Presentation on theme: "Workshop at Indian Institute of Science 9-13 August, 2010 Bangalore India Fire Safety Engineering & Structures in Fire Organisers:CS Manohar and Ananth."— Presentation transcript:
Workshop at Indian Institute of Science 9-13 August, 2010 Bangalore India Fire Safety Engineering & Structures in Fire 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: Fire Safety Engineering Methods Session JT10
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
Fire Control and Suppression
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 Damk ö hler Number Oxygen Displacement Reduces the Mass Transfer number Reduces the flame temperature Reduces the Damk ö hler Number Chemical Inhibition Affects the Chemistry Reduces the Damk ö hler 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/m 2 s Water Mists Water Discharge ~ lt/m 2 s 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
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 CF 3 Br Halon CH 2 ClBr Halon C 2 F 4 Br 2
Why is Halon so Effective? Combustion of Methane Halon Heat
Why is Halon an Environmental Problem?
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,ugTg,ug M, c p, A s 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 Alpert - “Calculation of response time of ceiling- mounted fire detectors” - quasi-steady fires Heskestad & Smith - Development of plunge test & RTI concepts Heskestad & Delichatsios - “Initial convective flow in fire” - t-squared fires NFPA 72E App. C Evans & Stroup - DETACT models Heskestad & Bill - Conductance factor added SFPE Task Group - Review bases of DETACT
Bases Heat balance at detector Convective heating only Lumped capacity analysis Negligible losses ( basic model )
Predictive equation for temperature rise Definition of detector time constant Time constant not really constant Solution
Response Time Index For cylinders in cross flow Implications Definition of RTI Predictive equation
RTI relationships Lower RTI Faster response Lower m or c p Lower RTI Higher h c or A s Lower RTI In limit, as RTI 0, T d T g
RTI determination (1) Plunge test T g = constant u g = constant T act = known Analytical solution
DETACT formulation Euler equation for T d Substitute equation for dT d /dt Evaluation requires RTI, T g (t) and u g (t)
Detector activation Fixed temperature devices Rate-of-rise devices Typical value of dT act /dt: 8.3ºC (15 ºF) /min
Gas parameters - T g, u g Alpert correlation (unconfined ceiling jet) TemperatureVelocity
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” (T f ) lower than 37.8 o C Subdivided in: Class IA:T f <22.8 o C (ambient temperature), T e <37.8 o C Class IB:T f 37.8 o C Class IC: 22.8 o C 93 o C T e =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 (Q d )
Flow Through a Sprinkler: “K” Factor Factor-K Nominal gpm/(psi) 1/2 Factor-K Range gpm/(psi) 1/2 Factor-K Range dm 3 /min/ (kPa) 1/2 % Over Nominal Discharge with K-5.6 Thread /2 in. NPT /2 in. NPT /2 in. NPT /2 in. NPT in. NPT in. NPT
Sprinkler Density Sprinklers per m 2 : “n” Total number of sprinklers: “N” N=n.A
Activation Temperature The decision is based on the fuel load/occupancy
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 S T-A STST SASA S SPSP SCSC SCSC
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….