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

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


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.


A very good description of the incident is given in the bookMAJOR CHEMICAL HAZARDS

AUTHOR: V.C. MARSHALL

(In the UCC Library under Classification 660.28)

The course notes for PE 2002 should also be consulted especially the material dealing with pressure vessels.


An illustration of the truck in question


Picture shows remains of rear axle of truck





Map of Location of Accident


PRODUCT DESCRIPTIONPROPENE (also known as Propylene)

H H H| | |

H – C – C = C C3H6| H H

Colourless Gas

Boiling point - 48 °C at Patm (1bar)

Explosive limits (in air) 2 % to 11 %

In the main Propene is similar to Propane C3H8 (main constituent of LPG) i.e. the commercially available gases such as Calor Gas, Flogas.


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.


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


LIQUID THERMAL EXPANSION CALCULATIONCoefficient of volumetric thermal expansion of the liquid

Β = 3.13 x 10 -3 /°C [strictly speaking is a ƒ(T)]

Volume change with temperature can be approximated linearly by

Note for water β = 0.207 x 10-3 /°C

Expansivity of propene is15 times greater!

VTV


LIQUID HYDRAULIC PRESSURE RISE CALCULATION

K is the Bulk Modulus of Elasticity of the Liquid

Compressibility = 1/K

K = 1.0 x 109 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

V

VKP

TKP


CONTAINMENT DESCRIPTIONThe Road Tanker was effectively a long, horizontal, cylindrical pressure vessel carried on the back of the truck.

Volume V = 45,000 l = 45 m3

Length L = 10.37 m Diameter D = 2.2 m

Maximum allowed fill to 80 %

i.e. working volume = 45 x 0.8 = 36 m3

ullage = 45 x 0.2 = 9 m3

Wall thickness, t of 8 mm (Note t/R ratio is = 0.00073)

L

D


PROPERTIES OF TANKER MATERIALThe road tanker was built from a grade of structural steel with the following approximate properties

Tensile Strength TS = 730 MN/m2

Young’s Modulus E = 200 GPa

Poissons Ratio = 0.3

Linear Coeff. Of

Thermal Expansion α = 12 x 10-6 / °C


STRESS ANALYSIS OF TANKER WALLThe 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/m2)

Test pressure 30 bar (max = 413 MN/m2)

Pressure at which rupture will occur

= 53.1 bar ESTIMATE!

1.1

008.010730 6x

R

tP TS

RUPTURE


VESSEL PRESSURE RELIEFThere 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).


DESCRIPTION OF INCIDENTAt the refinery 23,470 kg of C3H6 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/m3

Filled volume = 23470 = 43.63 m3Percentage fill 97 %

538

Legal limit of 80 % 19,368 kg of product


DESCRIPTION OF INCIDENTWHY 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 !!


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 wall3. 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

TeTTTt

Cm

AU

ip)(


HEAT TRANSFER – TEMPERATURE RISE WITH TIME

m = 23,470 kg, Cp = 2,250 J/kg K A= 86.9 m2

Take U = 65W/m2K

Can calculate the rise in temperature of the propene versus time

Time Temperature C

0

1

2

3

4

5

4

11.35

16.36

19.6

22.1

23.6


HEAT TRANSFER COEFFICIENTTotal Thermal Resistance

R = External Convection/Radiation + Tank Wall Conduction + Internal Convection

Air 27 °C

Q (heat) Propene 4°C

Geometry: Di = 2.2 mDo = 2.216 m L = 10.37 m

ii

io

oo AhLk

DDIn

AUR

1

2

)/(1


PRESSURE RISE WITH TIME (RIGID WALLED VESSEL)

At t = 0 T = 4 °C, = 538 kg/m3 Tank is 97 % full, Ullage = 1.37 m3 i.e. vapour space

Tank internal pressure P = Pvp = 7 bar

∆V = 1.37 m3

= 10 °C

i.e. when temperature T = 14 °C, the tank is full and Pvp = 8.75 bar

63.431013.3

37.13x

San Carlos Camping Site DisasterPRESSURE 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 10-3 (15.4 – 14) 45 = 0.193 m3

This is prevented by the tank walls i.e. the tank walls must develop a pressure against the liquid.

= 44.4 bar45

193.010035.1 9x

V

VKP


PRESSURE RISE WITH TIME (RIGID WALLED VESSEL) This hydraulic pressure combines with the vapour pressure

At T = 15.4 °C P = 8.75 + 44.4 = 53.2 bar

Maximum membrane stress in tank is the circumferential (hoop) stress in the cylinder

= 737 MN/m2

σ > σ TS FRACTURE!!

008.0

108.1.102.53 5x

t

RP


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).

)(3)45(2

)( iiv TTvEt

PRTT

K

P

3

)3()45(2

v

viTK

Pv

Et

PR

T


Graphs of propene temperature, volume and pressure versus time can be drawn to show the inevitability of the incident.


CONSEQUENCES OF LOSS OF CONTAINMENT

Tank wall ruptured at some point.

23.5 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).


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 !

fgVPL hmCm 484.15

kgmm VL 23470

TCph

TCpm

fgV

470,23


VAPOUR DISPERSIONAt 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 C3H6 as a fireball!.


FIRE BALL (BLEVE) CALCULATIONSModel the instantaneous combustion of the escaped vapour. Duration of burning of fire ball is

The radiative power of the fire can be calculated from

QR Radiative power W

HC Calorific Value J/kg

333.046.0 Mtd

td Duration of fire ball sM Mass of fuel in fire ball kg

d

CR t

HMQ 3.0


FIRE BALL (BLEVE) CALCULATIONSA point source model of the fire gives the radiative heat flux as

Radiative flux W/m2

QR Radiative power of flame W

r Distance from source m

In turn the thermal radiation dosage can be calculated as

L Thermal radiation dosage (kW/m2)1.33s

Intensity of radiation (radiation flux) kW/m2

t Duration of exposure s

24 r

QR

tL 3

4

tL 3

4


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 with

Hence can estimate how close people must have been to the fire to have been killed or injured.

tL 3

4

Dosage, L Severity of Burns Fatalities

(kW/m2)1.33s - - 90 Pain Threshold 100 First Degree 1000 1 % 1200 Second Degree 2100 50 % 2500 Third Degree 6500 99 %


FATALITIES

215 Total

Causes of Death

Mechanical injury - Blast wave / tank splinters

Freezing - Propylene liquid (15.5 t)

Asphyxiation - Propylene vapour prior to combustion

Burns - Fireball


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/m2


SEQUENCE OF ACTIONS PREVENTION ACTION Overfilling Pump cut-out / penalties

Temperature rise Insulation of tank

Pressure rise Relief valve

Stress increase Stronger steel

Fracture(Loss of Containment)

Documents

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