San Carlos Camping Site Disaster
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 660.28)
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
San Carlos Camping Site Disaster
San Carlos Camping Site Disaster
San Carlos Camping Site Disaster
Map of Location of Accident
San Carlos Camping Site Disaster
PRODUCT DESCRIPTION
PROPENE (also known as Propylene)
H
|
H– C –
|
H
H
|
C
H
|
=
C
C3H6
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.
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 10 -3 /°C
[strictly speaking is a ƒ(T)]
Volume change with temperature can be approximated linearly by
V   T V
Note for water
β = 0.207 x 10-3 /°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 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
P   K
V
 P  K  T
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.
L
D
Volume V = 45,000 l = 45 m3
Length L = 10.37 m
Maximum allowed fill to 80 %
i.e.
working volume =
ullage
=
Wall thickness, t of 8 mm
Diameter D = 2.2 m
45 x 0.8 = 36 m3
45 x 0.2 = 9 m3
(Note t/R ratio is = 0.00073)
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
=
Young’s ModulusE
=
200 GPa
Poissons Ratio

=
0.3
Linear Coeff. Of
Thermal Expansion
α
=
12 x 10-6 / °C
730 MN/m2
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/m2)
Test pressure 30 bar
(max = 413 MN/m2)
Pressure at which rupture will occur
PRUPTURE
=
53.1 bar
 TS t
6
730 x 10 0.008


R
1.1
ESTIMATE!
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 C3H6 was pumped in from a bulk storage tank at a
temperature of 4 °C.
Assume adiabatic pumping process 
Liquid Temp

Liquid Density =
Filled volume = 23470
538
Legal limit of 80 %
=
43.63 m3
=
4 °C
538 kg/m3
Percentage fill
19,368 kg of product
97 %
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
T  (Ti  T ) e
U A
t
m Cp
 T
San Carlos Camping Site Disaster
HEAT TRANSFER – TEMPERATURE RISE WITH TIME
m = 23,470 kg,
A= 86.9 m2
Cp = 2,250 J/kg K
Take U = 65W/m2K
Can calculate the rise in temperature of the propene versus time
Time
0
1
2
3
4
5
Temperature C
4
11.35
16.36
19.6
22.1
23.6
San Carlos Camping Site Disaster
HEAT TRANSFER COEFFICIENT
Total Thermal Resistance
R = External Convection/Radiation + Tank Wall Conduction + Internal Convection
In( Do / Di )
1
1
R


U o Ao
2 k L
hi Ai
Air 27 °C
Q (heat)
Geometry: Di = 2.2 m
Do = 2.216 m
Propene 4°C
L = 10.37 m
San Carlos Camping Site Disaster
PRESSURE RISE WITH TIME (RIGID WALLED
VESSEL)
At
t=0
T = 4 °C,
 = 538 kg/m3
Ullage = 1.37 m3 i.e. vapour space
Tank is 97 % full,
Tank internal pressure
P = Pvp = 7 bar
∆V = 1.37 m3
1.37
 
3.13 x 10 3 43.63
=
10 °C
i.e. when temperature T = 14 °C, the tank is full and Pvp = 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 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.
P  K
V
0.193
 1.035 x 10 9
V
45
= 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 °C
P
= 8.75 + 44.4 = 53.2 bar
Maximum membrane stress in tank is the circumferential (hoop) stress in the
cylinder
P R 53.2 x 10 5. 1.108
 

t
0.008
=
737 MN/m2
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
P
PR
  v (T  Ti ) 
(5  4v)  3 (T  Ti )
K
2t E
PR
P
(5  4v) 
 Ti ( v  3 )
2t E
K
T
 v  3
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.
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).
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.
m L C P 15.4   48  mV h fg
mL  mV  23470 kg
23,470 Cp T
mV 
h fg  Cp T
=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 C3H6 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
t d  0.46 M 0.333
td
M
Duration of fire ball
Mass of fuel in fire ball
s
kg
The radiative power of the fire can be calculated from
M HC
QR  0.3
td
QR
HC
Radiative power
Calorific Value
W
J/kg
4
L  3 t
San Carlos Camping Site Disaster
FIRE BALL (BLEVE) CALCULATIONS
A point source model of the fire gives the radiative heat flux as


QR
r
QR
4 r 2
Radiative flux
Radiative power of flame
Distance from source
W/m2
W
m
In turn the thermal radiation dosage can be calculated as
4
L  3 t
L

t
Thermal radiation dosage
Intensity of radiation (radiation flux)
Duration of exposure
(kW/m2)1.33s
kW/m2
s
4
L  3 t
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
Dosage, L
(kW/m2)1.33s
90
100
1000
1200
2100
2500
6500
Severity of Burns
-
Fatalities
-
Pain Threshold
First Degree
1%
Second Degree
50 %
Third Degree
99 %
Hence can estimate how close people must have been to the fire to have
been killed or injured.
San Carlos Camping Site Disaster
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
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/m2
San Carlos Camping Site Disaster
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)