combination with an ignition source, can lead to an explosion. Indoor area filters are therefore protected with a flameless venting devices, outdoor filters with explosion vents. If transport or traffic routes are located in the vicinity of the explosion venting, add-on modules for explosion vents are used to deflect the flames and pressure wave into non-critical areas (Figure 5 and 6). 

The overall situation is decisive

Each piece of equipment in a facility must be considered separately. However, a generic safe and economical concept can only be achieved with a holistic view of the facility, which takes into account the interaction of the equipment and the individual arrangement of the respective production plant.

Dr.-Ing. Johannes Lottermann, Chief Business Development Officer, REMBE
Dr.-Ing. Johannes Lottermann, Chief Business Development Officer, REMBE

Even though professional explosion safety has its price, unprofessional over engineering or poor protection can be much more expensive in the long run. Not just in terms of money. In the worst-case, people will pay with their lives. Independent experts therefore recommend plant operators to work with experienced professionals who establish a holistic view of the situation, and create a comprehensive, individual protection concept for the facility. Even the purchase of turnkey plants never relieves the future operator from the duty to implement sufficient explosion safety. Cooperation with experts is therefore a logical step right from the very beginning, for example, during the initial planning phase.

About the author:

Dr.-Ing. Johannes Lottermann, Chief Business Development Officer, Explosion Safety, REMBE GmbH Safety + Control, has spent most of his life studying, researching and developing new concepts and approaches to combustible dust explosion protection. In addition to his doctoral thesis, “Approaches to Integrated Fire and Explosion Safety…”, Johannes has worked for notified bodies charged with certifying compliance with ATEX certifying requirements in Europe, governmental regulatory bodies equivalent to OSHA, standards associations equivalent to NFPA, as well as conducting risk analyses in manufacturing facilities. As Chief Business Development Officer Explosion Safety, Johannes spends most of his time on the road, visiting manufacturing facilities, evaluating their risks and recommending solutions to best protect against the potentially catastrophic consequences of combustible dust explosions.


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