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The extinguishment and control of full surface flammable tanks fire demands more commitments in terms of equipment resources and human logistics. Owing to the incurred loss potentiality, the fire protection industries have significantly improved the methods used to effectively extinguish and control fires encountered in outsized storage tanks.
These techniques are incessantly updated. In this paper, various types of atmospheric storage tanks, the encountered fire types, safe storage tanks code requirement, preplanning alongside the applicable firefighting methods are examined.
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. For instance, cases of flammable storage tanks fire have been experienced in the U.S., the East Coast, West Coast as well as other cities found in between.
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, Cole and Markley 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. In fact, fires which involve outsized aboveground flammable storage tanks proved to be very costly with respect to business interruption, public opinion, environmental damage and property damage (Shelley, Cole and Markley 47).
Fire protection methods used in flammable storage tanks
The kind of storage tank used in storing combustible and flammable liquids heavily relies on the stored product physical characteristics and the location of the tanks. Various flammable storage tanks and the adopted methods used to combat fire incidences are as discussed below.
Fixed roof tanks: These are perpendicular cylinders bearing lasting attached roofs. The roofs of these tanks are slightly domed or flat to permit vapor spaces and inhibit water accumulation between the roof underside and liquid surface. Such tanks have frail rooftop shell seams and are constructed as per the API standards (American Petroleum Institute 11).
When an internal overpressure occurred from explosion, the design permits the roof to detach from the perpendicular shell to avert the bottom seams failure which may cause the tanks to propel or rocket upward.
The tanks incorporate some kinds of venting ability which allows the storage tanks to breathe in extreme temperature variations, during loading and unloading. The pressure vacuity vent permits the accruing pressure within the storage tanks to remain idem to the outside 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.
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. Besides, these flammable storage tanks have rim seals that help in preventing the escaping vapours.
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 above fixed roofing has an open air vent that allows the internal above space roofing to breathe.
The vent allowance for the fixed roofs assumes this strategy given that their vapour spaces are deemed to be less than the flammable limits (Institution of Chemical Engineers 19). Within the rim lid spaces, seals are normally provided to inhibit any escaping fugitive emissions. Such tanks are used typically in storing complete high flammable products like gasoline.
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External floating domed storage tanks: The tanks are constructed via retrofitting a dome which covers the standing overall external floating roofed tank. The main purpose of the dome is to offer protection from the ensuing elements and similarly provide environmental fugitive emission control.
When firefighting operations are in the early stages, the panels at these tanks must melt away whereas the supporting framework is expected to the only barrier (Shelley, Cole and Markley 47). The supporting framework is expected to fold in and consequently fall on the surface of the burning fuel while the domed storage tanks fires continue to burn.
Safe storage tanks code (SSTC) requirement
SSTC requires that all flammable storage tanks must be well designed and constructed as per the stipulated standard distance. Tank diameters and application rates must be properly constructed and fire extinguisher media well installed. There must be well trained firefighting personnel and industrial emergency tasks force deployed at every site.
There must always be sufficient resources and accessibility to the fire scenes. All third party contractors who specialize in fire incidents need to be well identified and frequently communicated with (American Petroleum Institute 17). The requisite foam quantities and adequate water supply must be availed to aid during firefighting.
Firefighting methods applicable to flammable storage tank
The methods used to fight flammable storage tank fires depend on the level of severity which range from complete liquid surface flammable tanks fires to simple vent fires. The firefighting methods are as subsequently discussed.
Vent fires: These fires are linked to the fixed roof tanks including the internal floating and cone roof tanks. Vent fires are mainly caused by lightning strikes which ignite the fugitive vapours that might be present at the flammable storage tanks vents. However, it is a diminutive austere fire type that is often extinguishable via reducing the accumulating pressure inside the storage tanks or using dry chemical fire extinguishers.
Overfill ground fires: Also dubbed as dike fires, these types of fires emanate from tank or piping leakage. Habitually, overfill ground fires result from other causes including equipment malfunction or operator errors.
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, Cole and Markley 51).
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 appears to be the primary ignition cause even though as per the floating roofed tanks, electrically induced charge that have no direct lightning hit might also occur.
To successfully extinguish the rim seal fires, the protective rim seal fire systems including the foam chambers are normally installed. To access the fire area in order to apply the fire extinguishing media, firefighters use the access covers or vents.
Obstructed full liquid external fires: 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 vapour and ultimately extinguish fire (American Petroleum Institute 21).
Auxiliary aids and appliances having foam systems that are situated within the premises usually aid firefighters in curbing incidences of fire. In case the fully or semi static appliances are not installed, firefighters might utilize transferable equipment to effectively extinguish such fires. These may include the use of monitors and hose-lines in filling the rim-seal areas with foam or water solutions.
American Petroleum Institute (API). API 2021a, Interim study: Prevention and suppression of fires in large aboveground atmospheric storage tanks. Washington, DC: API, 1998.
Hildebrand, Michael and Noll Greg. Storage tank emergencies: Guideline and procedures. Annapolis, MD: Red Hat, 1997.
Institution of Chemical Engineers (IChemE). BP process safety series fire safety booklet liquid hydrocarbon tank fires: Prevention and response. Rugby, United Kingdom: IChemE, 2005.
Shelley, Craig, Antony Cole and Timothy Markley. Industrial firefighting for municipal firefighters. Tulsa, OK: Fire Engineering, 2007.