Fires and Explosions

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 AssociationFlammability 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 Pressure @ 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 LFLmixusing Le Chatelier’s relationship:

23. Mixture Flammability Limits • You can also calculate an Upper Flammability Limit of the mixture UFLmixusing 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 16.04 212.79 Ethane 30.07 372.81 Propane 44.09 526.74

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)