San Carlos Camping Site Disaster 11th July 1978 Over 200 people killed in a camp site in Spain when a truck carrying propylene exploded as it passed the site. Thanks to Ann-Marie McSweeney &John Barrett Department of Process Engineering, UCC

San Carlos Camping Site Disaster Disaster Overview The road tanker was loaded with chilled liquid propene at a refinery near Barcelona and was travelling down the coast road to a customer near Valencia. The tanker had been over-filled with propene and as its temperature rose, it expanded. When the volume of propene equalled the tank volume, free expansion was no longer possible. The subsequent temperature rise, caused enormous pressures to be developed in the tank which in turn produced membrane tensile stress in the wall that exceeded the tensile strength of the wall material. This mode of failure is termed Hydraulic Rupture. The tank burst open, the liquid propene flashed off and dispersed as a vapour over the campsite. Lying within the flammable range and finding a source of ignition, it engulfed the whole area in a fireball.

San Carlos Camping Site Disaster A very good description of the incident is given in the book MAJOR CHEMICAL HAZARDS AUTHOR: V.C. MARSHALL (In the UCC Library under Classification ) The course notes for PE 2002 should also be consulted especially the material dealing with pressure vessels.

San Carlos Camping Site Disaster An illustration of the truck in question

San Carlos Camping Site Disaster Picture shows remains of rear axle of truck

San Carlos Camping Site Disaster

Map of Location of Accident

San Carlos Camping Site Disaster PRODUCT DESCRIPTION PROPENE (also known as Propylene) HHH || | H – C – C = C C 3 H 6 |H Colourless Gas Boiling point- 48 °C at P atm (1bar) Explosive limits (in air) 2 % to 11 % In the main Propene is similar to Propane C 3 H 8 (main constituent of LPG) i.e. the commercially available gases such as Calor Gas, Flogas.

San Carlos Camping Site Disaster The Vapour Pressure Curve for Propene is From this chart, knowing the temperature of the propene, its vapour pressure (i.e. tank internal pressure) can be found.

San Carlos Camping Site Disaster The density of propene falls with increasing temperature showing that its volume expands. The slope of the line is the coefficient of thermal expansion for the liquid, β

San Carlos Camping Site Disaster LIQUID THERMAL EXPANSION CALCULATION Coefficient of volumetric thermal expansion of the liquid Β = 3.13 x /°C [strictly speaking is a ƒ(T)] Volume change with temperature can be approximated linearly by Note for water β = x /°C Expansivity of propene is15 times greater!

San Carlos Camping Site Disaster LIQUID HYDRAULIC PRESSURE RISE CALCULATION K is the Bulk Modulus of Elasticity of the Liquid Compressibility = 1/K K = 1.0 x 10 9 Pa approximately for Propene Can determine the pressure that will be developed in a closed vessel or tank if the free expansion of liquid propene is prevented

San Carlos Camping Site Disaster CONTAINMENT DESCRIPTION The Road Tanker was effectively a long, horizontal, cylindrical pressure vessel carried on the back of the truck. Volume V = 45,000 l = 45 m 3 LengthL = mDiameter D = 2.2 m Maximum allowed fill to 80 % i.e. working volume=45 x 0.8= 36 m 3 ullage=45 x 0.2= 9 m 3 Wall thickness, t of 8 mm (Note t/R ratio is = )

San Carlos Camping Site Disaster PROPERTIES OF TANKER MATERIAL The road tanker was built from a grade of structural steel with the following approximate properties Tensile Strength  TS =730 MN/m 2 Young’s ModulusE=200 GPa Poissons Ratio =0.3 Linear Coeff. Of Thermal Expansionα=12 x / °C

San Carlos Camping Site Disaster STRESS ANALYSIS OF TANKER WALL The maximum stress in the wall will be the hoop or circumferential membrane stress due to the internal propene pressure. Design pressure 18 bar(  max = 248 MN/m 2 ) Test pressure 30 bar(  max = 413 MN/m 2 ) Pressure at which rupture will occur =53.1 barESTIMATE!

San Carlos Camping Site Disaster VESSEL PRESSURE RELIEF There was no pressure relief valve present in the tank. At the time this was not mandatory in Spain though it would be now. While a safety valve would have prevented the accident, such valves have some drawbacks Weakening of shell (due to development of stress concentration at valve hole in shell) Common source of concentrated loads (people tying ropes onto the valve to gain a purchase on the tank) Valve leakage and flammable vapour escapes (many valves leak and the flammable propane vapour could subsequently ignite) Vessel design pressure is twice maximum expected pressure (i.e. no apparent real need for the valve).

San Carlos Camping Site Disaster DESCRIPTION OF INCIDENT At the refinery 23,470 kg of C 3 H 6 was pumped in from a bulk storage tank at a temperature of 4 °C. Assume adiabatic pumping process  Liquid Temp=4 °C  Liquid Density=538 kg/m 3 Filled volume = 23470=43.63 m 3 Percentage fill97 % 538 Legal limit of 80 %19,368 kg of product

San Carlos Camping Site Disaster DESCRIPTION OF INCIDENT WHY was the tank over-filled?  Poor Accuracy of weighing? (not likely)  Poor Accuracy of material data for density? (not likely)  Greed / Economic pressures (Contractor Driver)? (most likely) Daily shipment, 5 days a week at 80 % full.5 x 0.8 = 4 Every Friday transporting air up and down the route (wages, fuel, vehicle depreciation, etc.) TEMPTATION on driver not to abide by 80 % fill rule !!

San Carlos Camping Site Disaster HEAT TRANSFER – PROPENE TEMPERATURE RISE WITH TIME Tanker was travelling along the road. The sun was shining down on it and the outside air was warm. Hence the propene began to heat up. Effective outside air temperature (Spain) T  = 27  C Heat Transfer has three sequential stages: 1.Convection plus radiation from air to tank wall (external H.T) 2.Conduction through tank wall 3.Convection from wall to propylene (internal H.T) Can subsequently determine an overall heat transfer coefficient, U. Assume tank contents are well mixed and at a uniform temperature. Propene temperature with time can be found from

San Carlos Camping Site Disaster HEAT TRANSFER – TEMPERATURE RISE WITH TIME m = 23,470 kg,C p = 2,250 J/kg KA= 86.9 m 2 Take U = 65W/m 2 K Can calculate the rise in temperature of the propene versus time Time Temperature  C

San Carlos Camping Site Disaster HEAT TRANSFER COEFFICIENT Total Thermal Resistance R = External Convection/Radiation + Tank Wall Conduction + Internal Convection Air 27 °C Q (heat) Propene 4°C Geometry: D i = 2.2 mD o = mL = m

San Carlos Camping Site Disaster PRESSURE RISE WITH TIME (RIGID WALLED VESSEL) At t = 0 T = 4 °C,  = 538 kg/m 3 Tank is 97 % full, Ullage = 1.37 m 3 i.e. vapour space Tank internal pressure P = P vp = 7 bar ∆V = 1.37 m 3 =10 °C i.e. when temperature T = 14 °C, the tank is full and P vp = 8.75 bar

San Carlos Camping Site Disaster PRESSURE RISE WITH TIME (RIGID WALLED VESSEL) At some time later, say propene temperature rises to T= 15.4 °C Unrestrained (free) expansion of the liquid if this was possible ∆V = 3.13 x (15.4 – 14) 45 = m 3 This is prevented by the tank walls i.e. the tank walls must develop a pressure against the liquid. = 44.4 bar

San Carlos Camping Site Disaster PRESSURE RISE WITH TIME (RIGID WALLED VESSEL) This hydraulic pressure combines with the vapour pressure At T = 15.4 °CP = = 53.2 bar Maximum membrane stress in tank is the circumferential (hoop) stress in the cylinder =737 MN/m 2 σ > σ TS FRACTURE!!

San Carlos Camping Site Disaster PRESSURE RISE WITH TIME (REAL VESSEL) A more sophisticated analysis would take into account that as the pressure and temperature of the vessel contents rise, it must be taken into account that the vessel will expand or stretch due to: 1. Strain due to the mechanical load (pressure) 2. Strain due to thermal expansion Using this more sophisticated analysis the vessel contents must rise to 17.5 °C to produce a pressure of 53.2 bar (as opposed to 15.4 °C).

San Carlos Camping Site Disaster Graphs of propene temperature, volume and pressure versus time can be drawn to show the inevitability of the incident.

San Carlos Camping Site Disaster CONSEQUENCES OF LOSS OF CONTAINMENT Tank wall ruptured at some point tonnes of liquefied propylene at 53 bar is suddenly depressurised to 1 bar (atmospheric pressure) where it is at a gaseous state. Huge release of stored pressure energy causes: 1. shattering of the tank into fragments (~20 % of energy) 2. blast wave (~80 % of energy) Nothing chemical above this, the first explosion. (A non-flammable liquefied gas such as Nitrogen would behave similarly).

San Carlos Camping Site Disaster VAPOUR FLASHING Calculate of the proportion of liquid propylene that vapourises (flashes off). Considering an adiabatic energy balance - energy consumed in evaporating off vapour is provided by cooling of the liquid fraction. Maximum cooling is from 15.4 °C down to – 48 °C. =7,568 kg Almost a third ! Two phase discharge from vessel !

San Carlos Camping Site Disaster VAPOUR DISPERSION At the time and place of rupture, vapour concentration is close to 100%. - Too rich a mixture to burn and could actually extinguish fires! Density of propylene vapour greater than air. Propylene cloud will disperse away from the point of rupture and as its concentration falls at the edges, it moves into the flammable limits with air. When concentration falls below 11 % propylene / air, an ignition source will start a fire that will consume the 7.5 tonnes of C 3 H 6 as a fireball!.

San Carlos Camping Site Disaster FIRE BALL (BLEVE) CALCULATIONS Model the instantaneous combustion of the escaped vapour. Duration of burning of fire ball is The radiative power of the fire can be calculated from Q R Radiative powerW H C Calorific ValueJ/kg t d Duration of fire balls MMass of fuel in fire ballkg

San Carlos Camping Site Disaster FIRE BALL (BLEVE) CALCULATIONS A point source model of the fire gives the radiative heat flux as  Radiative fluxW/m 2 Q R Radiative power of flameW rDistance from sourcem In turn the thermal radiation dosage can be calculated as LThermal radiation dosage(kW/m 2 ) 1.33 s  Intensity of radiation (radiation flux)kW/m 2 tDuration of exposures

San Carlos Camping Site Disaster FIRE BALL (BLEVE) CALCULATIONS Note the duration of exposure is equal to the duration of the fire ball. Damage to people exposed to the fire can be quantified with Hence can estimate how close people must have been to the fire to have been killed or injured.

San Carlos Camping Site Disaster FATALITIES 215Total Causes of Death Mechanical injury-Blast wave / tank splinters Freezing-Propylene liquid (15.5 t) Asphyxiation-Propylene vapour prior to combustion Burns-Fireball

San Carlos Camping Site Disaster POSSIBLE ACCIDENT PREVENTION STRATEGIES 1.Installation of a pressure relief valve – set to lift at circa 18 bar (vessel design pressure). 2.Thermal insulation on vessel exterior – slow down the heat transfer and temperature rise. 3.Prevention of over-filling - Automatic pump cut out - Financial penalties for overloading 4.Stronger material of construction. - Already  TS ~730 MN/m 2

San Carlos Camping Site Disaster SEQUENCE OF ACTIONSPREVENTION ACTION OverfillingPump cut-out / penalties Temperature riseInsulation of tank Pressure riseRelief valve Stress increaseStronger steel Fracture (Loss of Containment)