Endüstriyel tesislerde toz patlamaları, modellenmesi ve risk azaltılması
Definition, modelling and risk reduction of dust explosions occur in industrial plants
Yazar: MERYEM MÜGE ASANA
Danışman: YRD. DOÇ. DR. HİKMET İSKENDER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Patlamaların tek bir tanımı olmamakla birlikte, ani bir basınç dalgası sonucunda ortaya çıkan kimyasal enerji ile meydana gelmektedir. Gaz ve toz patlamalar olmak üzere ikiye ayrılmakla birlikte, yaygın bir inanışa göre, gaz patlamalar kadar tehlikeli olarak görülmemektedirler. Bunun yanı sıra, prosesinde toz bulunan sanayi dallarının da pek çoğunda, kullanılan tozun yanıcılık özellikleri bilinmemektedir. Ancak bu inanışın aksine, yanıcı tozları proseslerinde bulunduran sanayi dalları ve işletmelerde gerekli önlemler alınmadığı takdirde büyük maddi ve manevi zarara yol açabilecek toz patlama riskleri bulunmaktadır. Örneğin, un, şeker, nişasta gibi ürünlerin üretimleri, metanol, propanol gibi yanıcı gazların üretiminin aksine, ilk bakışta patlamaya sebebiyet vermeyecekmiş gibi gözükse de, bu ürünlerin patlayıcı özellikte olmaları sebebiyle ciddi bir toz bulutu oluşturma ve buna bağlı olarak da toz patlama riski teşkil etmektedir.
Toz patlamaların meydana gelmesi için gerekli olan koşullara bakılırsa, bunun için, kapalı bir ortamda patlamaya uygun toz-hava karşımı, tutuşturma kaynağı, yakıt ve oksitleyici bulunası yeterlidir. Bu aşamada, çalışılan tozun patlayıcılık özelliklerine bağlı olarak sınıflandırılmasının incelenmiş olması büyük önem taşır. Çeşitli sınıflandırmaların bulunuyor olmasıyla birlikte, bunların en yaygın olanları CC (Combustible Class) ve Kst sınıflandırmalarıdır.
Toz patlama riski olan tesislerde, bu riski minimize etmek için patlamaya uygun bir tasarım yapmak gerekmektedir. Uygun tasarım için de modelleme yazılımları kullanılabilir. Uygun tasarımın ardından, yürürlükte olan yönetmeliklere uymak gerekmektedir. Dünya çapında, toz patlamaları önlemek ve oluşması durumunda da risklerini azaltmak için pek çok yönetmelik üzerine çalışılmaktadır. Türkiye'de de 2016 Ocak itibariyle yürürlüğe girecek olan SEVESO III Direktifleri ve Amerika'nın kabul ettiği NFPA (National Fire Protection Association) Standartları en kapsamlı olanlarıdır.
Toz patlamaları incelerken göz önünde bulundurulması gereken en önemli parametre ise partikül boyutudur. Tasarım ve prosesin işlerliği aşamalarında kullanılan tozun partikül boyutuna dikkat edilmelidir.
Endüstriyel tesislerde yaşanan ve yaşanması olası toz patlamaların doğasının incelendiği bu çalışmada, risklerin azaltılması için alınabilecek önlemler ve patlamanın gerçekleşmesi durumunda da, zararlarının azaltılabilmesine yönelik alınması gereken eylemler üzerinde durulmuştur. Yaşanmış büyük toz patlamalarından örnekler verilirken, dünya çapında geçerliliği olan yönetmelikler baz alınarak, toz patlaması modellerinin tasarımındaki önemli eksiklikler vurgulanmıştır.

Abstract:
Any solid material that can burn in air will do so with a violence and speed that increases with increasing degree of subdivision of the material [1]. In consequence, dust explosion occurs. 
In a general conception, dust explosions are considered less important and alongside a very narrow  industrial application areas' problem in comparison with gas explosions. The flammability and explosibility of many kinds of dusts present in industry are not known with respect to fluids like gasoline and methanol. Vast majority of dusts used or generated in the industry shows flammable or explosive behaviour. Even nutrients like flour, sugar, cacao, tea may generate dust clouds causing big explosions under appropriate conditions. 
Industries, which work with combustible dusts, are always under risk of dust explosion. At this point, defining dusts are essential.  Particle size must be the most important parameter of design. After classifying the dust as exposible, then, due to its particle size, using some modelling softwares, appropriate desings should be taken place. In some industries, ALOHA, ASPEN, FLUCS, EFFECTGIS type softwares have been using. Making the design with using these kind of simulators provides safer desings. 
In industrial applications, designs made without taking proper precautions and  neglecting dust explosion parameters cause loss of life and property, halt of production, negative effects on production continuity and most importantly, severe or permanent damages in the environment [2].
There are four main structural protective measures against dust explosions which are: designs resistant to explosions, ventilation, suppression and insulation.
The explosion resistant designs allow controlled damage during the explosion. It is practically difficult for the installations in large facilities nevertheless it is possible to apply in equipment basis. The cost-effectiveness and heavy weight is considered to be a disadvantage of explosion reisistant designs.
Ventilation is one of the most important factors for reducing risk potentials. It is essential to construct proper ventilation in every unit and channel in the facility where there is a risk of explosion. The European standard  EN14492 and American NFPA 68 are the most common ventilation standards among others. 
Explosions, before occuring, reach to the explosion point in a certain time period. The initial measurements can give the signals of explosion to the sensors in this period and  the extinguishers can be activated meantime. With this suppression method, flame can be eliminated thus it prevents the upcoming fire. The advantages of suppresion are that it lowers the probability of toxical and corrosive materials exposure; suppression also provides flexibility for the mobility of the equipments. The investment and maintenance costs are the disadvantages in comparison with ventilation systems.
The reason of explosion insulation is to prevent secondary explosions from happening directly or undirectly linked to the primary explosions. With the rapid system shut-down designs, the valves in the pipes which connect the tanks, can be activated by explosion sensing pressure detectors. The explosions can be insulated either by a chemical's release or a mechanical mechanism.
Besides appropriate design, following the prepared legal directions is a must. There are some kind of regulations for decrasing the potential risks and hazards in big industrial accidents, which include explosion safety rules and standards, as well. Most well-known and applied directives ase Seveso Directives, United States of America's National Fire Protection Association (NFPA) Standards and ATEX Directives. 
Seveso Directives are the regulations which are prepared to prevent large industrial accidents and minimize risks where it is inevitable to prevent the accidents. Seveso II Directive is directly applicable to the companies that keep enough quantity of dangerous materials in their production process, for sales or only for storage, which may cause accidents and only related with the presence of such dangerous materials. Seveso III, partially effective starting from 2015 and will be fully replacing with Seveso II on July, 1st of 2015, is simplified to ease the applicability and configured in accordance with the European Classification, Labelling and Packaging (CLP) Directive. This direction also will be fully applied to Turkish regulations in January, 2016.
Another stantards are NPFA Standards, which is current legislation of United States of America. NFPA Standards are applied in the processes where production, processing, blending, transportation with conveyor belts, packaging and use of all flammable solid particles take place. NFPA determines the structural necessities of the facilities, equipment types to be used, choosing and designing of the proper explosion prevention systems. There are two main NFPA standards, which are totally related with dusts and dust explosions: NFPA 484 applies to fine particle sized metals like aluminum and magnesium. During an explosion, these fine dusts release very big amounts of energy so the accident prevention systems have rather different standard then NFPA 654, which has almost all kinds of dusts other than the specific dusts examined in NFPA 484.
The abbreviation of "atmosphere exposible" ATEX is effective since 2003 in the US and it consists of chapters such as classifications for products and facilitiess in the explosive environments. ATEX Directive takes into account of possibility and frequency of explosive environment formations, identifies risk zones. Also, the equipments used in these zones are grouped. These groups are named as "Conformity Categories".
Following these regulations provides continuos improvement, risk awareness and risk minimizing. 
According to analysed dust explosion cases, it can easily be said that, not making design appropriate to explosions, neglecting the regulations and being not aware of dust explosions are the main causes of the accidents. If during designing plants, modelling were used, then management could be aware of potential risks and reduce risks with appropriate design. Preparing quality systems in the companies are essential to create corporate culture, which includes continuous improvement, risk awareness, education, proficiency and creating know-how. 
In this article, industrial dust explosions have been analysed throughout some cases and for risk reduction, prevention and protection methods have been defined. According to global directives, some inadequacies on dust explosion modelling desing have been identified and solutions have been suggested. It has been aimed to reduce risks of the dust explosions occur in industrial plants with this article.