INDUSTRIAL FIRE JOURNAL / October 2005

REFINERY PROTECTION

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facilities have piping laid in trenches to contain anyleaks. ESTI has a 19m2 L-shaped trench to simulatejust such an LNG pipeline.

The new facilities at ESTI, referred to as “the props”,consist of four parts - the trench; two 1.2m deep pitswith areas of 9.3m2 and 65m2; and a third pit, 2.4mdeep covering some 45m2. The three pits representtypical impounding basins in an LNG facility.

The third pit includes a simulation of an LNGmanifold on an LNG tanker and the steel deck andhull of a ship simulating the facilities used duringLNG offloading. The added depth enables a waterbase to be used that simulates the sea.

Vapour dispersionIf LNG spills on to the ground, it starts to vaporiseinstantly. The cold LNG vapour condenses moisture inthe air to produce a white vapour cloud. One optionis to allow this process to continue, provided you cancontrol the vapour and there is no ignition sourcenearby.

Alternatively, the flammable vapours can bedispersed away from potential ignition sources morequickly by warming up the LNG. Water or lowexpansion foam should not be applied directly on toLNG to do this, since the heat transfer from the watercauses a severe reaction as a result of the LNGvaporising too quickly. This was graphicallydemonstrated in the ‘marine pit’ where LNGvaporisation rates on water were around five timesthat on land.

Water curtains can be used to control the drift ofthe LNG vapour. But these can be difficult to placecorrectly with changing wind direction, and thewater must not be allowed to come into contact withthe LNG spill.

M ike Willson explains, “Natural gas is thecleanest burning of all fossil fuels, andglobal demand for it as an energysource is growing rapidly. The tests

were organised to re-evaluate current fire protectionequipment and techniques in realistic fire scenarios”.

“The venue for the tests was the new world-class LNG testing and training facility developedand sponsored by BP in collaboration with TexasA&M University Emergency Services TrainingInstitute (ESTI).”

Cold gasWhen natural gas is chilled to -164°C at atmosphericpressure, it condenses into a cryogenic liquid, takes620 times less space, and can be economicallyshipped around the world aboard ocean-goingtankers, just like oil. Once landed, it is transferred tostorage tanks, and then returned to its gaseous formbefore being fed into pipelines to reach end users.

When LNG is warmed up and turned into naturalgas it is flammable within a very limited range. If themixture of natural gas with air contains less than 5per cent natural gas, it cannot burn because it is toolean. If the mixture contains more than 15 per centnatural gas, it is too rich to burn.

However, the big problem with LNG is that if itcatches fire it gives off twice the amount of heat ofan equivalent-sized gasoline fire! Heat emissions arethe principal cause of damage from LNG fires,capable of causing severe damage to personnel,structural steelwork, plant and adjacent facilities ifleft unchecked.

LNG terminals and facilities follow practices thatare different in some ways from those in otherindustrial installations. For example, many LNG

©

In this report,exclusive to IFJ,

Mike Willsonhighlights the

results of aseries of

pioneeringlarge-scale

tests on theeffectivenessof foam and

applicationequipment on

LiquefiedNatural Gas

(LNG) fires byleading foam

manufacturer,Angus Fire.

Angus LNG Turbexapplies Expandol high

expansion foamdespite the searing

heat of the LNG fire.

New large-scale LNG fire tests

23INDUSTRIAL FIRE JOURNAL / October 2005

REFINERY PROTECTION

A good quality high expansion foam is a far moregentle and effective way of warming LNG andcontrolling vaporisation. The highly aerated foamprovides a thick, light blanket with much lower watercontent per unit volume.

The 65m2 pit was used to simulate an unignitedLNG spill from pipework or a bulk storage tank. Ahigh expansion foam was observed to reduceground-level vapour concentrations within secondsto well below the Lower Flammable Limit of 5%. Alayer of frozen foam was formed at the LNG/foaminterface that supported several feet of additionalfoam.

Ice tubes also formed where the vapours boiledthrough the foam blanket. As the vapours ascendedthrough the foam, they were warmed, becamelighter than air, rose upwards, and dissipated safely inthe air above potential sources of ignition.

Fire controlIf vapour finds an ignition source it is likely to beoutside the white vapour cloud, and the flames willburn back to the liquid pool and generate intenseradiant heat. A high performance dry chemicalpowder like Monnex can put the fire out quicklyprovided it is not too large and does not have anyobstructions.

However, this is not always desirable since apotentially flammable vapour cloud maysubsequently build up above the liquid and pose arisk of reignition. Should the LNG vapours enter asemi-confined space, damage could result from theresulting ignition.

The accepted approach is to use high expansionfoam of expansion ratio around 500:1 to achieve acontrolled burn-off and in the process greatly reducethe radiant heat emissions. The principle is to apply itfast enough and at a high enough rate to get controlquickly and avoid potential risk to personnel, plantand equipment.

As the lower portion of the foam freezes, ice buildsup within the foam blanket, venting the vapour in acontrolled way, producing flames of greatly reducedintensity on the surface. The foam bubbles insulatethe LNG from the heat source above, controlling therelease of vapour. As the heat breaks down the foamblanket, more foam must be applied regularly as aseries of pulses until all the LNG has burnt away.

Foam and application techniquesInitial small-scale tests were carried out to assessthe effectiveness of different types of foam andapplication technique on a range of LNG fires. Avariety of application techniques were usedincluding low, medium and high expansion as wellas low expansion compressed air foam (CAF)systems. Low expansion foam delivers too muchwater to be effective. Dry CAF performed slightlybetter when applied gently, but the logistics andequipment required to produce the foam wasimpractical.

Slow-draining Tridol ATF foam applied gentlythrough a hand-held medium expansion foambranchpipe was observed to control the fire andachieve a reduction in heat radiation of over 90%.This was only considered suitable for small-sizedspills due to the high application rates required.

Large-scale tests were then carried out to simulatea major spill fire in a containment pit or the bunded(diked) region surrounding an LNG storage tank. LNGwas placed in a huge 65m2 test pit, the largest of itskind in the world.

The procedure to ignite the LNG was to allow avapour cloud from the LNG to extend out from thepit over a distance of open ground. A white cloud wasobserved where the cold LNG vapour condensed

moisture in the air. The flammable vapours werethen ignited using a torch on a 3m long pole.Flammable vapour levels of 5 to 15% are normallyexpected on the fringes of the visible cloud, butportable gas monitoring equipment detectedpockets of flammable vapours up to 150 metres fromthe visible cloud.

Once the vapour cloud was ignited, flames shot inall directions consuming and mixing the gas, whichburned through the extended cloud back to thesource, turning the pit into a burning cauldron of fire.The flames reached over 30 metres into the air andgave off so much heat that personnel were forced toretreat to a safe distance.

Historic test data using various foam applicationrates with outdated LNG storage and handlingtechniques provides no margin for safety in themodern world. Only foam and equipment proven tobe effective on the ESTI fire ground for prolongedperiods of use are now acceptable to BP. Only aminimum application rate of 10 l/m2/min throughwater turbine-driven foam generators delivering500:1 expansion ratios and capable of 90% radiationreductions within 60 seconds proved effective underrealistic site conditions.

Intense heat emissions from LNG fires means thatordinary high expansion foam equipment is totallyunsuitable. It quickly distorts, buckles and ceases togenerate any foam. That is why two speciallyengineered LNG Turbex high expansion foamgenerators from Angus Fire were used throughoutthe test programme. The test programme iscontinuing this year with the construction ofadditional LNG fire testing facilities. As LNG becomesmore prominent in the world’s energy supply mix,Angus Fire’s latest test data will help emergencyresponders world wide to prevent and combat LNGfires.

Make sure you talk to Angus Fire beforefinalising your LNG facilities upgrade or new buildto obtain a reliable and effective system for whenyou need it most. ❙

To protect fixed rooftanks (domed coned roof)for full surface fire, thereis the option of the toppourer foam system orsub-surface system wherefoam is injected into thebase of the tank andallowed to ‘float’ to thesurface of the fuel.