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

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INDUSTRIAL DUST EXPLOSIONS

Mini Project Done by

Dhineesh-P (G.E.C, Kozhikode)

June 2006 Industrial Dust Explosion 2

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


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


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


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


Types of Dust Explosion

Dust Explosions are of two types


Explosible Concentration

• Dust cloud explosions can only occur if the dust concentration is within certain limits


Explosible Concentration

• In general the lowest concentration of dust that can give a dust explosion is around 50-100g/m3 and the maximum is 2-3kg/m3

• 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/m3

A cloud of this concentration is unlikely to be in the workplace, and should only be found in process vessels


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 discharges6. Smouldering or burning dust


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)


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


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


Avoiding Dust Explosions

Prevention falls into two categories, preventing ignition preventing the formation of a dust cloud


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


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



1. Add inert gas to the atmosphereLowering 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



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



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


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


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


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


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


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)

< 345 1 / 6.1

345 - 690 1 / 4.6

> 690 1 / 3.1


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 NH4H2PO4, although for some powders superheated steam is quite good too


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


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


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:


Choice of a Safety Solution Hazard analysis1. 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


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


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


Thank You

Thank You

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

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