GAS FIRES

Helen Verstraelen

FIRES: what you need to remember • Fire safety aspects: prevention, protection, detection, combat • Transport of heat • Mecanism of a fire • Flamability, speed of reaction and explosion limits • Fire triangle, fire tetrahedron • Fire risk • Fire development • Fire classes • Causes of fire, fire prevention • Fire detection • Fire fighting: material, methods, installations,…

DIFFERENT TYPES OF GASES • LNG: liquefied natural gases: especially methane; relatively stable • LPG: liquefied petroleum gases: especially butane and propane relatively stable • Chemical gases: liquefied inflammable gases: unsaturated and possible unstable ex: ammonia, VCM

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CRITICAL TEMPERATURE / PRESSURE • Critical temperature: the temperature above which a gas can not be liquefied, no matter what the pressure. There is no difference between the liquid and the gas: “supercritical fluid” • Critical pressure: pressure required for liquefaction at the critical temperature • Liquefaction possible by: • Cooling down • Compressing (not above critical temperature) • Combination cooling and compressing

CRITICAL TEMPERATURE / PRESSURE

CRITICAL TEMPERATURE / PRESSURE gas

critical temperature (°C)

NH3

132

critical pressure (atm) 111.5

O2

-119

49.7

CO2

31.2

73.0

H2O

374

217.7

1 atm = 1.01325 bar

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TRANSPORTATION • Full pressure method: pressurised at normal temperature. Cargo holds are spherical or cylindrical • Full refrigerated method: low T° and pressure just above atmospherical pressure. Material strength at low temperature is important • Semi-refrigerated (semi-pressurised) method: at the boiling temperature and corresponding pressure

TRANSPORT

TRANSPORT

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METHANE • CH4 • Most important component of LNG • Transported at -165°C (boiling temperature) / 1.04 kg/cm² • S.G. liquid: 0.474 • S.G. gas: 0.554 • Explosion limits: 5.3 - 14% • Flashpoint: low • Self ignition temperature: 595°C

LNG TRANSPORT Safety and Risks of LNG Transportation • LNG shipping so far has an excellent safety record • No shipboard fatalities over the life of the industry associated with cargo • No major losses of cargo and only one minor LNG on board fire (lightning strike near vent riser, cargo tanks not affected) • Two groundings resulting in major hull breaches without cargo loss BUT you should never rest on your laurels. R&D programs should continue to search for improvements.

PROPANE • • • • • • •

C2H8 Transported at -43°C / 1.04 kg/cm² S.G. liquid: 0.583 S.G. gas: 1.55 Explosion limits: 2.1 - 9.5% Flashpoint: - 105°C Self ignition temperature: 470°C

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BUTANE • • • • • • •

C4H10 Transported at - 1°C / 1.04 kg/cm² S.G. liquid: 0.602 S.G. gas: 2.9 Explosion limits: 1.8 - 8.5% Flashpoint: - 60°C Self ignition temperature: 406°C

LPG • Mixture of gases, gases, mainly propane and butane • Stored under pressure to keep it in a liquid state • Boiling point of LPG varies from about −44°C to 0°C, so the pressure required to liquefy it is considerable • LPG is an attractive fuel for internalinternalcombustion engines because it burns with little air pollution and little solid residue, residue, it does not dilute lubricants, lubricants, and it has a high octane rating.

ETHYLENE OXIDE • • • • • • •

C2H4O Transported at -11°C / 1.04 kg/cm² S.G. liquid: 0.913 S.G. gas: 1.52 Explosion limits: 3 - 100% Flashpoint: - 57°C Self ignition temperature: 429°C

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AMMONIA • • • • • • •

NH3 Transported at -32°C / 1.04 kg/cm² S.G. liquid: 0.683 S.G. gas: 0.597 Explosion limits: 14 - 28% Flashpoint: Self ignition temperature: 652°C

VINYLCHLORIDE • • • • • • •

C2H3Cl Transported at -14°C / 1.04 kg/cm² S.G. liquid: 0.965 S.G. gas: 2.20 Explosion limits: 4 - 22% Flashpoint: - 78°C Self ignition temperature: 472°C

GAS FIRE • Molecules of gas collide => energy is liberated • The higher the temperature, the more energy => molecules fall apart into atoms and free radicals => combine with new combustible molecules .... Chain reaction • Accelerated or branched chain reaction leads to an explosion

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GAS FIRE

EXPLOSION • Can be mechanical, chemical or nuclear • Mechanical: no chemical reaction. Ex: breaking of vessel containing compressed gas • Chemical: fast exothermic chemical reaction. Ex: polymerisation, decomposition, fast burning, nuclear..

• In case of leakage (gas, liquid) • Mixture gas-air and no ignition • Mixture gas-air and immediate ignition: fire • Mixture gas-air and postponed ignition: explosion

DEFLAGRATION / DETONATION • Deflagration most common • Speed of flames 1 to 1000 m/s (compared to stationary observer) • Pressure: some bars

• Detonation: • supersonic (compared to speed of sound in non combusted gas in front of the flames) • 1500 - 2000 m/s • Shock wave 15 à 20 bar

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DETONATION • Probability of detonation depends on type of fuel • Very reactive products like hydrogen acetylene or ethylene might explode when an accident occurs • Detonation of a pure mixture methane / air is not known

POSTPONED IGNITION • Depending on dispersion of gas cloud

SPECIFIC GRAVITY • Gases transported by ships: • Liquid: lighter than water • Gaseous: often heavier than air, except for methane, natural gas and ammonia

• ! Pay attention: influence of temperature: methane at –110°C is as heavy than air

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SPECIFIC GRAVITY gas

specific gravity liquid to water (at 25°C) or boiling point

specific gravity gas to air

Methane - CH4 / natural gas

0.464 at –164°C

0.5537 / between 0.6 and 0.7

Ammonia - NH3

0.823

0.596

Benzene - C6H6

0.873

2.6961

Butane - C4H10

0.599

2.0061

Ethylene (Ethene) C2H4

0.568 at –104.C

0.9683

Hydrogen sulfide - H2S

0.915 at –60°C

1.1763

Sulfur Dioxide - SO2

1.458 at –10 °C

2.264

Chlorine - Cl2

1.562 at –34°C

2.486

DISPERSION OF GAS

DISPERSION OF GAS Near deckhouses

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DISPERSION OF GAS Near deckhouses

CONTRIBUTING FACTORS • The consequences of a gas fire/explosion depend on: • • • • • • •

Type of fuel and atmosphere Dimensions gas cloud and concentration Mixture Place of ignition Power of ignition Dimension, position and type of venting Position and dimension of equipment and structural elements

CONSEQUENCES GAS LEAKAGE Garrisson 1988

• 42 % explosion of gas clouds (inside and outside) • 35 % fire • 22 % explosions (uncontrolled reactions, BLEVE’S, mechanical explosions, explosions in equipment • 1% evaporation / dilution in air (by wind)

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PRESSURE JET FIRE •Fire of gas leakage under pressure : “pressure jet fire”

VENT MAST FIRE

POOL FIRE •Fire of pool of liquid gas “pool fire”

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POOL FIRE • Refrigerated transport • About 30% of the leaked product will evaporate; what’s left will form a pool • Low temperature may give cracks in the vessels construction: Spray water to protect the metal and to avoid heating of the construction • Gas fire: speed of flames 3 x speed of fuel fire

POOL FIRE • The flames will be blown away by the wind • Methane: height flames approximately 3x to 4x diameter pool • Important: reduction of heat effect • Heat will be dangerous over 100°C (electrical isolation will melt at 130°C)

POOL FIRE

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POOL FIRE • Pay attention for convection heat (air or water) • Pay attention for radiation heat: • Near seat of fire 170 kW/m² • Skin problems at 4.7 kW/m² • Second degree burns at 6.2 kW/m² • Most flammable products catch fire at 12 kW/m²

FLAMABLE GAS SENSORS

FLAMABLE GAS SENSORS • Two ceramic beads (pellistors) with embedded platinum coils are heated to ~450°C. One pellistor is impregnated with a catalytic material that, at the given temperature, oxidizes the gas (O2 must be present!!!!) and thus forms additional heat which can be detected by measuring the resistance of the platinum coil. Using a Wheatstone bridge with a second, deactivated pellistor as a reference, the bridge current is approximately proportional to the gas concentration in the 0%–100% range of the lower explosive limit (LEL).

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FLAMABLE GAS SENSORS • Advantages: can be calibrated for very large range of flammable gasses. Methane is often used for calibration, calculations give results for other gasses, using enthalpy (heat released during combustion) • Mixture of flammable gasses: calibrate for least sensitive gas. That way the detector will err on the side of safety • Disadvantage:in the presence of lead, sulphur, silicone vapours, halogens the sensor can lose all sensitivity

FLAMABLE GAS SENSORS

FLAMABLE GAS SENSORS • The IR measuring principle is based on the fact that gas molecules are excited by IR light of a certain wavelength and so produce vibrations while partly absorbing energy from the light. • Compared to the original IR light intensity, the attenuated intensity within a defined fixed optical path is a measure for the gas concentration. A second beam with a wavelength not absorbed by gas can be used to measure the original IR light intensity.

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FLAMABLE GAS SENSORS • Fluctuation of power of the IR source, contamination of the mirror or window, as well as faults caused by dust or aerosol in the air, affect the 2 detectors similar • Can be used in a O2 low or free admosphere!!

FIRE FIGHTING

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FIRE FIGHTING

FIRE FIGHTING

Water deluge system for protection

FIRE FIGHTING

Water monitors

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FIRE FIGHTING

Powder extinghuishing

FIRE FIGHTING • DO NOT EXTINGUISH !!!!! Except when the seat if the fire is isolated and the surface is reducing • OR • To safe human life • When the outpouring of the gas can no longer be controlled by water fog • When fear for expansion exist

FIRE FIGHTING • Goal of fire fighting is the cooling down of: • Seat of fire • Dangerous zone downwind (turning of the vessel might be necessary • Equipment exposed to radiation heat

• Determine in advance the area’s that need cooling • Attack the fire from the weather side

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WATER • Seldom used for extinguishing, more for protection. • Never to be used on a liquefied gas pool (spreading) • Use large jet • Spray a water film on the surfaces that need protection • Good housekeeping of the installation is essential (corrosion, ice, salt,...)

EFFECT OF WATER • Extinguishing effect on fire: • Cooling: very limited when flashpoint is lower than the water temperature • Suffocating: volume steam = 1700 x volume liquid • Emulsification: when not soluble in water • Dilution: when soluble in water. Use 8 à 20 l/min/m²

EFFECT OF WATER • Control of combustion: • Even when the fire can not be extinguished, it can be controlled • Continuous supply of water is necessary: 20 l/min/m² of the surface on fire

• Protection of surfaces: • • • •

containers: > 10 l/min/m² exposed surfaces superstructure: > 4 tot 10 l/min/m² cables: 12 l/min/m² others: > 10 l/min/m²

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EFFECT OF WATER • Prevention of expansion of the fire: • Depending of type of fuel: about 10% of what is necessary to extinguish the fire • Use the fog position of the nozzles • Immediate availability important • Closed valves need also be cooled • Try to direct the gas cloud and avoid contact with sources of ignition.

POWDER • 3 types often used: • Sodium bicarbonate • Potassium bicarbonate • Urea potassium bicarbonate

• Very effective for small LNG and LPG fires • Use the powder until all the flames are extinguished • Keep cooling down after extinguishment

POWDER • Gas codes require fixed dry powder system which can deliver powder to any part of cargo area with fixed monitors and hand held hoses • Also jetty manifolds often protected with portable or fixed dry powder systems • Effective for • Gas fires on deck • Jet fires on holed pipes • Used for vent mast fires

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POWDER • Every powder has its own properties • When used with foam: make sure that the powder does not degrade the foam • When used with water: make sure that the powder does not disolves in the water

MECHANISM • FIRST: • inhibiting chain reaction (absorption of free radicals in combustion process) • Start the reaction by the generation of active radicals CH2, OH, O and H • Na en K salts combine with H and OH to make them rare • Ammonia salts provoke an endothermic polymerisation reaction

MECHANISM • SECOND: • Decrease evaporation by reducing the radiation heat • Inerting by reducing the oxygen level and with the production of CO2 • Slowing down the fire due to the formation of a surface film

• Negligible cooling effect. Beware for re-ignition. cool down hot surfaces with water before extinguishing with dry powder.

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POWDER ON BOARD • Never change the type of powder without consulting the constructor / designer • Never mix deferent types of powder • Powder can not always be used (not almighty): not for metal fires, cellulose nitrate, class A fires if the complete surface can not be covered with the fire (no cooling effect)

DISADVANTAGES • When used on electronically equipment, an almost irremovable membrane can be formed at higher T° (above 127°C) of humidity > 50% • Erosion is possible • Visibility almost zero while using powder • Powder can cause breathing difficulties. When used on large surfaces, evacuation is needed • Exercises are necessary for the correct use of powder

FOAM • Mostly used on surface of pool fires (when confined in bunded area): • • • •

Reduces vaporisation rate Intensity of the pool fire is limited Foam depth at least 1 or 2 meter is needed High expansion foam of about 500 to 1 expansion rate has proven to be the best for this purpose

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FOAM • Used on un-ignited pool fires: • Reduces extend of gas cloud • Stable foam can freeze at the interface of the foam-gas cloud and will so reduce vaporisation rate. Make sure the right foam is used. If it breaks down into the liquid, it may increase the vaporisation rate.

• Foam will not extinguish a liquefied gas fire. • Needs to be applied to substantial depth. This is not easy on ships and therefore only found on terminals and not on gas carriers.

INERT GAS • Preventional measures with IG generator:

METHANE

• Permanently inerting of interbarrier spaces or cargo related spaces

CO2 • High pressure bottled CO2 in case of fire in enclosed space • Ventilation stopped, space closed • Evacuation needed • CO2 injection produces electrostatic charging (be aware when used as precautionary measure in flammable atmosphere)

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STATIC ELECTRICITY CO2 Incidents: • While attempting to inert a jet fuel tank, by using a portable CO2 fire extinguisher, an explosion occurred and killed two navy firemen. • On board a tanker, four people were killed in an explosion while inerting a naphtha tank using CO2 cylinders. • In Bitburg, Germany, twenty-nine people were killed as a result of an explosion while witnessing the demonstration of a newly installed CO2 fire-extinguishing system for a partially filled jet fuel tank.

STATIC ELECTRICITY CO2 • When liquid CO2 expands up to absolute pressures of less than approximately 5 bars, the result is the formation of small particles of solid CO2 (dry ice). As the two-phase solid/gas flows through the piping, static charges are produced by the particles rubbing against other particles, between themselves, piping and equipment. • These charges accumulate in the zones that are not earth/grounded at the end of the pipelines, most often in valves and nozzles. The size of these fields, can reach values of between 50 and 180 kV/m. • Similarly, static electricity can be generated by the dry ice particles after they leave the discharge nozzle. • The pressure and impurities in the CO2, equipment materials in transfer line hoses, etc. all influence the generation of static electricity.

CO2 • Used into safety relief valves or vent mast fires (after shut down) • CO2 is not a cooling agent. Boundaries must be cooled with water as re-ignition is possible with the introduction of oxygen.

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EMERGENCY PROCEDURES • Ship procedures, communication is vital • Plans should deal with: • • • • • •

Missing or trapped personnel Collision Grounding water leakage into hold or interbarrier space Cargo containment leakage Cargo connection rupture, pipeline fracture or cargo spillage • Fire in non cargo area’s • Fire following cargo leakage • Fire in compressor or motor room

EMERGENCY PROCEDURES • Terminal procedures: • Less standardised than on ships • Command of an on-site incident controller, often overtaken by port authority • Incident plans • Cargo spillage or fire on board a ship alongside a jetty • Cargo spillage or fire while loading or receiving cargo • Cargo spillage or fire not associated with loading or receiving cargo

EMERGENCY PROCEDURES • Emergency shut down (ESD) – ship/shore link • Exist on all gas carriers and large terminals • Communication is essential • Loading: first terminal ESD, then ship • Unloading: first ship ESD, then terminal

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EMERGENCY PROCEDURES • Emergency release systems (ERS) • Hand arms • First alarm between predetermined limits (movements of ships): safe shut down • Second alarm outside these limits: disconnection with limited spillage • 2 ball or butterfly valves close in 5 seconds • release coupling opened • arm swings by counter balance

• Break-away couplings for hoses For smaller terminals which work with hoses

EMERGENCY PROCEDURES

EMERGENCY PROCEDURES • Removal of ship from berth • Burning ship alongside is less a hazard if kept alongside where shore services can provide assistance • In case of an emergency within the terminal, safe practice to remove ship to prevent involvement • Consultation master, terminal, port authority

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EMERGENCY PROCEDURES • Ship-to-ship transfer • Communication between ship masters vital • One ship can use emergency resources of other • Sometimes separation better option to minimise overall risks and allow unobstructed access by fire tugs and salvage services

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