The oil and gas industry has "turned up the volume" on its production capabilities and facility designs, resulting in increased volumes of open floating-roof tanks. Consequently, fire protection requirements must also increase.
Most of the safety regulations we follow today were formed at a time when tank sizes throughout the industry averaged 60 feet in diameter, with outliers falling in the range of 100-150 feet. Today, tanks average 160 feet in diameter, while open floating-roof tanks used for crude oil storage commonly exceed 300 feet. As tanks have continued to double or even triple in size, the recommended minimum safeguards (standards and designs) written decades earlier must also evolve to adequately address the inherent risks associated with these tank design trends. When considering surface area, for example, doubling the tank diameter quadruples the surface area -- a critical factor when specifying effective application rates for a full-surface fire.
Tank size considerations
When comparing a 100-foot versus 300-foot-diameter storage tank, the most striking statistics relative to fire protection equipment and response are the required foam application volume for an NFPA-recommended, 65-minute foam application. Using a scalebased application rate (better suited to the incremental risk that jumbo tanks represent as their diameters increase), the foam solution flow rate on a 100-foot tank is 1,257 gallons per minute (gpm), compared to 15,551 gpm for a 300-foot tank. And the total volume of foam concentrate required respectively is 817 gallons versus 10,108 gallons.1
The risks associated with full-surface tank fires on these larger jumbo tanks have intensified and are now outpacing conventional methods used for tank fire protection.
Type II: Fixed/semi-fixed system technology
Traditional foam pourers and foam chambers used on smaller tanks struggle to deliver the adequate foam application density required to properly address the potential incident scale of today's jumbo tanks. When dropped from a pourer or chamber, higher expansion foam results in a slow horizontal velocity. A slower application velocity -- in conjunction with thermal characteristics of the burning surface, e.g., gross-radiant heat and upward thermal draft -- can result in foam blanket dehydration before the full-surface coverage necessary for extinguishment.
Type III: Over-the-top technology
Large-volume portable monitors have offered the greatest success rate in combating large-scale flammable liquid incidents, thanks to high-terminal velocity and the foam application density of a Type III response. However, as tank sizes increase, so do the logistical requirements for "over-thetop" foam delivery, many times exceeding the facilities' response capabilities.
Combining Type II and III
A recent development in a fixed-position attack from the tank rim has been to combine the technologies of Type II and Type III applications. The result is a comprehensive approach that allows an incremental systems design tailored to the tank scale and the risk it represents. This Compliance PLUS system provides more aggressive rim seal protection (rim fires), coupled with a Type III application projecting outward to the center of the tank for complete coverage (full-surface fire).
Both applications benefit from aggressive forward momentum that join forces within the tank for a more effective and timely extinguishment. This rim-based application has minimal fallout compared to a groundbased approach, resulting in reduction of overall applied foam volume. When designed and sourced properly, this system can greatly reduce manpower requirements and exposure, while also improving effective response time.
For more information, contact Brent Gaspard at (409) 365-4250 or brent. gaspard@tycofp.com.
