By: Mohammed Shaheen

Marine Based Fires and Their Suppression

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.

1. Gas FiresDiffusive 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.

Types of Offshore Fires:

2. Liquid FiresEnclosed 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 FiresBurning liquid flowing downwards surfaces or falling through the air.

Fire on the Sea Surfacelarge pool Oil fires on water form a major threat to the structure below an offshore platform.

Liquid Spray FiresThe spill of oil from high-pressure pipelines with a jet-like shape and performance.

3. Other Combustible MaterialsItems 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 non- conductive, 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.

• 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.

• Large open areas with high ventilation rates present a challenge to water mist technology.

• 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.

• 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).

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.

• 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.

Principal Action of Liquid Fire Suppressants:

• 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.

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).

Water discharges initially in the form of a solid jet then undergoes a transition to diffuse jet:

Modes of Application

•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 average size range from 100 to 1000 µm is of most interest for fire fighting.

The Spectrum of Droplet Sizes:

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-fightingEach water suppression mechanism requires different water dropletcharacteristics 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:

Where: - is the total heat of vaporization - β is the increase in enthalpy

Eq. (3)

• 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.

• 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 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.

• The behavior of spray droplets impinging onto horizontal heated surfaces is characterized in terms of the non-dimensional 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 fire-fighting 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.

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 tocompensate the fall out losses.

Designs of Water Mist Nozzles:

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

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 Work On Fire Suppression By Water Sprays

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

• 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.

Electronic Ultrasonic Water Mist

Mechanism of Electronic Ultrasonic Water Mist:

• 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

This water mist is produced by electronic ultrasonic device shown in Figure.

• 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 high frequency up to 2.5 MHz.

• 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.

The droplet size increases with an increase in the flow rate.

3. Effect of Flow Rate on The Droplet Size:

2. Effect Of Ultrasonic Power Dissipation:

The droplet size increases with an increase in the power of ultrasound.

4. Effect of Liquid Phase Viscosity on Droplet Size:

Viscosity Droplet size decreases with an increase in the

liquid viscosity

Surface TensionDroplet size decreases with a decrease in

surface tension

Correlation To Predict Droplet Size:

A good agreement had been observed between the predicted values from the Eq. and the experimental data.

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.