Transcript No Slide Title

Fire Safety Engineering & Structures in Fire
Workshop at Indian Institute of Science
9-13 August, 2010
Bangalore
India
Fire Safety Engineering Methods
Session JT10
Organisers:
CS Manohar and Ananth Ramaswamy
Indian Institute of Science
Speakers:
Jose Torero, Asif Usmani and Martin Gillie
The University of Edinburgh
Funding and
Sponsorship:
Suppression
 Water Suppression
Should Active Suppression be Used?
 Why can the decision of not using active
suppression be made?




Cost
Environmental Concerns
Damage of Property
Incompatibility with the purpose of the building
 Fire is a complex problem that requires a
“cost/benefit” analysis
The Problem
Fire Control and Suppression
Combustion Products
Contaminated Water
Suppression Agents
Fire Retardants
Contaminated Residues
i.e. lead from paint
chars, tars, soil degradation
Fire Prevention
 Early Detection
 Smoke Detectors, CO Detectors, IR Detectors,
UV Detectors, Motion Detectors
 Effective but not infallible
 Proper Material Selection
 Low Flammability Materials - not always
possible to use – many times are not cost
effective
 Fuels – aircraft, cars, ships
 Plastics – everyday use
 Metals – flammable under extreme conditions –
i.e. turbine engines
Fire Retardants
 Additives used to reduce the “flammability” of a
material
 Halogen-based retarded materials – i.e. PVC
 Inhibit gas phase combustion chemistry
 Produce contaminants during a fire
 Produce contaminants during recycling
 Phosphorous based charring materials




Formation of chars – reduces flow of fuel to flame
Produce contaminants during fire
Contaminate suppression water
Lead to smolder fires – very difficult to detect and suppress
 New environmentally friendly technologies
 Based on carbon fibers and nano-composites – still under
development
Fire Suppression





Water Sprinklers
Water Mists
Gaseous Agents
Foams
Dry Chemicals
Mechanisms of Flame Suppression
 Thermal Sink
 Reduces the Mass Transfer number
 Reduces the flame temperature
 Reduces the Damköhler Number
 Oxygen Displacement
 Reduces the Mass Transfer number
 Reduces the flame temperature
 Reduces the Damköhler Number
 Chemical Inhibition
 Affects the Chemistry
 Reduces the Damköhler Number
Water Based Systems
 Work on the basis of energy removal and
oxygen displacement
 Sprinkler Systems




Simple systems, Low Maintenance, Low Cost
Work by wetting the fuel surrounding the fire
Not a suppression technique, more a control system
Therefore: High Water discharge ~ 0.25 lt/m2s
 Water Mists
 Water Discharge ~ 0.00025 lt/m2s
 High penetration due high momentum injection
 Everything else is more complex due to high pressure
Foams
 Very limited applications
 liquid fuels
 protection of structures
 Need to produce a film that spreads across the
fuel lead to complex chemical composition
generally based on Fluorine and Iodide
 i.e AFFF Foams
F
F F
F
F
F
F
F
CH3
F C C C C C C C C SO2N(CH2)3 N CH3
F
F F
F
F
F
F
F
CH3
+
I-
Dry Chemicals
 Generally can only be discharged
once
 Reduced penetration
 Act as mostly as thermal sinks –
Less Efficient
 Chemical suppression only present if
dry chemical is “halogen” based
 Generally – highly corrosive
Gaseous Agents
 High effectiveness
 Chemically active – i.e. Halons
 Less effective
 Chemically inactive – extinction by reduction of
oxygen concentration or thermal sink
 Advantages
 No clean-up necessary, easy storage, low
weight/volume ratio, high penetration (total
flooding), electrically non-conductive, mostly
non-corrosive, etc., etc., etc.
Mechanisms of Flame Suppression
 Most effective is Chemical Inhibition
 Halons are extremely effective at
attacking “chain branching” reactions
in combustion processes
Halons
 Nomenclature
Halon 1301
Halon 1011
Halon 2402
C
1
1
2
F Cl Br
3 0 1
0
1 1
4
0 2
I
CF3Br
CH2ClBr
C2F4Br2
Why is Halon so Effective?
Combustion of Methane


CH 4  M  CH 3  H  M


H  O 2  O  OH


CH 3  OH   H 2  H 2 CO


H 2  O  H  OH



CO  OH  CO 2  H


Halon 1301 + Heat

CF3 Br  M  CF3  Br 
Br   H   HBr
HBr  OH   H 2 O  Br 
Why is Halon an Environmental Problem?

CF3 Br  UV  CF3  Br


Br  O 3  BrO  O 2

2BrO  2Br  O 2
Consequences
 The Montreal protocol banned the
manufacturing and use of Halon 1301
 No other alternative has proven to be as
effective as Halon 1301
Fact
 Halon is present in 98% of commercial aircraft
 In 2000 there where 178 Halon discharges in
commercial aircraft
 It has been estimated that of those 178
discharges more than 100 would have resulted
in generalized fires that would have crashedlanded the aircraft
Conclusion


Is it justifiable to ban the use of Halon 1301
for fire applications?
Is environmental protection a sufficient
“cost” to overwhelm the “benefits” of Halon
1301?
Fact
 The ozone depleting potential of all fire
related Halon 1301 deployments in a year is
equivalent to that of the emissions of 132
cars!
Water Suppression-Sprinkler Systems
 Water suppression is the most
commonly used mechanism of active fire
control in structures
 Among the different water suppression
systems, sprinklers are by far the most
commonly used
 Some design considerations will be
presented
Effect of Sprinklers (I)
Effect of Sprinklers (II)
 Increase the time to “Flash-Over”
 Decrease toxic product concentrations,
CO, HCN, etc.
 Decrease the room temperature
 Push the hot layer down slowing fire
growth Push the hot layer down slowing
fire growth
 Increase visibility “soot” dissolves in
water
Effect of Sprinklers (III)
 “sprinklers” are NOT designed to Extinguish
the fire
 “sprinklers” are designed to Increase the
time available to extinguish the fire
Fire Detector Activation
 A first order analysis for predicting fire
detector activation based on convective
heating and a lumped capacity analysis
 Principles of the DETAC Model
Tg,
ug
M,
cp,
As
Background
 1972 - Alpert - “Calculation of response time of ceilingmounted fire detectors” - quasi-steady fires
 1976 - Heskestad & Smith - Development of plunge test
& RTI concepts
 1978 - Heskestad & Delichatsios - “Initial convective
flow in fire” - t-squared fires
 1984 - NFPA 72E App. C
 1985 - Evans & Stroup - DETACT models
 1987 - Heskestad & Bill - Conductance factor added
 1998 - SFPE Task Group - Review bases of DETACT
Bases
 Heat balance at detector
 Convective heating only
qabs  qin  qout
qin  hc As (Tg  Td )
 Lumped capacity analysis
q abs
 Negligible losses (basic model)
dTd
 mc p
dt
qout  0
Solution
 Predictive equation for temperature rise
(Tg  Td )
dTd hc As

(Tg  Td ) 
dt
mc p
τ
 Definition of detector time constant
τ
mc p
hc As
 Time constant not really constant
Response Time Index
 For cylinders in cross flow
hc ~ u g
 Implications
τ ~ 1 ug
 Definition of RTI
 Predictive equation
τ u g  const
RTI  τ u g
ug
dTd

(Tg  Td )
dt
RTI
RTI relationships
 Lower RTI  Faster response
 Lower m or cp  Lower RTI
 Higher hc or As  Lower RTI
 In limit, as RTI  0, Td  Tg
RTI determination (1)
 Plunge test
 Tg = constant
 ug = constant
 Tact = known
 Analytical solution
ΔTd
t / τ o
 1 e
ΔTg
ΔTact
t act
 1 e
ΔTg
uo / RTI
Plunge test
1
0.9
0.8
ΔTd
t / τ
 1 e
ΔTg
 T d/ T g
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
t/
4
5
DETACT formulation
 Euler equation for Td
dTd
Td
 Td 
Δt
dt
 Substitute equation for dTd/dt
( t  Δt )
Td
( t  Δt )
 Td
(t )
(t )

ug
(t )
RTI
T
g
(t )
 Td
(t )
Δt
 Evaluation requires RTI, Tg(t) and ug(t)
Detector activation
 Fixed temperature devices Td  Tact  tact
 Rate-of-rise devices
dTd dTact

 t act
dt
dt
 Typical value of dTact/dt: 8.3ºC (15 ºF) /min
Gas parameters - Tg, ug
 Alpert correlation (unconfined ceiling
jet)
Temperature2 / 3
Tg , pl
Tg ,cj
Tg , pl
Velocity
Q
 16.9 5 / 3
H
u g , pl
0.32

2/3
(r / H )
u g ,cj
u g , pl
1/ 3
 Q 
 0.95 
H
0.2

5/ 6
(r / H )
General Information
 Based on NFPA 13 – National Fire
Protection Association Codes
 Sprinkler selection is based on the
rapidity with which the thermal sensor
operates - RTI
Sprinkler System Design
 The design of a sprinkler system consists of
the following steps:





Identification of the fuel load
Identification of the use of the building
Calculation of the sprinkler density
Determination of sprinkler placement
Definition of the different components of the system




Sprinklers
Pipes
Pumps
Valves
 Establishment of maintenance procedures
Procedures
 Classification of occupancy or
 Classification of the fuel load
 Determination of quantity of water
needed
 Determination of sprinkler type
 Water flow
 Activation temperature and RTI
Occupancy
 Light risk
 Moderate risk
 High risk
 Special Occupancy:
 I.e. Historic documents, film, art, nuclear power
plants, airports, etc.
Fuel Load
 Class I: Non combustible materials stocked on “wood
pallets” or in single thickness cardboard boxes
covered with a plastic film cover.
 Class II: Non combustible materials stocked on “wood
pallets” or in multiple thickness cardboard boxes
covered with a plastic film cover.
 Class III: Wood products, paper, natural fibers, C-Type
plastics.
 Class IV: A Type Plastics (between 5-15% of the
weight) and plastics of types B or C for the rest.
Liquids
 Flammable Liquid (Class I): “Flash Point” (Tf) lower
than 37.8 oC
 Subdivided in:
 Class IA:
 Class IB:
 Class IC:
Tf<22.8 oC (ambient temperature), Te<37.8oC
Tf<22.8 oC (ambient temperature), Te>37.8oC
22.8oC <Tf<37.8 oC
 Combustible Liquid (Class II): “Flash Point” (Tf)
greater than 37.8 oC
 Subdivided in:
 Class II Liquid:
 Class II A:
 Class II B:
37.8 oC<Tf< 60 oC
60 oC<Tf<93 oC
Tf>93 oC
 Te=Boiling temperature
Plastics
 Type A: ie. Polyethylene, polystyrene,
polypropylene, PVC, etc.
 Type B: ie. Fluoroplastics, natural
rubber, nylon, silicone
 Type C: ie. Melamine, fenolites, urea
Water Density (Qd)
Flow Through a Sprinkler: “K” Factor
Qi  K P
Factor-K
Nominal
gpm/(psi) 1/2
Factor-K
Range
gpm/(psi) 1/2
Factor-K
Range
dm 3/min/
(kPa) 1/2
1.4
1.9
2.8
4.2
5.6
8.0
1.3-1.5
1.8-2.0
2.6-2.9
4.0-4.4
5.3-5.8
7.4-8.2
1.9-2.2
2.6-2.9
3.8-4.2
5.9-6.4
7.6-8.4
10.7-11.8
11.2
14.0
16.8
19.6
22.4
25.2
28.0
11.0-11.5
13.5-14.5
16.0-17.6
18.6-20.6
21.3-23.5
23.9-26.5
26.6-29.4
15.9-16.6
19.5-20.9
23.1-25.4
27.2-30.1
31.1-34.3
34.9-38.7
38.9-43.0
%
Over
Nominal
Discharge
with K-5.6
25
33.3
50
75
100
140
200
250
300
350
400
450
500
Thread
1/2 in. NPT
1/2 in. NPT
1/2 in. NPT
1/2 in. NPT
1/2 in. NPT
1/2 in. NPT or
3/4 in. NPT
3/4 in. NPT
3/4 in. NPT
3/4 in. NPT
1 in. NPT
1 in. NPT
1 in. NPT
1 in. NPT
Sprinkler Density
 Sprinklers per m2 : “n”
Qd
n
Qi
 Total number of sprinklers: “N”
N=n.A
Activation Temperature
 The decision is based on the fuel
load/occupancy
Activation
Classification
Temperature (oC)
38
Ordinary
66
Intermediate
107
High
149
Extra-High
191
Very-Extra-High
246
Ultra High
329
Ultra High
Colour
Code
No-Colour
White
Blue
Red
Green
Orange
Orange
Distribution and Installation
 Sprinklers are distributed through the
protected space homogeneously
 The water pressure will be established by
the code and the sprinkler density
 Water pumps are many times necessary
 The total flow is established on the basis
of the number of sprinklers
Installation Details
ST-A
ST
SS
SA
SP
SC
SC
 NFPA 13 gives
details on how
to place
sprinkler heads
Special Sprinkler Types
 Regular Sprinklers: Direct 40-60% of the
water towards the fire







ESFR-Early suppression fast response
Extended Coverage
Large Drop Sprinkler
Open Sprinklers (no actuator)
Quick Response (QR)
Quick Response Early Suppression (QRES)
Residential Sprinklers (fast response sprinklers
rated for residential use), etc., etc., etc.,
Installation Types
 Wet Pipe System-Standard, water filled
pipes with sensor at the sprinkler head
 Circulating-Closed Loop System-combined wet
pipe sprinkler system with HVAC system
 Dry Pipe System-Pressurized air/nitrogen, its
release opens the water valve-for non-heated
environments
 Combined-Dry Pipe Pre-reaction System-thermal
sensor + fire detection system, for fast or
screened response
 Deluge System-Dry pipes with a fire sensor, no
thermal sensor (open sprinklers)
Limitations of this approach
 Effectiveness of the system is base on
empirical data for a reduced number of
configurations
 No quantitative estimate of the
“probability of success” can be stated
 No quantitative estimate of the potential
“outcome” can be specified
 This approach is being phased-out by
performance design….