Workshop at Indian Institute of Science 9-13 August, 2010 Bangalore India Fire Safety Engineering & Structures in Fire Organisers:CS Manohar and Ananth.

Workshop at Indian Institute of Science9-13 August, 2010BangaloreIndia

Fire Safety Engineering & Structures in Fire

Organisers: CS Manohar and Ananth RamaswamyIndian Institute of Science

Speakers: Jose Torero, Asif Usmani and Martin GillieThe University of Edinburgh

Funding and Sponsorship:

Fire Safety Engineering Methods

Session JT10

Suppression

Water Suppression

Should Active Suppression be Used? 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

C

F

F

C

F

F

C

F

F

C

F

F

C

F

F

C

F

F

C

F

F

C

F

F

F SO2N(CH2)3 N

CH3

CH3

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

C F Cl Br I

Halon 13011 3 0 1 CF3Br

Halon 1011 1 0 1 1 CH2ClBr

Halon 2402 2 4 0 2 C2F4Br2

Why is Halon so Effective?

Combustion of Methane

HCOOHCO

OHHOH

COHHOHCH

OHOOH

MHCHMCH

2

2

223

2

34

BrOHOHHBr

HBrHBr

BrCFMBrCF

2

33

Halon 1301 + Heat

Why is Halon an Environmental Problem?

2

23

33

OBr2BrO2

OBrOOBr

BrCFUVBrCF

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 crashed-landed 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

Tg, ug

M, cp,

As

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

Background

1972 - Alpert - “Calculation of response time of ceiling-mounted 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

Lumped capacity analysis

Negligible losses (basic model)

outinabs qqq

)( dgscin TTAhq

dt

dTmcq d

pabs

0outq

Predictive equation for temperature rise

Definition of detector time constant

Time constant not really constant

Solution

τ

TTTT

mc

Ah

dt

dT dgdg

p

scd)(

)(

sc

p

Ah

mcτ

Response Time Index

For cylinders in cross flow

Implications

Definition of RTI

Predictive equation

gc uh ~

guτ 1~ constuτ g

guτRTI

)( dg

gd TTRTI

u

dt

dT

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

oτt

g

d eT

T /1Δ

Δ RTIut

g

act oacteT

T /1Δ

Δ

Plunge test

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5

t/

T

d/

Tg τt

g

d eT

T /1Δ

Δ

DETACT formulation

Euler equation for Td

Substitute equation for dTd/dt

Evaluation requires RTI, Tg(t) and ug(t)

tdt

dTTT dtd

ttd Δ)()Δ(

tTTRTI

uTT t

dt

g

tgt

dtt

d Δ)()(

)(

)()Δ(

Detector activation

Fixed temperature devices

Rate-of-rise devices

Typical value of dTact/dt: 8.3ºC (15 ºF) /min

actactd tdt

dT

dt

dT

actactd tTT

Gas parameters - Tg, ug

Alpert correlation (unconfined ceiling jet)

Temperature Velocity

3/2,

,

3/5

3/2

,

)/(

32.0

9.16

HrT

T

H

QT

plg

cjg

plg

6/5,

,

3/1

,

)/(

2.0

95.0

Hru

u

H

Qu

plg

cjg

plg

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: Tf<22.8 oC (ambient temperature), Te<37.8oC Class IB: Tf<22.8 oC (ambient temperature), Te>37.8oC Class IC: 22.8oC <Tf<37.8 oC

Combustible Liquid (Class II): “Flash Point” (Tf) greater than 37.8 oC Subdivided in:

Class II Liquid: 37.8 oC<Tf< 60 oC Class II A: 60 oC<Tf<93 oC Class II B: 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

PKQi Factor-K Nominal

gpm/(psi)1/2

Factor-K Range

gpm/(psi)1/2

Factor-K Range

dm3/min/ (kPa)1/2

%Over Nominal

Discharge with K-5.6

Thread

1.4 1.3-1.5 1.9-2.2 25 1/2 in. NPT 1.9 1.8-2.0 2.6-2.9 33.3 1/2 in. NPT 2.8 2.6-2.9 3.8-4.2 50 1/2 in. NPT 4.2 4.0-4.4 5.9-6.4 75 1/2 in. NPT 5.6 5.3-5.8 7.6-8.4 100 1/2 in. NPT 8.0 7.4-8.2 10.7-11.8 140 1/2 in. NPT or

3/4 in. NPT 11.2 11.0-11.5 15.9-16.6 200 3/4 in. NPT 14.0 13.5-14.5 19.5-20.9 250 3/4 in. NPT 16.8 16.0-17.6 23.1-25.4 300 3/4 in. NPT 19.6 18.6-20.6 27.2-30.1 350 1 in. NPT 22.4 21.3-23.5 31.1-34.3 400 1 in. NPT 25.2 23.9-26.5 34.9-38.7 450 1 in. NPT 28.0 26.6-29.4 38.9-43.0 500 1 in. NPT

Sprinkler Density

Sprinklers per m2 : “n”

Total number of sprinklers: “N”

N=n.A

i

d

Q

Qn

Activation Temperature

The decision is based on the fuel load/occupancy

Activation Temperature (oC)

Classification ColourCode

38 Ordinary No-Colour 66 Intermediate White

107 High Blue 149 Extra-High Red 191 Very-Extra-High Green 246 Ultra High Orange 329 Ultra High 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

NFPA 13 gives details on how to place sprinkler heads

ST-AST

SA

SS

SP

SC

SC

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….