Safety Engineering - Fires and Explosions - M. Jahoda

Transcription

1 Safety Engineering - Fires and Explosions - M. Jahoda

2 Introduction 2 Fires and Explosions Statistically the most common type of accident in the chemical industry fire (31%) explosion (30%) release of toxic substance The most common source of explosion organic solvent vapors Example: burning (explosion) 1 kg of toluene the energy is released ~ 40 MJ can destroy the chemical laboratory can cause loss of life A chemical engineer must be familiar with the flammable properties of materials, the nature of the burning and explosion process, and ways to minimize hazard.

3 Introduction 3 Fire Fast exothermic oxidation to form a flame. Explosion The sudden release of energy associated with the increase in temperature and pressure, a sudden change in pressure is spread to the surroundings as shockwave. Fuel can be in different states but burning always takes place in the vapor phase. Liquids evaporate before burning and the solids decompose. The difference between the burning and the explosion is in the speed of the released energy. The burning is slow, while the explosion is fast (in one thousandths of a second). Burning can go on an explosion and vice versa.

4 Introduction 4 Fire triangle Gas O 2, F 2, Cl 2 Liquid H 2 O 2, HClO 3, HNO 3 Solid KClO 3 Gas - acetylene, methane, hydrogen, LPG Liquid - gasoline, acetone, ether, hexane Solid - plastics, flammable dust Initiating energy Heat, flame, sparks, static electricity,... Initiating energy

5 Fires 5 Division By area closed spaces open spaces By form of burning homogeneous (vapors, gases) heterogeneous combustion (solids) Fire Development

6 Burning Schemes 6 Homogeneous burning liquid heat flammable gas mixing air flammable vapors air mixing flammable mixture initiation fire flammable mixture fire initiation Heterogeneous burning carbonaceous residue heat air glowing flameless combustion heat solid pyrolysis flammable vapors air mixing flammable mixture initiation fire

7 Fires: closed spaces 7 Scheme

8 Fires: closed spaces 8 Rollover (flameover) In the initial phase of the fire, room temperature increases, the flame temperature is about 500 C. Higher amounts of heated flammable gases accumulate at the ceiling of the room and mix with air oxygen. If flammable gas concentrations reach between flammability, ignition and rapid fire spread. The flames spread at a high velocity below the ceiling, until the combustible gases emit or the oxygen concentration drops.

9 Fires: closed spaces 9 Flashover Total sudden ignition of flammable materials (rapid transition phase flashover fully engulfed fire) once. Sufficient amount of oxygen produced by a flame which cause turbulent flow of hot gases in the room. The temperature throughout the room approximates the values of selfignition of materials in the room (temperatures of about C under the ceiling). From the moment flashover fire is controlled by ventilation, since due to vigorous combustion occurs to decrease the oxygen concentration.

10 Fires: closed spaces 10 Backdraft It occurs when a sudden supply of oxygen enters a closed room containing hot flammable gases but no flaming burning. The temperature in the room is approaching the values of selfignition of materials in the room (temperatures of about 500 C under the ceiling).

11 Fires: closed/open spaces 11 Pool Fire combustion of vapor of liquid, which is bounded by solid walls (reservoirs)

12 Fires: closed/open spaces 12 Spill Fire burning vapors of liquid which is not bounded by solid walls (pool) Deepwater Horizon oil spill, 2010

13 Fires: closed/open spaces 13 Spill Fire burning vapors of liquid which is not bounded by solid walls (pool) Deepwater Horizon oil spill, 2010

14 Fires: closed/open spaces 14 Spill Fire hoření par kapaliny, která není ohraničena pevnými stěnami (kaluže) Deepwater Horizon oil spill, 2010

15 Fires: closed/open spaces 15 Fire Ball the result of rapid leakage and ignition of flammable gases under pressure (eg natural gas)

16 Explosion 16 Division according to the cause of local energy release Mechanical explosion when the gas or liquid pressure is exceeded in a closed container eg steam boiler explosion releasing otherwise bound kinetic energy eg by releasing the rotating flywheel inside the machine (turbine) Electric blast there is a jump of discharges between places with a high difference in electrical potential such as a high-voltage discharge or a storm flash Nuclear explosion consequence of a nuclear or thermonuclear reaction Chemical explosion rapid decomposition of chemical compounds associated with the release of large amounts of gases and heat Vulcan explosion sudden release of magma pressure in the Earth's crust volcanic eruption

17 Definition 17 Deflagration: explosion spreading at subsonic speed. Detonation: Explosion spread at supersonic velocity and characterized by shock wave. Explosive range: The range of flammable substance concentration in the air at which an explosion may occur. Flash point: The minimum temperature at which the liquid forms sufficient gas or vapor to ignite immediately after application of the initiator source. LEL: Lower explosion limit. Lower explosion point (LEP): The temperature of a combustible liquid at which the saturated vapor concentration in the air is equal to the lower explosion point. UEL: Upper explosion limit. Upper explosion point (UEP): The temperature of a flammable liquid at which the saturated vapor concentration in the air is equal to the upper explosion point.

18 Definition 18 Oxygen concentration limits: the highest concentration of oxygen in a mixture of flammable substance, air (oxidants) and inert gas that can not be exploded. Explosive atmospheres: a mixture of air and flammable substances in the form of gases, fogs or dusts under atmospheric conditions, in which, after initiation, the flame spread to the entire unburned mixture. Dangerous explosive atmospheres: an explosive atmosphere that, if exploded, causes damage. Hybrid mixture: mixture of air and flammable substances of different physical states; eg a mixture of methane, coal dust and air; a mixture of air and droplets of gasoline. Inerting: adding an inert substance to prevent explosive atmospheres. Minimum flash point of explosive atmospheres: ignition temperature of combustible gas or steam of combustible liquid or minimum ignition temperature of cracked dust. Explosion pressure: the pressure generated in a closed container when explosive atmospheres are exerted at a given concentration. Reduced explosion pressure: the pressure generated by the explosion of an explosive atmosphere in a container protected either by the decompression of the explosion or the suppression of an explosion.

19 Fire characteristics of the flammable substances 19 Classification of flammable substances ISO 3941 or EN 2: 1992 Class A flammable substances of solid state of organic origin wood, paper, straw, textiles, coal,... Class B liquid flammable, flame-burning petrol, kerosene, alcohol, ether, oils, paints, varnishes, fats, resins,... Class C combustible gaseous substances hydrogen, acetylene, methane, propane-butane, carbon monoxide, natural gas,... Class D flammable metals aluminum, magnesium, alkali metals, alloys,... Class F vegetable and animal fats used in the kitchen edible oils and fats

20 Fire characteristics of flammable liquids 20 Characteristic temperatures Point of ignition Burning point Ignition temperature Concentration range Explosion limits Limiting oxygen concentration A flammable liquid is considered to be a liquid, suspension or emulsion, meeting at atmospheric pressure of 101 kpa, and at the same time the following conditions: not at + 35 C solid or pasty, has a saturated vapor pressure of not more than 294 kpa at + 50 C, has a flash point of not more than C, the burning temperature can be determined.

21 Fire characteristics of flammable liquids 21 Flash Point the lowest temperature at which vapors above a volatile combustible substance ignite in air when exposed to flame. Flammable liquids are divided into four categories as follows (CZ) Category I (flash point up to 21 C) gasoline, toluene, acetone, methanol, carbon disulfide, nitroating agents and paints, alcohol... Category II (flash point up to 55 C) styrene, white spirit, synthetic thinners and paints,... Category III (flash point up to 100 C) naphtha, kerosene, phenols, naphthalene, some paraffins, resins,... Category IV (flash point up to 250 C) heating oils, aniline, some paraffins, nitrobenzene,...

22 Fire characteristics of flammable liquids 22 Flash Point - measurement There are two basic types of flash point measurement: open cup and closed cup. closed cup open cup In open cup devices, the sample is contained in an open cup which is heated and, at intervals, a flame brought over the surface. The measured flash point will actually vary with the height of the flame above the liquid surface and, at sufficient height, the measured flash point temperature will coincide with the fire point The amount of vapor produced when determining the flash point is approximately the value of the lower explosive limit.

23 Fire characteristics of flammable liquids Fire Point the temperature at which the vapor above the flammable substance after ignition burns at least 5 s higher than the flash point burning is permanent = produces heat for sufficient production of other vapors 23 Fire point is higher than the flash point. The difference between the two temperatures is very small for low boiling liquids, but increases with the decreasing volatility of the liquid. A low boiling liquid is considered to have a "normal pressure" ( Pa) burning temperature lower than 50 C.

24 Fire characteristics of solids 24 Dust particles

25 Fire characteristics of solids 25 Overview ash content, water, volatile flammability, fixed carbon medium grain size bulk density calorific value ignition temperature flash point lower explosive limit maximum explosion pressure maximum rate of pressure increase explosive class

26 Fire characteristics of solids 26 Selected characteristics Limiting oxygen content the highest concentration of oxygen, where the dust-free mixture is no longer capable of explosive reactions important in protecting the device against explosion hazard by means of inert gas Minimum initial energy spark energy (in order of joules) required to ignite the dusty mixture important in protecting the device from electrostatic discharge Determination of susceptibility to self-ignition information on the slopes of powder materials will self-ignite during storage important during transport and storage

27 Fire characteristics of substances 27 Autoignition Temperature = the lowest temperature of the hot surface at which the optimal mixture of vapors or gases of the substance ignited with air, when determining the ignition temperature, the ignition is only produced by the action of heat, not an open flame or a spark. Autoignition temperature C acetone 535 peat gasoline 220 lignite 410 wood 330 activated carbon 495 kerosene 265 straw 310 cereal dust 267 flour 440 asphalt 485 PVC carbon disulfide plexiglass 460 cyclohexane 270 white phosphorus ethylbenzene 430 diesel 220 coke 400 coal dust 260 tobacco 175 cotton 410 hay 333 paper rubber 340 toluene 535

28 Chemical explosions 28 Deflagration An explosion with a resulting shock wave, moving at a rate lower than the speed of the sound in a non-reactive environment. It occurs when the flame front speed is limited by molecular or turbulent diffusivity. Flame propagation rate is from 30 to m/s. Detonation Explosion with resulting shock wave, moving at a speed higher than the speed of sound in a non-reactive environment. There are two mechanisms in which a large amount of energy must be released in a small volume and in a short time. With the thermal mechanism, the released energy increases the reaction rate at the same time. The chain and branch mechanism generates free radicals (centers) that rapidly increase the number of elementary reactions. Flame spread rate up to m/s, overpressure up to 20 bar.

29 Chemical explosions 29 Explosion limits 100 % air x fuel 100 % vapor combustibles Non-Explosive Safety area Explosive Non-(Explosive) 50 % LEL (LEL, LFL) (UEL, UFL) Methane

30 Chemical explosions 30 Flammability limits, examples of the value (vol. % vapor) mixed with air acetylene 1,2-80,0 coal gas 5,8-63,0 ammonia 15,5-31,0 natural gas 5,3-15,0 carbon monoxide 12,5-75,0 hydrogen sulphide 4,3-45,5 methane 5,0-17,0 hydrogen 4,0-74,2 gasoline 1,1-6,0 acetone 1,6-15,3 butane 1,6-8,5 carbon disulfide 1,3-50,0 propane 1,9-9,5 As a non-hazardous concentration, it is possible to mark a concentration that does not exceed 50 % of the lower explosive limit. Mixtures of solids dust and air are dangerous explosion if the lower explosion limit is less than or equal to 65 g/m 3, and are particularly dangerous explosion if the lower explosion limit is less than or equal to 15 g/m 3.

31 Chemical explosions 31 Flammability limits - Measurement

32 Chemical explosions 32 Flammability limits, mixtures of substances - prediction Mixtures of vapor - Le Chatelier equation, empirical equations Lower explosive limit (vol. %) LFL UFL mix mix n 1 n 1 1 yi LFL Upper explosive limit (vol. %) 1 yi UFL i i y i is the molar fraction of the combustible (explosive) substance in the n-component mixture Assumptions: Constant heat capacity of products Similar adiabatic heat heating Similar kinetics of combustion

33 Rate of pressure increase(bar/s) Pressure (bar) Pressure of explosion (bar) Chemical explosions Explosion curve - after the initiation of the explosive mixture (time 0) an exothermic reaction occurs; the temperature rise is manifested by an increase in pressure 33 LEL Concentration (vol. %) UEL Time (ms) Explosion constant - the maximum rate of pressure increase in relation to the time of explosion in volume V for the design of anti-explosion protection elements (membranes, valves) Concentration (vol. %)

34 Chemical explosions 34 Minimum Oxygen Concentration (MOC) needed to promote burning MOC = LEL * m the mixture does not explode although it is within the range of explosiveness unless the oxygen content is at least equal to MOC reduction of oxygen content under MOC is possible by addition of inert = INERTISATION Substance MOC (vol. % O 2 ) Methane 11.6 Ethane 11 Hydrogen 4.8 Hexane 12.1 Propylene 11.5 Example: Substance MOC (vol. % O 2 ) Ethylene 10 Benzene 11.8 Propane 11.2 Coal gas 7 CO 5 Butane burning: C 4 H O 2 4CO 2 + 5H 2 O - the lower butane burning limit is at a concentration of 1.6% by volume (molar). MOC = 1.6 x 6.5= 10.4 %. Approximate estimate: Usually, this concentration ranges between 8-10% of molar oxygen

35 Chemical explosions 35 Flammability Diagram requires experimental data depends on temperature and pressure

36 Chemical explosions TNT-Equivalency Method = an equivalent amount of trinitrotoluene that causes the same pressure wave as the explosive amount of flammable substance 36 yield factor; 0,03 Amount of explosive substance, kg Heat of combustion of substance, kj/kg Blast Energy of TNT, kj/kg Equivalent-Charge weight of TNT, kg It turns out that probably only a relatively small portion of the total available energy of combustion is actually involved in the formation of shock waves. The measurement unambiguously confirmed that most explosions of vaporous combustible hydrocarbons in the form of pressure waves developed only between 1 and 3 % of combustion energy.

37 Chemical explosions - dusts 37 Explosive nature of flammable dusts In the form of dust, almost all substances burn except for purely inorganic materials such as dolomite, limestone and oxides and metal salts. Metal powders are dangerous aluminum powders that have the highest rate of pressure build-up and one of the highest values of maximum explosion pressure, magnesium, titanium, zinc, iron. All organic dusts are explosive: hay and wheat powder, starches and flour, sugar, cocoa, tea, tobacco..., soapy powders, dyes, pharmaceuticals. They are prone to electrostatic charge generation and have low lower explosion limits. WGBH Educational Foundation. Photograph courtesy of Valerie Briscoe About one third of dust explosions are associated with human failure.

38 Thermal characteristics 38 Heat Release Rate), HRR represents the energy released by flammable material per unit of time is the basic parameter for determining the intensity of combustion is time-dependent is not a simple material property (experimental finding) Q max conical calorimeter (solids) radiometer loss of material Q average pine wood Development phase Steady phase Phase of burning

39 Thermal characteristics - HRR 39 Approximate values of released heat - different sources Experimentally: eg water cooled radiometer type Schmidt-Boelter SBG01 Substance burning cigarette incandescent light bulb burning candle man in normal movement burning paper in a waste bin burning pool of gasoline, 1 m 2 wooden pallets stacked up to the height 3 m HRR, Q 5 W 60 W 80 W 100 W 100 kw 2.5 MW 7 MW

40 Thermal characteristics 40 Flame temperature Source Temperature [ C] burning match burning candle smoldering cigarette burning paper incandescent electric heater flame lighter bulb Substance Temperature [ C] peat, mazut wood, polystyrene, diesel black coal, rubber, gasoline alcohol methane hydrogen acetylene 2 325

41 Heat Transfer 41 Heat transfer is a major factor in the ignition, growth, spread, decay and extinction of a fire. It is important to note that heat is always transferred from the hotter object to the cooler object - heat energy transferred to and object increases the object's temperature, and heat energy transferred from and object decreases the object's temperature. Conduction is heat transfer within solids or between contacting solids where T is temperature (in Kelvin), and k is a constant that unique for different materials know as the thermal conductivity and has units of Watts/(meters*Kelvin)

42 Heat Transfer Convection is heat transfer by the movement of liquids or gasses 42 where T is temperature (in Kelvin), and h is a constant that is unique for different materials known as the convective heat transfer coefficient, with units of W/(m 2 K). These values are found empirically, or, by experiment. For free convection, values usually range between 5 and 25. But for forced convection, values can range anywhere from 10 to

43 Heat Transfer Radiation is heat transfer by electromagnetic waves. 43 where T is temperature (in Kelvin), α is the thermal diffusivity (a measure of how quickly a material will adjust it's temperature to the surroundings, in meters squared per second) and ε is the emissivity (a measure of the ability of a materials surface to emit energy by radiation).

44 Thermal characteristics - experiments 44 Lab scale UCT Prague - measurement of mass loss of flammable liquid - flame temperature and vessel walls - flame height Temperature measurement with thermal camera and spot thermocouples. The dish is placed on the scales, which are shielding plate from mineral fiber.

45 Thermal characteristics - experiments 45 Small-sized scale - room temperature measurement (ISO 9705 standard) - concentration of gaseous components (O 2, CO, CO 2, NO...) - flow velocit non-propagating liquid fire - heptane non-propagating gas fire - propane butane non-proliferating fire of solids - pine wood

46 Thermal characteristics - experiments 46 Large-size scale - room temperature measurement - concentration of gaseous components (O2, CO, CO2, NO...) the spreading fire of living quarters - kitchens and bedrooms

47 Thermal characteristics - experiments 47 Large-size scale - measurement of temperature profiles - concentration of gaseous components - velocity of gases (smoke, combustion gases) test fire in Valík tunnel

48 Thermal characteristics - experiments 48 Large-size scale - measurement of temperature profiles - resistance of structural profiles the spreading fire of an office/warehouse building

49 Thermal characteristics - experiments 49 Chinese Fire Training Center, Chongqing Municipality

50 Thermal characteristics - experiments 50 Chinese Fire Training Center, Chongqing Municipality

51 Reality Fire of the kitchen in an apartment house. Cause: When frying the meat oil has been ignited. fig: HZS KHK Fire of a child's room on the seventh floor of an apartment building. Cause: Portable DVD player in charging mode. fig: HZS MSK