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Title: 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
11
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
19
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
31
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.

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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.
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