LASTFIRE, which stands for Large Aboveground Storage Tank Fire, has established itself as the recognized industry forum on this subject. The LASTFIRE group includes international companies with operating facilities that store hydrocarbons and consists of storage tank and emergency response professionals who collect, share and evaluate data on incidents, determine risk, conduct research, evaluate technologies, and establish design and operation practices as means to prevent, control and extinguish storage tank fires.
Floating roofs are used on tanks storing high-volatility products for both fire safety and vapor emission reduction. In addition to a LASTFIRE study of external floating roof tank risk, in November 2010, member companies and selected industry experts considered hazards/risks associated with internal floating roofs. For risk mitigation, LASTFIRE identified desired design features are:
1. Surface contact with stored product.
2. Constructed with inherently buoyant material.
3. Constructed with material not consumed by fire.
Beginning in 2011, over two dozen floating roof models constructed of various materials were loaded and fire tested to perfect material selection, manufacturing and construction technique. Contact type floating roof, mechanical shoe seal and cellular glass technologies were selected for the composite internal floating roof (CIFR). The fire-resistant CIFR design is a highly buoyant cellular glass core combined with a stainless steel skin. The stainless steel skin reinforces the high-compression-strength glass core to form chemically inert assembled glass modules. Shop-fabricated buoyancy modules are field joined, similar to assembly of pressure-sealed aircraft cabin shells. Another industry-related application for the cellular glass is to insulate LNG tanks in cryogenic service.
LASTFIRE drafted fire test protocol for use with NFPA 11. In October 2014, fire testing was conducted at Centro Jovellanos in Asturias, Spain, with over-sight of LASTFIRE program management and members of NFPA TC11. The CIFR fire test was carried out in an 8-foot-diameter (2.44 meter) test pan. The CIFR tested included a 6-inch-thick (152-millimeter) cellular glass modules clad with a 0.0187-inch (0.0475-mm), 26-gauge stainless steel skin. The assembled CIFR included a formed stainless steel structural rim 0.05 inches (1.27 mm) 18-gauge thick by 12 inches (305 mm) high. Although not required by test protocol, a stainless steel mechanical shoe peripheral seal was used to center the roof and maintain typical peripheral rim space. To simulate damage and verify inherent buoyancy, a 24-inch (600 mm) penetration was installed at the center. The roof sample was placed in a test tank on water, and then fuel (heptane) was applied to float the roof 1 inch (25 mm) above water. Holding struts suppressed the inherently buoyant roof until the deck surface was fully submerged beneath 1 inch (25 mm). The total fuel layer was 8 inches (203 mm).
Testing reflects deck damage and a full-surface “spill on roof fire.” The surface fuel burns off quickly, followed by fire at the peripheral rim and center opening, with total burn duration of 81 minutes until all fuel is consumed. In January 2015, an NFPA 11 Internal Floating Roof Fire Resistance/Resilience Summary Test Report was issued to cover fire testing performed. The test’s “pass criteria” after fuel consumption are:
- No adhesion between steel holding strut pads and the roof.
- No damage to the roof at the holding struts’ point of contact.
- No ongoing combustion or flaming of the roof.
- Center penetration shall not increase more than 1 inch (25 mm) in diameter (none occurred with CIFR).
- No damage to the roof at the seal area.
The fire-tested CIFR is designed to meet or exceed minimum requirements of API 650 Annex H, and the completed assembly may be pressure tested using the procedures applied to a fixed roof of a tank designed to be gas tight (API 650 Section 7.3.8.1).
For more information, email George.Morovich@TankEnv.com or call (979) 247-4812.
