65742 Fire and Explosion Investigation

Tutorial 1.

1. What is the flash point ofa) Petrol -45 ◦Cb) Diesel >+45 ◦C

2. What is the autoignition temperature ofa) Petrol 246 ◦Cb) Diesel 210 ◦C

3. Explain the properties of petrol and diesel in terms of their intended use.Petrol is designed for use in an engine which is driven by a spark. The fuel should be premixed with airwithin its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel shouldnot preignite in the hot engine. Therefore, petrol is required to have a low flash point and a high autoignitiontemperature.Diesel is designed for use in a high-compression engine. Air is compressed until it is heated above theautoignition temperature of diesel and then the fuel is injected as a high pressure spray, keeping the fuel-airmix within the flammable limits of diesel. There is no ignition source. Therefore, diesel is required to have ahigh flash point and a low autoignition temperature.

4. Astrolite G, a secondary explosive, produces a shockwave with a speed of 8,600m/s. Suggest its main use.The brisance of an explosive is dependant on its speed of shockwave propagation. A speed of 8,600 ms−1

implies a very brisant explosive. As a result, this explosive is used for military applications.

5. A fire burning at 600 degrees Celcius is ventilated and the temperature rises to 850 degrees, without increas-ing the surface area of the flames. How does the radiation flux from the fire change?Radiation flux is proportional to the fourth power of the absolute temperature of the flames. If other factorshold constant, the radiation flux of the fire increases by a factor of10204/8704 = 1.89**1020 = 850+270. 870 = 600+270. Absolute temperature (Kelvin).

6. Residue from the seat of an intense fire tests postive for magnesium and chlorine, and negative for organiccompounds. Explain.These residues are typical for a deflagrating mixture where powdered magnesium is the fuel.

7. Why does polyurethane produce such toxic pyrolysis products?Polyurethane is a condensation polymer between a di-isocyanate and a di-alcohol. The structure of the ureth-ane functional group:

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The pyrolysis products from a condensation polymer are generally fragments of the functional groups andthe condensation units. Urethanes can produce carbon monoxide, hydrogen cyanide and cyanuric acid aspyrolysis products.

8. What is the structure of styrene monomer? What are its properties?

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Styrene

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Properties of styrene monomer:

• m.p. -35 ◦C• b.p. 135 ◦C• flash point 31 ◦C• flammable limits: lower 1.1, upper 6.1%• toxicology: acute respiratory toxin, chronic potential carcinogen.

Summary: Styrene monomer is a volatile, flammable and toxic compound. It is one of the most prominentpyrolysis products found at fire scenes.

9. Write a stoichiometric equation for the explosive decomposition of TNT. Does TNT have sufficient internaloxidiser to completely oxidise itself? What about tetryl?TNT: 2C7H5N3O6 → 3N2+5H2O+7CO+7CTNT is oxygen deficient. It does not completely decompose to CO2, water and nitrogen. Tetryl is closer tobeing oxygen-balanced, but is still deficient.Tetryl: C7H4N4O8 → 2N2+2H2O+6CO+C

10. Two vital firefighting tactics in compartment fires are:a) Ventilating the compartment.b) Concentrating water spray at the ceiling of the compartment, not at the flames.Why do firefighters do these things?Both of these actions are designed to prevent or delay flashover. Firefighters try to prevent flashover for theirown safety, to allow victims a chance of survival and to prevent structural damage.Ventilating the compartment removes the fire plume layer at the ceiling, preventing the build up of hot,radiating gases that cause flashover. Water spray at the ceiling is similarly aimed at reducing radiation fluxfrom high-level gases in the compartment.

11. Intense, localised damage is likely from what type of fire?A smouldering fire.

12. Where is there still a good chance of recovering flammable liquids, even after a flaming fire?In protected areas, such as carpet, upholstery, under furniture, etc.

13. You subsample a red liquid from a 5 liter container. The odour is of petrol. How should this be packaged forthe laboratory?In a glass container with a sealing lid which is non-permeable to hydrocarbons.

14. How quickly can an accidental fire on polyurethane foam furniture proceed to flashover in a domestic livingroom? Is a determination of arson, based on speed of flame development alone, a valid conclusion for domesticfires?Flashover can result in 3 –5 minutes in a domestic setting if a fire starts on PU foam furniture. The speed offlame development is not by itself a reliable indicator of arson in a domestic setting.

15. When can a severe/rapidly developing fire indicate arson?When the fire seat is in an area with a low fuel load, such as a corridor. Also, when the fire is in a locationthat leads to maximum fire spread / victims trapped. The investigator must determine if it is reasonable fora fire to start by coincidence at a point where maximum damage or loss of life will result.

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