1336227 Flashover, Backdraft, And Fire Gas Ignitions
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Transcript of 1336227 Flashover, Backdraft, And Fire Gas Ignitions
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GRA 5.8flashover, backdraught
and fire gas ignitions
Fire and Rescue ServiceOperational Guidance
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Generic Risk Assessment 5.8
Flashover, backdraught and firegas ignitions
August 2009
London: TSO
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Published with the permission of the Department for Communities and Local Government on behalf of
Her Majesty’s Stationery Ofce.
© Crown Copyright 2009
ISBN 978 0 11 754011 8
Copyright in the typographical arrangement and design rests with the Crown.
This is a value added publication which falls outside the scope of the HMSO Class Licence.
Applications for reproduction should be made to Ofce of Public Sec tor Information, Information Policy Team, Kew,Richmond, Surrey, TW9 4DU.
Published by TSO (The Stationery Office) and available from:
Online
www.tsoshop.co.uk
Mail, Telephone, Fax & E-mail
TSO
PO Box 29, Norwich, NR3 1GN
Telephone orders/General enquiries: 0870 600 5522
Fax orders: 0870 600 5533
E-mail: [email protected]
Textphone: 0870 240 3701
TSO@Blackwell and other Accredited Agents
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Tel 028 9023 8451 Fax 028 9023 5401
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Contents
SECTION 1 GRA 5.8 Flashover, backdraught and fire
gas ignitions 5
Scope 5
Significant hazards and risks 5
Flashover 5
Backdraught 5
Fire gas ignitions 6
Key control measures 7
Recognition of the signs of backdraught and flashover 7
Training 8
Pre-determined attendance 8
Personal protective equipment 8
Incident command and control 9
Additional equipment 9
Technical references 10
Glossary 10
Neutral Pressure Place 10
Deflagration 10
Pyrolysis 10
SECTION 2 Summary of GRA 5.8 Flashover, backdraught and
fire gas ignitions 11
The Generic Risk Assessments in this series only apply to England.
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5
SECTION 1
GRA 5.8 Flashover, backdraughtand fire gas ignitions
Scope
This generic risk assessment (GRA) examines the hazards, risks and controls that relate
to Fire and Rescue Service staff and others who may be affected when the phenomenon
of Flashover, Backdraught or Fire Gas Ignition is experienced.
As with all GRAs, this assessment provides a starting point for FRSs to conduct their ownassessments within the context of local conditions and existing organisational arrangements.
Significant hazards and risks
Flashover
In a compartment fire there may come a stage where the total thermal radiation from the
fire plume, hot gases and hot compartment boundaries (ceilings and walls) causes the
radiative ignition of all exposed combustible surfaces within the compartment. Where the
compartment is adequately ventilated, this sudden and sustained transition of a growingfire to a fully developed fire is known as a flashover .
Backdraught
A backdraught is where limited ventilation can lead to a fire in a compartment producing
fire gases containing significant proportions of partial combustion products and unburnt
pyrolysis products. If these accumulate, the admission of air when an opening is made to
the compartment can lead to a sudden deflagration. This deflagration moving through the
compartment and out of the opening is a backdraught.
There are two scenarios of which firefighters must be fully aware:
If the fire is still burning within a compartment when the door is opened,
especially if the combustion gases are not escaping, the incoming air will
mix with the gases and create an explosive mixture. If the gases within the
compartment are hot enough, they will auto-ignite and flame will spread
back into the compartment along with the fresh air. This would result in rapid
fire growth, but not necessarily in a backdraught. Alternatively, if the gases
are not sufficiently hot they will only be ignited once sufficient oxygen has
reached the gases surrounding the fire. The flame will then travel across the
compartment towards, and out of the doorway, driven by the expandinggases behind it.
S c e n a r i o 1
Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions
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6 Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions
A more dangerous situation can occur if the fire in the compartment has
almost died out. Once the door is opened air flows in and an explosive
mixture may be created. There is the potential for ignition of these gases
not to occur immediately. Once the firefighters enter the room however, and
start to disturb the contents (e.g. turning over), an ignition source may be
exposed and result in total flame engulfment. This is defined as a ‘delayed backdraught’.
S
c e n a r i o 2
Fire gas ignitions
Fire gas ignitions occur when gases from a compartment fire are ‘leaked’ into an adjacent
compartment and mixed with the air within this additional area. This mixture may then fall
within the appropriate flammable limits that if ignited, will create an increase in pressure
either with or without explosive force. Where this process occurs it is not necessary for an
opening to be opened for such ignition to take place. If an explosive force is experienced,
this is commonly termed a ‘smoke explosion’. Where an ignition occurs with much less
pressure, the term ‘flash fire’ is more appropriate.
The backdraught, flashover and fire gas ignition phenomena described above can have
potentially fatal outcomes. It should be recognised that although their outcomes may
be similar the phenomena themselves are in fact significantly different in terms of the
following:
• their pre-conditions
• evolution
• recognisable indicators.
At one incident, in the same structure, there may well exist the preconditions for
each phenomenon. Further detailed information of the nature and preconditions for
backdraught, flashover and fire gas ignitions is provided in the Fire and Rescue Service
Manual Volume 2 Fire Service Operations, Compartment Fires and Tactical Ventilation.
Experience has shown that standard personal protective equipment (PPE), in itself, will
not afford adequate protection against the effects of either flashover or backdraught.
Therefore, it is vital that FRSs consider all the key control measures in order to reduce the
risk to personnel to a tolerable level.
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7
Key control measures
FRSs should consider how they control the hazards and risks described above. Key
measures are as follows.
Recognition of the signs of backdraught and flashover
The first consideration that there is a possibility of a backdraught occurring will be
dependant upon the fire history. Therefore, as much information as possible should be
gathered in the early stages of the incident.
Fire in a compartment with limited ventilation:
• Fire has been burning for some time
• Fire gases being pushed out under pressure from gaps
• Windows blackened with no visible signs of flame
• Fire gases pulsing out from gaps.
External signs of backdraught
Fire in a compartment with limited ventilation:
• Low neutral pressure plane
• Darkening of fire gases
• Inrush of air and fire gases forced out, through the opening
• Pulsation of fire gases through an opening
• Deflagration through the opening.
Internal signs of backdraught
• Flames visible in the fire gases
• Combustible materials gassing off due to pyrolysis
• High temperatures, and increasing rate of combustion
• Neutral plane moving down
• Sudden increase in development of fire
• Pyrolysis at floor level in the compartment.
Signs of flashover
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Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions8
Training
The level and nature of training undertaken should be based upon an informed
assessment of the operational need for each Fire and Rescue Service and within agreed
guidance for competency based training (CBT).
Personnel engaged in an operational firefighting role should receive information instruction
and training which should include:
• chemistry of combustion
• the effects of walls and the ceiling on the fire plume
• signs and symptoms of backdraught, flashover and fire gas ignitions
• door entry and gas cooling procedures
• safe system of work for dealing with incidents where flashover and backdraught
may occur
• physiological stress and its management.
The outcomes of training must be evaluated to ensure that the training is:
• appropriate
• effective
• up-to-date
• meeting the identified operational need.
Pre-determined attendance
FRSs should ensure that the operational response to a building fire will be sufficient
to allow relevant safe systems of work to undertake the activity. This will initially be
determined by information received from the caller and followed and supplemented by the
assessment of the Incident Commander.
Personal protective equipment
PPE manufactured to the current accepted standard should be worn.
When choosing suitable protective garments, the standard of clothing worn beneath the
PPE should also be considered. Retained and auxiliary personnel may not have sufficient
protection afforded by this clothing and therefore appropriate guidance/restrictions should
be provided.
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9Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions
Incident command and control
The National Incident Command System should be adopted on arrival at the incident.
Where a high risk of backdraught is identified, consideration should be given to the initial
adoption of defensive firefighting tactics.
Detailed guidance on operational tactics is published in the Fire and Rescue ServiceManual, Volume 2, Fire Service Operations – Incident Command, 3rd Edition 2008.
Ventilation of the premises should be conducted in a controlled and considered manner
with due account taken of wind conditions. Effective communications are essential for this
to be achieved.
Ventilation points and exposed risks should be covered by water sprays to reduce the risk
of external fire spread.
Suitable communications will be needed to ensure that the Incident Commander and
sector commanders are able to communicate at all times.
The area outside the building on fire should be controlled ( Inner Cordon Management) to
reduce the number of persons at risk to the minimum necessary should a backdraught
occur. This may include the tactical positioning of breathing apparatus entry control
boards and fire appliances.
Additional equipment
An informed assessment of the risks within a FRS area will allow FRSs to consider the
provision, and use of any additional equipment. Equipment may include:
• thermal imaging cameras
• ventilation tools
• hose reel spray branches
• PPV fans.
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Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions10
Technical references
1 Table of contents, Volume 3 Guide to Operational Risk Assessment
2 Fire and Rescue Service Manual – Volume 2 – Compartment Fires and Tactical
Ventilation (1997)
3 Fire and Rescue Service Manual, Volume 2, Fire Service Operations – IncidentCommand, 3rd Edition 2008 – TSO
4 The principles of Operational Training Fire Service Circular 5/96
5 Institute of Occupational Medicine; Home Office, FRDG Fire Research Reports
and Memoranda, 18/96
6 Study of the Physiological Effect of Wearing BA (Institute of Occupational
Medicine). DFM letter and DCO letter 8/1997 (Management of Physiological
Stress)
7 GRA 3.6 Fighting Fires – Using PPV, HMSO 1998
8 BS EN 469 Protective clothing for firefighters & Personal Protective Equipment
at Work: guidance on Regulations: Personal Protective Equipment at Work
Regulations 1992
9 Dynamic Management of Risk at Operational Incidents, HMSO 1998
10 Home Office Technical Bulletin 1/97 Breathing Apparatus – Command and
Control
11 Euro Firefighter – Global firefighting Strategy and Tactics, Command and Control,
Firefighter Safety – Paul Grimwood FIFireE
GlossaryNeutral Pressure Plane
Fire in a compartment causes smoke and gases to rapidly expand with the hot gases
rising due to their lower density and greater buoyancy. This results in a higher (positive)
pressure in the fire room than the areas outside of the compartment. The forces, once
fully developed, create an equilibrium whereby negative pressure relative to the outside,
exists in the lower part of the room an positive pressure exists in the higher levels.
Where these two zones meet is known as the ‘Neutral Pressure Plane’.
Deflagration
To burn or cause to burn with great heat and intense light.
Pyrolysis
Transformation of a substance produced by the action of heat.
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11Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions
S E
C T I O N
2
S u
m m a r y o f G R A 5 . 8
F l a
s h o v e r , b a c k d r a u g h t a n
d f i r e g a s i g n i t i o n s
R e
f .
N o
T a s k
H a z a r d
R i s k
P e r s o n s a t r i s
k
C o n t r o l M e a s u r e s
1
E n t e r i n g a n o u t e r
c o m p a r t m e n t
p o t e n t i a l l y a f f e c t e d
b y a fi r e g a s
i g n i t i o n
R a p i d fi r e s p r e a d
T o t a l fl a m
e e n g u l f m e n t o f
fi r e fi g h t e
r s w i t h i n t h e fi r e
c o m p a r t
m e n t
B u r n s
a n d s c a l d s
F i r e a n
d e x p l o s i o n
B l a s t i n j u r y
W h o l e t i m e
D a y c r e w e d
R e t a i n e d
V o l u n t e e r s
O t h e r e m e r g e n
c y
s e r v i c e p e r s o n n e l
P u b l i c
O p e r a t i o n a l p r o c e d u r e s a s p e r F R S p o l i c y
a n d s u p e r v i s i o n
T r a i n i n g i n i d e n t i fi c a t i o n o f
i n d i c a t o r s a n d
m e t h o d s o f a t t a c k
P P E m a n u f a c t u r e d t o t h e
c u r r e n t a c c e p t e d
s t a n d a r d
2
O p e n i n g u p a
c o m p a r t m e n t
c o n t a i n i n g o x y g e n
s t a r v e d fi r e
R a p i d fi r e s p r e a d / b a c k d r a u g h t
B u r n s
a n d s c a l d s
F i r e a n
d e x p l o s i o n
B l a s t i n j u r y
W h o l e t i m e
D a y c r e w e d
R e t a i n e d
V o l u n t e e r s
O t h e r e m e r g e n
c y
s e r v i c e p e r s o n n e l
P u b l i c
O p e r a t i o n a l p r o c e d u r e s a s p e r F R S p o l i c y
a n d s u p e r v i s i o n
T r a i n i n g i n i d e n t i fi c a t i o n o f
i n d i c a t o r s a n d
m e t h o d s o f a t t a c k
P P E m a n u f a c t u r e d t o t h e
c u r r e n t a c c e p t e d
s t a n d a r d
3
W o r k i n g i n
c o m p a r t m e n t
w i t h fl a m m a b l e
g a s e s p r e s e n t a n d
e x p o s i n g a s o u r c e
o f i g n i t i o n
T o t a l fl a m
e e n g u l f m e n t o f
fi r e fi g h t e
r s w i t h i n t h e fi r e
c o m p a r t
m e n t
B u r n s
a n d s c a l d s
F i r e a n
d e x p l o s i o n
B l a s t i n j u r y
W h o l e t i m e
D a y c r e w e d
R e t a i n e d
V o l u n t e e r s
P u b l i c
O p e r a t i o n a l p r o c e d u r e s a s p e r F R S p o l i c y
a n d s u p e r v i s i o n
T r a i n i n g i n i d e n t i fi c a t i o n o f
r i s k s .
P P E m a n u f a c t u r e d t o t h e
c u r r e n t a c c e p t e d
s t a n d a r d
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Generic Risk Assessment 5.8 – Flashover, backdraught and fire gas ignitions12
R e
f .
N o
T a s k
H a z a r d
R i s k
P e r s o n s a t r i s
k
C o n t r o l M e a s u r e s
4
C a r r y i n g o u t
t a c t i c a l v e n t i l a t i o n
R a p i d fi r e s p r e a d / b a c k d r a u g h t
B u r n s
a n d s c a l d s
F i r e a n
d e x p l o s i o n
B l a s t i n j u r y
W h o l e t i m e
D a y c r e w e d
R e t a i n e d
V o l u n t e e r s
O t h e r e m e r g e n
c y
s e r v i c e p e r s o n n e l
P u b l i c
O p e r a t i o n a l p r o c e d u r e s a s p e r F R S p o l i c y
a n d s u p e r v i s i o n
T r a i n i n g i n i d e n t i fi c a t i o n o f
i n d i c a t o r s a n d
m e t h o d s o f v e n t i l a t i o n
P P E m a n u f a c t u r e d t o t h e
c u r r e n t a c c e p t e d
s t a n d a r d
5
O p e n i n g u p a
c o m p a r t m e n t
w h e r e t h e
t h e r m a l r a d i a t i o n
g e n e r a t e d b y
t h e fi r e i s h i g h ,
c a u s i n g t h e r o o m
c o n t e n t s t o g i v e
o f f fl a m m a b l e
g a s e s .
R a p i d fi r e s p r e a d / fl a s h o v e r
B u r n s
a n d s c a l d s
F i r e a n
d e x p l o s i o n
B l a s t i n j u r y
W h o l e t i m e
D a y c r e w e d
R e t a i n e d
V o l u n t e e r s
O t h e r e m e r g e n
c y
s e r v i c e p e r s o n n e l
P u b l i c
O p e r a t i o n a l p r o c e d u r e s a s p e r F R S p o l i c y
a n d s u p e r v i s i o n
T r a i n i n g i n i d e n t i fi c a t i o n o f
i n d i c a t o r s a n d
m e t h o d s o f a t t a c k
P P E m a n u f a c t u r e d t o t h e
c u r r e n t a c c e p t e d
s t a n d a r d
E x p l a n a t o r y K e y
D e
s c r i p t i o n
R e f e r e n c e n u m b e r f o r t h e r i s k
T a s
k – t h e s p e c i fi c a c t i v i t y b e i n g c a r r i e d o u t
H a
z a r d – H a z a r d p r e s e n t g i v i n g r i s e t o
t h e r i s k
R i s
k – t h e c h a n c e ,
h i g h o r l o w ,
t h a t s o m e b o d y c o u l d b e h a r m e d b y t h e s
e a n d o t h e r h a z a r d s ,
t o g e t h e r w i t h
a n i n d i c a t i o n o f h o w s e r i o u s
P e r s o n s a t r i s k – t h i s s h o u l d d e t a i l e i t h e r e m p l o y e e , a n d / o r m e m b e r o f p u b l i c
C o
n t r o l m e a s u r e s – T h a t c o u l d b e u s e d b y t h e S e r v i c e t o r e d u c e t h e r i s k