Scholarly article on topic 'Coal Dust Explosion Prevention and Protection Based on Inherent Safety'

Coal Dust Explosion Prevention and Protection Based on Inherent Safety Academic research paper on "Chemical sciences"

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{"Inherent safety" / "Dust explosion" / Prevention / Risk / "Coal dust"}

Abstract of research paper on Chemical sciences, author of scientific article — Yuan Chunmiao, Li Chang, Li Gang

Abstract In order of dust explosion prevention and protection, a dust explosion control framework was proposed based on four basic principles of inherent safety. The required conditions and consequents of dust explosion were considered in the framework. It was found that standards and procedures of non- electric apparatus for explosive atmospheres should be established currently in China. The thinking of multi-layer protection should be considered when taking engineered technical measures to control the dust explosion, and the measures must withstand experimental tests. The inherent safety of workers was very important for implements of explosion-proof standards or engineered technical measures. The feedback control mechanism, especially positive feedback, was very important to decrease human unsafe behaviors, and to increase the inherent safety of workers. The case study was shown the principles of inherent safety would be of value to combustible powder process industry as a guideline to making dust explosion risk reduction measures.

Academic research paper on topic "Coal Dust Explosion Prevention and Protection Based on Inherent Safety"

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Procedia Engineering

ELSEVIER

Procedía Engineering26 (2011) 1517 - 1525

www.elsevier.com/locate/procedia

First International Symposium oe Mine Safety Science red Engineering

Coal dust explosion prevention and protection based on

inherent safety

Yurn Chunmiro, Li Chrng , Li Gang

Fire & Explosion Prevention and Protection Lab of Northeastern University, Shenyang, China

Abstract

In order of dust explosion prevention and protection, a dust explosion control framework was proposed based on four basic principles of inherent safety. The required conditions and consequents of dust explosion were considered in the framework. It was found that standards and procedures of non- electric apparatus for explosive atmospheres should be established currently in China. The thinking of multi-layer protection should be considered when taking engineered technical measures to control the dust explosion, and the measures must withstand experimental tests. The inherent safety of workers was very important for implements of explosion-proof standards or engineered technical measures. The feedback control mechanism, especially positive feedback, was very important to decrease human unsafe behaviors, and to increase the inherent safety of workers. The case study was shown the principles of inherent safety would be of value to combustible powder process industry as a guideline to making dust explosion risk reduction measures.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong.

Keywords: Inherent safety; Dust explosion; Prevention; Risk; Coal dust

1. Introduction

Dust explosion was one of major industrial accidents. With the development of process industry, dust explosion has a higher probability and its consequent is much severer than before [1]. The hazard of dust explosion in coal mining was much greater than other production process. For example, on November 27, a

* Corresponding author. Tel.: +5-86-524-83687756 ; frx: +5-86-524-83684178. E-mail address: lc666666@126.com .

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.20n.n.2333

coal dust explosion occurred at Dongfeng mine in Qintanhe and caused 171 deaths. Therefore, it is very significant to prevent and mitigate dust explosion intrinsically with the thinking of inherent safety. Overton said that this thinking was helpful to prevent and control dust explosion in the process industries [2]. Amyotte have done many case studies about the application of inherent safety principles to dust explosion prevention and mitigation [3-8].

The aim of present work was to propose a dust explosion- proof framework and its implements, by linking the inherent safety principles of minimization, substitution, moderation and simplification with strategies for dust explosion prevention and mitigation. By paying attention to the initialization and development of dust explosion, this framework and its implements can reduce this risk inherently.

2. Principles of inherent safety

The concepts and principles of inherent safety were proposed firstly by Kletz, the frequently-used basic principles were listed in Table 1 [9]. So far, these principles have many applications in the design of electric apparatus for explosive atmospheres. The design of apparatus with thinking of inherent safety seek to remove ignition hazards in the first instance, instead of identifying the present hazards and then to control them.

Table 1. Principles of inherent safety

No. Principles Descriptions

1 Intensification/minimization Reduction in the quantity of hazardous materials

2 Substitution Use of safer materials

3 Attenuation/ moderation Running equipment at safety operation condition

4 Limitation of effects Changing equipment design and operations for less- severe effects

3. Dust explosion- proof framework

The first three principles in Table 1 were to prevent dust explosion by considering hazard materials or process conditions. Combustible dusts in industry often heaped as dust layer, or were suspended as dust clouds. The dust layer was easy to induce fire, and dust explosion often was induced by dust clouds. Therefore, an inherent safety design should consider firstly the required conditions of fire or explosion, and then control them. If an accident such as fire or dust explosion happened, the measures to reduce such risk should be taken according to the fourth principle of inherent safety. The cause analyses of dust explosion and its controlling framework was shown in Fig. 1.

dust stutus:

consuquent

cause conditions

cloud>

preventive measures:

protective measures:

2nd! ]1st explosion

fire „If,^ explosion

as an ignition source

, .. „ Explosion- pressure resistant construction

isolation, flow * . . , . „ . . control et al Explosion isolation, Explosion venting,

automatic explosion supression

Fig. 1. Cause analyses & controlling framework of dust explosion

4. Dust explosion- proof framework

What you don't have can't leak [10]. The hazard materials involved should be considered at first when taking any measures to prevent and mitigate the risks of dust explosion. The hazardous areas could be classified on a basis of frequent and period of present of combustible dust. According to the classification of hazardous areas and minimum ignition temperature of combustible dust, the suitable electric apparatus for explosive atmospheres can be determined [11,12]. Moreover, experimental test of explosion characteristics of combustible dust (Table 2) was required, with which suitable non-electric apparatus for explosive atmospheres can be determined. A comprehensive explosion- proof scenario was shown in Fig. 2.

Table 2. Explosive parameters of combustible dust

Dust layer Dust cloud

Paramet ers Minimum ignition temperature Minimum ignition temperature Lower explosion limit Minimum ignition energy Limiting oxygen concentration maximum explosion pressure, rate of explosion pressure rise

Abbrevi ation MITL MITC LEL MIE LOC Pmax, (dp/dt)max

St , GB/T 16430- GB/T 16429- GB/T 16498[18] FN GB/T 16426[19],

Stanaar 1996^3], IFC 1996[15], IFC 16425[16], FN mC ^l^ll 2 F4o34[17] ISO 6184/1 [20], FN

d 61241-2-1 [14] 61241-2-1 [14] 14034[17] 3FC4] 61241-2- 14034[17] 14034[17]

Apparat

Hot plate

G-G furnace

1m3, 20L

Temperature classification of Usage electric apparatus for explosive Inerting by solid atmospheres

Control of ignition

source T ,

, Inerting by , ior example

Electrostatic gas

discharges

Explosion- pressure resistant construction Explosion isolation, Explosion venting, automatic explosion suppression

Hazardous materials

Fig. 2. Comprehensive explosion- proof scenario

The successful implement of comprehensive explosion- proof scenario mentioned above depended on enforcement of standards or procedures, engineered technical measures or interventions based on experimental test, and risk management which focused on the inherent safety of employees.

4.1. Ehfdecamant

Currently, explosion- proof standards involved in China are as followed.

• Determination of explosion parameters of combustible dust (See Table 2)

• Guide for venting of deflagrations, explosion suppression and inerting system (See reference [21-24])

• Safety regulations for dust explosion prevention and protection (See reference [25])

• Classification of hazardous zones in present of combustible dust, and electric apparatus for explosive atmospheres (See reference [11-12])

• Explosion protection standards in special industries, work or trade (see reference [26])

From comparison between present explosion protection standards and content in Fig. 2, it was easy to find that the standards or regulations of non-electric apparatus for explosive atmospheres were scarce in China today. A comprehensive establishment of explosion protection standards should be enhanced.

4.2. Engineered tachnic/l ma/sueas

Explosion parameters from Lab have their limitation due to no further validation in industry. At the same time, electric and non- electric apparatus for explosive atmospheres have their probability of going wrong. Therefore, there were still some uncertainties of dust explosion risk in the actual process in industry. In order to reduce the probability of dust explosion risk, a thinking of multi-layer protection should be considered when taking engineered technical measures or interventions for the control of dust explosion, on the basis of the experimental studies (Fig. 3).

Fig. 3. Engineered technical measures with Three- layers protection

First layer: The required conditions to induce explosion were considered in this layer, such as dust concentration, oxygen concentration, ignition source and so on. The explosion- pressure resistant construction was also included in this layer.

Second layer: If an exception occurred in the first layer, automatic explosion suppression will be employed to further decrease the dust or oxygen concentration, so that the flame propagation could be resisted.

Third layer: If the exceptions in the first two layers happened at the same time, explosion venting system should be introduced to reduce the consequence of dust explosion, and explosion isolation system should also be considered to mitigate the situation from dust explosion.

4.3. Risk management

The staff, as a core of risk management, is very important for the performance of standards or procedures mentioned above, as well as the operation and maintenance of engineered technical measures. The control of human unsafe behaviors was the key of risk management. Statistics indicated that the casualty accidents caused by human factor directly or indirectly accounted for 70-90 percents of the total number during the process of production activities [27]. The factors which contributed to human unsafe behaviors were so complex that it was very difficult to control them [28-30]. However, these factors could be changed by administrative controls. The feedback tracking control for human behaviors was an effective way to control them. The feedback control mechanism included not only the traditional negative feedback (seek and analyze causes, and then punish responsible persons), but also included and reinforce the positive feedback (Fig. 4). The human correct behaviors during the process of production activities had a hope of or needed positive feedback. For example, when a worker was executing the operation of house keeping, an encouragement or reward for this operator should be offered so that he/she could maintain and enhance this safe operation or behavior forever. Human safe behaviors were approved and enhanced in this mechanism, at the same time; human unsafe behaviors were gradually weakened and diminished by the negative feedback. Therefore, the feedback control mechanism was helpful to improve human inherent safety, and then to keep apparatus and working environment in the status of inherent safety.

Fig. 4. Mechanism of feedback control for human behaviors

5. Case study

5.1. Explosion parameters of Coal powders

The bituminous dusts in Junde Mine of Longmry mining group Co, LTD was selected as a case study. Explosion parameters were shown in Table 3 whose median particle diameter was 63 ^m [31].

Table 3. Explosion parameters of Coal powders

Parameters MITL MITC LEL MIE LOC Pmax,(dp/dt)max

Experimental results >260 °C 580 C 15-20g/m3 50-100mJ 12% 0.74Mpa, 13Mpa/s

5.2. Preventing and protecting measures

According to the four principles of inherent safety in Fig. 1, and controlling measures in Fig. 2, the implement of scenario to prevent and protect dust explosion of Coal powder should firstly considered the corresponding safety standard and procedures, and enhancement of the safety management for workers. Human unsafe behavior should be weaken and diminished with feedback control mechanism, and then the inherent safety of workers could be improved. Secondly, the engineered technical measures with multilayer protection should be taken in the actual production process. The descriptions for each layer of protection were as follows.

First layer: The explosion- pressure resistant construction was suitable and reasonable for the protection of coal dust explosion, because the Pmax was less than 0.9Mpa. The MITL of coal dust was so low (its temperature class is T12) that the surface temperatures of most apparatus could reach 260 °C easily, and ignited the coal dust layer deposited at the corners. Then, the ignited coal dust layer may be the potential ignition source of dust explosion. Therefore, the coal dust layer deposited should be cleaned timely, at the same time, the selection of electrical apparatus for explosion atmosphere and electrostatic discharges protection should be considered in the production process in the coal mines. Because of the high MIE of coal dust clouds (far more than 10mJ in Table 3); the probability of dust explosion induced by the electrostatic spark was much lower than by hot surface. However, some electric sparks (for example electric arc) due to electrical accident (for example short circuit) were enough strong to ignite coal dust clouds.

Regardless of lump coal break process and coal powder production process, the maximum dust concentration suspended in the apparatus (or pipes) usually exceeded the LEL of Coal powders, 15 g/m3. Therefore, the inerting of dust cloud should be included in the production process. When the inert CaCO3 powder was added in the dust cloud, and its concentration reached 74%, a coal dust explosion would not happen [32]. The inerting by N2 or other rare gases could contribute to decrease the oxygen concentration in the coal dust cloud. It was not difficult to carry out an absolute inerting atmosphere by inert gas in practice, since the LOC of coal dust was enough high [33]. Even if an absolute inerting atmosphere was unavailable, a partial inerting should be considered to reduce the oxygen content in the atmosphere, and then there is a systematic decrease of both ignition sensitivity and combustion rate of the dust cloud (increase in MIE, MITL&MITC). In many cases the explosion hazard may be reduced markedly by only a moderate reduction of the oxygen content [34].

The second layer: It was very hard to control the LOC of Coal powders in practice. If the first layer failed to prevent dust explosion of Coal powders, an automatic explosion suppression system should be set as the second layer of protection, which can resist the propagation of dust explosion by spraying inert solids or gases.

The third layer: If the first and second layer failed to prevent dust explosion at a same time, explosion isolation and venting should be set to decrease the consequent of dust explosion as the third layer of protection. Because the implement of these measures depended on Pmax and (dp/dt)max of coal dust clouds, the partial inerting should be set to decrease the Pmax and (dp/dt)max in the first layer, and then to increase the efficiency of these measures. Fox example, the limestone content of coal powders had a great effect on

the reduction of Pmax and (dp/dt)max of coal dust explosion[35]. As Nitrogen was selected to inert coal dust clouds, the Pmax and (dp/dt)max decreased rapidly, and the explosion range narrowed[34].

Finally, if all the engineered technical measures mentioned above were considered in the actual process, the sole thing to leave was to decrease or diminish human unsafe behaviors, and to enhance the inherent safety of worker with feedback mechanism. These safety managements were helpful to carry out and maintain these measures very well.

6. Conclusions

The principles of inherent safety will be of value to combustible powder process industry as a guideline to making dust explosion risk reduction measures. The standards or regulations of non-electric apparatus for explosive atmospheres should be established today. A comprehensive and suitable establishment of explosion protection standard system was required currently in China. The engineered technical measures with multi-layer protection should be considered in the actual production process. These measures should be tested by experimental studies firstly. The inherent safety of staff was very important for the performance of standards or procedures, as well as the operation and maintenance of engineered technical measures. The feedback control mechanism, especially positive feedback, was very important to enhance the roles of workers in the risk management, and to decrease or diminish human unsafe behaviors.

Acknowledgements

This paper was supported by National Natural Science Foundation of China (Grant no. 51004026) and the Fundamental Research Funds for the Central Universities (Grant no. N100301001).

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