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Process Hazards

Process Hazards. SOURCE MODEL. RELEASE OF HAZARDOUS MATERIALS. Agents: Chemicals: organic solvent, pesticides, lead etc Physical (dusts, fibers, noise, and radiation) agents, e.g. Asbestos Carcinogenic, terratogenic , mutagenic Biological organisms Effect Acute Vs Chronic

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Process Hazards

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  1. Process Hazards

  2. SOURCE MODEL

  3. RELEASE OF HAZARDOUS MATERIALS • Agents: Chemicals: organic solvent, pesticides, lead etc • Physical (dusts, fibers, noise, and radiation) agents, e.g. Asbestos • Carcinogenic, terratogenic, mutagenic • Biological organisms • Effect • Acute Vs Chronic • Reversible vs irreversible • Local Vs Systemic

  4. Example: Bhopal • 40 tons Methyl Isocynate escape • Immediate cause : 500L seepage • Erupts and release fumes • 3000 died : respiratory failure • 500,000 suffer aftermath • USD470mil spent

  5. Source Models Source models represent the material release process Provide useful information for determining the consequences of an accident rate of material release, the total quantity released, and the physical state of the material. valuable for evaluating new process designs, process improvements and the safety of existing processes. Source models are constructed from fundamental or empirical equations representing the physicochemical processes occurring during the release of materials Results are often estimates since there are always many uncertainties – physical properties, designs etc

  6. Various types of limited aperture releases.

  7. Type of Releases • Wide Aperture • Release of substantial amount in short time • example: Overpressure of tank and explosion • Limited Aperture • Release from cracks, leaks etc Gas / Vapour Leak Vapour or Two Phase Liquid Vapour Gas / Vapour Liquid Liquid flashes into vapour Release of Gasses Release of Liquids

  8. Basic Models Flow of liquids through a hole Flow of liquids through a hole in a tank Flow of liquids through pipes Flow of vapor through holes Flow of vapor through pipes Flashing liquids Liquid pool evaporation or boiling

  9. 1 - Flow of Liquid Through Holes Liquid escaping through a hole in a process unit. The energy of the liquid due to its pressure in the vessel is converted to kinetic energy with some frictional flow losses in the hole.

  10. 2 - Flow of Liquid Through a Hole in a Tank A hole develops at a height hL below the fluid level. The gauge pressure on the tank is Pg and the external gauge pressure is atmospheric, or 0. The shaft work, Ws is zero and the velocity of the fluid in the tank is zero.

  11. 3 - Flow of Vapor through Hole External Surrounding Gas Pressurized Within Process Unit Po, To, Ūo = 0; Δz0 = 0 WS = 0 At throat, ūo < Sonic velocity

  12. 4. Flow of Vapour Through Holes External Surrounding Gas Pressurized Within Process Unit Po, To, ūo, Δz0, WS0 At throat, ūo < Sonic velocity

  13. 5. Flow of Vapour Through Pipes Vapor flow through pipes is modeled using two special cases : adiabatic or isothermal behavior. The adiabatic case corresponds to rapid vapor flow through an insulated pipe. The isothermal case corresponds to flow through an uninsulated pipe maintained at a constant temperature; an underwater pipeline is an excellent example. Real vapor flows behave somewhere between the adiabatic and isothermal cases.

  14. 6. Flashing Liquids Liquids stored under pressure above their normal boiling point will partially flash into vapor following a leak, sometimes explosively. Flashing occurs so rapidly that the process is assumed to be adiabatic. The excess energy contained in the superheated liquid vaporizes the liquid and lower the temperature to the new boiling point. If m is the mass of original liquid, Cp the heat capacity of the liquid (energy/mass deg), To the temperature of the liquid prior to depressurization, and Tb the depressurized boiling point of the liquid, then the excess energy contained in the superheated liquid is given by

  15. 7. Liquid Pool Evaporation or Boiling The case for evaporation of volatile from a pool of liquid has already been considered in Chapter 3. The total mass flowrate from the evaporating pool is given by Qm is the mass vaporization rate (mass/time), M is the molecular weight of the pure material, K is the mass transfer coefficient (length/time), A is the area of exposure, Psat is the saturation vapor pressure of the liquid, Rg is the ideal gas constant, and TL is the temperature of the liquid.

  16. DISPERSION MODEL

  17. Dispersion models Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release, the airborne toxic is carried away by the wind in a characteristic plume or a puff The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, due to turbulent mixing and dispersion of the toxic substance with air.

  18. Plume

  19. Puff Wind Direction Concentrations are the Same on All Three Surfaces Puff at time t1> 0 Puff at time t2> t1 Initial puff formed by Instantaneous release of Materials Puff moves down wind and dissipates By Mixing with fresh air

  20. Factors Influencing Dispersion Wind speed Atmospheric stability Ground conditions, buildings, water, trees Height of the release above ground level Momentum and buoyancy of the initial material released

  21. Dispersion Modelling

  22. FIRE & EXPLOSION MODEL

  23. Fuels Liquids gasoline, acetone, ether, pentane Solids plastics, wood dust, fibers, metal particles Gases acetylene, propane, carbon monoxide, hydrogen Oxidizers Liquids Gases Oxygen, fluorine, chlorine hydrogen peroxide, nitric acid, perchloric acid Solids Metal peroxides, ammonium nitrate The Fire Triangle AIR (OXYGEN) FUEL IGNITION SOURCE • Ignition sources • Sparks, flames, static electricity, heat

  24. Application of the Fire Triangle Fires and explosions can be prevented by removing any single leg from the fire triangle. Oxidant • However, Ignition sources are so plentiful that it is not a reliable control method. • It is better to prevent existence of flammable mixtures Fuel No Fire Ignition Source

  25. Some Properties SAME • Flammable / Explosive Limits • Range of composition of material in air which will burn • UFL – Upper Flammable Limit • LFL – Lower Flammable Limit • HEL – Higher Explosive Limit • 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) SAME

  26. Flammability Relationships FLAMMABLE REGION AIT FLASH POINT Upper Limit UPPER LIMIT AUTO IGNITION Concentration of Fuel Mist FLAMMABLE REGION VAPOR PRESSURE MIST Vapour Pressure CONCENTRATION OF FUEL Lower Limit LOWER LIMIT Temperature AIT TEMPERATURE • Temperature above which spontaneous combustion can occur without the use of a spark or flame. • Lowest temperature at which a flammable liquid gives off enough vapor to form an ignitable mixture with air

  27. Typical Values

  28. Minimum Ignition Energy (MIE) MIE is dependent on temperature, % of combustible in combustant, type of compound

  29. Autoignition Temperature (Some Data)

  30. Example of Flammability Diagram (Methane) Limiting O2 Concentration: Vol. % O2 below which combustion can’t occur LOC FLAMMABLEMIXTURES HEL LEL

  31. Jet Fire • High pressure release of gas from a vessel or pipeline ignites almost immediately. • This give rises to a giant burner of flame length tens of meters. • Danger from thermal radiation and also impingement on adjacent pressurized vessel, such as LPG vessel, heating the content followed by pressure build up causing ‘boiling liquid expanding vapor explosion’ (BLEVE). • Sometimes called Torch Fire

  32. Jet Fire Model • The jet fire model by Cook, Baharami and Whitehouse

  33. Pool Fire • Liquid spilled onto the ground spreads out to form a pool. • Volatile liquid (e.g. petrol) evaporate to atmosphere and soon form flammable mixture with air. • Upon ignition, a fire will burn over the pool. • The heat vaporizes more fuel and air is drawn in round to the side to support combustion. • Danger to people is by direct thermal radiation and burn.

  34. Pool Fire Model • Burgess and Hertzberg

  35. Pool Fire Model • The flame length can be estimated using Thomas Correlation where L is flame length (m), D is the pool diameter (m), g is the acceleration due to gravity is the ambient density (kg/m3)

  36. Flash Fire • Fire due to vapour cloud below explosive limit • Resulting from spillage of relatively volatile (e.g. propane, butane, LPG) material due to rapid evaporation • People at risk from thermal radiation effects. • Usually unexpected event and short duration

  37. Raj and Emmons Model (Flash Fire) The model is based on the observation; • The cloud is consumed by a turbulent flame front which propagates at a velocity which is roughly proportional to ambient win speed. • When a vapour cloud burns, there is always a leading flame from propagating with uniform velocity in the unburned cloud. The leading flame front is followed by a burning zone. • When gas concentrations are high, burning is characterized by the presence of a tall, turbulent diffusion, flame plume. • At point that cloud’s vapour had already mixed sufficiently with air, the vertical depth of the visible burning zone is about equal to the initial, visible depth of the cloud.

  38. Raj and Emmons Model (Flash Fire) • H is visible flame 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 • 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.

  39. w can be determine using the following equation; is a constant pressure expansion ratio for stoichiometric combustion (typically 8 for hydrocarbon), ø is a fuel-air mixture composition Øst 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 flame height is equal to the cloud depth, w = 0. the behaviour of the expression for w should smoothly reflect the transmition from one extreme condition to the other. • The model gives no solution for the dynamics of a flash fire, and requires an input value for the burning speed S. the burning speed can be estimated as follows S=2.3Uw where Uw is the ambient wind speed.

  40. Vapor Cloud Explosion • 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.

  41. Phillips Pasadena, USA • 23rd Oct. 1989, Vapour Cloud explosion • 23 Deaths 130 Injuries, Loss US$ 500 Millions

  42. Factors Favoring Overpressures of Vapor Cloud • Confinement • Prevents combustion products escaping, giving higher local pressures even with deflagration. • Creates turbulence, a precursor for detonation. • Cloud composition • Highly unsaturated molecules are bad due to high flammable range, low ignition energy, high flame speed etc. • Weather • Stable atmospheres lead to large clouds. • Low wind speed encourages large clouds.

  43. Factors Favoring Overpressures of Vapor Cloud • Vapor Cloud Size impacts on: • probability of finding ignition source • likelihood of generating any overpressure • magnitude of overpressure • 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

  44. Vapor Cloud Explosion (VCE) where I is the mass of vapor in the cloud (te), p is the peak overpressure (bar), and rc is the radius of the cloud (m) where RLFL is the distance to the LFL (m) and k* is a constant defining crosswind range 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

  45. BLEVE (Boiling Liquid Expanding Vapour Explosion) • Occurs when a vessel containing liquid above its normal boiling point and under pressure fails catastrophically • When the vessel fails, the pressure immediately drops to atmospheric, and the hot liquid rapidly boils, generating large quantity of vapor • Damage is caused by pressure wave from rapid expansion of the released vapor, and from flying pieces of the vessel and piping. If the liquid is flammable, it creates large fireball • BLEVE can be caused by excessive pressure in the vessel, exposure to external fire or mechanical impact on the vessel.

  46. Boiling Liquid Expanding Vapor Explosion (BLEVE) There are many models describing BLEVE. One such model is given below. Here t10 is the lift-off time in seconds, Wtot 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

  47. The Tragedy Of San Juanico, PEMEX, Mexico City, 19 Nov 84 • Pemex is a liquid petroleum gas ( LPG) distribution plant. • Pemex is located a few km. north of Mexico City (Pop = 16MM). • Plant was 25 years old and built to 1950 API standards of the U.S. • LPG gas is used for heating and cooking in almost every household. • 15 of 48 Vessels BLEVE in domino fashion • 550 people killed. 2,000 people receive severe burns. 7,231 people classed as injured. • 9 explosions recorded

  48. Dust Explosion *If any of these five conditions is missing there can be no dust explosion

  49. Imperial Sugar Explosion February 7, 2008 in Port Wenworth, Georgia, USA. 13 people killed and 42 injured

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