1. Fires and Explosions

2. Fires and Explosions Definitions Flammability
Flash Points
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

3. Introduction
We have been talking about source models for the release of materials and about dispersion models if the material is a toxicant.
Another concern is a release of flammable materials where we need to worry about fires and explosions.

4. Fire Triangle Most are familiar with the Fire Triangle.
In order for a fire to start or be sustained you need to have a Fuel, an oxidizer and an ignition source.
If one of the three components is eliminated, then there will not be a fire (or explosion)

5. Fuel Fuel must be present in certain concentrations.
Typical cases where fuel occur are if there is a leak, during filling operations, transfer operations, or excessive dusts.
Although we often cannot always eliminate these sources we can help by having good ventilation to keep vapors from building up.
Often we locate things out-doors, use grating on floors so vapors don’t build up.

6. Oxidizers
Oxygen is the most common oxidizer, especially that found in ambient air.
For oxygen, we often use “inerting” with nitrogen, helium blankets over flammable materials to reduce O2 content below that where you can have combustion.

7. Ignition Sources Heat is a common ignition source.
“Ignition sources are free!!!”
Although we can eliminate ignition sources, it is almost inevitable that an ignition source will be available if there is a large release of flammable material that cannot be diluted quickly.

8. Fire Tetrahedron
The fire tetrahedron or fire pyramid adds a fourth component—chemical chain reaction—as a necessity in the prevention and control of fires.
The free radicals formed during combustion are important intermediates in the initiation and propagation of the combustion reaction. Fire suppression materials scavenge these free radicals

9. Definitions
Combustion – a chemical reaction in which a substance combines with an oxidizer and releases energy.
Explosion – rapid expansion of gases resulting in a rapid moving pressure or shock wave.
Mechanical Explosion – due to failure of vessel with high pressure non reactive gas.

10. Explosions
Detonation – explosion (chemical reaction) with shock wave greater than speed of sound
Deflagration – explosion (chemical reaction) with shock wave less than speed of sound
BLEVE – Boiling Liquid Expanding Vapor Explosion – when liquid is at a temperature above its atmospheric boiling point. Vessel ruptures – flammable liquid flashes and results in a fire/explosion

11. Explosions
Confined explosion – an explosion occurring within a vessel or a building. Usually results in injury to the building inhabitants and extensive damage.
Unconfined explosion – an explosion occurring in the open. Usually results from spill of a flammable gas spill. These explosions are rarer than confined since dilution occurs.

12. Explosions
Dust Explosions - This explosion results from the rapid combustion of fine solid particles. Many solid materials become very flammable when reduced to a fine powder.

13. Fires and Explosions Definitions Flammability
Flash Point
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

14. Flammability
Flash Point (FP) – a property of material used to determine the fire and explosive hazard. The lowest temperature of a liquid at which it gives off enough vapor to form an ignitable mixture with air.
Needs to be determined experimentally.
Different methods to determine, open cup and closed cup. Open cup is usually a few degrees higher.

15. National Fire Protection Association Flammability classification
Flammable IA – Flash point < 73°F, boiling point < 100 °F
Flammable IB – Flash point < 73°F, boiling point > 100 °F
Flammable IC – 73°F < Flash point < 100 °F
Combustible II – 100 °F < Flash point < 140 °F
Combustible IIIA – 140 °F < Flash point < 200 °F
Combustible IIIB – Flash point > 200 °F

16. Mixture Flash Points
Flash Points of mixtures can be estimated only IF one of the components is flammable. If more than one is flammable then need to determine experimentally.
For mixtures:
Determine the temperature at which the vapor pressure of the flammable in the liquid is equal to the pure component vapor pressure at its flash point.

17. Mixture Flash Points Example
Methanol FP=54°F, Vapor 54°F is 62 mmHg
Determine the flash point of a solution that is 75wt% MeOH in water.
Solution:
Since only one component is flammable, can estimate mixture FP:

18. Mixture Flash Point Example Continued

19. Mixture Flash Point Example Continued

20. Flammability Limits
There is usually a range of compositions of a flammable vapor and air where combustion occurs.
Too little fuel (lean mixture) not enough fuel to burn.
Too much fuel (rich mixture) not enough oxygen to burn

21. Flammability Limits
Table 6-1 gives upper flammability limits and lower flammability limits for several common substances.
Experimentally determined.
LFL can be estimated from Flash Point:.

22. Mixture Flammability Limits
If you have a mixture of flammable components you can calculate Lower Flammability Limit of the mixture LFLmix using Le Chatelier’s relationship:

23. Mixture Flammability Limits
You can also calculate an Upper Flammability Limit of the mixture UFLmix using Le Chatelier’s relationship:

24. Flammability Limits – Temperature effect
Table 6-1 gives flammability limits for 25°C and atmospheric pressure. If you are at a different temperature you can modify flammability limits

25. Flammability Limits – Pressure effects
LFL is not affected by pressure
UFL does depend on the pressure
Procedure
Correct for Temperature
Correct for Pressure
Calculate for mixture

26. Fires and Explosions Definitions Flammability
Flash Points
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

27. Minimum Oxygen Concentration (MOC)
LFL is based on “air” but actually it is O2 that is important. Often in industry they “inert” to dilute the O2 concentration.
Below the MOC the reaction cannot generate enough energy to heat the entire mixture to the extent required for self propagation.

28. MOC

29. Fires and Explosions Definitions Flammability
Flash Points
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

30. Minimum Ignition Energy (MIE)
Minimum energy input needed to initiate combustion
Most hydrocarbons have low MIE~0.25 mJ
Whereas the “spark” from walking across the room is 22mJ (almost 100X too much)
Again, we always assume that an ignition source will exist
Table 6-2 gives MIEs for some substances

31. Fires and Explosions Definitions Flammability
Flash Points
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

32. Adiabatic Compression
When gases are compressed they heat up and can ignite (this is how a diesel engine works, also the cause of “knocking” in gasoline engines)
The adiabatic temperature rise is:

33. Fires and Explosions Definitions Flammability
Flash Points
Flammability limits
Mixtures
Temperature Dependence
Pressure Dependence
Minimum Oxygen Concentration
Minimum Ignition Energy
Adiabatic Compression
Ignition Sources

34. Ignition Sources Ignition sources are free!!!
Table 6-3 gives the results of a study by Factory Mutual Engineering Corporation who studied over 25,000 industrial fires to determine the source of ignition.

35. In Class Problem
What is the UFL of a gas mixture composed of 1% methane, 2% ethane and 3% propane by volume at 50°C and 2 atmospheres:
Data:
Component MW Heat of Combustion
(kcal/mol)
Methane
Ethane
Propane

36. Solution Procedure: Correct for temperature
Correct for pressure (only for UFL)
Find for mixture.

37. Solution
Correction for Temperature : UFL from Table 6-1

38. Solution cont.
Correction for Pressure (UFL only)

39. Solution cont.
Mixture calculation
Equation 6-2 for mixtures

40. Solution Continued
Since total combustibles in air 1+2+3=6 < 18 then the system is in the combustible range (below UFL)