by Srikanth Vengasandra, Ph.D., Product Manager, Flame & Gas Detection Group, Rosemount Analytical
On January 1, 2015, eight tanks at a truck unloading station in Alexander, North Dakota, caught on fire, setting ablaze approximately 1,600 barrels of crude oil1, 2. Although the cause of the fire remains under investigation, the incident underscores the need for preventive and alternative methods to protect against the risks of tank fires and explosions3.
Often the most common type of fire in floating roof tanks is the rim-seal fire. Rim-seal fires occur whenever crude oil vapors escape through worn seals, accumulating on tank skirts. These leaked vapors can be ignited, often by lightning, resulting in a fire around the tank perimeter. According to data obtained from the Large Atmospheric Storage Tank Fire (LASTFIRE) project, rim-seal fires occur at a rate of 0.3 to 21 x 10-3 incidents per tank year, with Nigeria, Thailand, and Venezuela reporting the highest frequencies3.
Suitable design, construction, maintenance of storage systems, and overfill and leak protection take precedence over any protective scheme, since they are preventive layers. They help prevent crude oil inventories from escaping in the first place. A host of mechanical integrity management standards address the provision for inherently safe design4.
A second priority is the establishment of leakage, overflow, and flame detection. These measures help to reduce the consequence of a material release after loss of containment has taken place. In particular, flame detectors should be used for monitoring bunded fuel tank areas and rooftops. Figure 1 illustrates a typical arrangement for protecting the latter. The CAD drawing shows the top view of a crude oil tank 77 meters in diameter and 16 meters tall with the overlapping coverage from six UV/IRS flame detectors, FD#1, FD#2, FD#3, FD#4, FD#5, and FD#6. The number of flame detectors is determined from the fuel source and the associated flame detector coverage angle of 120° in the horizontal plane at a distance of approximately 43 meters from the fuel source.
In addition to protecting rooftops, flame detectors are also used to mitigate the risk of fires produced from bottom leakage. These fires are common to fixed, floating room, and domed roof tanks and are treated like large pool fires. Rarer than rim fires, small and large bund fires occur at a rate of 9 x 10-5 and 6 x 10-5 incidents per tank year. Because dikes prevent the spread of tank contents, it is usual to install flame detectors in the dike perimeter, typically in corners and trained toward the base of the tanks5.
The main hazards associated with the storage and handling of combustible liquids are fire and explosion. As the size of tanks and inventories in terminals increase, so too has hazard severity. For floating roof tanks, rim-seals should be constantly monitored by the use of flame detectors, and similarly, flame detectors should provide a large zone of detection for bunded areas. Together with combustible gas detection, they offer an effective means to cope with fire hazards.
1 Eckroth, L. 2015. Oil Storage Tank Fires Extinguished. Bismarck Tribute, http://bismarcktribune.com/bakken/oil-storage-tank-fires-extinguished/article_6f3c8606-92a9-11e4-8750-e72b310f0c34.html
2 Scheyder, E. 2015. Oil Storage Tanks in North Dakota Catch Fire; No Injuries. Reuters, http://www.reuters.com/article/2015/01/02/us-enbridge-inc-fire-idUSKBN0KB00B20150102
3 Marsh. 2011. Atmospheric Storage Tanks. Marsh, https://uk.marsh.com/Portals/18/Documents/Atmospheric%20Storage%20Tanks_lowres.pdf
4 Holmes, A. 2009. Mechanical Integrity Management of Bulk Storage: Review of Standards, RR760, Health and Safety Executive, http://www.hse.gov.uk/research/rrpdf/rr760.pdf
5OGP Risk Assessment Data Directory, Report No. 434-3, March 2010. Storage Incident Frequencies, International Association of Oil & Gas Producers.