Mine explosions

Introduction

Worst explosions

Year loss of lives Place

1962 299 Germany

1963 457 Japan

1965 331 Japan

1965 306 India

1965 144 Yugoslavia

1968 78 USA

1970 51 Yugoslavia

Mine explosions

Requirement of an explosions

Explosive substance:

Methane

Coal dust

Water gas

Fuel oil vapours

Hydrogen

Mine explosions

Methane:

Accumulation

large airways

Goaf

Drainage

Coal dust

Chief factor in causing intensive explosions

Mine explosions

Factors causing coal dust explosions

Fineness

Incombustible content

Volatile content

Moisture content

Age of coal

Occluded flammable gas

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Sources of ignition:

Frictional sparking

Explosives

Open lights

Electricity

Electrostatic sparking

Fires

Mine explosions

Electrostatic charge

Static electricity is caused by the movement of electrons when dissimilar substances in contact with each other are separated.

Examples:

1. Moving conveyor belt

2. Non-conducting fluids flowing through hoses.

3. Gas issuing from a nozzle under high speed

4. Human body:

• Contact of shoes with floor

• Working close to a machine generating static electricity

Mine explosions

Characteristics of an explosion:

1. When flammable mixture of methane is ignited, temp. rises to 2000 degrees Celsius.

2. A shock wave is produced that moves ahead of the flame front and raises the dust cloud.

3. The most violent explosions occurs when mixture contains 9.8% methane. Flame speed in this case may go up to twice the speed of sound.

4. Speed of the flame front increases when it consumes layers of coal dust and it may reach to 1100 m/sec.

5. The air in front of the pressure wave is compressed adiabatically causing spontaneous ignition of dust particles.

Mine explosions

6. Spontaneous ignition is known as detonation and flame and shock wave travel at the same speed (2000 m/sec).

Relationship between flame Velocity and shock wave

Flame velocity (m/sec) Shock wave velocity (m/Sec)

50 375

260 500

400 600

650 800

1000 1100

Mine explosions

Control of explosions:

1. Control over ignition sources

2. Control over flammable material

Stone dusting Inert dust has the effect of slowing down the flame

by absorbing heat.

Difficulties:

1. Coal dust is more dispersible than stone dust

2. Intimate mixing of stone and coal dust is difficult.

Mine explosions

Barriers

1.Stone dust barriers

2.Water barriers

Stone dust barriers: Blast travels ahead of the flame. Barriers are designed

to be operated by the blast thus creating a dust cloud before the flame of explosion arrives

To ensure the dispersal of dust, the shelves are made collapsible.

Shelves should collapse at the right time, too early or too late collapse may severely reduce the effect of stone dust

The maximum spacing between two rows of shelves is equal to number of kilogrammes of stone dust on one shelf of the barrier.

The recommended amount of stone dust is 30 kg/m length of shelves for light barriers and 60 kg/m for heavy barriers.

Stone dust barriers may fail to arrest the flame if methane is present in the form of roof layer.

Mine explosions

Mine explosions

Factors to be considered in designing of barriers:

1.Total quantity of dust required

2.The stability of the shelves

3.The placement of the shelves

4.Position of the barrier

Storied barriers:

• Consist of two or more superimposed shelves. These barriers are more effective than single-layer shelf barrier

Mine explosions

Water barriers:

1. Water filled troughs operated on the same principle as stone dust barriers.

2. The water is dispersed throughout the general body of the area

3. Cooling effect is obtained by reducing heat.

4. Water troughs are made of pvc and of about 90 liter capacity

5. Water barriers are designed to shatter and to give an adequate dispersion of water.