Transcript Document

Water Spray Protection in the
Wildland/Urban Interface: The
Successes and Failures
Joseph W. Mitchell,
Ph.D.
Mark V. Potter
Regional Director
M-bar Technologies & Consulting
Ramona, CA
[email protected]
Country Fire Authority
[email protected]
Twitter: gipps_potzii
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Introduction
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Why external spray systems?
Types of systems
Science of water spray protection
Design requirements
Success and failures
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About the presenters
Joseph W. Mitchell, Ph.D.
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Particle physics research (1981-1996)
Wildland fire research (home ignition
prevention, power line fires 2002-present)
Published in Fire Safety Journal, Fire and
Materials, Engineering Failure Analysis
Mark Potter
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Regional Director, CFA Gippsland
Author of FPAA scholarship study on
water spray protection for bushfires
Represented AFAC for spray system
standards development
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Firewise principles
• Manage vegetation and other flammable
material in the home ignition zone to
reduce radiant heat and prevent flame
contact.
• Construct and maintain structures to
reduce the risk of piloted ignition from
embers (firebrands).
When is more than this appropriate?
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Water spray justifications:
• Redundancy:
– Hidden vulnerabilities
– Maintenance oversights
• WUI issues:
– Historical / vulnerable
structure
– Can’t control all home
ignition zone
• Security
– Entrapment potential
• Convenience
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Water for Fire Control
Cooling – Evaporation can
cool 2.4 kJ/g
Example: ¼ l/min ideally
removes 10kW/m2
(Toaster ~ 3 kW/m2)
FLAME and RADIANT HEAT
Hydration of Fuels
Wet fuels require extra energy
to ignite
EMBER ignition
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Types of water sprays
Distance from Fire Front
Ember Attack (100s of meters)
Flame
Radiant Heat
12.5 kW/m2
ignition threshold
usually < 50 m
Light Water Spray
19kW/m2
Deluge (AS 5414)
Much better:
FUEL CLEARANCE!
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Water spray design requirements
• Sufficient water (independent supply)
• Assume loss of electricity and other utilities
• Sufficient water flow for purpose (either deluge
or ember dowsing)
• Flow for full duration of threat
• All structure vulnerabilities covered
• Wind-resilience of spray (extreme weather)
• Spray system components protected
THE CATASTROPHIC IS TYPICAL FOR MOST HOME LOSSES
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How much water do you need?
To put out a wildfire,
a lot
To protect a home
against radiant heat
and flame, a lot
Cohen et al, 2004 –
13.1 kW/m threshold
Windows are the
most vulnerable
Separation from fuels is preferable!
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How much water do you need?
AJB Photography
To put out a match not so much.
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How much water do you need?
To put out a 10,000 matches not so much
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Ember protection sprays
Separate the problem of radiant heat &
flame protection
(answer: distance from fuel /
vegetation management)
from the problem of firebrand
protection…
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EMBERS ATTACK
“How we cheated the flames of death”
Gary Hughes
• Australian Columnist
• Black Saturday survivor
(barely)
• Had CFA training
• Failed “Stay & Defend”
“It comes at you like a runaway train. One
minute you are preparing. The next you are
fighting for your home. Then you are fighting for
your life…
But it is not minutes that come between. It's
more like seconds. The firestorm moves faster
than you can think, let alone react…
They call it "ember attack". Those words don't
do it justice…
It is a fiery hailstorm from hell driving relentlessly
at you. The wind and driving embers explore,
like claws of a predator, every tiny gap in the
house. Embers are blowing through the cracks
around the closed doors and windows…”
HIDDEN
VULNERABILITY!
The Australian Feb 9, 2009
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Ember ignition
• Is stochastic – i.e. “random”
• Mean number of ignitions
M ≈ Flux * Vulnerable area * Probability of ignition per brand
• Flux = embers per unit area per unit time
• Ignition Probablility
P ~ 1 – exp(-M)
• Prevention:
Remove vulnerable areas
Reduce ignitability (wetting)
Foote, et. al (Calfire & NIST)
Trampolines – Highly variable
ember density
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Water-spray protection
against ember attack
• If spray density is high enough, brands
can be directly extinguished.
• Water accumulates on surfaces and
around structures, creating a “moat”.
• Spray & vapor can hydrate light fuels.
Sensitive to wind disruption
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Re-evaluation of Paint Fire data
Ethan Foote thesis results on water spray
Destroyed
Survived
Total
Probability
Structures without
external sprinklers
32
148
180
Sprinklers before fire
4
17
21
0.89
Sprinklers during fire
1
37
38
0.01
Sprinklers after fire
1
33
34
0.01
Multivariate analysis found significance at >90% confidence level
(but not 95%)
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The Balance:
• Water spray needs to be active during and
after fire
• Needs sufficient spray density
Supply = Density X Area X Time
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When to apply water defense
Time
Fire Approach
Ember Attack
Radiant
Heat &
Flame
Light Water Spray
Heavy Deluge
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Area Sprays
Your Water Supply vs. Geometry
R
1/9
1/4
1
2R
3R
Water density
2
drops as 1/R
kKeep water
close to home
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Wind-Enabled Ember Dousing
(WEEDS)
CONCEPT: ACHIEVE WINDRESILIENT BRAND
PROTECTION BY DIRECTING
COARSE WATER SPRAY
OUTWARD FROM THE
STRUCTURE
• The wind blows it back onto
the structure
• Spray accumulates where
embers do
• Low spray densities needed to
protect from brands (as opposed
to radiant heat)
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J.W. Mitchell – Fire Safety Journal 2006
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WEEDS design features
• Low flow rate (~120 l/min)
• Agricultural spray nozzles
(2 mm mean droplet size)
• 5000 US gl water tank
(plus municipal supply)
• 12 kW generator (propane)
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• 1.5 kW pump
• 3-4+ hour protection window
• Potential improvements:
gravity feed, 10k gal tank,
automated or remote triggering
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Is it sufficient?
• Crib experiments suggest 1.5 4.0 gm/m2sec is sufficient to
extinguish cribs (reviews:
Novozhilov et al., Grant et al.)
• Simulation of droplet in wind

md ud   md g  Cd Ad ud  uwe ud  uwe
t


• Used similar nozzle for droplet
size distribution
• Achieves extinguishment zone
around the structure at nominal
design
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Wind resilience of spray
• Results conservative
– don’t take airflow
into account
• Overlap of spray
patterns to 50 km/hr
• 40% of spray onto
roof / eaves at high
wind speed
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Wind speed = 20 km/hr
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Testing of system
October 26, 2003
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Cedar Fire
Nominal operation
Apparent success
Structures lost on all
adjacent properties
• 60-70% loss rate / no
professional fire
protection
• Forensic evidence of
brands on property
Not proof, but a case study (Fire Safety Journal, Sept. 06)
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Alternative technologies:
• Water sprays
• Gels
• Foam
• Wraps
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Technology Comparison
Infrastructure
& Cost
Ease /
Speed
Water
Usage
Test /
Clean-up
Wind
Duration
Water
Spray
X
+
X
+
?
?
Gel
+
-
-
X
-
-
Foam
+
-
-
X
X
X
Wrap
-
X
+
+
X
+
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Manual Activation
+ Simple
+ Reliable
- Requires person
(late evacuation
or long duration)
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Automatic Activation
• Thermal detection – good for radiant heat /
flame but not ember attack
• Smoke – Where to set threshold?
• Mobile / Satellite activation – great if you
know where fire is and networks stay up
General issue – small market, R&D
insufficient to create guaranteed system
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Warning: Don’t Use Thermally
Triggered Sprinkler Heads!
Mark Potter (CFA) 2007:
• Advantage: Water at
maximum heat load
• Use ONLY for radiant
heat exposure
• Do NOT use for ember
exposure
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Mount Stromlo workshop
Conventional spray head – external
Heated windows shattered from spray
Traditional Sprinkler Heads
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Spray System Failures
• Unprotected
components
• Component
failures
• No regular testing
• Close vegetation
• Insufficient
duration
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Black Saturday
The 2009 Victoria Fires
173 deaths
> 2000 homes lost
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What is Known?
• Good statistical data buried in data set
• High failure rate for water spray systems
due to component failures (Justin Leonard,
CSIRO)
• Overall positive assessment by Victoria
Royal Bushfires Commission
• Details in Fatality Reports
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Victoria Royal Bushfire Commission
Final Report
“The raw data revealed that a much lower
proportion of houses fitted with [bushfire]
sprinkler systems were destroyed.”
Also reduced severity:
“‘If we didn’t have the sprinkler system, I believe
we would have been incinerated in the house in
less than two minutes. The sprinkler system
bought us time and absorbed the “hit” of the
firefront’”
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Bushfire CRC Data Collection
1065 Homes Assessed
CSIRO – Managed the survey/methods
CFA – Assisted with analysis
Much higher survival probability seen for structures with
sprinklers
BUT: Confounded with “Stay and Defend” and
other variables. Needs further analysis.
Stay & Defend 3.5X (Whittaker et. al., Journal of
Wildland Fire, 2013)
Needs additional work to extract meaningful interpretation
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VBRC – Sprinklers NOT a Guarantee
“The Commission also notes that there were
examples of people who had installed sprinklers
who died while sheltering in their homes during
the 2009 fires. It therefore cautions that
sprinklers should be seen as a supplement to
other measures and, in particular, are not a
substitute for active external defence of a
property. Reliance on a mechanical system
alone does not appear to be sufficient to provide
a satisfactory level of protection.”
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The VBRC Fatality Reports
• Every person who died had their story told
• Contains details of defensive measures,
including water spray systems
• Failure modes / Lessons learned
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From the Stories:
Of 173 fatalities, 40 had some sort of
water spray protection.
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From the Stories:
Sprinkler Type
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•
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Garden
Roof
Advanced
Unknown
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9
5
1
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From the Stories:
Sprinkler Activated
• Yes
• No
• Unknown
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2
10
40
From the Stories:
Vegetation
(Definitions not always clear)
• Heavy
• Heavy +
Cleared
• Cleared
• Unknown
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8
2
5
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From the Stories:
Failure Mode
• Pump failure /
inadequate
• Wind – roof
destroyed
• Overwhelmed /
flame contact
• Extreme speed
• Unknown
• Structure Survived
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3
2
3
8
1
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Embers a smaller factor:
“in comparison with other recent major fires, a
much lower proportion of houses were damaged
by embers only and a higher proportion of
houses were damaged by direct flame contact—
20 per cent destroyed by embers only and 13
per cent by flame contact”
Light water spray systems are most effective
against embers, not heat.
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How did “advanced” systems fail?
• Wind – Roof Destroyed
• Unknown
2
3
Wind damage was a factor in 13% or structure
losses.
LESSON – THIS APPLIES TO *ALL* DEFENSIVE
MEASURES
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AS 5414 – Bushfire Water
Spray Systems
• Australian Standard was
developed in 2012
• First of its kind in the world
• Aimed at ember attack
situations
• Protection against radiant
heat up to 19 kW/m2
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AS 5414 – Bushfire Water
Spray Systems
• The installation should be
preceded by an onsite evaluation.
• Focuses water spray onto areas
vulnerable to embers
• Spray nozzle performance
requirements established
• 10 l/m2-min windows
5 l/m2-min other vulnerable
surfaces
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AS 5414 – Bushfire Water
Spray Systems
• Potential shortfalls:
– 30 minute spray duration
– Use of petrol pumps (heat/smoke sensitive)
• Future knowledge needs:
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–
–
–
Automatic operation
Spray performance under varying wind conditions
Pump performance under fire conditions
Water density requirements for ember attack only
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Lessons:
• Water spray systems are a tool, not a
panacea
• Effective where heat load is low
• Couple with defensible space
• All system components designed for fire &
wind
• Further research needed – optimal flow
rates, reliable triggering, cost reduction
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Thank You
Slides available from:
M-bar Technologies and Consulting
Ramona, California
www.mbartek.com
[email protected]
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