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Process Safety > Explosions > Flashcards

Explosions Flashcards

1
Q

What is an explosion?

A

A sudden, violent release of energy

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2
Q

How do explosions compare to fire and toxic release for serious accidents?

A

Explosions cause fewer accidents than fire but more than toxic releases. However, when it does occur then it usually inflicts a greater loss of life than fire.

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3
Q

How do explosions compare to fire and toxic release for disaster potential?

A

Explosions have a greater disaster potential than fire but less than toxic release.

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4
Q

What forms of energy may be released in explosions?

A

Mechanical/potential energy (due to pressure in fluids or stresses in solids) or chemical energy (heats of reaction, latent heats) or may be due to nuclear sources.

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5
Q

What are the two classes of explosives?

A
  • Primary explosive

- Secondary explosive

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6
Q

Is RDX a primary or secondary explosive? Why?

A

It is a secondary explosive. It has a lot of energy but it is not easy to ignite.

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7
Q

Is blackpowder a primary or secondary explosive? Why?

A

It is a primary explosive. It is high energy and it is easy to ignite.

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8
Q

How is the explosivity of a material tested?

A

A hammer is dropped down on a sample of the relevant material and then measuring the impact energy to gauge the explosivity.

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9
Q

What is the main causes of explosions in chemical process industries?

A

Combustion of flammable gases.

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10
Q

What are the two types of explosions?

A
  • Detonations

- Deflagrations

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11
Q

Describe detonations.

A

The energy release is virtually instantaneous, producing a supersonic shockwave.

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12
Q

What is a hard explosion associated with detonation?

A

The pressure front moves at the speed of sound producing a sharp rise in pressure over a short duration.

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13
Q

Give an event in which detonation occurred.

A

PEPCON in Nevada, US. Rocket fuel for the NASA space shuttle program. Killed 2 people and 372 injured.

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14
Q

Describe the blast curve for detonation.

A

It is plotted on pressure versus time. There is a sharp decrease in pressure. The area under which is overpressure. Then the curve dips under the x axis which is the area of underpressure before once again exceeding the x axis.

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15
Q

What is the major consequence of detonations?

A

Shock waves which can decimate buildings.

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16
Q

Describe deflagrations.

A

The pressure rises in response to the slow propagation of a flame front through the cloud.

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17
Q

What is a soft explosion associated with deflagration?

A

Pressure rises more softly and slowly and the overpressure has a longer duration.

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18
Q

Give an event in which deflagration occurred.

A

Danlin Industries in Oklahoma, US. The plant produced chemicals for oil industry.

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19
Q

Describe the blast curve for deflagration.

A

It is plotted in pressure over time. The peak overpressure is lower and the duration is longer.

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20
Q

Are there booms in deflagrations?

A

No

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21
Q

Give an example of an explosive chemical which would cause detonations.

A

TNT

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22
Q

Give an example of substances which would cause deflagrations.

A

Vapour cloud explosions.

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23
Q

Which factors contribute to the damage?

A
  • Amount of energy released
  • Magnitude of overpressure
  • Duration of overpressure
24
Q

How is the overpressure estimated?

A

Using the TNT equivalence method.

25
Q

What are condensed phase explosions?

A

Condensed phase explosions are explosions involving conventional explosives, even when materials have not been designed for that specific purpose. It is an explosion of a high explosive.

26
Q

What do the materials in condensed phase explosions typically contain?

A

Their own source of oxygen.

27
Q

What is the reference explosive in condensed phase explosions?

A

TNT

28
Q

What do most incidents of condensed phase explosions involve?

A

Munitions, explosives, ammonium nitrates e.g. ship carrying ammonium nitrate as fertiliser.

29
Q

How does an explosion resulting from a runaway reaction occur?

A

It occurs when an exothermic reaction gets out of control.

30
Q

How does reaction rate relate to temperature for an exothermic reaction? How does this relate to explosions?

A

The reaction rate increases with temperature leading to larger releases of heat of reaction and then further increases with temperature. This can lead to explosions depending on the species involved in this reaction.

31
Q

Why do physical explosions occur?

A

Simply overpressures can cause vessels to explode without any ignition. For example when a vessel is overpressurised beyond its mechanical strength.

32
Q

What is the Brode Equation to estimate the energy released by a physical explosion?

A

Energy = ((initial pressure - final pressure)*Volume vessel)/ (ratio of specific heats - 1)

33
Q

What is the additional energy that will be released in physical explosions?

A

That due to the strain of the metal.

34
Q

What type of explosion is associated with confined explosive materials?

A

Explosions from combustion

35
Q

What is the maximum energy available for explosions in combustion?

A

The total heat of combustion. However much energy is “lost” and does not produce overpressure.

36
Q

What is the yield or explosion efficiency in explosions from combustion?

A

The fraction of the total energy evolved which manifests as overpressure.

37
Q

What is the typical explosion efficiency for

(a) gas explosion inside a vessel
(b) vapour cloud explosion

A

(a) 50 %

(b) 3 - 10 %

38
Q

When do “Vapour Into and Explosion in Buildings” (VEEB) occur?

A

An explosive gas is released into a building and then explodes.

39
Q

Why are VEEBs particularly destructive?

A

Because many people are generally in buildings and because of damage to the structure this can also cause injuries and death.

40
Q

When does a Boiling Liquid Expanding Vapour Explosion (BLEVE) occur?

A

This occurs when a vessel containing a liquid above its normal boiling point fails, releasing the liquid. The superheated liquid then boils vigorously and produces a shockwave.

41
Q

What is the most common cause of BLEVE?

A

External fire

42
Q

What are the two regimes that exist when a vessel containing liquid is heated?

A
  1. Below the liquid level

2. Above the liquid level

43
Q

What happens above the liquid line when a contained liquid is heated?

A

There is no cooling, the metal rises to the flame temperature.

44
Q

What happens below the liquid line when a contained liquid is heated?

A

The vessel is cooled by the boiling liquid, the metal temperature will be approximately the boiling temperature.

45
Q

What is the result of these two regimes on the vessel?

A

Even if the pressure is maintained by a safety valves then the metal will weaken and the vessel will fail below its design pressure.

46
Q

What are the hazards associated with BLEVE?

A
  • Fireball
  • Missiles and fragments
  • Rocketing vessel parts
  • Overpressure from shockwave
47
Q

What is a fireball?

A

Intense radiation, wide area engulfed in flame

48
Q

What are the equations for the diameter and duration of fireball?

A

Diameter: D = 5.8 W ^ (1/3) in m
Duration: t = 0.45 W ^ (1/3) in s
where W is the mass of fuel in kg

49
Q

What are the limitations for the equations of diameter and duration of fireball?

A

W is less than 37000

50
Q

What is the equation for the duration of fireball if W > 37000?

A

t = 0.30 W ^ (1/3)

51
Q

What is the maximum diameter for the fireball?

A

1 m

52
Q

Why is W difficult to determine? And what fraction of W should we use for calculations?

A

Some fuel will be lost through the safety and not all will rise in the fireball, usually 90 % of the inventory.

53
Q

How do we prevent BLEVEs?

A
  • Layout: avoid pool fires below vessels by sloping the ground and separate vessels appropriately
  • Protect vessels by a water deluge
  • Insulate with fire proof insulation
54
Q

When do dust explosions occur?

A

A cloud of flammable dust ignites and a flame and pressure front propagates.

55
Q

In which industry are dust explosions most common?

A

Food industry including starch, flour, sugar.

Chemical industries are more highly regulated. However there are still dust explosions associated with coal.

56
Q

In summary what are the seven types of explosions?

A
  • Condensed phase explosions
  • Explosions from combustion
  • Dust explosion
  • Vapour Escape and Explosion in Buildings
  • Boiling Liquid Expanding Vapour Explosions
  • Explosions from Runaway reactions
  • Physical Explosions

Process Safety (10 decks)

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