Transcript Marine Based Fires and Their Suppression

Marine Based Fires and Their
Suppression
By: Mohammed Shaheen
Contents
• Introduction
• Types of offshore Fires and Suppression Systems
• Water Mist Suppression Mechanisms and Modes of
application
• Water Suppression Systems
• Desirable droplet characteristics for fire-fighting
• Experimental Work On Fire Suppression By Water Sprays
• Development In Water Mist Technique Used In Fire
Extinguishment
• Conclusion
• References
Introduction
- Offshore Vs Onshore Fires.
- Enclosed Vs Open Fires.
Offshore Vs Onshore Fires:
• Fires on offshore process installations
are different from those onshore by:
– The effect of enclosure on fire
severity for liquid pool fires and
gas fires.
– Effect of impinging jet fires from
high pressure gas leakages.
– Threat to people from smoke and
heat and difficulty of evacuation
• To make the picture complete, the drilling operations are often located
together with the hydrocarbon processing area, and a helicopter port is
located on top of all.
Enclosed Vs Open Fires:
• The environment offshore is rough, and most of the process areas are
enclosed or shielded against weather. A fire inside such a confinement
develops differently from an open fire.
• The restriction in air supply results in a severe under-ventilated fire
environment develops.
• Burning of hydrocarbons under such
conditions is more intense than in open
fires:
-
-
The mixture of air and fuel may be close
to a stoichiometric mixture.
Heat loss from the flames to the
surroundings is reduced, leading to
higher flame temperatures.
Under-ventilated fires produces carbon
monoxide (CO) rather than carbon
dioxide (CO2).
Types of offshore Fires and
Suppression system
- Types of Offshore Fires.
- Offshore Fire Suppression Systems.
- Water Mist as an Alternative for Halon.
Types of Offshore Fires:
1. Gas Fires
Diffusive Gas Fires:
• Formed by low velocity gas escape
fires cause overpressure in tight
rooms.
• Structures of an offshore platform
can withstand overpressures.
Jet Fires:
• Formed by escape of flow with sonic
speed through pipes.
• Identified as one of the most severe fire
hazards.
• Theoretical calculations presenting heat
loads of the order of 1000 kW /m2.
2. Liquid Fires
Enclosed Pool Fires:
Fuel evaporation rate from enclosed liquid pool
fires may be at least twice as high as in open pool
fires.
Running or Falling Liquid Fires
Burning liquid flowing downwards surfaces
or falling through the air.
Fire on the Sea Surface
large pool Oil fires on water form a major threat
to the structure below an offshore platform.
Liquid Spray Fires
The spill of oil from high-pressure pipelines
with a jet-like shape and performance.
3. Other Combustible Materials
Items in offices and kitchens, wall lining materials may catch fire, and the smoke
produced even by small fires may lead to a critical situation for people on board.
Offshore Fire Suppression Systems:
• Halon:
Used in the protection of engine rooms, auxiliary machinery
spaces, electrical equipment areas, and flammable liquid
storage rooms on ships.
• Inert Gases:
Argon , Nitrogen and their mixture, the same way as Carbon Dioxide in
reducing the Oxygen. However, unlike Carbon Dioxide, they are not toxic.
• Halocarbon Gases:
Chemical agents contain chlorine and fluorine, they are electrically nonconductive, vaporize and leave no residue, however they all have
environmental impacts.
• Water Mist
Very effective in extracting heat from a fire and producing steam that
displaces oxygen.
Water Mist as an Alternative for Halon:
• Water mist fire-suppression systems
have become an important area of
study since bromine-based chemical
fire suppression agents, such as
halons, were banned by Montreal
Protocol in 1995 being responsible for
ozone layer depletion.
• The environmentally friendly non-toxic properties of water
mean that it is an attractive alternative to other fire-fighting
media.
• Some of the first places where water mist fire suppression
systems were implemented were offshore oil and gas rigs.
• Water mist fire-fighting systems perform well in enclosed
spaces with limited ventilation, similar to turbine hoods,
pump rooms and emergency generator rooms.
• Large open areas with high
ventilation rates present a challenge
to water mist technology.
• The experience in small enclosures is that large fires are
extinguished within seconds after water release, but that it takes
longer to extinguish smaller fires.
• This phenomenon is called ‘the paradox of water mist’ and is
caused by the physical properties of water.
• Water needs a certain temperature to stay as a gas inside an
enclosure ( water boils at 100°C while CO2 has a boiling point of -78.5
°C, and will stay as a gas at normal ambient temperature).
• Water vapor forms an inert gas concentration of 30 % at a temperature
of 70 °C. So if the atmosphere is cooled down too quickly to
a temperature below 70 °C, the water vapor will re-condense and form
droplets instead of gas.
• At that point, if all fires in the enclosure haven’t been extinguished,
small fires could continue to burn.
Aim of the Project:
The aim of the research project is to establish the current
state-of-the-art regarding the use of water sprays for the
suppression and extinguishment of fires and to identify
where gaps exist in the current knowledge.
Water Mist Suppression Mechanisms
and Modes of application
-
Suppression Mechanisms.
Modes of application.
Principal Action of Liquid Fire Suppressants:
• liquid fire suppressants, such as water, remove heat from fires
through their heat capacity and latent heat of vaporization.
• Water enters as droplets, which are heated by the hot gases they
entrain. Depending on the size and the flow pattern, droplet
may:
– Evaporate or partially evaporate before it hits a surface.
– Survive as a droplet and hits a surface.
•
When a droplet hits a surface, it may:
– Evaporate.
– Form a water film which is taking heat out of the surface.
– Run off the surface.
• The cooling power of water is approximately the same per degree
in both a liquid or vapor state (4kJ/liter °C).
• In liquid state, 418 kJ/liter of heat is absorbed by water to be
heated from 0°C to 100 °C.
• In vapor state, 800 KJ/liter of heat is absorbed from 100 °C to 300 °C.
• In the transition from liquid to vapor at 100 °C 2257 kJ/liter of heat
is absorbed.
Water Suppression Mechanisms:
• Cooling The Fuel Surface:
Reducing the pyrolysis rate and the rate of fuel supply to the flame
zone, reducing the heat release rate.
• Cooling The Flame Zone Directly:
Disrupting the chemical reactions of combustion by abstracting
heat in evaporating the liquid water.
• Volumetric Displacement Of The Oxidant (flame smothering):
Production of (inert) water vapor within the combusting
environment that displaces oxygen.
Modes of Application
Water discharges initially in the form of a solid jet then
undergoes a transition to diffuse jet:
Solid Jet:
• It is a tube-like flow of large water droplets, its stability depend on
the nozzle exit pressure, high pressure preserves the solid nature of
the jet.
• Calculation of jet trajectory is simplified by assuming that the fluid
stream behaves in a similar manner to a solid projectile.
• The maximum throw of a jet is achieved with an initial angle of 32o.
• The maximum vertical height of throw, is achieved with an initial
discharge angle of 80o.
Diffusion jets:
• Due to solid jet instability, water flow shifts towards a diffusion jet,
of fine water droplets (separated flow characteristic).
• Large water Droplets:
- Possess the momentum required to penetrate the flame.
- A small fraction of the large droplets participates in heat extraction
through evaporation, while the majority remain in the liquid phase
and runoff.
• Fine water Droplets:
- Fine droplets promote rapid evaporation.
- They don’t possess the momentum required to penetrate.
Theoretical measurement of drop size:
• In practice, single sized droplets sprays (mono disperse) are rare and most
sprays of are poly-disperse (containing a wide distribution of droplet sizes).
• Poly-disperse sprays have undergone intense experimental investigation
to find simple empirical equations which characterize the mean droplet
diameter and size distribution.
• A standard notation for defining mean diameters has been suggested by
Mugele and Evans (1951):
values of a and b depend on the phenomenon under investigation.
Practical methods for measuring drop size distribution:
• Modern optical techniques have two main advantages over other older
methods (Mechanical and electrical) in that they are non-intrusive and
they allow measurements over very short and/or very sharply defined
time intervals.
The Spectrum of Droplet Sizes:
• The average size range from 100 to 1000 µm is of most interest
for fire fighting.
Desirable Droplet Characteristics
for Fire-Fighting
-
Cooling of gaseous combustion products
Cooling of solid fuel surfaces
Thermal radiation by water droplets
Spray penetration
Optimum droplet size
Desirable Droplet Characteristics For Fire-fighting
Each water suppression mechanism requires different water droplet
characteristics
Characteristics Needed for Cooling of Gaseous Combustion Products:
• Heat required to vaporize water droplets is transferred to the
droplet surface by conduction and convection from the
surrounding hot gas.
• The rate of heat transfer to the droplet depends upon:
– Surface area.
– Heat transfer coefficient ().
• The heat transfer coefficient for a spherical droplet in a
quiescent atmosphere, can be calculated from:
Where:
- k is the thermal conductivity of the surrounding gas.
- d is the droplet diameter.
- Nu is the Nusselt number.
• Ranz and Marshall (1952) performed experiments on droplet
evaporation in air at temperatures up to 220 Co, for drop
diameters in the range 600–1000 µm , the following expression
was found to correlate the experimental data:
Eq. (1)
The range of validity was given as:
• Herterich (1960) found that, water vapor has an insulating effect
as it passed through the boundary layer surrounding the drop,
tending to reduce the rate of evaporation where the measured
times were consistently some 60% greater than those predicted
by Eq. (1). Herterich reported an alternative corrected form:
Eq. (2)
• Guttler (1972) employed Eq.(2) to estimate the
total quantity of heat transmitted to water sprays
using the following expression:
where:
–  is the heat transfer coefficient for an
individual droplet.
– O is the total surface area of the spray per unit
volume of water
• He estimated the drop size from the discharge
velocities of fire-fighting sprays :
•
Rasbash (1962) proposed the expression:
Eq. (3)
Where:
-  is the total heat of vaporization
- β is the increase in enthalpy
• Rasbash used Eq. (3) to plot the rate of
convective heat transfer as a function
of drop velocity for drop sizes ranging
from 50 to 2000 mm, assuming a flame
temperature of 1000 Co .
• Higher droplet velocities and smaller
droplet diameters were found to
increase the heat transfer rate.
• He was able to estimate the droplet
penetration distance into the flame
prior to evaporation.
Characteristics Needed for Cooling of Solid Fuel Surfaces:
• In addition to absorbing heat from the fire gases or flames, water sprays
also abstract heat from hot solid surfaces including: Burning fuel,
Unburned fuel, Non-combustible surfaces such as brick or metal
structural elements.
Definition:
Saturation Temperature of Liquid (Tsat): is the temperature of the liquid–
vapor interface at the local pressure.
Excess Surface Temperature (Ts): is difference between the surface
temperature (Ts) and the saturation temperature of liquid (Tsat).
• Bejan (1993) identified four distinct regimes of boiling heat transfer for
water at atmospheric pressure, depending on the temperature of the
solid surface Ts these are:
– Natural convection boiling.
– Nucleate boiling.
– Transition boiling.
– Film boiling.
• The initial application of water
(Ts 200–300Co) produces film
boiling on the fuel surface.
• A continuous film of water vapor
is formed between the solid
surface and the liquid water
droplets.
• The decrease in Ts (or  Ts ) is
accompanied by a gradual
reduction in heat flux.
• This continues until a temperature called Leidenfrost temp. is reached,
corresponding to a minimum heat flux leaving the surface.
• At this point the vapor film collapses, causing a sudden increase in
.
• The behavior of spray droplets impinging onto horizontal
heated surfaces is characterized in terms of the nondimensional Weber number which is the ratio of the
inertial force to the surface tension force:
• For We < 30, droplets rebounded immediately from the
heated surface without disintegrating.
• For 30 < We < 80, they tended to spread over the surface,
forming a thin vapor layer on the underside, before
contacting with the hot surface and finally rebounding.
• For We > 80, impinging droplets form a thin spreading
liquid film upon collision, which subsequently
disintegrated into smaller droplets.
• Rymkiewicz and Zapalowicz (1993), assuming three main
system variables presented a qualitative illustration of these
droplet-surface interactions: droplet size, impact velocity
and initial surface temperature.
Thermal Radiation by Water Droplets:
• Water has the ability to absorb radiant heat.
• Rasbash (1962) stated that radiative heat transfer to firefighting sprays depends mainly on the temperature and
emissivity of the flame.
• Emissivity of a flame depends on its thickness.
• A 1 m thick flame radiates (for a flame temperature of 1000
Co) approximately 150 kW/m2, compared with the
convective heat transfer rates calculated for water sprays
(1.7–2.5 MW/m2).
• The radiative transfer is negligible and its contribution to
flame extinguishment is ignored.
• Spray Penetration: is strongly dependent upon the drop size
distribution due to:
- The kinetic energy of a droplet is proportional to its
mass, which in turn is proportional to the cube of its
diameter.
- The aerodynamic resistance offered by the atmosphere
to the forward motion of a droplet is proportional to its
diameter.
- For 400–1500 µm drops (nozzle pressure 28–40 bar) the
penetration is 9 m.
- For 100–200 µm drops (nozzle pressure 14–20 bar), the
penetration is 2 m.
• Evaporation: is enhanced by maximizing the surface area
per unit volume of fire-fighting water.
Relationship Between Droplet Mean Diameter and the Total
Surface Area of the Spray:
• Consider the atomization of one liter
of water into a number of droplets
of equal diameter:
• So the diameter of each droplet is
given by:
• And the corresponding total surface
area per liter of the resulting spray
is:
• The plot shown illustrates that the
increase in surface area is achieved
with effective atomization.
Optimum Droplet Size:
• Given the importance of droplet size in fire-fighting sprays, it is
reasonable to ask whether there exists is an optimum droplet
size.
• In trying to answer this question, recall the three main
mechanisms for fire extinguishment by water:
– flame cooling.
– fuel cooling.
– Inerting the atmosphere through the production of water
vapor.
• If only one of these mechanisms dominates in the process of
fire suppression, then it might be possible to stipulate an
optimum drop size.
It was observed that different fire scenarios required different
fire-fighting tactics and there is no single optimum drop size for
fire-fighting.
• If flame cooling was the single most important factor, then a fine
spray would be preferable, the large surface-area to volume ratio
would promote efficient heat transfer and droplet evaporation,
leading to speedy fire extinguishment.
• If cooling the fuel was the primary objective, then a coarser spray
would be a better choice in order to ensure droplet penetration
through the fire.
Water Suppression Systems
-
Water Spray Systems.
Water Mist systems.
Water Atomization:
• Water has a high surface tension.
• Water droplet takes a spherical shape to satisfy the minimum
surface energy which makes it difficult to atomize.
• Atomization occurs only when the magnitude of the external
forces exceeds the surface tension force.
• This physical nature of water presents a fundamental problem in
nozzle design.
• According to the droplet size we can extinguish between spray
and mist water suppression systems.
Water Spray Systems:
Definition:
• The spray nozzle should provide droplet diameter 500 – 1500 µm with
a flow rate from 100 to 400 l/min, at pressures of 5 bar.
Methods of Spray Production:
• Pressure atomizers:
Water is moved within the nozzle and the ambient
air is still.
• Gaseous atomizers:
Water is stationary and the gas which effects the
atomization moves rapidly within the nozzle.
• Rifling nozzles:
The nozzle remains stationary, while the water is
given a forward motion and also a rotational motion.
The leading edge of the liquid takes the form of a
hollow cone.
Water Mist Systems:
Definitions of water mist:
• Is a water distribution of fine drops having a mean diameter of 80–200
µm and (V99) 99% of the droplets diameter less than or equal to 500
µm measured at the coarsest part of the spray in a plane 1 m from the
nozzle, at its minimum operating design pressure.
• Mawhinney and Solomon (1997)
proposed a classification system
which distinguishes between mist
and water spays.
• In practice, Class 1 and Class 2 sprays
are suited to the suppression of
liquid pool or spray fires or where
splashing of the fuel is to be avoided.
• Class 3 sprays are a better choice where fuel wetting is tolerable.
Designs of Water Mist Nozzles:
1. Single-fluid Low-pressure Systems:
• Operate below 12 bar, with larger orifice
producing larger droplet sizes ( 90 – 100 µm
at 5 bar) at high flow rates.
Advantage:
large orifice reduces the need for corrosion
prevention and water supply filtration.
Disadvantages:
- larger droplet sizes have higher velocity resulting in a higher
fall out rate of droplets from the mist reducing the amount of
mixing throughout the space.
- larger droplet sizes reduce the systems capabilities against
obstructed fires.
That is why the low pressure systems utilize water flow rates to
compensate the fall out losses.
2. Single-fluid High-pressure Systems:
• The single-fluid high-pressure systems operate at
pressures up to 210 bar which generates high
concentrations of small droplets.
Advantages:
• Small droplets improve mixing and heat transfer
characteristics due to greater surface area to
volume ratios.
• Increases in capabilities against obstructed fires
and allows the systems to utilize less water.
Disadvantage
• High cost due to the need for high pressure
system components (pipes, fittings, valves,
pumps).
3. Twin-fluid Systems:
• Twin-fluid systems require two fluids, water and
an atomizing fluid, both being supplied to the
nozzle using separate piping networks.
• These nozzles utilize a high velocity stream of air
or nitrogen to shear the water into small
droplets ( 30 µm ).
Advantage:
• Produces large quantities of small water droplets at low
operating pressures, usually less than 7 bar.
Disadvantage
• Additional piping, storage volume, and associated cost of the
atomizing fluid.
Experimental Work on Fire Suppression
by Water Sprays
Experimental Work On Fire Suppression By Water Sprays
The Scaling Problem:
• Most experimental data on fire loads and fire development are
based on reduced-scale experiments, one single fuel type or
fuel condition is normally investigated in each test set-up.
• In the process industry, fires may occur on such a large scale and
with such complex fuel mixtures that it is impractical or very
costly to replicate real scenarios in controlled experiments.
• There are problems in extrapolating the results of small-scale
fire tests to the full-scale situation.
• The advantages of laboratory-scale tests are economy and the
degree of control that may be exercised on the test conditions.
Experimental Suppression Tests On Unconfined Fires:
• Experiments performed on suppression of open fires of alcohol,
benzole, petrol, kerosene (fire point 60–68 Co ), gas oil (fire point 104–
115 Co ) and transformer oil (fire point 175–180 Co ).
• Different sprays were employed, with mean diameters in the range
(157–250 µm).
Results:
• The finer sprays promoted the most rapid extinguishment on
volatile fuel fires (alcohol, petrol, benzole).
• The coarser sprays were better suited to extinguishing the less
volatile fuels (gas oil, transformer oil).
• The primary function of water in unconfined fire suppression is to
remove heat from the body of the fuel.
Suppression Tests on Confined Fires:
• The extinguishment of room fires by spray jets was
investigated as a function of the water flow rate and the size of
the room.
Results:
• Increasing the room ventilation increases water quantity
required for fire extinguishment.
• Reducing the ventilation decreases the extinguishment time
significantly because water vapor assists in smothering the fire.
Studies concerned with confined fire scenarios in the offshore
and process industries has recommended the mechanisms of
flame cooling and atmospheric inerting by the production of
water vapor.
Development in Water Mist Technique
Used in Fire Extinguishment
Electronic Ultrasonic Water Mist
• The way to get water mist consists of small water drops is
mainly high pressure water mist and twin-fluid water mist,
which is direct high mechanical energy.
• Recent Researches began to concentrate on using electronic
ultrasonic water mist fire suppression technology.
• Electronic ultrasonic water mist fire suppression technology
is different from common water mist techniques in that it is
produced under ambient-pressure and don’t need so much
direct high mechanical energy.
Mechanism of Electronic Ultrasonic Water Mist:
This water mist is produced by
electronic ultrasonic device
shown in Figure.
• The circuit output high frequency ultrasonic signal to the device,
the piezoelectric transducer will then convert the electricity
energy into high frequency mechanical vibrations.
• The water above the piezoelectric transducer is rarefied and
compressed at a high frequency cycle, and water mist is
produced in this process
• Adiga K C and Adiga R (2006), invented an improved method and
apparatus for producing an extremely fine micron size water (14
and 30 µm diameter ) mist using an electronic ultrasonic device
that produces the mist at ambient-pressure.
• The apparatus produces
frequency up to 2.5 MHz.
high
• They found that the average extinguishment time of a 120 kW
heptane fire was around 5 min with six mist units operating at a
total of 0.66 l/min.
• A 70 kW methanol fire was extinguished in approximately 8 min.
Researches of Electronic Ultrasonic Water Mist:
• Kiran and Aniruddha (2012) measured the droplet size distribution
in an ultrasonic atomizer using photographic analysis with an
objective of understanding the effect of different parameters on
the droplet size distribution :
– Equipment parameters: operating frequency, power dissipation.
– Operating parameters: flow rate and liquid properties.
1. Effect Of Vibrational Frequency Of Atomizer:
The obtained results in the Fig. show
that the droplet size decreases with
an increase in the frequency.
2. Effect Of Ultrasonic Power Dissipation:
The droplet size increases with an
increase in the power of ultrasound.
3. Effect of Flow Rate on The Droplet Size:
The droplet size increases with an
increase in the flow rate.
4. Effect of Liquid Phase Viscosity on Droplet Size:
Viscosity
Droplet size decreases with an increase in the
liquid viscosity
Correlation To Predict Droplet Size:
A good agreement had been observed
between the predicted values from the
Eq. and the experimental data.
Surface Tension
Droplet size decreases with a decrease in
surface tension
Conclusion
• The ban on halon and the environmental friendly nature of water has
made it an attractive alternative to other fire-fighting media.
• Recent research are interested into the effectiveness of water mist for
fire suppression.
• Some of the first places where water mist fire suppression systems was
implemented were offshore oil and gas rigs.
• Development in using water mist in offshore application, requires more
full-scale demonstrations and tests.
• Water mist with very fine droplet diameter can be produced using
electronic ultrasonic technology.
References
1. A. Jones and P. F. Nolan, “Discussions on the use of fine water sprays or mists
for fire suppression”, J. Loss Prev. Process Ind., 1995, Volume 8, Number 1.
2. G. Granta, J. Brentonb, D. Drysdale, “Fire suppression by water sprays”,
Progress in Energy and Combustion Science 26 (2000) 79–130.
3. Liu Jian-yonga, Liang Donga, Zhao Zhea, Dong Wen-li, “Progress in Research
and Application of Electronic Ultrasonic Water Mist Fire Suppression
Technology”, the 5th Conference on Performance-based Fire and Fire
Protection Engineering (2011).
4. Kiran. A. Ramisetty, Aniruddha. B. Pandit, Parag. R. Gogate, “Investigations
into ultrasound induced atomization”, Ultrasonics Sonochemistry 20 (2013)
254–264.
5. Ragnar Wighus, “Fires on offshore process installations”, J. Loss Prev. Process
Ind., 7994, Volume 7, Number 4.