Vapour Cloud Explosions

1
FACULTY OF ENGINEERING School of Process
Environment and Materials Engineering

• Vapour Cloud Explosions
• Presentation to
• Engineering Excellence Forum 2008
• 8th April 2008, Abu Dhabi Mens College.
• Professor Gordon E. Andrews
• Professor of Combustion Engineering
• Energy and Resources Research Institute
• School of Process, Environment and Materials
  Engineering
• Faculty of Engineering
• University of Leeds.
• Leeds, UK.

2
FACULTY OF ENGINEERING School of Process
Environment and Materials Engineering

• Vapour Cloud Explosions
• or how to lose a 2B refinery in 2 seconds!
• Vapour cloud explosions occur when there is a
  large leak of hydrocarbons which are heavier than
  air. The leak must continue for some time before
  ignition occurs the cloud size must be large,
  several 100m diameter.
• The leak may occur through a pipe failure, but
  generally occurs through human error, as we will
  see in the examples. Overfilling of storage or
  reactor vessels is another common cause and this
  is entirely due to bad safety management and
  human error!

3
Professor Gordon E. Andrews Vapour Cloud
Explosions

• The incidents we will look at illustrate various
  aspects of human error and safety management
  failures.
• Inadequately designed plant modification
  Flixborough (UK).
• Failure of permit to work system and failure to
  protect against explosion hazards only fire
  hazards were protected Piper Alpha.
• Overfilling of a process vessel (Texas City) or
  overfilling of a storage vessel (Buncefield UK).

4
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Buncefield and human error!
• In the Buncefield incident in Dec. 2005
  Armageddon resulted from the failure of a level
  control system. It was noticed that the level was
  not changing, even though gasoline was still
  being pumped into the tank. No one tool any
  action bad training!
• There was CCTV coverage of the site that showed a
  cloud of vaporising gasoline spreading across the
  site no one looked at the CCTV image bad
  training!
• The leakage was gt100tonnes and no one noticed and
  took remedial action!

5
Professor Gordon E. Andrews Vapour Cloud
Explosions

• The filling of the storage vessel was controlled
  by the refinery NOT by the receiving storage
  depot. It is now recommeded that this is changed.
• Even the HSE and EU thought that one level
  control system was adequate even when the
  consequences of failure are a major disaster. No
  one thought that direct metering of the contents
  was justified inadequate regulation by HSE and
  EU.
• This was a COMAH site and no one had put a
  possible vapour cloud explosion in the risk
  analysis!! The inspection and maintenance of the
  level controllers was inadequate, but so was the
  whole safety regulation.

6
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

7
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Introduction
• In this presentation I want to concentrate on the
  technical aspects of vapour cloud explosion
  rather than the safety management failures that
  led to them.
• The destructive force of large scale explosion
  that are not fully confined, especially those
  that are nominally unconfined, are very difficult
  to explain and are not fully understood today. I
  will review the latest understanding.
• The technical enquiry in the recent Buncefield
  explosion in England does not have a satisfactory
  explanation of how the very large overpressures
  could have occurred. This is gt30 years after
  major incidents of this type started to occur in
  large chemical plants around the world.

8
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Flixborough 1st June 1974
• This was a large vapour cloud explosion that
  followed the failure of a 0.5m diameter temporary
  pipe connecting two distillation vessel.
• The cyclohexane was at 10 bar and 423K, its
  boiling point at atmospheric pressure was 354K.
  The fracture of the pipe caused a massive leak of
  flash vaporising cyclohexane.
• About 30 50 tonnes escaped in about 50 s or 1
  tonne/s. Once the pipe fractured both ends split
  away and two openings in each vessel, 0.7m
  diameter, resulted in offset opposing jets of
  very high velocity. This created very high
  turbulence. There were plenty of ignition sources
  around including a diesel vehicle and a furnace.
• There was also other plant and pipework around
  and the congestion around the leak was a factor
  in the explosion.

9
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
9
Flixborough
The temporary pipe was inadequately designed.
10
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
10
Flixborough 1st June 1974. Complete destruction
of a major chemical plant through
an explosion. 28 dead lucky it was a
Sunday. Normally many more would be on site and
all would have died.
Burgoynes
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PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
11
These photos and the last slide show the effects
of a high static pressure that crushed the
vessels and a road tanker at the centre of the
event. Predicting this is a major
engineering problem even today.
Burgoynes
12
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
12
Gugan Unconfined vapour Cloud explosions 1979,
IChemE
This was 100m from the reactors.
The presssure to cause this damage was estimated
to be in excess of 10 MPa (10 Bar). 15 bar and
higher were estimated closer to the centre of
the explosion.
The static pressure necessary to fracture this
cast iron manhold cover, plus that necessary to
crush the equipment in the previous slides can be
calculated.
13
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
13

• A feature of the crushed vessels was that many of
  them were open to the atmosphere as there was a
  pipe opening. Thus if the pressure had increased
  slowly, there would have been no crushing as the
  pressure would have been the same inside and
  outside.
• It was concluded that the pressure must have
  risen very fast to account for this. Pressure
  rise rates of 100 - 175 bar/s were estimated,
  even though there was no confinement.
• To achieve such rates of pressure rise
  hydrocarbon/air flame speeds of the order of 250
  m/s were estimated. These are very fast turbulent
  flames but NOT detonations (speeds of 2 km/s).
• Explaining the physics of such flames has been a
  challenge, the University of Leeds in ERRI has
  explosion research facilities that can safely
  reproduce such flame speeds. High turbulence
  generated in the explosion is the explanation of
  the very fast flame speeds.

14
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
14
Piper Alpha 15s after the explosion 22.00 6th
July 1988 167 died The Cullen Report Nov. 1990
DoE
15
PREN 2080 Safety ManagementModule Leader Prof.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
15
The Piper Alpha explosion was caused by a leak
of LPG. This occurred as a result of a failure
of the management of the permit to work
system. This is an experimental reconstruction of
the event in a wind tunnel and the concentration
contours determined in terms of the size of the
cloud that was flammable at the
Lower Flammability Limit (LEL). Note the
congestion of pipes and process equipment around
the leak.
30s after the start of leak
100 kg/min
16
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Pipe Alpha was a 2B loss and the UK Government
  lost 10B in oil revenues from the N.Sea.
• This led to a major reorganisation of offshore
  safety and to intensive investigations of how a
  major offshore oil production facility could be
  completely destroyed in an explosion.
• The Pipe Alpha explosion occurred in a congested
  module with side wall venting into the sea. It
  was not an unconfined explosion but a congested
  volume vented explosion. However, it led to vast
  sums of money being spent onto research into
  explosions in congested volumes.
• I will review some of this research, but its
  application to unconfined relatively uncongested
  explosions may be limited.

17
Prof. Gordon E. Andrews Vapour Cloud Explosions
The test method is to cover in plastic film,
fill with stoichiometric premixed gas/air and
to ignite in the centre.
Typical congested volume offshore modules that
have been the main subject of large industrial
explosion investigations.
The geometries studied in the MERGE project.(Mercx
1994) Highly congested.
18
BP Refinery Explosion and Fire Texas City 23.3.05
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BP Refinery Explosion and Fire Texas City 23.3.05
The Guardian 29.6.05
20
Of the 15 killed, 11 worked for companies
servicing a different part of the Refinery
entirely.This photo shows the temporary offices
at the bottom, and a storehouse near the top of
the photo. Several people died in this
temporary office trailer The temporary offices
were within 50 metres of the blast zone
21
It is believed that a car being started may have
provided the source of ignition for the
explosive vapour cloud
22
Guardian 8.12.06
The total cost of this incident is expected to
reach US 1 Billion
BP May 12th 2005
BP has set aside 1.6 Billion to resolve
outstanding Litigation The Guardian 10/11/06
23
Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005
The Guardian 10.5.06
24
Professor Gordon E. Andrews Vapour Cloud
Explosions
40m dia
gt1bar
25m dia.
25
Prof. E. Andrews Vapour Cloud
Explosions
http//www. buncefieldinvestigation .gov.uk 9th
May 2006
26
Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005 Explosion and Fire

• Buncefield was the 5th largest petroleum storage
  depot in the UK with the capacity to store 273
  million litres of petroleum products.
• It was part of the UKs oil pipeline network and
  half its storage capacity was set aside for the
  aviation industry, mainly Gatwick and Heathrow
  and Luton airports. The rest was mainly petrol
  for the SE.
• On 11th December 2005 at 6.03 am the first in a
  series of massive explosions erupted and this was
  heard over 100 miles away. Subsequent explosions
  occurred at 06.27 and 06.28. The explosion
  measured 2.4 on the Richter scale. It was the
  biggest explosion in the UK in peacetime.
• Subsequently to the explosion 20 massive oil
  fires occurred and the flames were 100m high. 16
  Brigades were involved.

27
Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005

• Amazingly no one died and only 43 injuries were
  reported. Nearby office blocks were the most
  severly damaged and had all their windows and
  doors blown inwards. A warehouse 800m away had an
  entire wall removed by the overpressure and at
  least one house had its roof removed. Cars
  parked in nearby streets were set alight by
  radiation from the fires.
• The initial report of the investigation was
  published on 13th July 2006.
• The incident began when at 7pm the night before
  tank 912 started receiving unleaded petrol from a
  filling pipeline. At midnight the terminal was
  shut and stock checks about the site showed
  everything was normal in the tanks. From 3am
  onwards tank 912s level gauge did not change
  despite continued filling and no one noticed and
  it was allowed to be filled further.

28
Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005

• At 5.30 am tank 912 would have been full. At this
  point the high level alarm within the tank should
  have been activated and automatically shut off
  the filling pipe.

BBC News Websit www.bbc.co.uk/news Official
government inquiry http//www.buncefieldinvestigat
ion.gov.uk/reports/initialreport.dpf
29
Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005

• The vapour cloud extended almost 300m to the west
  and north of the tank. The extent of the damage
  meant it was not possible to determine the exact
  source of ignition, but investigators did not
  believe that it was not caused either by the
  driver of a fuel tanker, as had been speculated,
  or by anyone using a mobile phone. It appears to
  have been located in a car park perhaps a
  generator or the nearby pump house.
• The resulting fire engulfed 20 other large tanks
  on the site.
• Findings The level float got stuck and did not
  register as the tank continued to fill.
• The high level alarm should have been triggered
  but didnt.
• Material sourced by Carl Sherwood MSc 2006.

30
Prof. Gordon E. Andrews Vapour Cloud
Explosions

Pump House
Fuji
Tank 12
915
Pre incident layout of of Buncefield depot
and Immediate surroundings
gt912
Northgate
910
Generator
HOSL East site tanks had little damage
Tank 4
3-Com
RO
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
Boundary Way
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Buncefield - Hertfield Oil Storage Ltd.Owned by
Total UK Ltd (60) and Texaco (40)11th December
2005
32
Prof. Gordon E. Andrews Vapour Cloud Explosions
Ignition CCTV evidence indicates that two
explosions occurred with a time interval of one
or two seconds. The second explosion was the more
severe. These could have been separate explosions
within two buildings, but other explanations are
possible. The first ignition is considered to
have occurred at a source in the emergency
generator cabin. This was considered to
be probably a fuel rich ignition. An alternative
scenario is that the first ignition was in the
emergency pump house. One of the empty fuel
tanks suffered an internal explosion. Further
work is required to identify the ignition source.
Buncefield Major Incident Investigation Board,
Explosion Mechanism Advisory Group Report 08/07.
33
Prof. Gordon E. Andrews Vapour Cloud Explosions
The emergency generator cabin
View looking north towards The Northgate and
Fuji Buildings with the Buncefield Deport off
right.
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
34
Prof. Gordon E. Andrews Vapour Cloud Explosions
Emergency generator cabin
close to the SE corner of the Northgate Bld.
viewed from the NE.
The NE corner of 3-Com bld. is on left.
Buncefield Major Incident Investigation
Board, Explosion Mechanism Advisory Group
Report 08/07.
35
Prof. Gordon E. Andrews Vapour Cloud Explosions
Emergency pump house on the HOSL west site, from
its NE corner.
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group
Report 08/07.
36
Professor Gordon E. Andrews Vapour Cloud
Explosions

• These two possible ignition sources are assumed
  to result in a vented explosion so that a highly
  turbulent jet flame emerges from these buildings
  and caused turbulence in the flammable vapour
  cloud, ignition and subsequent fast turbulent
  explosions.
• This is know as bang box ignition of vapour
  clouds.
• This could be the start of the first explosion
  with the subsequent main explosion being fast due
  to the storage vessels being very large scale
  obstacles of about 20m diameter and turbulent
  length scales of 10m.
• This scenario has NOT been considered in the
  Buncefield investigation, where no current
  explanation for the high overpressures has been
  offered and more experimental work is advocated.

37
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 37
Eckhoff
Bang Box type ignition. A primary ignition
explosion followed by the major cloud explosion.
Venting of a polypropylene/air dust explosion
from 5.8m3
38
Prof. Gordon E. Andrews Vapour Cloud Explosions
Damaged car in RO building car park. The car was
facing south. Note the crushing due to high
static pressure.
Buncefield Major Incident Investigation Board,
Explosion Mechanism Advisory Group Report
08/07.
39
Prof. Gordon E. Andrews Vapour Cloud
Explosions
Damaged cars at the SW of the Northgate
building car park Boundary way is in the
background. Behind is the blast damage to the NW
corner of the 3-Com building.
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
40
Prof. Gordon E. Andrews Vapour Cloud Explosions
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
Damaged car at S end of the Fuji Bld. car park,
adjacent to the hedge with the Northgate building
car park.
41

Prof. Gordon E. Andrews Vapour Cloud
Explosions
Map of the area around Buncefield showing
the Approximate over- Pressure isobars. These
are based on the Distribution of damage Caused to
buildings, tanks and vehicles. The red area
contains Strong forensic damage Evidence that the
Overpressure was gt 1
bar
Flow blockage is 60 in this plane.
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
42
Prof. Gordon E. Andrews Vapour Cloud Explosions
ltSteel post in the west car park of the Northgate
building. The post shows abrasion marks on its
south face.
Abrasions to the base gt of a tree in the
Northgate building west car park, viewed
from the south.
The abrasions could be caused by the explosion
moving outwards or by air being sucked inwards to
feed the rich gas
Buncefield Major Incident Investigation Board,
Explosion Mechanism Advisory Group Report
08/07.
43
Prof. Gordon E. Andrews Vapour Cloud Explosions

• There has been relatively few studies of large
  scale low blockage explosions.
• The Texas city refinery and Buncefield gasoline
  storage explosions have emphasised our lack of
  knowledge in the physics of such explosions.
  Texas city had some confinement with smaller
  pipework obstacles, but Buncefield had only a few
  trees in addition to the large storage vessels.
• They both involved large scale obstacles in the
  form of the storage vessel, but there were only a
  few of them so that interactions were low. Hence
  studies of single obstacles of low blockage are
  warranted, where the initial velocity might be
  relatively low, but the flame acceleration is
  high due to very large length scales.

44
Prof. Gordon E. Andrews Vapour Cloud Explosions

• No explosion studies have been carried out with
  length scales of the order of those of large
  petroleum storage vessels and yet this is what is
  required if the high overpressures at Buncefield
  are to be understood. The work of Andrews and
  Phylaktou at Leeds is unique in trying to
  characterise the behaviour of explosions across a
  single obstacle.
• One obstacle with a high velocity upstream flow,
  can create sufficient turbulence to accelerate a
  flame to gtgt100m/s. Also one large scale obstacle
  with a relatively low upstream flow velocity can
  also create sufficient flame acceleration as
  will be shown in the lecture. This is the
  Buncefield situation.

45
Professor Gordon E. Andrews Vapour Cloud
Explosions

• A key uncertainty in the Buncefield explosion
  mechanism advisory group report (08/07) was that
  they could not agree on which of two mechanism of
  explosion occurred
• The expansion of burnt gases in the explosion
  induced an outward flow of unburnt gas/air
  mixture, that interacted with obstructions to
  accelerate the flame. This in turn increased the
  outward explosion induced wind, which created
  more turbulence. The Buncefield report concludes
  that the incident had overpressures in excess of
  1 bar and flame speed of gt400m/s may have
  occurred.
• This is the classic mechanism investigated
  extensively following the Piper Alpha and
  Flixborough explosions.

46
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 46
If there are more than one obstacle the flame can
keep accelerating as the increased wind
velocity ahead of the flame creates more
turbulence at the next obstacle and greater flame
acceleration then occurs.
The problem at Buncefield is the lack of
congestion in the obstacles but the report
does not mention the storage vessels as possible
large scale obstacles. The turbulence downstream
could extend gt200m.
V
47
Prof. E. Andrews Vapour Cloud
Explosions
Looking East across the top of Tank 912. Tank 12
is in the background, across Cherry Tree Lane.
Were the storage vessels the obstacles that
created the turbulence at Buncefield?
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group
Report 08/07.
48
Prof. Gordon E. Andrews Vapour Cloud Explosions
View south along Buncefield Lane showing sooting
of lower parts of telegraph poles and tree. These
trees could have formed obstacles that generated
small scale turbulence in the explosion. Note
that the trees survived.
Buncefield Major Incident Investigation
Board Explosion Mechanism Advisory Group Report
08/07.
49
Professor Gordon E. Andrews Vapour Cloud
Explosions

• 2. The release of the large volume of flammable
  gasoline vapour had an initial ignition that
  caused a massive influx of air or induced wind.
  Every kg of gasoline leaked requires 15 kg of air
  to burn. The precise amount of gasoline that was
  leaked has been estimated in the Buncefield
  report to be gt100 tonnes. The vapour cloud was
  reported as 300m diameter and say 2m high. This
  would have a vapour mass of 100KTonnes and
  require 1.5MTonnes of air to burn. The whole
  explosion event lasted a few seconds and it is
  possible that large inflow wind conditions
  occurred. This is what happened at the Ufa
  explosion in Russia the worlds largest
  explosion incident.
• However, there are no reports of hurricane force
  winds towards the site after the first explosion.

50
Prof. Gordon E. Andrews Vapour Cloud Explosions
G. Makhviladze, U. Leeds, CPD, Explosion
Mitigation, 2007
51
Prof. Gordon E. Andrews Vapour Cloud Explosions
1224 people died
G. Makhviladze, U. Leeds, CPD, Explosion
Mitigation, 2007
52
Prof. Gordon E. Andrews Vapour Cloud Explosions
The trees were felled by the explosion and all
pointed inwards
53
Prof. Gordon E. Andrews Vapour Cloud Explosions
54
Professor Gordon E. Andrews Vapour Cloud
Explosions

• It is possible that at Buncefield and similar
  incidents, that both mechanisms were involved.
  Air inflow induced by the large mass of vapour
  that was leaked after the first ignition would
  occur first and this inflow would create
  turbulence due to flow over obstacles and around
  the storage vessels.
• The initial explosion would then be followed by a
  more severe highly turbulence explosion using the
  turbulence generated by the air inflow and then
  creating high velocity outflow in the now
  premixed gas/air explosion which accelerated over
  the large scale obstacles.
• This is my interpretation of events and is not
  the scenario put forward in the Buncefield
  investigation, which did not consider the storage
  vessels as obstacles or that both events would
  occur.

55
Prof. Gordon E. Andrews Vapour Cloud Explosions
Other incidents similar to Buncefield - 1
56
Prof. Gordon E. Andrews Vapour Cloud Explosions
Other incidents similar to Buncefield - 2
The Buncefield Investigation Third progress
report, 9th May 2006 http//www.buncefieldinvestig
ation.gov.uk
57
Prof. Gordon E. Andrews Vapour Cloud Explosions
Incidents similar to Buncefield - 3
What the Buncefield investigation seems to have
overlooked is that the storage tanks represented
a very large scale obstruction and there was an
array of them. The issue is therefore not one of
a search for obstacles but of a mechanism to
induce a high velocity to flow over
the obstacles. The turbulent length scale was
probably gtgt1m in this case.
58
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Key Problem Area at Buncefield Type Incidents
• High static overpressures required fast turbulent
  flames, as reviewed later. However, turbulence
  generated by outward flow over obstacles required
  high velocities approaching the obstacles how
  does this occur when there is no initial
  turbulence in the vapour leak and there was no
  wind on the day. Flame acceleration mechanisms
  will be reviewed.
• If the storage vessels were obstacles then the
  length scales involved are well outside those of
  any experimental work. However, experimental work
  at Leeds U. and elsewhere on the length scale
  effect does indicate that this could be a major
  influence in the Buncefield explosion.

59
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

60
Prof. Gordon E. Andrews Vapour Cloud Explosions
H.N. Phylaktou, PhD Thesis, U. Leeds, 1993.
P (2?M2)/(1M) Taylor 1946 M Flame speed Mach
No. Flame Speed EST Where E is the
expansion Coefficient ?u/?b Tb/Tu ST
Turbulent burning velocity
Harris, R.J. and Wickens, M.J.,
1989, Understanding vapour Cloud explosions
an Experimental study, Inst. Gas Engineers,
55th Autumn meeting Communication 1408.
Taylor, G.I., 1946 Proc. Roy. Soc, A186, p.273.
An overpressure Of 1 bar requires A flame
speed 300m/s ST 40 m/s ST/SL 100
61
Prof. Gordon E. Andrews Vapour Cloud Explosions
C.L.Gardner, PhD, U. Leeds 1998
Measurements made on a 0.5m dia 12m long end
vented pipe with an obstacle after 3m. Scale of
the obstacle and blockage was varied.
P (2?M2)/(1M) Taylor 1946
62
Prof. Gordon E. Andrews Vapour Cloud Explosions
P (2?M2)/(1M) Taylor 1946
P0.0019 (Sf/10)1.83 bar Catlin, 1991, Combustion
and Flame, 83, 399-411 Catlin and Johnson, 1992,
Combustion and Flame v.88, p.15
Many models assume Pmax Sf2 which is not
correct.
Pmax Sf1.6 Is a close fit to both Eqs.
C.L.Gardner, PhD, U. Leeds 1998
63
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

64
Explosion Mitigation, Fire Flammability and
ExplosionsProfessor G.E. Andrews, ERRI, SPEME,
U. Leeds. 64
HC/air flames have a maximum laminar burning
velocity of 0.4-0.5. This includes gasoline
vapour and cyclohexane. Only ethylene
(0.8m/s) and acetylene (1.6 m/s) are outside this
range. The highest burning velocity is for
hydrogen at 3.5 m/s.
65
Explosion Mitigation, Fire Flammability and
ExplosionsProfessor G.E. Andrews, ERRI, SPEME,
U. Leeds. 65
Flame speed and induced wind
66
Explosion Mitigation, Fire Flammability and
ExplosionsProfessor G.E. Andrews, ERRI, SPEME,
U. Leeds. 66
Andrews and Bradley, Combustion and Flame 1972
The flame speed was measured here in a closed
vessel explosion, using high speed
photography. The expansion of burnt gases
creates a movement of the unburnt gas ahead of
the flame, Sg. This has been measured here
using a fast response velocity probe a hot
wire anemometer. The burning velocity UL Ss - Sg
67
Explosion Mitigation, Fire Flammability and
ExplosionsProfessor G.E. Andrews, ERRI, SPEME,
U. Leeds. 67

• Ss E Su where E is the expansion ration ?u/?f
  Tf/Tu
• Ss Su Sg E Su
• Hence, Sg Su (E-1)
• Thus the induced unburnt gas velocity due to the
  burnt gas flame kernel expansion or explosion
  wind can be calculated from Su and E.
• Also Sg Su(E-1) (Ss/E) (E-1) Ss (1 1/E)
• For 10 methane air E 7.49
• Hence Sg 0.866 Ss for adiabatic conditions
• If the burnt gases are subject to heat losses
  through contact with wall then E is reduced.
  Experimentally we have found that Sg 0.8 Ss
  but in high velocity flames the time for heat
  losses is low and Sg 0.9 Sf, close to the
  adiabatic condition.

68
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

69
Professor Gordon E. Andrews Vapour Cloud
Explosions

• It is clear that laminar spherical explosions of
  hydrocarbon/air mixtures have a flame speed of
  the order of 3 m/s and an induced wind ahead of
  the flame of the order of 2.5 m/s.
• These are much lower that the gt250m/s necessary
  to generated the observed overpressures in large
  vapour cloud explosions.
• Hence, the flames must be accelerated in the
  incidents and there are two mechanisms
• Self acceleration due to cellular flames which
  may continue as flame acceleration to turbulence
  and then to detonation (Karpov Russia, but NOT
  generally agreed)
• Turbulence flame acceleration by interaction of
  the explosion induced wind with obstacles.

70
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 70
Cell size 10mmgt
Andrews PhD Thesis U. Leeds 1972
Cracking (LHS) and fully developed cellular
flames (RHS) for A 9.5 methane air flame in the
later stages of the explosion in a 300mm diameter
cylinder. Window in top half of vessel.
71
Prof. Gordon E. Andrews Vapour Cloud
Explosions
Dia. 12.4m
120m3 balloons
Mike Johnson, British Gas Transco, Leeds U. CPD
Explosion Mitigation 2007.
5. Harris, R.J. and Wickens, M.J., Understanding
Vapour Cloud Explosions An experimental study.
Inst Gas Engineers 55th Autumn Meeting, Comm
1408, 1989. 6. Blackmore, Eyre et al. (Shell)
Refrigerated Gas Research, AGA Transmission
Conf. Atlanta, 1981.
72
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 72

• Bradley (1998) has shown that at the
  self-turbulising transition from cellular to
  self-turbulising, for a wide range of mixtures,
  the turbulent to laminar burning velocity ratio
  is given by
• UT/UL 3.1 or ST/SL 3.1 as the density ratio
  is the same.
• Thus the maximum cellular flame acceleration is
  3. For natural gas this would give a flame speed
  of 8 m/s close to the 7 m/s observed by Harris
  and Wickens (1989).
• In the self turbulising regime
• UT/UL 3.1 (Pe/Pet) for PegtPet
• Now Pe R/dL and hence UT/UL 3.1 R/Rt
• Hence the turbulent burning velocity continuous
  to increase with radius of the flame after the
  critical cellular to turbulence transition.

D. Bradley, Proc. 2nd Int. Sem. Fire Exp.
Haz.,1998, p.51-59
73
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 73

• Very large spillages of flammable gases have been
  studied by Karpov et al. Proc. 1st Int. Se. Fire
  and Exp. Haz. 1995
• Moscow p.429, this work was undertaken after Ufa
  in 1992.
• They found that the flame velocity increased
  continuously and they studied this regime of self
  turbulised flames through to acceleration to
  spherical detonation.
• The maximum flame velocity, Wmax, was
  proportional to the initial volume of the mixture
  Vo to the power 1/6
• Wmax Vo1/6
• An acceleration factor f (Wmax Uns) / (Uns)
• Where Un laminar flame velocity with no
  curvature
• s ?u/?b
• The experimental results are shown in the next
  slide.

74
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 74
f (Wmax Uns) / (Uns) (UT/UL 1)
Karpov et al., Proc. 1st Int. Sem. Fire Exp.
Haz. Moscow, 1995.
lt Wmax Uns(10.01 Re0.5)
Re Un Ro / ? Ro initial raidus of the gas
cloud.
75
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 75

• Wmax Uns(10.01 Re0.5)
• This correlation equation yields the following
  critical dimension Rc of a cloud when the flame
  velocity attains the sound speed when
  detonation would occur.
• Rc 70m for hydrogen air
• Rc 3.5 km for propane air
• Rc 5 km for methane air
• For mixtures with oxygen
• Rc 3.5m for hydrogen oxygen
• Rc 10m for methane oxygen
• The acceleration to detonation will not occur in
  any process vessel as these are never large than
  about 10m diameter (524 m3)
• At Buncefield the gasoline vapour cloud was 300m
  diameter and this is 1/20 of the distance
  required for this mechanism to account for the
  events that occurred. However, self acceleration
  would result in significant flame acceleration to
  at least x3 of the laminar flame speed and this
  was the source of the wind velocity into the
  obstacles.

Note that in Russia they had a leak in the
trans-siberia natural gas pipe line the led to a
gas cloud of gt5km size and a devastating
explosion ignited by a passing train on which
1200 died.
76
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

77
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 77

• How does turbulence increase the combustion
  burning rate?

U m/s
Mean Velocity
u
UT/UL AT/AL Assuming that UL is constant for
all of AT
Turbulence intensity u/U
Time s
Instantaneous flame Area AT
Turbulent flame Surface extended by the action
of turbulent velocity fluctuations.
Laminar flame Flat area AL
78
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 78
Lefebvre, Gas Turbine Combustion, Hemisphere,
p.54, 1983.
Turbulent flames burning in the wake of an
obstacle in a gas turbine reheat
burner configuration. The edge of the flame
shows the extended surface area due to
turbulence fluctuations. Gas turbine
main combustion is high turbulence like that for
u30.5 m/s
u 3.1 m/s
u30.5 m/s
79
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds 79
Phylaktou and Andrews IChemE Symp. Ser. No. 130
1992
Experimental data supports the
simple correlation ß ST/SL 1 2 u/SL
80
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou, Alexiou and Andrews, U.
Leeds Interaction of fast explosions with an
abstacle HSE Offshore Technology Report OTI 94
625, HSE Books, 1995, ISBN 0-7176-0934-0
The Le and? terms are small and 1 for HC/air
81
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou, H.N., 1993, PhD Thesis, U. Leeds.
Data range ST 0.38-116 SL0.16-3.61 u 0.07-68 all
in m/s. L 145mm Le 0.42.9 (?/?a) 0.64-1.02
Turbulent burning velocities gt50 m/s are possible
for HC/air. This is x100 of the laminar
burning velocity
Note ST L0.33
The Phylaktou and Andrews correlation
STSL20.9u0.79L0.33SL0.47Le-0.42(?/?a)0.61 (R2
96)
82
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou, H.N., 1993, PhD Thesis, U.Leeds.
Data range ST 0.38-116 SL0.16-3.61 u 0.07-68 all
in m/s. L 145mm Le 0.42.9 (?/?a) 0.64-1.02
(ST/SL-1) 0.67(u/SL)0.47RL0.31Le0.4(?/?a)0.95
(R2 93)
83
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou and Andrews 1995, Trans. IChemE Vol.73,
PartB, p.3-10
The constant C in the correlation is different
for each data set. 0.67 for the Leeds data. The
functional dependences do not change, but the
differences in the measurement techniques
change the value of C.
RL uL/? ?kinematic viscosity
(ST/SL-1) C(u/SL)0.47RL0.31Le0.4(?/?a)0.95
84
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou and Andrews Trans IChemE Vol. 73, Part
B, 1995.
Hjertager, B.H. 1993 Gas explosions
in Obstructed vessels Leeds CPD course 1993
(ST/SL-1) 0.67(u/SL)0.47RL0.31Le0.4(?/?a)0.95
85
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou and Andrews Trans IChemE Vol. 73, Part
B, 1995.
Hjertager, B.H. 1993 Gas explosions
in Obstructed vessels Leeds CPD course 1993
(ST/SL-1) 0.67(u/SL)0.47RL0.31Le0.4(?/?a)0.95
86
Prof. Gordon E. Andrews Vapour Cloud Explosions
O. Gulder, 1990 Turbulent premixed flame
propagation models for Different combustion
regimes, 23rd Int. Combustion Symp., p.473
Phylaktou, Alexiou and Andrews, U.
Leeds Interaction of fast explosions with an
abstacle HSE Offshore Technology Report OTI 94
625, HSE Books, 1995 ISBN 0-7176-0934-0
ST/SL 0.62(u/SL)0.5RL0.25
Gulder
87
Prof. Gordon E. Andrews Vapour Cloud Explosions
Abdel-Gayed, R.Ggt, Bradley, D and Lawes, M.,
1987, Turbulent burning velocities a General
correlation in terms of straining rates. Proc.
Roy. Soc. A414, p.389. Bray, K.N.C., 1990,
Studies of turbulent burning velocity, Proc. Roy.
Soc., A431, p.315.
Phylaktou, Alexiou and Andrews, U. Leeds,
Interaction of fast explosions with an
abstacle HSE Offshore Technology Report OTI 94
625, HSE Books, 1995, ISBN 0-7176-0934-0
88
Prof. Gordon E. Andrews Vapour Cloud Explosions
Phylaktou, Alexiou and Andrews, U.
Leeds Interaction of fast explosions with an
abstacle HSE Offshore Technology Report OTI 94
625, HSE Books, 1995, ISBN 0-7176-0934-0
(ST/SL-1) 0.67(u/SL)0.47RL0.31
This is used in UVCE modelling
Bradley used a fan stirred explosion vessel with
no variation of L.
The turbulence was isotropic which never occurs
in obstacle flow
89
Prof. Gordon E. Andrews Vapour Cloud
Explosions
Bradley, D., Lau, A.K.C. and Lawes, M., Phil.
Trans. Roy. Soc., A338, p.359
ltUT/UL12u/UL
The key parameters are KaLe and RL/Le As well
as u/SL But for HC Le1 and so the
key parameters are Ka and RL as well as u/UL.
ST/SL
Quench KaLe6
u/SL
20
15
10
5
90
Prof. Gordon E. Andrews Vapour Cloud
Explosions
Puttock, Yardley and Cresswell, Shell Global
Solutions, J. Loss Prevention in the Process
Industries, V. 13, 419, 1999
0.1
0.21
UT/u
KaLe
Solid lines expts. Bradley 1984 For lower u and
the Computations of Bradley 1992 For high u.
0.14
0.43
0.3
0.63
1.0
Dashed lines are UT/UL1.60(KaMa)-1/3u/UL using
LeMa/4.1 in UT/u 0.88(KaLe)-0.3 with -0.3 gt
-0.33 or -1/3
UT/UL12u/UL
UTL0.167
u/UL
MaMarkstein No.
91
Prof. Gordon E. Andrews Vapour Cloud
Explosions
Bradley, D., Lau, A.K.C. and Lawes, M., Phil.
Trans. Roy. Soc., A338, p.359
Form of UT correlation must Be UT UL const.
u As UTUL when u0. However, if ugtgtUL then UT
const u And UT/u 0.88(kaLe)-0.3
UT/u 0.88(KaLe)-0.3
Each symbol is a different u/UL range. Overall
range 0 - gt30
92
Prof. Gordon E. Andrews Vapour Cloud
Explosions
A microscale Karlovitz number can also be defined
as Ka (dL/SL)/(?/u) Where ? Taylor
microscale and R??u/? which is related to RL by
R?6.36 RL0.5 Also ?2/L40.4 ?/u The constants
in the above are taken from Abdel-Gayed and
Bradley Proc. Roy. Soc. A301, p.1.
Ka Karlovitz Number Ka Chemical
Lifetime/ Turbulent lifetime
Ka(dL/SL)/(L/u) dLlaminar flame
thickness C?/SL
Ka (udL)/(?SL) Ka (uC?/SL)/(40.4L?/u0.5
SL) C/40.40.5 (u/SL)2RL-0.5 Ka
0.157C(u/SL)2RL-0.5 C (u/SL)2 R?-1 The Eq.
for Ka f RL was first used by Abdel-Gayed and
Bradley with C1, which is incorrect as C30 as
dL1mm and ?/SL0.035mm for stoichiometric
methane/air.
93
Prof. Gordon E. Andrews Vapour Cloud
Explosions

• Ka 0.157C(u/SL)2RL-0.5 C (u/SL)2 R?-1
• The Bradley et al. correlation at low Ka gives a
  high dependance of UT on u. This occurs if SL is
  low, such as for lean combustion in gas turbines
  or lean explosions. Also if RL is high then Ka is
  low.
• UT/u 0.88(KaLe)-0.3
• For hydrocarbons Le 1 and if we substitue the
  above Eq for Ka with C1 then
• UT/u 1.53 (u/UL)-0.6RL0.15
• This equation has been used by Catlin (British
  Gas Transco- Centrica) for explosion scaling.
  It predicts that
• UT L0.15u0.55UL0.6
• If Pmax Sf2 then Pmax L0.3, which is a lower
  dependence than found experimentally by a factor
  of 2.

94
Prof. Gordon E. Andrews Vapour Cloud Explosions

• (ST/SL-1) 0.67(u/SL)0.47RL0.31 (Phylaktou and
  Andrews)
• This can be simplifed by substituting the terms
  in RL and using the kinematic viscosity of air
  (15.6 x 10-6) to give
• ST/SL 1 20.7 u0.78 SL-0.47L0.31
• L 10mm this reverts to 4.97 u0.78 SL-0.47
• L 100mm the constant in the above Eq. becomes
  10.1
• And for L1m it is obviously 20.7.
• The experimental data for Ut/Ul is mainly for
  small scale turbulence and the constant is likely
  to be 4.97, as shown by the comparison with
  experiments in the next slide.
• However, in large scale explosions such as at
  Buncefield, where the obstacles were as large as
  the rows of tanks of 25m diameter with L gt1m!

95
Unconfined Vapour Cloud ExplosionsProf. Gordon
E. Andrews, ERRI, SPEME, U. Leeds 95
1000gt

?

Phylaktou and Andrews IChemE Symp. Ser. No. 130
1992
Experimental data supports the simple
correlation ß ST/SL 1 2 u/SL
However, this has no influence of L.
Overlaid on the expt.results are the
Predictions of the correlation ST/SL1 20.7
u0.78 SL-0.47L0.31 with 0.45m/s laminar burning
velocity for 4 length scales from 1mm to 1m. Most
of the literature data for UT is at small L.
?

ST/SL 100 For L1m u/SL10 u4m/s
?

?
ST/SL


?
L 1m 0.1m 10mm 1mm
?


?
?


u/SL
96
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 96
Pressure loss is the energy loss that is
dissipated as turbulence energy
Turbulent kinetic energy
C is a calibration const.
Blockage B or BR B 1 A2/A1
97
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 97
The calibration of the turbulence constant can
be made if measurements of turbulence and
pressure loss for grid plates are made.
Phylaktou and Andrews, Prediction of the Maximum
turbulence intensities generated By grid-plate
obstacles in explosion Induced flow. 25th Symp.
On Combustion The Combustion Institute, 1994, p.
103-110
A1
A2
U1
98
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 98
99
PREN3170 Advanced Aerospace Propulsion
Professor Gordon E. Andrews, ERRI, SPEME, U.
Leeds 99
Phylaktou and Andrews, Prediction of the Maximum
turbulence intensities generated By grid-plate
obstacles in explosion Induced flow. 25th Symp.
On Combustion The Combustion Institute, 1994, p.
103-110
100
PREN3520 Gas and Dust Explosion ProtectionProf.
Gordon E. Andrews, ERRI, SPEME, U. Leeds
100
Phylaktou, Alexiou and Andrews, U.
Leeds Interaction of fast explosions with an
obstacle HSE OTI 94 625, HSE Books, 1995 ISBN
0-7176-0934-0
Gas velocity measured using the obstacle as an
orifice plate and measuring ?P.
Gas Velocity 0.9 Flame Speed This is close to
the adiabatic value of 0.87 from Sg (E-1)/E
ST where ST is the turbulent flame speed.
As turbulence makes the flame propagate faster
the wind ahead of it
increases in
proportion.
101
Prof. Gordon E. Andrews Vapour Cloud Explosions

• There has been relatively few studies of large
  scale low blockage explosions.
• The Flixborough, Texas city and the Buncefield
  explosions involved large scale obstacles in the
  form of the storage or process vessels, but there
  were only a few of them so that interactions were
  low. Hence studies of single obstacles of low
  blockage where the initial velocity might be
  relatively low, but the flame acceleration is
  high due to very large length scales.
• No studies have been carried out with length
  scales of the order of those of large petroleum
  storage vessels and yet this is what is required
  if the high overpressures at Buncefield are to be
  understood.
• Andrews, Phylaktou and Gardner at Leeds U. Have
  extensively investigated the scale effect in
  several experimental configurations.

102
Prof. Gordon E. Andrews Vapour Cloud Explosions
The two closed vessel test rigs
had different flame speeds upstream of the
obstacle which is why the ST are lower on the
larger rig. Strong influence of length scale.
(ST/SL-1) 0.67(u/SL)0.47RL0.31 Part of the
data set that gave the above correlation.
103
Prof. Gordon E. Andrews Vapour Cloud Explosions
Rig 3 0.5m dia
ltRig 2 0.162 dia
ltRig 1 0.076 dia
40 m3 2.5 dia 8m long
Rig 4 (not shown) 1.5m dia and 6m long 10 m3
The University of Leeds, Explosion
laboratory Energy and Resources Research
Institute (ERRI)
104
40 m3 Dump
Rigs 12 Connections
Rig 4 2.5m dia 6m long
Rig 3 connects here
Part of Rig 3 0.5m dia. 12m long
105
Prof. Gordon E. Andrews Vapour Cloud Explosions
C.L. Gardner, PhD Thesis, U. Leeds, 1998.
Rig 3 0.5m dia
The larger gas velocities in Rig 3 were due
to higher flame speeds at the greater pipe length
scale
106
Prof. Gordon E. Andrews Vapour Cloud
Explosions
C.L. Gardner, PhD Thesis, U. Leeds, 1998.
The turbulent length scale was investigated
using different sized obstacles with the same
blockage. Large scales were one bar or one
orifice or one disc. Smaller scales were multiple
bars or holes in grid plates.
107
Prof. Gordon E. Andrews Vapour Cloud Explosions
C.L. Gardner, PhD Thesis, U. Leeds, 1998.
C.L. Gardner, H.N. Phylaktou and G.E.
Andrews,1998, IChemE Symp. Ser. No. 144
108
Prof. Gordon E. Andrews Vapour Cloud Explosions
C.L. Gardner, H.N. Phylaktou and G.E.
Andrews,1998, IChemE Symp. Ser. No. 144 .
109
Prof. Gordon E. Andrews Vapour Cloud Explosions
C.L. Gardner, H.N. Phylaktou and G.E.
Andrews, 1998, IChemE Symp. Ser. No. 144
110
Prof. Gordon E. Andrews Vapour Cloud Explosions
Compare with (ST/SL-1) 0.67(u/SL)0.47RL0.31Le0.
4(?/?a)0.95 same RL0.31
ST Sf/E where Sf is the measured flame speed
C.L. Gardner, PhD Thesis, U. Leeds, 1998.
111
Prof. Gordon E. Andrews Vapour Cloud Explosions
ST L0.39 If Pmax Sf2 Then Pmax L0.78 This
is one of the very few experimental variations
of length scale. Used large tube burners. x10
variation in scale.
Phylaktou and Andrews Trans IChemE Vol. 73, Part
B, 1995.
112
Prof. Gordon E. Andrews Vapour Cloud Explosions
The EU MERGE project, 1994 Mercx, Final Report
Contract STEP-CT-0111 (SSMA)
SL m/s 0.4 0.45 0.45
Number of rows of obstacles
C.L. Gardner, PhD Thesis, U. Leeds, 1998.
L D/2
113
Prof. Gordon E. Andrews Vapour Cloud Explosions
The EU MERGE project, 1994, Mercx, Final Report
Contract STEP-CT-0111 (SSMA)
Note the very large Sensitivity to n and SL n
varied 8-30 SL varied 0.40.65 mainly.
Mercx assumed Pmax Sf2 Leeds work for one
obstacle gives Pmax L0.62 SL1.9
Pmax 3.906 x 10-4 n3.1BR1.93D0.72SL3.12 L D/2
so Pmax L0.72 if other terms are const
114
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Contents
• Vapour Cloud Explosion and related incidents
• Flixborough, Pipe Alpha, Texas City, Buncefield,
  Ufa.
• High static overpressures require fast turbulent
  flames
• Laminar flames are slow
• Self acceleration of laminar explosion flames by
  x3.
• Review of the experimental influence of length
  scale on turbulent burning velocity, flame speed
  and overpressure.
• The Buncefield incident little congestion but
  very large length scales, can turbulent burning
  velocity explain this recent incident.

115
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Conclusion in relation to a Buncefield type
  incident
• L could be as large as 1-10m if the 25m diameter
  storage vessels were the obstacles.
• If the overpressure was 1 bar, as indicated, then
  a flame speed of 300m/s is required or a
  turbulent burning velocity of 43 m/s which for
  a SL 0.4 gives ST/SL 100.
• If it is assumed that self acceleration accounts
  for ST/SL3
• Then turbulence has to account for an increase by
  a factor of 33.
• (ST/SL-1) 0.67(u/SL)0.47RL0.31
• ST/SL 1 20.7 u0.78 SL-0.47L0.31 and for
  ST/SL 33 and L1m this gives u 1.0 m/s. This
  is a relatively low u and the flame speed after
  self acceleration will be 9m/s with an induced
  wind of 8 m/s. Thus u/U 0.125 and for sharp
  edged obstacles this requires a blockage of 20
  or 50 for rounded obstacles. The blockage was
  50-60 at Buncefield based on the distance
  between storage vessels.

116
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Thus where large length scales apply a single
  line of large obstacles of low blockage can, in
  combination with self acceleration of about a
  factor of 3, account for the large overpressures.
• It is not necessary under these circumstances to
  have high turbulence.
• If the above calculation was repeated for L0.01m
  the the required turbulence would be 6.26 m/s and
  u/U would be 78. This would required a blockage
  of about 70 for sharp obstacles and gt90 for
  round and this is too large for a single row of
  obstacles such as the tanks at Buncefield, as
  they were not that close together.
• Thus if the scale was small, congestion would be
  required to give interactions between obstacles.
  This was not present
• at Buncefield.

117
Professor Gordon E. Andrews Vapour Cloud
Explosions

• Thus the Buncefield incident can be explained by
  a combination of flame self acceleration and the
  turbulence induced by the flow ahead of the self
  accelerated flame interacting with the storage
  tanks as a single row of very large scale
  obstacles.
• There is no need to invoke the Ufa scenario of a
  large inward wind and the spillage was probably
  not large enough for this mechanism to be
  significant.
• Also it is not necessary for there to have been a
  bang box ignition source.

118
Professor Gordon E. Andrews Vapour Cloud
Explosions

• These conclusions are crucially dependent on
  the Phylaktou and Andrews turbulent burning
  velocity correlation, as other correlations such
  as that of Bradley require higher turbulence
  levels to give the required flame speeds.
• The MERGE data and that of Khitrin and
  Goldenberg give length scale exponents for the
  overpressure of 0.72 and 0.78 respectively,
  compared with the Phylaktou and Andrews exponent
  of 0.62.
• The MERGE data was for large arrays of pipes
  with a minimum number of interacting obstacles of
  8. The Khitrin and Goldenberg data was for tube
  flow turbulence and very large bunsen burner
  flames. Only the work of Phylaktou and Andrews
  was for single obstacles with the length scale
  increased at constant blockage and hence constant
  u.

119
Professor Gordon E. Andrews Vapour Cloud
Explosions

• All of these experimentally based length
  scale exponents are gt than those based on theory.
  
• Bradley et al and the Shell approach both use
  an exponent of 0.3 and the Gulder exponent is
  0.48.
• The 0.3 exponent has been used by the
  Buncefield explosion investigation team together
  with the Bradley turbulent burning velocity
  correlation. This approach failed to predict the
  observed overpressures.
• The present work indicates that the Phlaktou
  and Andrews turbulent burning velocity
  correlation together with the assumption of an
  initial self acceleration of the flame to x3 of
  the laminar flame speed, to give 8m/s explosion
  wind approaching the large scale storage vessel
  obstacles can explain the observed damage in the
  incident.