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2 presents INDUSTRIAL DUST EXPLOSIONS Mini Project Done by Dhineesh-P (G.E.C, Kozhikode)

3 June 2006 Industrial Dust Explosion2 Objective What is a Dust Explosion? When it happens? Types of materials that can cause Dust Explosions How to avoid/isolate Dust Explosions? Choice of a safety solution Case study

4 June 2006 Industrial Dust Explosion3 What is a Dust Explosion? A dust explosion occurs when a combustible material is dispersed in the air forming a flammable cloud and a flame propagates through it The maximum pressure in a dust explosion is typically around 5-12bar

5 June 2006 Industrial Dust Explosion4 When it happens?  If all of the following criteria are fulfilled then a dust explosion can occur  the dust must be combustible  the dust must be capable of becoming airborne  the dust must have a size distribution capable of flame propagation  the dust concentration must be within the explosive range  an ignition source must be present  the atmosphere must contain sufficient oxygen to support and sustain combustion

6 June 2006 Industrial Dust Explosion5 Types of Dust Explosion Dust Explosions are of two types Primary Explosion –The concentrations needed for a dust explosion are high inside of process vessels/equipment, so most severe dust explosions start within a piece of equipment, such as mills, mixers, screens, dryers, cyclones, hoppers, filters, bucket elevators, silos, aspiration ducts, and pneumatic transit systems Secondary Explosions –Secondary explosions are caused when dust lying outside of equipment is disturbed by the primary explosion and forms a second dust cloud, which then is ignited by the heat released from the primary explosion

7 June 2006 Industrial Dust Explosion6 Types of Dust Explosion Dust Explosions are of two types

8 June 2006 Industrial Dust Explosion7 Explosible Concentration Dust cloud explosions can only occur if the dust concentration is within certain limits

9 June 2006 Industrial Dust Explosion8 Explosible Concentration In general the lowest concentration of dust that can give a dust explosion is around g/m 3 and the maximum is 2-3kg/m 3 As a general rule of thumb it is said that if a 25W bulb can be seen through 2m of the dust cloud then the dust concentration is below 40g/m 3  A cloud of this concentration is unlikely to be in the workplace, and should only be found in process vessels

10 June 2006 Industrial Dust Explosion9 Ignition Sources  A cloud of dust, within its flammable concentration limits, will not burn unless sufficient energy is provided to ignite it. Possible ignition sources include: 1.Open flames (welding, cutting, matches, etc) 2.Hot surfaces (dryers, bearings, heaters, etc) 3.Heat from mechanical impacts 4. Electrical discharges 5. Electrostatic discharges 6. Smouldering or burning dust

11 June 2006 Industrial Dust Explosion10 Materials that can cause Dust Explosions  Natural Organic materials (grain, linen, sugar, etc)  Synthetic Organic Materials (plastics, organic pigments, pesticides, etc)  Coal and Peat  Metals (aluminium, zinc, iron, etc)

12 June 2006 Industrial Dust Explosion11 Materials that can not cause Dust Explosions  The only materials that are not stable oxides can be involved in dust explosions. Therefore the following chemical compound types are unable to produce dust explosions:  Silicates  Sulphates  Nitrates  Carbonates  Phosphates  Therefore there is no risk of dust explosions in industries such as cement manufacture, sand quarrying, limestone excavation etc

13 June 2006 Industrial Dust Explosion12 Factors Affect Dust Explosion 1.Dust Chemistry and Moisture Content 2.Particle Size and Specific Surface area 3.Dust Concentration 4.Turbulence 5.Oxygen Content of Oxidizing Gas 6.Degree of Dust Dispersion 7.Initial Dust Cloud Temperature & Pressure 8.Combustible Gas Mixed with Dust Cloud

14 June 2006 Industrial Dust Explosion13 Avoiding Dust Explosions  Prevention falls into two categories,  preventing ignition  preventing the formation of a dust cloud

15 June 2006 Industrial Dust Explosion14 Preventing Ignition Sources  Prevent the following ignition sources  Smoking  Open flames  Open light (bulbs)  Welding  Cutting  Grinding  Heat from mechanical impacts  Electric sparks and electrostatic discharges

16 June 2006 Industrial Dust Explosion15 Preventing Explosible Dust Clouds  There are three major ways to avoid an explosible dust cloud without modifying the dust itself: 1.Add inert gas to the atmosphere 2.Ensure that the dust is outside of the combustible concentration limits 3.Add inert dust

17 June 2006 Industrial Dust Explosion16 Preventing Explosible Dust Clouds 1.Add inert gas to the atmosphere Lowering the oxygen content in a process area/vessel can minimize the chance of a dust explosion, the easiest way to achieve this is to add an inert gas to the system. Possible choices include:  Nitrogen  Carbon Dioxide  Flue Gases  Water Vapour  Rare Gases

18 June 2006 Industrial Dust Explosion17 Preventing Explosible Dust Clouds 2.Ensure that the dust is outside of the combustible concentration limits  This is extremely difficult to do in practice and is not generally used as a technique as the dust concentration in vessels is too unpredictable and very hard to measure accurately

19 June 2006 Industrial Dust Explosion18 Preventing Explosible Dust Clouds 3.Add inert dust  This method of avoiding forming explosive dust clouds is used successfully in coal mines  A layer of rock dust in the working area is entrained by the blast wave and, as it forms an incombustible atmosphere, it extinguishes the flame  This means of inertion is not usually available due to the contamination of product that it causes

20 June 2006 Industrial Dust Explosion19 Dust Explosion Isolation  There are a number of plant items that will avoid the propagation of flame throughout the system. These include choked screw conveyers, rotary airlocks, and special bursting disk systems that involve flow reversal

21 June 2006 Industrial Dust Explosion20 Dust Explosion Isolation Methods 1.Pressure Resistant Equipment 2.Explosion Venting 3.Automatic Dust Explosion Suppression 4.CV Technology 5.Good Housekeeping 6.Dust Control by Addition of Liquid 7.Plant Layout 8.Human Factors

22 June 2006 Industrial Dust Explosion21 Pressure Resistant Equipment  The best way to contain the primary dust explosion is to have process equipment that is strong enough to withstand the explosion  Most vessels at risk of dust explosion are designed so that under the maximum pressure in an explosion they will distort, but not rupture  Occasionally if the risk of a dust explosion is very high, the vessel may be built to withstand the pressure of the entire explosion as though it were continuous

23 June 2006 Industrial Dust Explosion22 Explosion Venting  Venting is the one of the cheapest, most effective ways to relieve pressure in an explosion situation  If the powder in the process is toxic; venting to atmosphere is not possible, however venting to a sealed area may be possible

24 June 2006 Industrial Dust Explosion23 Explosion Venting  Vent Area Sizing  There are some rules of thumb for sizing vents, you can use a vent ratio from this table  The vent ratio is the ratio of vent area to vessel volume Max Rate of Pressure Rise (bar/s) Vent Ratio (m -1 ) < 3451 / / 4.6 > 6901 / 3.1

25 June 2006 Industrial Dust Explosion24 Automatic Dust Explosion Suppression Fast fire suppression systems can be used to stop dust explosion The agent used for suppression is typically a powder based one such as NH 4 H 2 PO 4, although for some powders superheated steam is quite good too

26 June 2006 Industrial Dust Explosion25 CV Technology  CV Technology Inc. is a Florida based corporation devoted to the prevention, protection, and mitigation of industrial dust explosions and related fires  The company has established itself as a leader in protection technologies to major U.S. industries STEPS 1.Dusts are tested and processes evaluated to determine where there is risk for fire or explosion 2.the alternatives for protection need to be fully explored 3.The best solutions are installed, accompanied by a well- orchestrated implementation process

27 June 2006 Industrial Dust Explosion26 Other Isolation Methods  Good Housekeeping:- Careful removal of lying dust can eliminate the possibility of secondary dust explosions  Dust Control by Addition of Liquid:- Oil is often added to grain to help to minimize the amount of fine dust  Plant Layout:- Generally dust processing plants should be away from other buildings. Buildings should be as low as possible, preferably of one storey to minimize the chance of building collapse in an explosion.Other features that must be included are safe escape routes in case of an explosion and fire, fire resistant materials of construction, fire-resistant doors and high quality electrical insulation

28 June 2006 Industrial Dust Explosion27 Choice of a Safety Solution  Having seen in the previous section the many ways of avoiding and minimizing the dust explosion hazard, engineers need to identify what is needed for their plant  There are several ways of doing this, which usually come down to a cost versus safety analysis, to see whether a solution is practicable  Hazard analysis always takes a large part in the choosing of the correct safety system for a plant. They are:

29 June 2006 Industrial Dust Explosion28 Choice of a Safety Solution Hazard analysis 1.Hazard surveys :- These should be run to see how much dangerous material is on site before startup 2.HAZOP:-A HAZOP analysis (or hazard and operability studies) is a group effort that allows the analysis of the safety of a project 3.Fault tree analysis:-This will allow designers to consider what combinations of conditions could lead to an explosion or other hazardous situations 4.Safety audits:-Safety audits should be run to help keep abreast of any problems and identify anywhere where complacency is occurring

30 June 2006 Industrial Dust Explosion29 Dust Explosion Case Study Spontaneous Combustion in a Municipal Sewage Treatment Plant  A dust explosion occurred in a large storage silo holding dried sewage sludge  Explosion started due to a smoldering nest in the silo  The damage was limited by the bin vent

31 June 2006 Industrial Dust Explosion30 Conclusion  We concluded this Mini Project as, the Industrial Dust must be handled with care, and the peoples must be aware of Dust Explosions and take safety measures at the right time

32 June 2006 Industrial Dust Explosion31 Thank You Thank You For more slides log on to

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