compartment fire

Compartment Fire

Reference: D. Drysdale, 2003

Fire Dynamics Series

Ir. Yulianto S. Nugroho, MSc., PhDDepartment of Mechanical EngineeringUniversity of Indonesia

This module is intended as a general introduction to fire safety

Dr. YS. Nugroho – DTM FTUI

OUTLINE

IntroductionFire InitiationBurning Objects / ItemsFire Growth StagePre-Flashover FireFlashoverPost-Flashover

Ventilation controlled burningFuel controlled burning

Design FiresStructural Safety


Pendahuluan (1)

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Kerugian akibat kebakaran


Pendahuluan (2)

The term “compartment fire” is used to describe a fire which is confined within a building.


The fire triangle

Fuel + Oxidant = Combustion productsFuel + Oxidant = Combustion products

CHCH4 4 + O+ O2 2 => CO=> CO2 2 + 2H+ 2H2200

Reaction occurs when Oxygen/fuel mixture hot

enough

HeatHeat

OxygenOxygenFuelFuel


Burning Objects


Fire Initiation

Fire initiation includes ignition and the development of a self-sustaining combustion reaction.

There are many possible sources of ignition both deliberate and accidental. The ignition source is commonly very small and has low energy, but if it affects combustible materials it is often sufficient to start a fire. In many cases, ignition events have not started a significant fire because a small fire did not become self-sustaining and died out.

Ignition normally takes place in one of three ways:

Pilot ignition, normally initiated in a flammable vapor / air mixture by a "pilot", such as a flame or an electrical spark.


Fire Initiation

(Auto-ignition) where flaming develops spontaneously due to a sufficiently high temperature within a flammable vapour/air mixture in the absence of a pilot flame or spark.

Spontaneous combustion in bulk fuels. This is a less common means of fire initiation and is caused by self heating in bulk solids as a result of biological processes, chemical reactions or heating due to oxidation of drying oils, which can lead to smoldering combustion, normally starting deep within the mass of fuel.


Flammability Limits


Heat Release Rate

• Heat release rate (HRR) is one of the most important information in fire safety. Heat release rates for many items of furniture can be measured using furniture calorimeters.

• The heat release rate for selected items of furniture are shown below.


Flame growth


Flame spread (1)

Propagation of premixed flame through a flammable mixture in a duct following ignition (∗) at the closed end.


Flame spread (2)


Flame spread (3)


Flame spread (4)


Flame spread (5)


Flame spread (6)


Growth to Flash over


Compartment Fire


Growth to Flash over


Growth to Ventilation

Fuel + Oxidant => Combustion productsFuel + Oxidant => Combustion products

CHCH4 4 + O+ O2 2 => CO=> CO2 2 + 2H+ 2H2200


Fire Growth Stage:t-squared Fire

There are several approaches to estimating the growth rate for a particular design fire. The most popular is the t2 (t-squared) fire growth rate with three categories for fire growth; slow, medium, and fast.

These definitions are simply determined by the time required for the fire to reach 1.05 MW. A slow fire is defined as taking 600 seconds (10 minutes), a medium fire 300 seconds (5 minutes) and a fast fire less than 150 seconds to reach 1.05 MW (rounded to 1.00 MW in the calculations that follow).


t-squared Fire

The t2 fire growth can be thought of in terms of a burning object with a constant heat release rate per unit area in which the fire is spreading in a circular pattern with a constant radial flame speed. Obviously more representative fuel geometries may or may not produce t2 fire growth.

However, the implicit assumption in many cases is that the t2 approximation is close enough to make reasonable design decisions.


An alternative formulation to describe the heat release rate Q (MW) for a t2 fire is by:

Q = αt2

where α is the fire intensity coefficient (MW/s2).

The terms α and k are directly related by

α = 1.055 / k2

t-squared Fire


Typical growth times for t-squared fire


Heat release rates for t-squared fire


Pre flashover for t-squared fire


Fig below shows three heat release rate curves for a fire in office furniture.

Pre flashover for t-squared fire

Typical detector activation time, shown on HRR curve for a fast fire


The t-squared fires described above can be used to construct pre-flashover design fires, as input for calculating fire growth in rooms.

Fire can spread from the first burning object to a second object by flame contact if it is very close, or by radiant heat transfer if it is further away. The time to ignition of a second object depends on the intensity of radiation from the flame and the distance between the objects.

There may be many more items involved, and the resulting combination may itself be approximated by a t-squared fire for simplicity.

Fire Spread to Other Items


For example the first burns with medium growth rate for 10 minutes, followed by 1 minute of steady burning at its peak heat release rate of 4.0 MW. The second object ignites after 3 minutes, burning with fast growth rate for 4 minutes followed by steady burning at 2.5 MW for 2 minutes. The figure below shows the combined heat release rate curve for the two objects.

Fire Spread to Other Items


Growth to Flashover

The plume above the fire carries smoke and hot gases into the upper layer along with a considerable volume of entrained air. Temperatures in the upper layer rise rapidly due to the heat of the combustion products carried up in the plume. When the plume reaches the ceiling, hot gases travel along the ceiling, moving radially away from the fire. This flow of hot gases is known as the "ceiling jet", which will trigger operation of heat detectors or sprinklers.

Once the temperature in the upper layer reaches approximately 600oC and the direct radiation at floor level reaches about 20 kW/m2, all exposed combustible surfaces ignite rapidly and burn fiercely. This transition is known as flashover


Post-Flashover Fires

The behavior of the fire changes dramatically after flashover. The flows of air and combustion gases become very turbulent. The very hightemperatures and radiant heat fluxes throughout the room cause all exposed combustible surfaces to pyrolize, producing large quantities of combustible gases, which burn where there is sufficient oxygen.

The most important information for structural design is the temperature in the room during the post-flashover fire. Sometimes the burning rates are also useful.

The objective of design for a post-flashover fire is to contain the fire and prevent structural collapse, as necessary to meet the performance requirements. In the post flashover phase of a fire all of the combustible objects in the compartment are burning and the heat release rate is limited either by the fuel surface area or the available air supply.


Design Fires

When designing a structure to resist exposure to fire, it is often necessary to select a design fire. Alternative methods to obtaining design fires include hand calculations, published curves or parametric fire equations.

HAND METHODA very simple, but crude, method is to assume the fire has a constant temperature throughout the burning period. Such a time-temperature curve is sufficiently accurate for simple design.


Published Curves


1.00.90.80.70.60.50.40.30.20.10

1 2 3 4

1000°C800°C

20°C

200°C

400°C

600°C

Strain (%)

Normalised stress

Concrete also loses strength and stiffness from 100°C upwards.

Does not regain strength on cooling.

High temperature properties depend mainly on aggregate type used.

Concrete stress-strain curves at high temperatures

Ref. EC3 and EC4


Structural Safety

September 11, 2001


Stages of a natural fire - and the standard fire test curve

Cooling ….

ISO834 standard fire curve

Ignition - Smouldering

Pre-Flashover

Heating

Post-Flashover1000-1200°C

Natural fire curve

Time

Temperature

FlashoverRef. EC3 and EC4


Strain (%)0.5 1.0 1.5 2.0

Stress (N/mm2)

0

300

250

200

150

100

50

20°C

200°C300°C

400°C500°C

600°C

700°C

800°C

Steel softens progressively from 100-200°C up.

Only 23% of ambient-temperature strength remains at 700°C.

At 800°C strength reduced to 11% and at 900°C to 6%.

Melts at about 1500°C.

Steel stress-strain curves at high temperatures

Ref. EC3 and EC4


Compartment

Tem

pera

ture

Load

-bea

ring

resi

stan

ce

Time

Time

Fire severity time equivalent

Used to rate fire severity or element performance relative to furnace test.

Matches times to given temperature in a natural fire and in Standard Fire.

Fire resistance time equivalent

Standard fire

Natural fire

Element

Time-equivalence

Ref. EC3 and EC4


Furnace tests on structural elements

Fire TestingLoad kept constant, fire temperature increased using Standard Fire curve.

Maximum deflection criterion for fire resistance of beams.

Load capacity criterion for fire resistance of columns.

ProblemsLimited range of spans feasible, simply supported beams only.

Effects of continuity ignored. Beams fail by “run-away”.

Restraint to thermal expansion by surrounding structure ignored.

Ref. EC3 and EC4


100

200

300

0 1200 2400 3600Time (sec)

Deflection (mm)

Standard fire resistance furnace test

Ref. EC3 and EC4


Standard fire resistance furnace test

100

200

300

0 1200 2400 3600Time (sec)

Deflection (mm)

Span2/400dIf rate <

span2/9000d

Standard Fire

Span/30

Ref. EC3 and EC4


Structural fire protection:Passive Protection

Insulating BoardGypsum, Mineral fibre, Vermiculite.Easy to apply, aesthetically acceptable.Difficulties with complex details.

Cementitious SpraysMineral fibre or vermiculite in cement binder. Cheap to apply, but messy; clean-up may be expensive.Poor aesthetics; normally used behind suspended ceilings.

Intumescent PaintsDecorative finish under normal conditions.Expands on heating to produce insulating layer.Can now be done off-site.

Ref. EC3 and EC4


Thank you

Corresponding address:

Ir. Yulianto S. Nugroho, MSc., PhDDepartment of Mechanical EngineeringMT (S2) Program in Fire Protection EngineeringUniversity of IndonesiaKampus UI Depok 16424, IndonesiaE-mail : [email protected]

compartment fire

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