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Powder processing under dust and gas explosion conditions

Explosive dusts often occur during the processing of powders. To prevent dust explosions, specific measures must be taken that go far beyond avoiding electrical sparks. Charge separation occurs in flowing powders due to static electricity. Charge equalization generates sparks that can be dangerous. But this problem is manageable: With a system that sucks dust-free powder into the process under controlled conditions so that it can be safely processed further.

ystral TDS-Induction Mixer The TDS suction mixer generates a vacuum in its mixing head, which it uses to suck powder directly under the surface of the liquid

When working with powders in industrial production, there is usually also dust. In many companies, this is a safety problem, because with all organic or organically coated powders, as well as with some metal powders, this dust is in principle combustible and also explosive in the corresponding concentration, a risk that must not be neglected. However, this risk does not only exist in the chemical industry. On the contrary: far more combustible powders are processed in the manufacture of food, pharmaceutical or cosmetic products. But how great is this risk? And how can the danger be contained?

Where is the danger? 
To assess how high the risk is, you have to look at the specific situation. Whether an explosion can occur depends on how high the concentration of combustible or dust-explosive powders in the air is. If this concentration is between the upper and lower explosion limit, this is referred to as an ignitable dust-air mixture. However, such an ignitable mixture can even occur with a non-flammable and non-explosive powder if the powder is processed in a hybrid mixture in the presence of flammable gases or vapors. In this case, the explosive gas atmosphere between the powder particles causes the risk. In this case, it does not matter how high the dust concentration is. It always becomes particularly dangerous when a powder - whether flammable or not - is poured from above into a process container with "flammable liquids". Of course, experts know that there are no flammable liquids, because only the gases and vapors above the liquid are flammable.

And this is precisely the critical area through which the powder is poured. In any case, the powder must pass through the zone in which the gas-air mixture is ignitable. Ultimately, only one thing is missing to trigger an explosion: an ignition source with sufficient ignition energy. And this ignition source could be the flowing powder itself.

How does ignition occur?
The required minimum ignition energy (MIE) is a product parameter of the powder or combustible gas, measured in millijoules (mJ). A tiny amount of energy is often enough to ignite the hybrid mixture of powder, vapors and gases or the dust cloud itself. It is often assumed that ignition is only caused by sparks in electrical devices or on surfaces that are too hot. An underestimated and often overlooked source of danger is the risk of an electrical discharge in a flowing powder. Under the production conditions described, this risk is almost always present. Powder becomes charged when it flows out of a bag or hopper or flows through a pipe. This effect can be felt impressively when you reach your hands into a funnel from which wax powder is flowing. You can feel powerful spark discharges on every single fingertip. Any handling of non-conductive powders or pouring powder from non-conductive or conductive containers is therefore risky. When transporting non-conductive powders, increased requirements for the electrical conductivity of the containers and the pipes through which the powder flows must be observed. Appropriate conductive protective clothing must be worn during manual handling The use of unsuitable transport hoses is particularly risky. It is precisely in this area that a great deal has happened in recent years in terms of safety. Hoses that were considered suitable a few years ago may no longer be used today. What many users and valve manufacturers are not even aware of is the risk posed by flow-through valves in the powder path, where the ball or flap is mounted electrically insulated from the pipeline. Flowing powder charges these parts differently and sparks are created.

How to avoid dust clouds with intelligent technology
To ensure safety in production, it must therefore be a top priority to avoid all risks associated with dust formation. Outside of a powder-processing plant, a suitable and efficient extraction system is sufficient to reliably avoid such critical dust concentrations. It is somewhat more difficult when the powder is fed into a process container from above. Regardless of whether the powder is dosed into the container via a screw conveyor, for example, or poured directly from sacks or big bags, a critical concentration quickly develops inside the container. The size of the explosive volume depends, among other things, on the speed of addition and the dust formation of the powder. This makes this factor difficult to calculate. It also means that the risk cannot always be reliably controlled.

The solution is to change the process technology. The powder must not be added to the liquid to be mixed with it from above. Instead, it must be sucked directly into the liquid. This is possible with the principle of the TDS suction mixer. This creates a local vacuum in the liquid. The suction mixer is installed directly in the tank. It mixes the liquid and simultaneously generates the suction vacuum in its mixing head. It uses this vacuum to suck the powder directly under the surface of the liquid into the container. However, this principle can also be applied outside the mixing container. In this case, the liquid is conveyed in a circle and the powder is sucked in dust-free. The inline TDS machine used for this can also work with significantly higher viscosities and different container sizes or fill levels. It can be set up away from the process container and can even be connected to several containers at the same time. It works independently of the container size and its fill level and can begin powder application even at low starting fill levels. There is no dust cloud in the container above the liquid. This is an enormous advantage, not only for safety reasons, but also from a hygiene and cleaning point of view. When processing powders in "flammable liquids", there is no contact of the powder with flammable gases or vapors above the liquid, as the powder is sucked directly into the liquid. The machines are designed and certified in accordance with all dust and gas Ex directives and use only approved components suitable for dust and gas Ex. The systems are available in various ATEX classifications, and of course also in IEC-Ex and NEC.

Processing of dust-Ex powders in non-hazardous areas
A special Ex classification is aimed at all users who process dust-Ex powders in areas not classified as Ex zones. This occurs in the chemical industry, but is particularly common in the manufacture of food, cosmetics and pharmaceutical products. Countless organic powders are processed there, which are potentially hazardous to Dust-Ex.

The user has to process these, but does not want to or cannot set up an explicitly designated dust Ex zone for this purpose. The risk outside the processing systems can be eliminated by taking appropriate measures. As soon as the powders are completely wetted with liquid, there is no longer a dust explosion risk. Inside the systems, however, just before the powder is incorporated into the liquid, it always passes through a dust explosion zone. Under these conditions, this is a defined, demarcated area inside the machine. Machines are therefore available for such applications that are classified as Dust Ex on the inside, but are intended for installation in a non-Ex area on the outside. Only with these machines may dust-ex powders be processed in non-hazardous areas. Such systems can be found in the pharmaceutical and food industries, for example for vacuuming starch, sugar, dextrins or stabilizers. They are also used in the production of cosmetics, where organic thickeners, vitamin powders or sunscreens are used as additives. In the chemical industry, they include waxes, plastic powders, organically coated fillers, cellulose fibers and gas-filled lightweight fillers.

Why covers do not solve the problem
In the production of food, cosmetics or pharmaceutical products, stainless steel covers are often used over the electric motors or the entire machine for hygiene reasons. However, these are extremely critical as protection when used in Ex applications or in Ex zones. The Ex approvals for motors and other electrical equipment generally apply to the temperature range from -20 oc to +40 °c. The temperature inside the cover must be monitored and must not exceed 40 °C during operation, otherwise the Ex approvals for electrical components installed inside are no longer valid. However, this temperature is difficult to maintain. Measurements on pumps and other units have shown that temperatures of 60 to 80 °C often prevail even under relatively open covers over motors. This means that the Ex approval is no longer valid. But even in non-hazardous areas, the motor suffers from reduced cooling. This limits its effectiveness. In principle, the same applies to sound insulation hoods, although these insulate much more tightly and strongly than simple cover hoods. In order to be able to use hygienic covers and sound insulation in hazardous areas, the cover would have to be cooled internally. Internal coolers alone are often not sufficient due to the limited cooling surface available. In this case, ventilation is used. In the case of sound-insulating hoods, this ventilation is provided either via silencers or directly from external rooms. The air inside the cover is then exchanged via a sound-insulated ventilation system.

ystral Fachartikel Pulververarbeitung

Magazine: Technical Safety Vol. 9
Issue: 07/08-2019
Author: Dr. Hans-Joachim Jacob

About the author

Dr. Jacob is Senior Expert Process and Applications at ystral. Dr. Jacob, who studied mechanical engineering, joined the company in 1990 as a process engineer and has since been responsible for our key accounts worldwide. His professional passion is the mixing and dispersion of powders in liquids. During his long career, he has gained experience in handling thousands of powders from a wide range of industries and is happy to share his expertise in various technical articles, online seminars and lectures.

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With our vast knowledge and many years of experience in Process- and Application Engineering we offer targeted, customer-oriented solutions across industries - from lab equipment to production machines or plants. Together with you, we develop concepts and implementations for your individual applications, which mean mmediately realisable and quantifiable added value for you.

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