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atex in grinding coal mill

yara, thorwesten vent, robecco - successful, reliable explosion protection in coal grinding plants acc. to atex

yara, thorwesten vent, robecco - successful, reliable explosion protection in coal grinding plants acc. to atex

The companies Yara, robecco and Thorwesten Vent are well grounded in this particular field. Yara, robecco and TV are working in a close partnership for decades gaining a lot of experience over the past years. This partnership is going to reduce interface problems during the order processing phase as a result of a close coordination of each project. Limits of supplies are agreed in a very early stage in order to avoid any kind of problems during the erection phase on site. Commissioning and training of customer staff have a share in successful operation of such specific equipment.

The European Union has covered all aspects of Explosion Protection and Prevention in the ATEX (ATmosphre EXplosibles) guideline adopted by many other countries in the meantime. Furthermore the position of cement companies operating coal grinding plants has changed significantly by including ATEX requirements in their tender documents right from the beginning.

should an explosion nevertheless occur which could directly or indirectly endanger persons and, as the case may be, domestic animals or property, to halt it immediately and/or to limit the range of explosion flames and explosion pressures to a sufficient level of safety.

Explosions can be prevented by lowering the oxygen concentration in an atmosphere below critical levels. The emergency inerting technology can be used as a safety measure for coal grinding systems and many other applications. The selection of an approriate system depends on numerous factors, including environmental and infrastructure conditions.

As the components fuel and ignition cannot be eliminated in coal grinding or storage the one and only possibility is the reduction of oxygen concentration under critical levels, so called LOC (Limiting Oxygen Concentration).

In addition to the LOC, knowledge of the CO value during operation plays an important role in the context of preventive explosion protection. Increased CO values respectively increasedcorrespondingtemperaturesvalues within a coal grinding plant may lead to the conclusion of the presence of a smolderingor glowing fire. Smoldering or glowing fires can be extinguished only provided the oxygen concentration is reduced to 2-3%! Based on this fact storage tanks of CO2 have to be designed for a suitable volume in order to repeat the exchange of system volumes several times, if necessary. In addition it is deemed as mandatory to have a reliable monitoring system (monitoring and supervision of CO, O2 and temperature) in place. Triggering of an effective working emergency inerting system is based on a well operating monitoring system.

Dust explosions in coal grinding systems and pulverized fuel silos represent a significant hazard. The presence of combustible dust-air mixtures distributed in different areas of the coal mill plant reflects a permanent risk for an explosion. Therefore such a situation needs a careful implementation of appropriate monitoring and control equipment.

In this regard robecco takes care of the integrated explosion safety principle no. 1, prevention of formation of explosive atmosphere. In this regard detection of an explosive atmosphere realized by proper control equipment is the major topic.

As a result that emergency inertings systems and monitoring systems are linked up closely the knowledge of CO and O2 parameters measured during normal operation of a coal grinding plant constitues relevant preconditions to realize preventive explosion protection effectively.

Monitoring and control in coal grinding systems and pulverized fuel silos are required in order to maintain an inert atmosphere and to generate indications of potentially-explosive atmospheres as early as possible. Fastest triggering of preventative, effective actions contributes to avoid heaviest explosion incidents connected to fatal consequences.

Control systems are characterized by the adjustment of oxygen and carbon monoxide limit concentrations in relation to different type of coals. Varying properties of coal fuels influence safety limits and adjustments of the control system significantly. For this purpose volatile contents of the fuel as well as process temperatures need to be known in order to fix the corresponding set points. The evaluation of the measured values and an alignment with typical trial processes has to be assured. Relevant operating indications are generated, e.g. detection of leakages or prevention of further ingress of oxygen into the mill system, pulverized fuel silos or other related aggregates.

Recent explosion incidents in coal grinding plants caused by improper designed respectively non working constructional explosion protection have demonstrated the extent of explosions entirely. Therefore the consideration of construction explosion protection measures is of major significance.

Design and determination of required venting areas need to be assessed always in conjunction with the corresponding pressure shock resistance of the specific enclosures e.g. bag houses and pulverized fuel silos in coal grinding plants. Suitable and properly designed explosion vents based on self re-closing technology are provided for the protection of such enclosures.

Furthermore the risk of explosions due to dust explosion transmission through pipes and ducts of interconnected plant units is covered by consideration of constructional explosion protection measures. Major topic in this regard reflects the effective mitigation of flame front propagation in ducts. Suitable and properly designed explosion protection concepts by means of strategically positioned explosion vents of self re-closing technology are provided for this specific applications.

Generally, regardless of the described portfolios various terms and rules e.g. ATEX Directives, VDI & CEN Guidelines as well as DIN EN norms have to be considered compulsory. Safety equipment certified according to ATEX has to pass various tests conducted by accredited, notified bodies. Only tested equipment makes sure that any kind of disintegration during normal operation is excluded. Equipment without being tested represents a potential hazard because it might be turned into missiles!

comprehensive explosion protection of technological equipment in a coal mill - wolff group

comprehensive explosion protection of technological equipment in a coal mill - wolff group

Due to the specificity of technological processes that take place in production equipment such as silos, air filters or built-up transports, the elimination of the formation of explosive atmospheres inside these equipment can be not only complicated and expensive, but sometimes simply unfeasible.

The explosion risk assessment carried out at the cement plant indicated that appropriate measures were needed to protect the technological equipment and to contribute to reducing the level of risk. Explosive safety work was commissioned to GRUPA WOLFF specialists.

Modernisation of the coal mill in terms of explosion safety was necessitated the hazards posed by the coal dust/air mixture at the time of contact with e.g. hot surface, mechanical and electrostatic sparks, open fire source as a source of ignition. The risk was related mainly to the equipment involved in storage, grinding and pneumatic transport processes.

The task of our engineers was to select appropriate technical solutions to protect the equipment forming the entire installation, i.e. coal dust container, mill with dust duct, technological filter and coal conveyors system against the effects of a possible explosion.

As part of the order, our engineers developed the project, as well as delivered and assembled explosion-proof systems. Knowledge and experience in the field of explosion safety and knowledge of the processes carried out by the cement plant helped in the efficient course of work.Below, we demonstrate how the protection of the individual devices was ensured:

As part of the work carried out, the conveyor belt supplying coal dust from the coal hall to the coal dust tank was partially covered and the transfer of coal dust was reinforced. This enabled the installation of an explosion decoupling system by means of HRD cylinders on the hopper.

The coal dust tank itself had not previously been protected against the effects of explosion. If an explosion occurred, the tank would probably be torn apart, and the explosion could spread to the other apparatus of the production plant. Therefore, the tank was secured with the HRD explosion suppression and decoupling system on the side of the conveyor belt.

The mill is equipped with a CO2 inerting system and an inerting system using mineral dust the purpose of which is to reduce the risk of ignition of the product. However, in the event of ignition, these installations would not be able to protect the device from explosion effects.

In view of the above, the coal mill was secured by explosion suppression and decoupling systems a total of 6 HRD cylinders. Five of them were mounted on a coal mill and one on a dust duct transporting dust from the mill to the filter (cutting off the explosion).

The pipeline transporting coal dust vertically downwards from the mill to the filter was additionally protected by a decompression panel on the diverter. In the event of an explosion, the construction will provide a reduction of the explosion pressure in the pipeline and also reduce the risk of explosion transfer from the filter to the mill and vice versa.

Until then, the fabric filter inside the mill had been protected by uncertified explosion relief dampers. As part of the modernisation, they were replaced with certified decompression panels which would ensure even more reliable protection of the device. In the event of an explosion inside the filter, other apparatus of the system are also at risk. For the apparatus protection, the explosion isolation/decoupling system was applied by means of HRD cylinders on the dust channel (transport of coal dust from the mill to the filter). One of the HRD cylinders was used for protecting a small buffer tank located between the filter and the screw pump.

The modernization of the coal mill was subjected to the as-built explosion risk assessment prepared by our ATEX specialists. It was aimed at assessing the technical condition, effectiveness and usability of the technical solutions applied.

Undoubtedly, the modernisation of the coal mill has significantly improved the explosion safety of the plant and therefore no further risk reduction measures are required. It should be noted here that in the event of changes within the area covered by the modernisation that are important from the point of view of security, it will be necessary to update ATEX documents.

Design office Fire protection solutions Engineering systems designs Fire safety Fire detection and alarm system Spark extinguishing systems Extinguishing systems Smoke removal and ventilation system Sound alarm systems Ex-Signal and escape sign luminaires Explosion safety Explosion protection Explosion suppression Explosion relief (venting) Explosion decoupling (isolation) Spark detection and extinguishing system Electrostatic earthing Flame arresters ATEX case studies and ATEX training ATEX training Explosion Safety Audit Determining Explosion Risk Zones Explosion Risk Assessment Explosion Protection Document CONTACT +48 12 2018 100 [email protected] ABOUT US Blog Express Przemysowy GENERAL TERMS AND CONDITIONS OF SALES Electrical Equipment Electrical connectivity EX ATEX Ex-Junction boxes and terminal enclosures ATEX Ex-Control units and control stations ATEX Ex-Safety and main current switches ATEX Ex-Control and distribution systems ATEX Ex-Cable glands ATEX Optical-acoustic signaling EX ATEX EX lighting EX-linear light fittings ATEX Ex LED lighting ATEX EX-Ceiling pendant light fittings and floodlights ATEX EX-Signal and escape sign luminaires ATEX Portable EX-hand lamps ATEX EX Heating Room heaters Fan heaters Line liquid heaters Process media heaters Cabinet and enclosure heaters EX Thermostats Industrial Equipment Screening machines Mills for powders Valves / gates / dampers Sampling of loose material Granulators, dryers, coolers Magnetic separators Transport equipment Bead mills / mixers / deaerators Sludge and suspensions dryers / flakers Liquid samplers Industrial fittings Bursting discs Safety valves Reducing valves Breather valves Flame arresters Modernisation / Installation / Service Industrial systems and equipment service Industrial automation Modernisation and installation of equipment and steel structures Systems relocation Electric works for the industry Offer ATEX Explosion Risk Assessment Explosion Hazard Zones Explosion Protection Document EPD ATEX training HAZOP studies safety analysis of process systems ATEX audits and expert opinions Definition WOLFF GROUP provides specialised engineering works for broad industrial applications. Our activities include: explosion and process safety, turn-key construction of industrial systems, production and supply of process equipment and instruments as well as transfer of new technologies. Over 25 years of operation we have been trusted by hundreds of companies thank you. Copyright 2014 - WOLFF GROUP. All rights reserved.

WOLFF GROUP provides specialised engineering works for broad industrial applications. Our activities include: explosion and process safety, turn-key construction of industrial systems, production and supply of process equipment and instruments as well as transfer of new technologies. Over 25 years of operation we have been trusted by hundreds of companies thank you.

coal mill safety

coal mill safety

Global Cement (GC): Please could you introduce your experience in the cement sector to date? Vincent Grosskopf (VG): I started working in the bulk handling arena in the early 1970s until 1992, when I joined Thorwesten. At that point Thorwesten Vent had begun to work with explosion vents, predominantly for its own coal silos. We were then approached by others, such as Polysius and FL Smidth, which wanted to licence vents for their own designs. Learning about the use of explosion vents in collaboration with such firms led to applications in coal grinding equipment, including bag filters. On the surface this appears to be quite simple but the situations are quite different. The inside of a silo is big and wide, which means the explosion front propogates relatively slowly. By contrast, inside the narrow ducts of a coal grinding and de-dusting system, explosions move much more rapidly and with greater pressure. At Thorwesten Vent we found solutions around these differences and other issues, gaining unrivalled experience in explosion venting.

GC: What led you to establish Coal Mill Safety? VG: I established Coal Mill Safety (CMS) as a consultancy after I retired in 2011. If a cement producer wants to install a new coal grinding system, they can commission CMS to look at the suppliers design and probe it from a safety angle. If it has an existing system, it can ask CMS how it can improve it.

GC: How would you characterise the state of coal mill system safety in the cement sector in 2019? VG: At best, coal mill safety is not well understood and, at worst, it is ignored. When it comes to coal mill systems, most cement plant operators just presume that the supplier of the equipment knows all of the standards and rules and is 100% capable of making a system that conforms to these and is therefore safe. However, this is not the case.

GC: Why is this not the case? VG: The suppliers designs have undergone relatively little development over time from a safety standpoint. They contain legacy solutions. Even some of the best European suppliers lack the necessary expertise to really maximise coal mill safety in-house. There is no reason why an old design should be re-used just because it is convenient for the supplier and may be the most cost-effective solution for the user. There is too much at stake and the hidden costs, for example in excessive use of steel and concrete and poor maintenance access, quickly eat into the perceived economic advantages. Suppliers also say that a tailored solution will take longer. This is a major reason that old designs are repeated over and over again.

On top of this, suppliers are generally large companies that are not particularly dynamic, the designs take a lot of time and money to update and, frankly, there are more interesting projects to work on. Coal mill systems and their safety have taken a back seat for decades.

Of course, where there is not a focus on explosion protection, the supplier can create systems that are really dangerous. There are numerous suppliers all over the world, but particuarly in Asia, that do not understand the safety principles and, some might say, dont particularly want to.

Despite the use of oil and gas in many regions and the rapid rise of alternative fuels, coal remains the major cement production fuel. It must be safely handled when ground and fed into the kiln and, even though it is a difficult topic to broach, the onus to have the conversation is on the cement producer. (Picture: GlobalCementMagazin)

GC: What are some of the common faults? VG: These include: Explosion pressure shock resistance and explosion isolation issues on the inlet side of mills; Incorrectly protected vertical roller mill reject discharges; Incorrectly designed mill-to-bag house riser duct configurations; Incorrectly protected main bag houses with their downstream equipment for conveying pulverised fuel; Incorrectly designed and protected pulverised fuel silos; Incorrectly designed/installed gas analyser configurations and; Incorrectly configured emergency inerting systems. The methods and means of protection of raw coal stockpiles against fire are rarely organised and the designs of filters for the de-dusting of raw coal conveyor belt transition points are almost always wrong, from both fire and explosion protection points of view.

GC: Why have users not demanded improvement? VG: Producers do not have the expertise and very often dont have the time to ask the right questions or put their finger on the design flaws. The fact that the designs are so old lulls users into thinking that they must be safe, creating the perception that theres no need to act.

GC: What are some of the common ways that people get injured? VG: Many people are killed and maimed as a result of a coal dust explosions but often you wont hear about it. Even if you do hear about it, you wont get any details, which makes analysis of wider trends really difficult. A very common incident is when people open the system in, for example, a baghouse, with the expectation of fighting a fire thats inside. Oxygen enters the relatively oxygen-poor environment inside the system, there is a backdraft and anyone in the way is killed, or at least very badly burned.

A coal grinding system with a mill-to-bag house riser duct (marked with red line) that is very long. Through it, unmitigated flame front propagation could reach a velocity too high for the installed protection to effectively protect the bag house. This is a typical situation and it needs to be corrected.

GC: What data exists on the number of injuries and/or deaths caused by these systems? VG: There is pretty much no centralised data on this subject, which means we dont really know how bad things actually are. What we do know is that in many places around the world there are fatalities and maimings with alarming regularity. Some might reach the local news but there are many more that dont.

Even in developed markets, there are injuries and deaths as the result of explosions. They will be reported to local safety authorities but its very hard to get a picture of the scale of the situation beyond that.

GC: Its not possible to say how much improving safety would reduce harm then, is it? VG: Even if there was a good set of data, I still think it would be hard to act on, especially in regions where safety is not a major concern. Perhaps a major association could collate the data, but there are many other jobs, monitoring environmental performance for example, that demand their attention. I am pessimistic that this situation will change soon.

GC: Do those with first-hand experience of coal explosions take them more seriously afterwards? VG: When things go wrong with coal grinding systems, consultants like CMS will get called. On day one after the explosion, the plant staff will be very concerned and ask, What happened?, How do we stop it happening again? and so on By day three, the plant managers downtime clock is ticking louder and louder and the onus returns to production. The plant then carries on, with many of the same flaws in place and a possible repeat of the incident on the cards.

GC: Where is coal mill safety the best in the world? VG: This is not a question that can be answered geographically. There is no best or worst country at the moment, even when you look at litigious markets like Europe and the US. Id even go so far as to say that there isnt one completely safe cement sector coal mill system, anywhere in the world.

There may be some marginal improvements coming in Germany, where some inspections are now finally taking place, after ATEX Directives were transposed into national law. In Egypt, the ATEX Directives will have to be complied with by all coal-using industries very soon. This will be a very interesting process to observe.

A cyclone of an indirect firing coal mill system that hardly could have been laid out worse. The cyclone has been installed inside a building, which disallows protection by means of explosion venting. Equipping it with explosion vents has been aborted, as evidenced by the blind covers that have been installed in place of explosion vents. The explosion pressure shock resistance will be very low, if present at all. Flame front propagation would run into the cyclone completely unmitigated, since no explosion de-coupling upstream of the cyclones dirty inlet is installed. The configuration shown here is disastrous, since disintegration of the cyclone could cause a dust cloud inside the building, which, if ignited, could blow up the building itself.

GC: Can a coal mill system actually ever be safe? VG: Absolutely! If you combine all of the knowledge available to properly design and engineer your system, operate it correctly and maintain it, there is no reason why the system cannot be completely safe. This is why it is such a shame that the reality is so far from the situation we could have. If an explosion were to happen in such a system, there would be no loss of life, no injury and no major system damage.

GC: What can be done to improve the situation? VG: It starts with the cement plant operator asking the right questions during the design phase. To do that they may need the help of a consultant like CMS. Whoever is asking the questions, they need to have the power to actually demand changes to the design. Otherwise there is no point. Once, a major European cement multinational asked me to help negotiate the purchase of a coal system from a Chinese supplier. However, I was not given authority by the purchaser in that situation and the result of my efforts were negligable. The cement producer needs to understand that being the customer means they should be knowledgeable enough to not accidentally get the somewhat flawed 40 year old design the supplier wants to sell! You also need to operate the system safely and know how it needs to be used. It needs to be maintained properly too. Otherwise the system will become unsafe within three or four years. Even if the plant staff are really on the ball there will still be a place for experts. I was once at a plant in the Philippines where an explosion had occurred during the night before I visited. The plant staff were poring over their computers and control systems to try and find out about the incident. They could, for example, work out where the temperature rose too far and where there was too much oxygen in the system, but, looking at the damage quickly proved that their efforts to understand the effects of the explosion and why their protection had failed went nowhere. 45 years experience allows you to understand that part, without computers.

GC: Does it surprise you that after 45 years, an expert such as yourself is still needed at the plant? VG: No, Im not surprised. Plant staff in the cement industry need to focus on producing cement. Fire and explosion protection for coal grinding is a highly specialised field. You cannot expect that plant staff recognise flaws in the system that has been put in front of them, normally with no or very little input from their end.

GC: Will there be a brain drain in this area as consultants like yourself leave the field? VG: Thats a risk, yes. I just have to pass on as much information as I can in the remaining time that I can have in the sector.

GC: Are attitudes gradually changing? VG: Overall, no. Nothing is really changing at this point. Some producers are making sporadic efforts to understand this area and improve, but such large companies move so slowly. Many suppliers are listening to Thorwesten Vent, which is good. However, Thorwesten Vent can only influence certain aspects of fire and explosion protection of coal grinding systems, not everything.

GC: Could the standards be improved? VG: The standards and codes are very complicated and difficult to follow. They are always referred to but not understood. In some cases the standards leave a lot of room for interpretation. So, you see, you cannot even blame the engineers for misinterpreting the situation they are doing their best! The information from ATEX or the EN codes tell you all kinds of interesting information but they are not, and cannot, be exhaustive in terms of engineering solutions. You wont find answers to all the workarounds you need, most probably because it hasnt been needed before. There may be warnings at best. Ive already mentioned flame front propagation through a duct. That is something that the standards speak of, but they dont say how to deal with it.

GC: Is that because those writing the standards also dont know? VG: Its not that this is unknowable, but there are no standards with a focus on indirect firing coal grinding systems, which typically have some special conditions. NFPA 85, in spite of its pro-forma applicability to the indirect firing coal grinding systems of the cement industry, in reality only covers direct firing for the power generation industry, almost completely neglecting the elements that would form the basis of correct fire and explosion protection of indirect firing grinding systems.

GC: Would you advocate that a cement group standardise its coal mill safety solutions? VG: Yes. It would be good to issue a group guideline covering both design specifications/requirements and best practices. Compliance needs to be part of each plants quality management, with strong monitoring by the groups management. However, such an approach has become more difficult in the past decade or so, with the closures and downsizings of some groups technical centres. Lafarge, Holcim, HeidelbergCement and others used to have several of their own technical centres that would have some degree of in-house know-how and responsibilities, which certainly improved situations in the group. They would look at selected new and existing situations, but were not able to support, let alone control the safety of all the systems of their large groups. Now the big groups have closed or downsized several of their technical centres and delegated responsibilities to their plants management, where the necessary know-how will definitely be insufficient.

An awkwardly designed and installed explosion vent on a pulverised coal silo. The silo has been installed in a building, which disallows explosion venting without special measures that control the blasts exit from the building. These are not present. The explosion vent is of a design that will not reclose, due to design faults that will cause its hinged lid to deform and not to fall back after the explosion, leaving it open to ingress of O2 and uncontrolled losses of inerting medium that will make firefighting impossible. Explosion pressure shock resistance of the silo and the explosion vent are lacking. The explosion effects will affect the silos in-feed drop chutes. When venting, the blast will hit the concrete ceiling, which is far too close to prevent the flame bodys dangerous deflection, spread and expansion into the building.

GC: What kinds of producers are most proactive in coal mill safety? VG: The multinationals are starting to move in the right direction on paper, but its really slow. They dont help themselves with constant personnel changes. I have been in a situation where Ive been training say 8-10 individuals across a group. Everything goes well and then, six months later, I try to reconnect with them to see their progress. The problem is, theyre almost certainly in a different role by then! Theyve most likely forgotten everything they ever knew about coal mill safety and probably didnt transfer knowledge to the next person in their old role.

GC: What is the one easiest thing to do to improve an existing system with poor safety? VG: Sometimes the best solution is to rip it out and start again. That way you have a clean slate and can avoid so many of the common mistakes. When thats not possible, there is no easy win. Its all hard work! All situations are different in any case.

GC: It seems that your final answer sums up the whole issue VG: Indeed. Improving coal mill safety in the cement sector is a continuous and varied challenge. I hope that by highlighting some of the most common problems and failings in these pages in terms of systems, attitudes and regulations I can make others aware of how they can influence this area for the better. This will help the suppliers, cement producers and, most importantly, the men and women that risk their lives working with these unsafe systems.

coal mill atex zoning example

coal mill atex zoning example

For each project scheme design, we will use professional knowledge to help you, carefully listen to your demands, respect your opinions, and use our professional teams and exert our greatest efforts to create a more suitable project scheme for you and realize the project investment value and profit more quickly.

38 4 ATEXPrevention With lignite (brown coal) for example, the LOC amounts to approximately 12 % by volume using nitrogen (N 2) as inert gas, and approximately 14 % when using carbon dioxide (CO 2). In practice, the 2 2 ...

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2019/02/01 GC: How would you characterise the state of coal mill system safety in the cement sector in 2019? VG: At best, coal mill safety is not well understood and, at worst, it is ignored. When it comes to coal mill systems, most cement plant operators just presume that the supplier of the equipment knows all of the standards and rules and is .

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38 4 ATEXPrevention With lignite (brown coal) for example, the LOC amounts to approximately 12 % by volume using nitrogen (N 2) as inert gas, and approximately 14 % when using carbon dioxide (CO 2). In practice, the 2 2 ...

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2016/11/23 Understanding ATEX Zones for Gas & Dust This week we look at ATEX zones for gases and dust to dig a little deeper into ATEX understanding. If you are not sure whether you need an explosion proof ATEX fan or need to know the ATEX zone in which your application is classified, please consult an official authority in your country.

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ATEX Simplified White Paper Explosion Prevention White Paper Declan Barry 7 Cranford court Woolston, Warrington, Cheshire WA14 RX, UK Page | 1 Introduction A statement from the Irish factories Act in 1955 sets out its legal

2019/02/01 GC: How would you characterise the state of coal mill system safety in the cement sector in 2019? VG: At best, coal mill safety is not well understood and, at worst, it is ignored. When it comes to coal mill systems, most cement plant operators just presume that the supplier of the equipment knows all of the standards and rules and is .

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For example, a mill or pneumatic conveying system. Zone 21 Primary grade of release gives rise to Zone 21 - a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation.

(for example, matches and lighters) and the risks from electrostatic discharges. An international standard, BS EN 60079/10, explains the basic principles of area classification for gases and vapours, and its equivalent for dusts was

For example, a mill or pneumatic conveying system. Zone 21 Primary grade of release gives rise to Zone 21 - a place in which an explosive atmosphere, in the form of a cloud of combustible dust in air, is likely to occur occasionally in normal operation.

The coal mill can also grind petroleum coke and anthracite down to a fineness below 5% +90 micromillimeters (mm) when coupled with a variable speed mill motor. No sticky situations The ATOX Coal Mill can grind and dry raw coal .

For example, an analysis of the dust explosion figures in the American grain ..... since the inside of a mill is usually ATEX zone 21 because it is "very full" and there ... Coal, being an ore, does not come in a pure state and the

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emergency inerting systems for coal-grinding applications

emergency inerting systems for coal-grinding applications

Title image - Emergency inerting systems for coal-grinding applicationsWhen coal is ground in a mill, explosions are a significant risk. Inerting systems are an effective way to remove oxygen from the system, thus preventing explosions.High-pressure CO2 inerting system for HeidelbergCement's new Tula Cement plant in Russia.Low-pressure CO2 inerting system at Lafarge's Bamburi Cement plant in Kenya.Visualisation screen-shot of the inerting system installed by Yara Industrial at AS Cimento's Bucak Plant in Turkey.New sewage sludge silo at Titan Cement's Thessaloniki plant in Greece.High-pressure N2-pack inerting system with valve station installed at Lafarge's Rezina Cement plant in Moldova.

There is always a potential risk of explosions when handling combustible bulk solids and powders. Process technicians and engineers know how to estimate and minimise this risk. Nevertheless, devastating dust explosions occur too frequently. So-called 'hot-spots,' sudden spontaneous combustion and explosion risks lurk in every stage of the storage, processing and transportation of combustible powders. In the cement industry, this has relevance to coal-grinding systems. Here, Achim Rott from Yara Industrial GmbH explains the different types of inerting systems that can be used to prevent such explosions in the cement industry.

To have an explosion there needs to be oxygen (O2), a fuel source and an ignition source in the same place at the same time. In the case of coal-grinding or storage (as one might find in a cement plant) it is not possible to remove the fuel (coal) or ignition source (grinding energy, heat, static charges) and so one has to concentrate on removing the third necessary component - O2.

This fact has given rise to inerting systems that rely on the use of inert gases. Inert gases have a low level of reactivity and reduce oxygen concentration to below critical levels. By doing this, they prevent the occurrence of critical operating conditions and consequently any resulting explosions or fires. Different inert gases are effective to different extents and it is often not absolutely necessary to replace all of the O2.

Generally, in terms of the European Directive 94/9/EG (Atex 95a) inerting systems are not seen as protection systems and hence are not subject to compliance with the requirements for this directive. An installation not in range of a possible dust explosion zone, in accordance with the European Directive 99/92/EG (Zone 20, 21, 22), is thus strongly recommended.

Inerting systems avoid dust explosions and smouldering fires in silos, coal mills and filter equipment by creating an inert atmosphere. In the case of abnormal levels of carbon monoxide (CO), oxygen or heat, the inerting process is initiated automatically through a process-control system. Constant and accurate monitoring of conditions is therefore essential.

In normal operation inerting occurs by using the exhaust of the rotary kiln or from hot gas generated during the operation of the coal mill plant. In case of an emergency shutdown, the coal mill plant inert gas is injected.

The goal at all times is to reduce the limiting O2 concentration (LOC) so that explosions can no longer take place. The LOC is the highest oxygen:inert gas ratio at which explosion is not possible regardless of the dust concentration. The LOC depends on the kind of coal that is used and needs to be determined separately in consultation with an expert. With lignite (brown coal) for example the LOC amounts to approximately 12% by volume using N2 and approximately 14% when using CO2. The maximum allowed O2 concentration (MAOC) is an operational parameter that is set approximately 2-3% below the LOC.

Extinguishing smouldering fires is only possible at an O2 concentration as low as 23%. To prevent these, the inerting process has to be repeated up to three or four times depending on the LOC when inerting is first started.

In the flushing inerting method, the inert gas is introduced at the highest possible speed into different areas of the system to be inerted. As a result of the strong turbulence that is produced, the gases undergo thorough mixing and optimum inerting because pockets of high O2 concentration are avoided.

This high-speed is reached through special nozzles in connection with the adjusted inert gas pressure at the valve station. The number and size of nozzles is calculated according to the geometrical empty volume of the space that has to be protected.

Yara's inerting systems have been designed in accordance with the following criteria, which with regards to the safety standards in coal grinding plants was developed in collaboration with the leading cement manufacturers (including Lafarge, Holcim, Cemex and HeidelbergCement) as well as European and Chinese engineering and coal-grinding plant manufacturers.

3. The necessary inert gas capacity is calculated according to geometrical volumes of all components to be inerted. Calculation is based on the total geometrical volume of the coal grinding system (75% of silo net volume in case of two or more silos).

High-pressure CO2 tanks have high inert gas capacities and compact tank dimensions. They are mainly used in countries with large seasonal temperature fluctuations, such as those in Europe, Russia, Central Asia and parts of the Americas.

The installation consists of a cylindrical container with a maximum design pressure of 80bar. The installation is operated in the range from 50-70bar. To keep tank operation pressure in winter between 50-65bar, up to three heaters with a maximum heating capacity of 19kW each are installed.

During the summer the high pressure tank is cooled either with cooling water or by being contained within an air-conditioned room. Tanks are available with capacities of 3-15t. When using high inert gas volumes, regeneration and pressure build up time is necessary, which depends on ambient temperature and installed heater capacity. CO2 temperature and pressure are dependant on each other as shown below.

The tank is always filled with deep cold CO2 from a low pressure tanker (maximum pressure 2025bar), which corresponds to a liquid CO2 temperature of -20C to -29C. Monitoring of the CO2 level and tank pressure is done using sensors that take weight and pressure measurements.

The gas withdrawal valve of the CO2 vessel is connected to the valve station by a flexible high-grade steel corrugated hose and high-grade steel high-pressure pipe. The valve station is a framework rack with integrated pressure-reduction and the individual electromechanical and manually-controlled valves for the inerting endangered parts of the system, flow meter and CO2 gas detection system.

During the inerting process, gaseous CO2 is withdrawn from the CO2 tank and is taken into the valve station. The flow is monitored at the entry to the valve station using sensors. The CO2 then flows into the pre-selected areas of the system and displaces the O2. Inerting is triggered by the higher-ranking programmable logic control (PLC) in the control room, which permanently monitors the CO level, temperature and O2 concentration during the grinding process and storage of coal dust. Inerting of the individual pieces of equipment is started by opening the corresponding electromechanical ball valves. The pressurised CO2 flows into the system by means of nozzles once the appropriate ball valves are opened.

The electrical cabinet controls and monitors the CO2 tank and the valve station and is designed according to individual customer specifications. Electrical control of the tank is self-regulated and is monitored by control circuits.

At the inlet pipe a flow sensor is mounted to detect leaks during stand-by operation. During normal operation the valve station and tank are controlled by the PLC. Additionally local control is integrated so that a manual inerting process is possible in emergency situations like a power shutdown. The position of all electromechanical valves is indicated by limit switches.

Low-pressure CO2 inerting systems combine the advantages of controlled storage in combination with the most up-to-date technology. Their functionality is comparable with high-pressure systems. The main difference is that they feature deep cold storage of liquid CO2 with the help of an integrated refrigeration unit. For maximum inert gas discharge, the tanks are equipped with heating equipment to compensate for the drop in pressure.

Because the CO2 is taken in liquid form out of the vessel it has to be transformed into gaseous inert gas with the help of an evaporator. The most suitable kind of evaporators are ambient or atmospheric type because these are independent from any artificial power supply. Electrical evaporators or water-baths, which use power, risk malfunction in the event of a power shutdown. When one is attempting to make an inert atmosphere in a potentially explosive area, such malfunctions cannot be allowed.

Low-pressure inerting systems are mainly used in countries with constant temperatures above +5C, for example the Middle East, Africa, South Asia, Australia and Central America. The inert gas capacity is mainly dependant on the size of the evaporator and ambient temperature. Tanks are available in a range of sizes from 428t. The capacities and number of ambient evaporators needed are decided on an individual basis.

They are very compact and have many similar technical characteristics as discussed above including weighing sensors, pressure sensors and communication systems. Batteries are used with standard CO2-steel cylinders for inert gas discharge and may be used in nearly all countries of the world. Weather conditions do have to be considered however, meaning that a small housing structure and/or heating may be necessary.

These systems are used in countries with infrastructure that severely limits the availability of CO2 by road. N2 high pressure packs are provided with nine to 12 standard 200bar N2 steel cylinders for inert gas discharge. Again, they can be used in nearly in all countries of the world.

The number of packs depends on the inert gas volume required. Multiple lines may be used and Yara designs feature automatic switching to ensure uninterrupted gas flow. In such a set-up, each line can provide sufficient inert gas for the entire coal-grinding system. Empty packs will be indicated by pressure sensors and have to be exchanged immediately. Pressure fluctuations caused by climate conditions do not affect N2-pack based inerting systems, so erection outside with a protective roof is generally sufficient.

This meant that the new 365m3 sewage-sludge silo is now provided with inert gas by a single valve station erected near to the silo. Several injection points are used including at the top of the silo, at the ring pipe around the flat bottom of the silo and in the Schenck Multiflex dosing system.

During start-up, inert gas volumes have been successfully tested together with Titan's engineers and integrated into the PLC system. Based on this experience, Yara is now working on another sewage-sludge project with Titan Cement at a plant in Bulgaria.

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