The suppression and control of full surface combustible tanks fire demands more commitments in terms of equipment, resources and human logistics. In fact, given the intensity of the experienced losses, fire safety industrial units have considerably enhanced the techniques used to successfully extinguish and control fires faced in the outsized inflammable storage tanks.
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These techniques are persistently updated since the major fires and explosions which might cause earthquakes, lightning, demises or injuries arise from inapt safety practices adopted when flammable or combustible liquids are being handled.
Thus, the drastic measures assumed while using, handling and storing flammable chemicals or liquids necessitate suitable actions which might guarantee the protection of individuals from any kind of explosion or fire.
Applicable terms used
Given the nature of the subject being handled, it becomes imperative to highlight and shade light on different phrases that relate to the topic. First, the liquid vapor pressure as a term tends to designate the force that vapor exerts when chemicals or liquids are sealed in a vessel.
Second, according to Shelley et al., the liquid boiling point (BP) tends to define the measurable temperature where both the atmospheric pressure of a liquid and its saturated vapor pressure are equal (p.63).
Finally, NFPA defines the liquid flash point as the least temperature that allows liquids to produce sufficient vapor which forms a combustible fusion with any air that is near the surface of the liquid. This implies that, the term flash point is quite essential when classifying both flammable as well as combustible liquids.
NFPA 30 classification of combustible and flammable liquids
Basically, any liquid whose vapor pressure is equivalent to or not more than forty pounds per square inch at 37.80C and equally has a flash-point which is less than 37.80C (1000C) can be termed as flammable liquid (NFPA 6-198)(NFPA 30–4.2.3).
On the other hand, liquid mixes consisting of ninety-nine percent or even greater total mixtures and whose flash point constituents surpass 1000F are deemed nonflammable liquids. From this information, inflammable liquids are apparently grouped into three categories as discussed below:
- All liquids whose flash points either surpass or are equivalent to 1000F but less than 1400F are categorized under class II.
- All liquids having flash points that are less than 2000F but surpass or equal 1400F are grouped under class IIIA, and
- All liquids having flash points that either equals to or exceed 2000F are grouped under class IIIB.
Furthermore, the American Petroleum Institute asserts that combustible and flammable liquids can be classified under two major classes’ namely polar solvent (soluble) and hydrocarbon (insoluble) (p.6).
From these classifications, polar solvents comprise of soluble liquids including alcohols, ketones and esters. Hydrocarbons in contrast encompass petroleum products that are insoluble for example jet fuels, crude oils and gasoline.
Flammable storage tanks
In each and every industrial facility, there is a storage tank specifically designed to store combustible liquids and chemicals. Such industries always comprise of automobile manufacturing plants, oil or petroleum factories alongside power plants.
Based on NFPA Code 30, quite a number of such complexes usually hoard combustible liquids within their respective atmospheric storage tanks.
However, these inflammable storage tanks simply vary in dimensions and have breadths stretching from three hundred and fifty feet. Hildebrand and Noll claim that such a tank can hold flammable liquids with capacity above one million five hundred thousand barrels (p.18).
Storage tanks can be classified founded on the kind of roofing they have. For instance, at present, there are different kinds of flammable storage tanks including fixed roof tanks, external open top detached or floating roof tank, covered internal floating roofed tanks and external floating domed storage tanks (Institution of Chemical Engineers 12).
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Irrespective of the varieties, it is advisable that when storage tanks are being selected, critical factors should be taken into consideration. The factors include the location of tanks and the kind of liquid the storage tank will contain. It is worth noting that combustible liquids should be stowed in outsized coned roof storage tanks.
In case this is impossible, they should be stored in low pressurized tanks or underground storage tanks. Shelley et al. asserts that, when in large capacities, combustible liquids ought to be stockpiled in tanks that have open-top floating roofs or internal floating roofs (p.63).
The small quantities can however be stored in either vertical or horizontal low pressured tanks as well as in underground tanks
Various flammable storage tanks and the adopted methods used to combat fire incidences are as discussed below.
Fixed roof tanks
Basically, these flammable storage tanks are perpendicular cylinders in nature and they bear lasting attached roofs. The rooftops of these inflammable tanks are marginally hemispherical or flat to let in air spaces and obstruct water accretion amid the underneath rooftop and the inflammable liquid surface.
Such tanks have frail rooftop shell seams and are constructed as per the API standards (American Petroleum Institute 11). When an interior overpressure ensued from an outburst, the design lets the rooftop to disengage from the perpendicular shell to forestall the bottommost seams failure which might cause the inflammable tanks to impel or fly upwards.
The inflammable tanks integrate some sort of emitting capacities which let the inflammable storage tanks to puff in great temperature disparities during packing and unpacking.
The pressure space outlet lets the amassing pressure within the inflammable storage tanks to stay idem to the exterior atmospheric-pressure. From the flammable storage tanks location, the vents are sometimes equipped with flame diverters or arresters and environmental controls which aids in capturing fugitive emissions (Shelley et al. 64).
External open top detached or floating roof tank
These storage tanks are upright (steel) cylinders having roofs which float in the tanks liquid surface. They are open to the above atmosphere given that they do not have the overhead fixed roofs.
In fact, the open top floating roofed tanks have double decks which aids in the liquid surface floatation and pan which floats on the pontoons (Hildebrand and Noll 29).
Often, the roofs fall and rise as the liquid levels change. Moreover, these inflammable hoarding tanks have rim closures that aid in averting the evading gases and are regularly utilized in the stowage of rudimentary lubricants or crude oils.
Covered internal floating roofed tanks
These tanks have self-sustaining fixed roofs or upright supports inside the fixed roofed tanks. The pan which is the internal roof floats on the liquid surface while rising and falling as the flammable liquid level changes. The pan either has a double floatation deck or floats on the pontoons.
The overhead immovable roofing has a vulnerable air outlet that lets the thatched interior space to take in air. The vent allowance for the fixed roofs assumes this strategy given that their vapor spaces are deemed to be less than the flammable limits (Institution of Chemical Engineers 19).
In the rim cover spaces, stoppers are customarily offered to deter any evading quick discharges. Such inflammable storage tanks are characteristically utilized in packing widespread and highly inflammable products including gas.
The causes and preventions of storage tank fires
It is usually reported in the trade journals and newspaper clipping that petroleum product storage tanks have caught fire. Such news is common both locally and internationally.
Despite the reported cases of global flammable storage tank fires that are rampant, the causes are attributable to; natural sources, simple flammable storage tanks designs, unsuitable safety policy measures and errors made by human beings. The next section highlights numerous bases for the storage fires as well as the suitable fire prevention methods.
Vent fires are linked to the fixed roof tanks including the internal floating and cone roof tanks. These categories of fires are primarily triggered by lightning strikes which kindle the elusive fumes that could be available at the inflammable packing tanks chimneys.
They majorly occur when the flammable storage tanks are being filled (Hildebrand and Noll 35). Apart from lightning, vent fires might be caused by human activities, electrical arcing and static charges that occur close to the flammable storage tanks location.
Vent fires may transpire in kind of flammable storage tank except in the externally suspended cone roofed storage tanks. However, it is a diminutive yet austere fire type that is often extinguishable via reducing the accumulating pressure inside the storage tanks or using dry chemical fire extinguishers.
NFPA guidelines (NAFP 20-217) also stipulates that vent fires can be suppressed through close monitoring of human activities which take place near flammable storage tanks and by installing lightning arrestors to inhibit the occurrence of static charges.
Rim seal fires
They involve a vast bulk of external floating roof tanks fires yet occur also in domed roof or internal floating roofed tanks. Lightning seems to be the prime detonation source albeit according to the detached roofed inflammable storage tanks, electrically prompted charges which have uninterrupted lightning hit could similarly arise. Studies indicate that lightning cause 95% of the total rim seal fires.
To successfully extinguish the rim seal fires, the protective rim seal fire systems including the foam chambers are normally installed. National Fire Protection Association 780 also necessitates that rim seal fires can be suppressed through installing roof shunts.
This will help in dispelling lightning strikes energy and consequently suppress fire (p.12). Further, rim-seal fires might be barred by fixing constricted stoppers to contain gas from skirting from the inflammable storing containers.
The submerged ground cables should be properly set up to aid in connecting the roof of the storage tank and the shell. Firefighting and monitoring systems must be installed to assist in quick detection and rapid response in case of rim seal fires.
Overfill ground fires
Also dubbed as dike fires, these types of fires emanate from tank or piping leakage. Any flammable storage tank that is situated above the ground is prone to these types of fires. Habitually, overfill ground fires result from other causes including equipment malfunctioning or the errors of the operators.
Such errors and failures cause the combustible liquids to spill and form dikes above the surface. It is recommended that when a leakage ensues devoid of an ignition, caution should be exercised and all ignition sources must be isolated.
When an ignition occurs, the fire should be treated as an outsized pool fire. Dike fires are apparent in internal, domed, external floating, fixed cone and internal floating roof tanks (Shelley et al. 51).
Unobstructed and obstructed full surface external fires
According to Hildebrand and Noll claims, these fires take place when whole surface of the flammable liquid storage tanks catch fire (p.37). They can occur either as unobstructed or obstructed full surface external fires. As regards to obstructed-full surface fires, it becomes difficult to access the flaming parts because they are blocked by the pan or roof.
When the pan or the roof sinks, these fires are bound to occur. The roof might sink when rain accumulates on top of it thus surpassing the flammable tank design standards. It can also sink if unsuitable firefighting materials are used in case of rim seal fires. Finally, filling the platform with rust and other materials from the tank may cause the roof to sink.
These types of fire pose significant challenge to firefighters. This is because of presence of the pan or roof blocks to enter the burning flammable storage tank surface. Thus, the pan or roofs are made such that they can sink or tilt to cease any fires.
Alternatively, large water supply could be used to cool, suppress vapor and ultimately extinguish fire (American Petroleum Institute 21). Regular inspection should be conducted to aid in the identification of platform letdowns and drain plugins.
When people are freely allowed to access flammable tank surfaces, unobstructed bursting surface fires are likely to arise. They are commonly found in fixed roof tanks which lack the internal roofs or pans.
These fires cause the weak welding amid the shell and the roof to break causing the two to separate. They can equally be witnessed when the storage tanks capacity is overawed by heavy rains as is the case with external floating roof flammable tanks.
Case study: Flammable storage tanks
Barton Solvents on fire
An explosion accompanied with fire was witnessed in Barton Solvents Wichita complexes in Kansas Valley Center on July 17th 2007. A firefighter and eleven people were injured and got medical treatments.
Basically, this incident considerably interrupted businesses in Barton, destroyed the farms where the tanks were located and triggered Valley Center evacuations where nearly six thousand residents were displaced.
When an investigation was carried out by the United States Chemical and Investigation Board (CSB), it was found that the first explosion took place right inside the perpendicular but above ground flammable storage tank which at that time was being packed with VM&P (Varnish Makers and Painters naphtha. VM&P is a flammable liquid classified under Class IB.
Due to its low level of electrical conductivity, the liquid accumulated dangerous static electricity and produced ignitable air-vapor mixtures inside the tanks. It was a result of these that the first explosion happened when a tank farm administrator initiated the transfer of VM&P naphtha into a fifteen thousand gallon above the ground inflammable storage tanks.
Buncefield oil depot
Cases of flammable storage tanks fire have been experienced in the U.S., East Coast, West Coast and other cities. A very spectacular fire case occurred in the Hertfordshire Oil Storage Terminal fire which erupted at Buncefield Oil Depot, England.
Despite the incessant overlook of the common element namely volunteer firefighters and industrial hazard training that flammable storage tanks ought to have, the storage tanks failed the close monitoring by firefighters (Shelley et al. 47).
Failure to put in place the preplan and construction features required to respond to fire incidences exposed the petroleum industry to immense storage tank fires. While the petroleum industry full-fledged, it demanded fire protection, better construction, design and upgrading of standards and codes intended to curb flammable storage tank fires.
Regardless of such measures, it is worth noting that despite the decrease in storage tank fire frequencies, the tank sizes had increased. This presented austere hazards in fire events.
Fires which embroil immense aboveground inflammable storage reservoirs attested to be actually very pricey in regard to commerce disturbance, public judgment, ecological destruction and assets loss (Shelley et al 47). The fire took nearly five days to be extinguished burning down twenty three flammable storage tanks. There were no deaths even though 43 people got injured.
Management of fire risk
It is indeed very difficult to eliminate incidences of inflammable storage tanks fires. However, such incidences can be abridged via proper operations, design and well-articulated maintenance practices. It is deemed necessary to execute fit inspections based on the API 635 standards (Shelley et al. 68).
These standards requires once-a-month routine visual operational inspection to check incidences of corrosion, leakages shell alterations and tint crust. The formal operational external inspection should entail obligatory inspections done on storage tanks after 5 years if the rusting allowance is below twenty years.
The focus under this kind of inspection relates to fire protection systems in place, internal floating surfaces, coverings, foundations, conduits, airstream guiders, the entrance constructions and shell accessories. Report should be prepared on the operational capacities and conditions to gauge the levels of safety.
Internal inspections named out of service needs to be performed once after twenty years to determine the rate of storage tanks corrosion. If there are no signs of corrosion, this kind of inspection must be performed after ten years (American Petroleum Institute 86).
These inspections need to be performed after emptying and cleaning the storage tanks. Besides, a blend of leakage testing, magnetic fluxes and ultrasonic breadths should be carried out on top of the visual inspections. These might help in identifying and evaluating the status of tanks as regards to rust, leakages and incidences of failures.
Evidently, the stored combustible and inflammable liquids tend to bring with them innate risks of fire threats which follow explosions. This calls for the classification of combustible and inflammable liquids into three classes to aid in determining the suitable facilities that could be used to store such liquids.
Quite a number of tanks have however been designed to help store the inflammable liquids and all these storage tanks are susceptible to particular kinds of fire risks.
While most risks are pooled to all types of storage tanks, maintenance practices, numerous designs, inspections as well as standards like NASP that have established should be adopted to minimize the risks of storage tanks explosions and fires.
Entities are advised to choose from a list of the developed firefighting systems and equipment to be certain that all combustible and inflammable liquids are stored safely.
To avoid incidences of flammable storage tank fires, proper inspection should be conducted based on the instituted standards given that it is the best suppression technique.
This technique warrants that flammable storage tanks incessantly appraised in order to give room for amendments prior to loss of lives and emergence of disasters. There should also be planned emergency techniques to assist in curbing storage tank fire incidences.
American Petroleum Institute (API). Prevention and Suppression of Fires in Large Above Ground Atmospheric Storage Tanks. Washington, DC: API, 1998. Print.
Hildebrand, Michael and Noll Gregory. Storage Tank Emergencies: Guideline and Procedures. Annapolis, MD: Red Hat, 1997. Print.
Institution of Chemical Engineers (IChemE). BP Process Safety Series: Liquid Hydrocarbon Tank Fires, Prevention and Response. Rugby, United Kingdom: IChemE, 2005. Print.
National Fire Protection Association (NFPA). Fire Protection Handbook. Quincy, MA: NFPA, 2008. Print.
Shelley, Craig, Antony Cole and Timothy Markley. Industrial Firefighting for Municipal Firefighters. Tulsa, OK: Fire Engineering, 2007. Print.