innovative entrepreneurial global 1

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1 innovative entrepreneurial global 1

2 Oxidizers Liquids Gases Oxygen, 7luorine, chlorine hydrogen peroxide, nitric acid, perchloric acid Solids Metal peroxides, ammonium nitrate IGNITION SOURCE Ignition sources Sparks, 7lames, static electricity, heat Fuels Liquids gasoline, acetone, ether, pentane Solids plastics, wood dust, 7ibers, metal particles Gases acetylene, propane, carbon monoxide, hydrogen innovative entrepreneurial global 2

3 Fires and explosions can be prevented by removing any single leg from the Bire triangle. Problem: Ignition sources are so plentiful that it is not a reliable control method. No Fire Ignition Source Robust Control: Prevent existence of Blammable mixtures. innovative entrepreneurial global 3

4 Flammable / Explosive Limits Range of composition of material in air which will burn UFL Upper Flammable Limit LFL Lower Flammable Limit SAME HEL Higher Explosive Limit SAME LEL Lower Explosive Limit Minimum Ignition Energy Lowest amount of energy required for ignition. Minimum Oxygen Concentration (MOC) Oxygen concentration below which combustion is not possible. Expressed as volume % oxygen Also called Limiting Oxygen Concentration (LOC) innovative entrepreneurial global 4

5 UPPER LIMIT Upper Flammable Limit CONCENTRATION OF FUEL MIST FLAMMABLE REGION LOWER LIMIT AUTO IGNITION Lower Flammable Limit TEMPERATURE AIT FLASH POINT Lowest temperature at which a flammable liquid gives off enough vapor to form an ignitable mixture with air AIT Temperature above which spontaneous combustion can occur without the use of a spark or flame. innovative entrepreneurial global 5

6 LFL (%) UFL (%) Flash Point o C AIT o C DH C KJ/mol Hydrogen Methane Propane Butane Pentane Benzene Methanol Ethanol Gasoline kj/g Diesel kJ/g Ammonia innovative entrepreneurial global 6

7 LOC Limiting O 2 Concentration: Vol. % O 2 below which combustion can t occur FLAMMABLE MIXTURES HEL LEL innovative entrepreneurial global 7

8 MIE is dependent on temperature, % of combustible in combustant, type of compound innovative entrepreneurial global 8

9 L E L, % innovative entrepreneurial global 9

10 HEL line Natural Gas, volume% Natural Gas LEL In line Air at 28 o C For Natural Gas Lower flammable limit, L(vol %)= log P(atm) Higher flammability limit, U(vol %)= log P(atm) innovative entrepreneurial global 10

11 Source Percent of Accidents Electrical 23 Smoking 18 Friction 10 Overheated Materials 8 Hot Surfaces 7 Burner Flames 7 Cutting, Welding, Mech. Sparks 6 Static Sparks 1 All Other 20 innovative entrepreneurial global 11

12 Flash 7ire is the non explosive combustion of a vapour cloud resulting from a release of 7lammable material into the open air, which, after mixing with air, ignites. The cloud then burns as a 7lash 7ire, and its major hazard is from the effect of heat from thermal radiation. Flash 7ire is particularly dangerous in enclosed space, due to heat and smoke innovative entrepreneurial global 12

13 The model is based on the observation; The cloud is consumed by a turbulent 7lame front which propagates at a velocity which is roughly proportional to ambient win speed. When a vapour cloud burns, there is always a leading 7lame from propagating with uniform velocity in the unburned cloud. The leading 7lame front is followed by a burning zone. When gas concentrations are high, burning is characterized by the presence of a tall, turbulent diffusion, 7lame plume. At point that cloud s vapour had already mixed suf7iciently with air, the vertical depth of the visible burning zone is about equal to the initial, visible depth of the cloud. innovative entrepreneurial global 13

14 H = 20d S 2 gd ρ o ρ a H is visible 7lame height in m, S is constant velocity (burning speed) in m/s, d is cloud depth, r is stoichiometric mixture air fuel mass ratio, g is gravitational acceleration ρ 0 and ρ a is fuel- air mixed and air density. w is represent the inverse of the volumetric expansion due to combustion in the plume, is highly dependent on the cloud s composition. wr 2 ( 1 w) 3 innovative entrepreneurial global 14

15 W can be determine using the following equation; w = φ φ st α(1 φ st ) for φ > φ st α is a constant pressure expansion ratio for stoichiometric combustion (typically 8 for hydrocarbon), ø is a fuel- air mixture composition Ø st is stoichiometric mixture composition. If the cloud consist of pure vapour, w represents the inverse of the volumetric expansion resulting from constant pressure stoichiometric combustion: w = 1/9. If the mixture in the cloud is stoichiometric or lean, there no combustion in the plume; the 7lame height is equal to the cloud depth, w = 0. the behaviour of the expression for w should smoothly re7lect the transmition from one extreme condition to the other. The model gives no solution for the dynamics of a 7lash 7ire, and requires an input value for the burning speed S. the burning speed can be estimated as follows S=2.3U w where U w is the ambient wind speed. innovative entrepreneurial global 15

16 Eisenberg, Lynch and Breeding (See Lees, 1996) innovative entrepreneurial global 16

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18 The jet fire model by Cook, Baharami and Whitehouse L [ ( Δ ] = m! Hc R s = [ log ( / )] s L s ΔH is the heat of combustion (J/kg) L is the length of flame(m) m is the mass flow (kg/s) s is the distance from the source of release R s c is the radius of the flameat distance s (m) along with the centre line. innovative entrepreneurial global 18

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20 Burgess and Hertzberg. m = 0.001ΔH c ( ) ΔH v + c p T b T a Here, m! is the mass burning rateinkgm ΔH ΔH C T T b a P C V is the heat of combustion of fuel at its boiling point (kj/kg) is the heat of vaporisation of fuel at its boiling point (kj/kg) is the heat capacity of the liquid(kj.kg is the liquidboiling temperature (K) is the initial temperature of the liquid(k) -2 s -1-1.K -1 ) innovative entrepreneurial global 20

21 The flame length can be estimated using Thomas Correlation. L D = 42 m ρ a gd ( ) where L is flame length (m), D is the pool diameter (m), g is the acceleration due to gravity ρ is the ambient density (kg/m 3 a ) innovative entrepreneurial global 21

22 Lees (1996) prefers to use Brode equation for the estimation of the explosion energy of gas-filled vessels. The following version of the Brode equation in terms of TNT equivalence has been given in the Second Canvey Report (HSE, 1981) ( P E = 1.43x P 0 )V γ 1 E is the explosion energy (ton of TNT), P 1 is the initial pressure in the vessel (kpa) P 0 is the pressure of the surrounding (kpa) V is the volume of the vessel (m 3 ). The volume can be calculated using Ideal gas law. innovative entrepreneurial global 22

23 There are many models describing BLEVE. One such model is given below. t 10 = 0.60(W tot ) 1 6 Here D = 1.836( W ) 1 3 t 10 is the lift-off time in seconds, W tot is the total weight of combustibles and air in kg, D is the maximum diameter of fireball in m and W is the weight of combustibles in kg innovative entrepreneurial global 23

24 Cloud will spread from too rich, through flammable range to too lean. Edges start to burn through deflagration (steady state combustion). Cloud will disperse through natural convection. Flame velocity will increase with containment and turbulence. If velocity is high enough cloud will detonate. If cloud is small enough with little confinement it cannot explode. innovative entrepreneurial global 24

25 1. Confinement Prevents combustion products escaping, giving higher local pressures even with deflagration. Creates turbulence, a precursor for detonation. 2. Cloud composition Highly unsaturated molecules are bad due to high flammable range, low ignition energy, high flame speed etc. 3. Weather Stable atmospheres lead to large clouds. Low wind speed encourages large clouds. innovative entrepreneurial global 25

26 4. Vapor Cloud Size impacts on: probability of finding ignition source likelihood of generating any overpressure magnitude of overpressure 5. Source flashing liquids seem to give high overpressure vapor systems need very large failures to cause UVCE slow leaks give time for cloud to disperse naturally without finding an ignition source high pressure gives premixing required for large combustion equipment failures where leak is not vertically upwards increases likelihood of large cloud innovative entrepreneurial global 26

27 VCE is an explosion resulting from combustion of a vapour cloud resulting from a release of flammable material into the open air, which, after mixing with air, ignites. Considine and Grint (1985) estimation of peak over pressure due to VCE p o = 138 I 1/3 r C r < r c ; 0.05 < p o < 1 where I is the mass of vapor in the cloud (te), p is the peak overpressure (bar), and r c is the radius of the cloud (m) r c = 1.2k * 5/3 R LFL π 1/2 where RLFL is the distance to the LFL (m) and k* is a constant defining crosswind range innovative entrepreneurial global 27

28 *If any of these 7ive conditions is missing there can be no dust explosion innovative entrepreneurial global 28

29 Dust explosion occurs only if the dust is dispersed in air, but transition from a fire to an explosion can occur, and vice versa Some typical industrial dusts/powders wood, coal, food (e.g., starch, flour, sugar, cocoa, feed- stuffs), chemicals (e.g., drugs, dyestuffs), plastics (e.g., urea formaldehyde resin, polyethylene, polystyrene) and metals (e.g., aluminum, magnesium). If a burning dust is disturbed, a dust suspension may be formed and ignited. This initial explosion may generate further dust clouds, which in turn explode On the other hand, burning particles from an explosion may act as the source of ignition for a fire of other flammable materials. In dust explosions, the combustion process is very rapid. The flame speed is high, comparable with that in gas deflagrations. Maximum explosion pressures are often close to the theoretical values calculated assuming no heat loss during the explosion Can be conveniently modelled using TNT equivalent method innovative entrepreneurial global 29

30 TNT equivalency is a simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT. The approach is based on the assumption that an exploding fuel mass behaves like exploding TNT on an equivalent energy basis. Determine the total amount of flammable material involved in the explosion. Estimate the explosion efficiency and calculate the equivalent mass of TNT. Use the scaling law to estimate the peak side on overpressure. Estimate the damage for common structures and process equipment using table guide. innovative entrepreneurial global 30

31 m η m ΔE TNT ΔH TNT C m TNT = ηmδh C E TNT is the equivalent is the empirical explosion efficiency (unitless) is the mass of is the energy of is the heat of mass of hydrocarbon (kg) explosion of TNT (kg) combustion (kj/kg) TNT (4686kJ/kg) Another typical value for energy of explosion of TNT is 1120 calories/gram. The heat of combustion for the flammable gas can be used in place of the energy of explosion for the combustible gas. innovative entrepreneurial global 31

32 innovative entrepreneurial global / TNT e m r Z = Scaled Distance, in m/kg can be calculated from r in m, and m TNT in kg: Having Ze, scaled overpressure Ps can be estimated from the following equation. The overpressure can then be computed based on whatever units of atmospheric pressure Pa = e e e e s z z z z p a s O p p p =

33 INVENTORY (tons) UVCE BLEVE FIRE Distance in Meters innovative entrepreneurial global 33

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