Dry chemical
As the construction of new LNG facilities continues throughout the world and is constantly in progress, fire protection equipment will be on the forefront in the supplying.
The volatility of LNG liquid is such that when spills occur in depth or liquid spillage is diverted to containment, the vapours cannot be completely suppressed.
The high expansion foam is the recommended method for dealing with unignited LNG spillage: it reduces vaporization and thereby vapour size and migration, but cannot completely prevent vaporiztion
The only effective method for extinguishing an LNG fire is to use dry powder.
Dry chemical (BC ones in the case of LNG facilities) is a proven firefighting agent whether it is used in portable extinguishers or in fixed pipe fire suppression systems.
Advantages of using dry chemical powder are several:
While internationally accepted guidelines like NFPA 17[[1]] and EN 12416-2+A1[[2]] designs perfectly the fire-protection equipment, they currently avoid making any define recommendations on the extinguishing powder type: they let the choice in the use of dry chemical.
Thereby, as several kind of extinguishing powder are available on the market; how to choose the right one? May I choose a potassium-based powder with silicone protection? And what about sodium based dry chemical?
To select right, you need to ensure the efficiency on fire, the price of raw material, the sustainability of the powder and its flowdity in the piping.
First, let’s discuss how extinction occurs with the use of BC dry chemical powder.
Basically, when dry powder is correctly applied to a fire, it attacks and extinguishes it in two ways: chemically and physically.
Firstly, dry powder works by interfering with the reaction of combustion of the fuel by trapping some free radicals and by cooling the media with the transformation of the grain into gases. (free radical inhibition effect and cooling effect).
Secondly, by generating a fine powder cloud over the surface of the fire. This excludes the oxygen and so has a smothering effect on the fire (inerting effect and grid effect).
The combination of these processes leads to very quick knockdown of flames and stopping the combustion process: the impact/proportion of each effects is depending on how the powder is used and which is the mean size of the grain.
The Cooling Effect and Free Radicals inhibition are depending of the chemistry of the dry powder.
Hence, concerning the cooling effect, in one hand, the transformation of the potassium-based powder leads to form steam, CO2 and caustic potash. And in another hand, the transformation of the sodium-based powder leads to form steam, CO2 and caustic soda.
These decompositions cost energy to occur (few kJ/g for Na-based or for K-based versus 57 kJ/g for water); causing a lowering of the temperature and a slowdown in the combustion reaction.
Imagine now this fine powder cloud from a solid point of view: a “cold” (ambient temperature) particle of powder is projected in the hot atmosphere of a fire. Hence, a heat transfer occurs from the air towards inside of the grain. This thermal conduction is reducing the global temperature, and one again, slowdown the combustion. Here again, the composition of your particle enhances this phenomenon.
These endothermic reactions and this thermal absorption are more efficient than the exchange surface between the grain and the fire is important. So, the cooling effect is promoted with small grains.
Free Radicals can explain why dry chemical extinguishes fires. Combustion is an exothermic chemical reaction characterized by a chain reaction. These reactions are particularly suited to explain, firstly, the growth (in exponential form) of the combustion reactions, and secondly, the inhibiting effects on the extinguishment.
Radical reactions are often chain reactions; after a relatively slow period in which are formed the first free radicals. Then, reaction spreads very rapidly to a large number of molecules. then stops by roughly coincidental disappearance of radicals (it is known that free radicals are deactivated by collision and contact with a wall or with another molecule; which results in the formation of stable molecules and interruption of the chain).
The action of extinguishing powder on the flame can be explained by an inhibing action[3]. In this case, a quick raising of surface temperature of the grain causes a transformation of the components (and particularly metallic sodium or potassium ions ...) which could then react with the free radicals in the flames.
In summary, the efficiency of a dry chemical is related to its particle size and its composition.
The inerting effect is provoked by the both addition of solid (grain) and gas (expellant gas) in a steady-state combustion.
Like said previously, powder cloud, by its mass and its presence, causes changing in the proportion of fuel and oxidizer. Large is the particle, larger is the dilution.
As 0.03 kg of CO2 (or 1.5 Nm3 of N2) is necessary to transport 1kg of powder (that is roughly the expellant gas ratio), the presence of extra-gas () will disrupt the combustion reaction.
These contribute to dilute the proportion of fuel vapor and available oxygen in the air.
In the same time, grains of the cloud act as a 3D grid which blanket the fire. In that case, they produced mesh screen (same phenomenon used with the Safety Lamp used in the coal mines, as Davy lamp). acts as a flame arrestor. The grains slow and shrink the flames until they disappear.
These inerting/diluting effect and grid effect are more efficient with powder with a large particle size.
Thus, you need a dry chemical powder whose particle size is finely controlled according to the use made of it to promote the impact of the expected effect.
Intuitively, the effectiveness against the fire shall be consider. But ‘caking’ resilience, flowdity in pipes and environmental issues should be also pondered and taken into consideration.
While the standards do not make any differences between sodium or potassium bicarbonate agents for the design of the equipment, variances exist like application rate, raw material price.
According to the NFPA 17 standard [1], on class B and C hazards, dry chemical powder extinguishing system must use sodium or potassium bicarbonate agents, and users must refer to manufacturer recommendations to define an application rate (with a duration of discharge within 30 seconds).
Meanwhile, the EN standard [2] proposes application rates based on sodium bicarbonate powder (evolving between 0.6 kg/m3 and 1.2 kg/m3 for enclosed volume and local application).
As we saw in the background, application rate should vary per type of fire. ‘Slow fires’ (case of pool fire) and ‘dynamic fires’ (like torch fire) must not be considered with the same way.
Thus, if we based 1960’s pool tests on LNG fire done in Marinette [[4]], the chemistry of the powder takes an important part in the suppression of fire: potassium bicarbonate powders are more efficient to extinguish fires than the sodium bicarbonate (roughly 1.5 more efficient). But in the same time, if you consider the apparent specific gravity of the compounds (0.98 for Na-based and 0.88 for K-based), the equipment in Na-based compounds should be only 35% larger.
Now, if you consider “dynamic fire” (for instance, for protection of Pressure Release Valve vent on the top of LNG storage), tests done in 2014 [[5]] show the high efficiency of the sodium bicarbonate-based powder.
Moreover, raw material prices (with an extinguishing agent grade) present a large differential. NaHCO3 powders are low cost of produced: KHCO3 is between 3 and 5 times more expensive than NaHCO3.
In conclusion, the equipment load with sodium bicarbonate is are roughly 2.5 times cheaper than one with potassium loading at same fire extinguishing effectiveness.
The “caking” phenomenon occurs when moisture chemically reacts with a dry chemical. This reaction results in materials that, being hydrated by moisture, stick together to form a large agglomerate, or what is commonly referred to as lumps. Poor maintenance or maintenance in damp conditions can lead to powder caking. This in turn, can lead to the failure of equipment (e.g. clogged valves) at a crucial time.
Then, be sure that your chemical powder has a special treatment/coating strong enough to provide a resistance to moisture absorption (water-repellant) and preserve the extinguishing agent from an inappropriate maintenance.
Failure of dry chemical safety system can also result from the packing down of the powder inside a tank body. This is a particular problem for vehicle equipment because the vehicle vibration will cause the powder compress under its own weight.
Hence, be sure that your dry chemical powder has a fluidizing capability (even after long term storage). This ability is primordial and essential when the powder shall travel far (mostly if flexible pipes), through the pipes and fitting to the final nozzle.
Furthermore, the system acceptance tests require a discharge of the powder within 30 seconds (with a minimum of powder remaining present in the tank, i.e. few percents of the total amount); which means that the powder used shall have a high sliding ability.
Thus, be sure that your chemical powder has a special additive which provides a free-flowing into the pipes.
Within the context of COP21, the environmental aspect of the extinguishing powder should be considering: production of raw materials – impact after using – end of life of unused powder.
Basically, sodium bicarbonate and potassium bicarbonate are produced by reacting carbonate with carbon dioxide, then recrystalizing it (better known, since mid-19th century, as Solvay Process). Some of the processes involved in the Solvay Process are exothermic, they release heat (because of this mild condition, water is used during the cooling processes without causing disruption to aquatic organisms). Saving energy to produce.
Bicarbonate based powder, unlike multipurpose dry chemical, are known to be no abrasive, no corrosive and without toxic effects: meaning that, after a fire extinguished by BC powder, spilled unused grains will not impact saved equipments and cleaning staff (particles can be easily vacuumed). Thereby, you save cost by a light cleaning and in re-equipement.
As dry chemical powder contains some effective and durable additives (as water-repellant), its lifetime can reach a decade (and more).
In conclusion, saving lifetime cuts by less purchases and refills.
The dual agent application is the most effective tool for mitigating in extreme risk areas: the benefits of the extreme effectiveness of the dry chemical are added at a foam application (the foam secures the hazard by laying a vapor-suppressing blanket on the fuel and helping to cool the fuel).
But be ensure that your synthetic foams concentrate (such as AFFF) or your protein foam concentrate do not harm by the presence of powder (foam effectiveness should be identical or inside the 75% of effectiveness). Indeed, most dry chemicals contain metal stearates or silicone molecule to waterproof them, but these will tend to destroy the foam blanket.
Require a powder with a compatibility with foam approval.
GRANITO extinguishing powder, made by Ai GROUP, is ideal to be used in units which have been designed and built from the ground up for offshore applications and extreme hazard areas and suited for Offshore Oil & Gas Facilities, Marine Vessels, LNG plants, Aviation, Heliports, LNG Vessels and bunkering stations.
[1] NFPA 17- 2013 Edition : Standard for dry chemical extinguishing systems
[2] EN 12416-2+A1 – september 2007 edition : Fixed firefighting systems - Powder systems - Part 2 : design, construction and maintenance -
[3] ANPI – avril 1981
[4] Fire Protection Solutions for Liquefied Natural Gas - Ansul
[5] Unpublished results from INERIS for Ai GROUP