Presentation on theme: "Feyzin Oil Refinery Disaster"— Presentation transcript:
1Feyzin Oil Refinery Disaster Feyzin Oil Refinery (near Lyons), France4th January A large storage tank at an oil refinery holding liquefied propane exploded and killed a number of fire fighters. The incident was an important lesson for the hydrocarbon industriesThanks toAnn-Marie McSweeney, John Barrett & Jacinta Sheehan WareDepartment of Process Engineering, UCC
2Feyzin Oil Refinery Disaster A fire developed in a tank farm at an oil refinery. No person was in apparent danger.The fire service was called out but as the fire had already taken hold they decided to simply monitor it until it safely burnt itself out.However the firemen appeared unaware of the significance of the enormous radiant heat flux from the fire that was impinging on adjacent pressurized storage tanks and spheres.This was raising their temperature and the temperature of the products within them.More importantly it was weakening the integrity (tensile strength) of the wall material (material tensile strength falls with higher temperature).When the membrane stress in the storage vessels due to the raised internal pressure exceeded the reduced tensile strength of the wall material, the vessels burst open.The contents of the vessels then ignited in a fireball and killed the fire fighters.
3Feyzin Oil Refinery Disaster A very good description of the incident is given in the bookSAFETY & LOSS PREVENTIONAUTHOR: FRANK LEES(In the UCC Library under Classification )The course notes for PE 3005 should also be consulted especially the material dealing with the temperature dependence of material strength. Similarly the notes of PE 2003 dealing with pressure vessel analysis.
5Feyzin Oil Refinery Disaster PRODUCT DESCRIPTIONThe material in the storage tanks was PropaneC3H8Third member in the saturated hydrocarbon group known as the alkanes. Commonly used as a fuel.
6Feyzin Oil Refinery Disaster Thermodynamic PropertiesBoiling Point is – 42 °C at Patm.Colourless Gas at Room Temperature and Atmospheric PressureMolecular Weight M = 44Gas Constant R = 189 J/kgKCalorific Value = 47 MJ/kgFlammability Limits (in air) 2.5 % to 9.5 %
7Feyzin Oil Refinery Disaster Thermodynamic PropertiesLiquid Density = 588 kg/m3 (at 1bar)Vapour Density = kg/m3 (at 1bar)It can be seen that the liquid is lighter than water and the vapour is heavier than air.Specific Heat Cp= 1679 J/kgKRatio of Specific Heats = Cp/Cv = 1.126Latent heat of evaporation = 428 kJ/kg
8Feyzin Oil Refinery Disaster Vapour Pressure Curve (actually that of Propene)From this chart, knowing the temperature of the propane, its vapour pressure (i.e. tank internal pressure) can be found.
9Feyzin Oil Refinery Disaster CONTAINMENT DESCRIPTIONThe vessel in question was a large outdoor spherical vessel resting on vertical legs designed for the bulk storage of liquefied propane. The vessel was amongst other similar storage vessels. There was a pressure relief valve (safety valve) at the top on a pipeline leading to a flare. In emergencies this valve would open and the escaping vapour flared off. No information on the diameter of the safety valve orifice (subsequently assume it is 100 mm).Vessel GeometryVessel Diameter D = 14 m Vessel Radius, R = 7 mTotal Volume Vtotal = 4/3πR3 = m3Ullage (i.e. free space) set at 20 %Working Volume Vworking = 0.8 x = 1150 m3
10Feyzin Oil Refinery Disaster Illustration of storage sphere showing vessel supports and pressure relief systemSafety ValveFlareSupport Legs
11Feyzin Oil Refinery Disaster Vessel GeometryMaterial of construction is structural steel with a densitys = 7800 kg/m3Wall thickness, t = 45 mm· Mass of tank wall = 4πR2 t s = 216 tonnes· Surface area of tank = 4πR2 = 616 m2· Projected area of tank = πR2 = 154 m2
12Feyzin Oil Refinery Disaster Vessel Pressure Stress AnalysisTensile Strength (maximum strength) of structural steelTS = 620 MN/m2Rupture Pressure can be estimated from knowledge of the membrane stress in a spherical vesselPR = 80 barUnder normal conditions, the vessel would not be expected to rupture until the internal (propane) pressure reached 80 bar.Vessel was un-insulatedPropane temperature = Ambient outside temperature.
13Feyzin Oil Refinery Disaster Vessel Pressure versus Ambient TemperatureStorage pressure (i.e. propane vapour pressure) varies with ambient (i.e. propane) temperature. For this location: °C < TAMB < 40 °CHence can tabulate the normal pressures that might exist within the gas storage spheres.
14Feyzin Oil Refinery Disaster Vessel Pressure AnalysisThus normal tank internal pressure is well below the tank failure pressure. To prevent internal pressure for whatever reason rising to and reaching the rupture pressure, the safety valve was set to lift (i.e. open) at 20 bar; corresponding to a propane temperature of about 60 °C.Thus the maximum membrane stress that could be developed in the tank wall would be when internal pressure was 20 bar.
15Feyzin Oil Refinery Disaster Reduction in Steel Tensile Strength with TemperatureThe mechanical strength of metals depends on their temperature; as the temperature rises, the strength falls off. Unless otherwise stated, any quoted mechanical strength value is the value that exists at ambient temperatures.sTSMN/m2100T°C200700600500400300
16Feyzin Oil Refinery Disaster Significance of the Previous ChartAt ‘cold’ i.e. ambient temperatures, the vessel can contain internal pressures of up to 80 bar because the tensile strength TS = 620 MN/m2 (note it could even be higher!).However, if the vessel wall temperature rises to 700 °C, in which case the steel tensile strength, TS falls to 150 MN/m2, then the vessel will rupture even with the safety valve open.PSET = 20 bar => σ = 155 MN/m2σ > σTS RUPTURE!
17Feyzin Oil Refinery Disaster INCIDENT DESCRIPTIONWhat follows is a simplified account of what actually happened.At the date in question, there were 400 tonnes of propane in the tank.Volume occupied V = 680 m3Given the working volume VW = 1150 m3Tank was approximately 60 % full.
18Feyzin Oil Refinery Disaster INCIDENT DESCRIPTIONAn adjacent hydrocarbon storage tank at the depot caught fire and burnt fiercely (the actual incident was a good deal more involved).=> Propane storage sphere exposed to intense radiant heat.=> Temperature of propane (and steel wall) will rise.=> Vapour pressure of propane rises.When the vapour pressure reaches relief valve pressure setting, PSET of 20 bar, the safety valve lifted and propane vapour was expelled from the vessel and sent to the flare.
19Feyzin Oil Refinery Disaster INCIDENT DESCRIPTIONAssuming the pressure inside the vessel henceforth remains at 20 barWe have a kind of controlled equilibrium; boiling off propane vapour at a constant pressure of 20 bar.The fire fighting strategy was to let the vessel empty itself over time much like a kettle boiling itself dry; at the end all that would be left would be a burnt out empty vessel.The fire fighters that had been called to the scene remained in proximity to the fire.
20Feyzin Oil Refinery Disaster INCIDENT DESCRIPTIONWhat the firemen didn’t know!The lower part of the tank wall in contact with the boiling liquid propane will remain at something close to the liquid propane temperature (60 °C at 20 bar) due to the very high heat transfer coefficient (say 10,000 W/m2K) between a boiling liquid and metal wall.However the upper part of the tank wall in contact with the vapour receives no such cooling (H.T.C. of 100 W/m2K) and it will rise towards the of the radiant flame temperature. This would be an upper theoretical limit of about 1300 °C.
21Feyzin Oil Refinery Disaster INCIDENT DESCRIPTION A race is on!If the wall of the vessel reaches 700 °C before the vessel has emptied itself, the wall will rupture and the remaining propane will go up in a fireball!Two times must be calculated1] Time for upper surface of tank wall to reach 700 ºC due to radiant heat transfer from adjacent fire.2] How much propane will have left the sphere through the open safety valve in this time.
23Feyzin Oil Refinery Disaster Radiant Heat Transfer CalculationEarly morning in January, so initial temperature of Propane tank Ti 0 °CHow long will it take for upper tank wall to reach 700 °C?Do a very crude energy balance!Radiant heat flux: QR = ..A.(TFlame4 – TWall4) Take TFlame = 1300 °C = 1600 K TWall = ½(0 °C °C) = 350°C = 620 KA is the projected area of the sphere = 154 m2Stefan-Boltzman Constant = 5.67 x 10-8 W/m2KEmissivity (really a fudge factor), Take = 0.5QR = x (16004 – 6204)QR = 28 MW i.e. 28 MJ/s
24Feyzin Oil Refinery Disaster Temperature Rise in SystemCalculate the amount of thermal energy needed to produce the corresponding temperature rise of the system so that the upper wall reaches 700 C. Note the total heat in has sensible heat transfer and latent heat transfer components:1. Bring 400 tonnes of propane from 0 °C up to 60 °C.2. Bring approximately half of tank wall (108 tonnes of steel) from 0 °C up to 60 °C.3. Bring other half of tank wall from 0 °C up to 700 °C.4. Evaporate off some portion (say half) of the propane.Note the specific heat capacity of steel cp = 450 J/kgK
25Feyzin Oil Refinery Disaster Temperature Rise in SystemQ = 400 x (60 – 0) (1)+ 108 x (60 – 0) (2)+ 108 x (700 – 0) (3)+ 200 x x (4)Q = GJ = 163 GJDividing the total heat requirement by the heat flux to obtain timet = 5821 s = 97 minutesSo, very roughly we might expect that one hour and a half after the outbreak of the initial fire, the tank wall temperature will reach 700 °C.
26Feyzin Oil Refinery Disaster Rate of Vessel EmptyingHow much vapour has been expelled through the safety valve after an hour and a half (and assuming the safety valve lifts soon after the fire starts)?Require the mass flux through the safety valve; model the process as isentropic expansion of an ideal gas across a nozzle with choked flow at outlet.J Mass flow rate kg/sP Vessel pressure barA Valve outflow area m2T Propane vapour temperature (absolute) K
27Feyzin Oil Refinery Disaster Rate of Vessel EmptyingP = 20 bar, T = 60 °C = 333 K0.008 m2 = 1.126, R = 189 J/kgK (Propane)J = 38.3 kg/sTotal outflow of propane in 5821 sJ = 38.3 x 5821 = 223 tonnesThus after an hour and a half, the amount of propane remaining in the tank is400 – 223 = 177 tonnes.
28Feyzin Oil Refinery Disaster Accident ProgressionPlotting tank wall temperature and tank internal pressure versus time
29Feyzin Oil Refinery Disaster Accident ProgressionPlotting mass of propane in vessel, wall membrane stress and wall tensile strength versus time.
30Feyzin Oil Refinery Disaster CONSEQUENCESAn hour and a half or so after the commencement of the fire:· Wall temperature reached 700 °C· Membrane stress was at 155 MN/m2· Steel tensile strength had fallen to 150 MN/m2· Hence the Storage Sphere ruptured· 177 tonnes of propane exploded into a fireball· 17 people killed
31Feyzin Oil Refinery Disaster Disaster FIRE BALL (BLEVE) CALCULATIONSModel the instantaneous combustion of the escaped vapour. Duration of burning of fire ball isThe radiative power of the fire can be calculated fromQR Radiative power WHC Calorific Value J/kgtd Duration of fire ball sM Mass of fuel in fire ball kg
32Feyzin Oil Refinery Disaster FIRE BALL (BLEVE) CALCULATIONSA point source model of the fire gives the radiative heat flux as Radiative flux W/m2QR Radiative power of flame Wr Distance from source mIn turn the thermal radiation dosage can be calculated asL Thermal radiation dosage (kW/m2)1.33s Intensity of radiation (radiation flux) kW/m2t Duration of exposure s
33Feyzin Oil Refinery Disaster FIRE BALL (BLEVE) CALCULATIONSNote the duration of exposure is equal to the duration of the fire ball.Damage to people exposed to the fire can be quantified withHence can estimate how close people must have been to the fire to have been killed or injured.
37Feyzin Oil Refinery Disaster STORAGE SPHERES AFTER THE INCIDENT
38Feyzin Oil Refinery Disaster POSSIBLE ACCIDENT PREVENTION STRATEGIES1. Install a larger pressure relief valve so if the tank is exposed to fire, its contents can be flared more rapidly.2. Space the tanks further apart so that an outbreak of fire in one does not impose excessive heat radiation on adjacent units.3. Place thermal insulation on tank.4. Spray water over the top of tank to keep it cool and hence maintain its mechanical strength.5. Choose a different material of construction for the tank; specifically a heat resistant steel that can maintain substantial mechanical strength even at elevated temperatures.