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26 Spring 2019 Once the current is conducted to the perimeter of the domed roof, it must be transferred to the tank shell. There are several sections contained in NFPA 780 offering guidance. If the attachments between the roof and tank shell are considered equivalent to down conductors in a conventional lightning protection system, the requirement is for bonding the roof to the rim of the tank shell at intervals not to exceed 100 ft (NFPA 780, 4.9.10.1), or at intervals not to exceed 60 ft, if considered structural metal framework connection to ground (NFPA 780, 4.19.4.1). This spacing requirement is supported by the 100 ft maximum spacing of bypass conductors contained in NFPA 780, 7.4.3.2.2.4 (2). These bonding conductors should be as short and straight as is practical, and allow for full motion of the tank roof on the tank shell rim. Grounding Once the lightning energy reaches the base of the tank shell, it must be safely dissipated into ground. For lightning protection purposes, flat-bottom metal tanks and other structures are considered inherently self-grounding, per API 545, A.2.1 and API 2003, 5.4.1. This applies whether or not a containment liner or barrier is installed either in or under the tank or structure, per API 545, A.2.2. There are several different types of grounding electrodes allowed by the standards, including ground rods, ground plates, ground loop conductors, plant grounding grids and buried metal piping. One way to look at the grounding properties of a flat-bottom steel tank is to compare it to a flat plate grounding electrode, as allowed in NFPA 780, 4.13.6. The tank bottom has a much larger area than a flat plate electrode, and is pressed against ground by the weight of the tank and stored product. Therefore, it is an effective connection to ground. If a ground grid is installed at a site, each tank should also be grounded to that grid at a minimum of two locations, and preferably at additional intervals not to exceed 100 ft around the tank perimeter. Metal piping attached to a tank and buried for a minimum length of 10 ft may also be considered ground. This does not address grounding for purposes other than lightning protection. Grounding for other purposes, such as AC power grounding, should be considered by the owner/operator. Reality check The main concern with this design is lifting the heat of a direct attachment off of the geodesic dome panels. This could cause a burn-through and result in molten roof material falling onto the internal floating roof where, if the seals are not adequately maintained, an explosive atmosphere may be present. Another concern is to prevent arcing, particularly at the juncture of the geodesic dome and the tank shell rim where an explosive atmosphere may also be present. The use of the system described above reduces these possibilities. Although not required by any standard, some operators have, as best practice, installed bypass conductors between the internal floating roof and the tank shell as described in API 545 and 2003 and NFPA 780. Some may consider this unnecessary, but it can help to eliminate arcing between the floating roof and tank shell. Certifications Often an owner or operator will desire a third-party certification of a completed lightning protection installation. Underwriters Laboratories (UL) is the nationally recognised testing laboratory (NRTL) in the lightning protection industry, and should be consulted if third party certification is desired. UL converts NFPA 780 into materials and installation guidelines. UL 96 covers lightning protection components and UL 96A covers installation requirements. Based on compliance with these documents, UL can issue certifications of completed installations, the best known of which is a UL Master Label. However, the UL documents specifically exclude from their scope any "structures used for the production, handling, or storage of ammunition, explosives, flammable liquids or gases, and other explosive ingredients including dust". Therefore, in place of a Master Label, UL will issue a Letter of Findings or Engineering Inspection Report, which have the same import as a Master Label but reference this exclusion. Conclusion In addition to other, more prosaic risk factors, the following conditions should also be considered: Problematic: sites that have experienced a previous static or lightning event. High exposure: sites located in exposed locations such as hilltops or historically high-lightning locations. High consequence: sites located in environmentally sensitive locations or populated areas. Accessibility: sites with limited or difficult access for remediation and repair. Regulation: sites where a local/municipal/regional construction code requires lighting protection. High visibility: facilities that are in a particularly sensitive area (urban or otherwise high-profile). Operational importance: high production or otherwise operationally important sites. High investment: sites with significant equipment investment. If tanks, by virtue of these or any other factors, exceed the owner or operator's risk threshold, they may consider adding air terminals and bonding to their geodesic dome tank. As things stand, the best way to do so and to meet standards is to apply a structural lightning protection system as described above.

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