When lightning struck on or near a petroleum tank at the Magellan Midstream Partners distribution terminal in Kansas City, Kansas, in 2008, the tank, containing approximately 1.2 million gallons of unleaded gasoline, caught fire, sending a large plume of smoke across portions of the greater Kansas City metro area.
About one-third of all petroleum tank fires are due to lightning strikes. Floating roof tanks (FRTs), like the one that caught fire due to lightning in Kansas City, can be especially vulnerable. The costs can be catastrophic: from loss of product, equipment, and production; to loss of life, business, and goodwill; to lawsuits and increased regulatory scrutiny. In fast developing, lightning-prone areas such as Florida, China, Malaysia, and Singapore, the risks are highest.
To reduce the risk of tank fires, the American Petroleum Institute (API) recently issued API RP 545, Recommended Practice for Lightning Protection of Above Ground Storage Tanks for Flammable or Combustible Liquids.
There are some new options to make satisfying API RP 545 fast, economical, and safe. Forward thinking, proactive companies in the petroleum industry are heeding the RP, which is expected to become a standard in the near future. Those who don’t can only hope that lightning doesn’t strike once, let alone twice.
Two key findings resulted from the API technical committee’s research and testing. First, when lightning current passes through shunts at the roof-shell interface, it will result in arcing under all conditions. Second, it is the slow component of the lightning stroke which ignites flammable vapors. Therefore, when the slow component of a lightning stroke passes through any existing roof-shell interface, if flammable vapors are present, they will likely be ignited.
API RP 545 recommends three modifications to FRTs:
1) Install submerged shunts between the roof and shell every three meters around the roof perimeter, and remove any existing above-seal shunts.
2) Electrically insulate all seal assembly components (including springs, scissor assemblies, seal membranes, etc.) and all gauge and guide poles, from the tank roof.
3) Install bypass conductors between the roof and shell no more than every 30 meters around the tank circumference. These bypass conductors should be as short as possible and evenly spaced around the roof perimeter.
Since modifications #1 and #2 require the overhaul of new and existing tank designs and can cost up to $1 million per tank, implementing these is less than ideal. To implement submerged shunts, for instance, each tank has to be emptied and personnel must go inside both above and below the roof to make modifications. Because the shunts are submerged, they would be hard to inspect and maintain. Insulating seal components and poles would also require substantial design changes and field modifications, with inspection and maintenance issues.
Because modification #3, the installation of bypass conductors, can be implemented immediately and costs less than $10,000 per tank, it’s a readily viable option for petroleum companies. Existing tanks can be retrofitted with bypass conductors while still in service regardless of roof level; and since bypass conductors are external, they are easy to inspect and maintain.
To meet the bypass conductor requirements, tank owners can choose between a traditional fixed-length conductor and an innovative retractable conductor, wound on a spring-tensioned reel.
Fixed-length conductors such as loose cable have the drawback, however, of high impedance and poor conductivity when the FRT roof is high. An FRT is most at-risk from lightning when the roof is high since when the tank is full or near full, lightning current flows are concentrated in the shunts directly below the strike location. When the tank roof is low, lightning current disperses and is more evenly distributed among the available roof-shell bonds.
Moreover, during high-roof conditions when the tank is most at risk, conventional fixed-length cable will coil and randomly bunch up on the tank roof. If the loose cable conductor is not insulated, accidental sparking may occur where it contacts itself and other parts of the roof.
In contrast, retractable bypass conductors will always be as short as possible, and offer substantially less impedance when the FRT roof is high. For instance, the Retractable Grounding Assembly (RGA) by Boulder, Colorado-based Lightning Eliminators & Consultants (LEC), can offer just one-sixth the impedance of traditional fixed-length cable on a 50-foot tall tank.
The Benefits of Retractable Cable Roof-Shell Bonding
The ideal bond between the FRT roof and shell would have low impedance across a wide range of frequencies; be easy to install on new tanks and retrofit onto existing tanks; and be easy to inspect, test, and replace if necessary.
To satisfy these requirements, LEC developed the RGA which provides a very low impedance, direct connection between the tank roof and shell, using a wide thick-braided wire cable, spring-loaded on a heavy stainless steel reel. It is easy to install on new and existing tanks, as well as easy to inspect, test, and maintain.
With impedance of one ohm or less − compared to shunts or walkway ladders with impedance as high as 500 ohms − the RGA offers a reliable, full-time grounding connection that can help prevent lightning or static discharge-related petroleum fires.
The RGA’s path of impedance is kept to a practical minimum by a combination of the shortest path, wide braid, and constant spring tension. Its spring-loaded reel extends cable as the roof descends, and retracts it as the roof rises − so the line remains taut at the minimal distance needed for grounding, assuring minimal impedance and faster, more reliable grounding.
Unlike traditional roof-shell bonding methods, the RGA’s wide braid maximizes surface area and therefore conductivity, since high frequency electrical charges (electrons) actually travel most effectively along the surfaces of wire conductors. The RGA cable, constructed from 864 strands of #30 AWG copper wire, is braided together to form a strap 1.625” wide by 0.11” thick, and is tinned for extra corrosion protection. With over twice the surface area of typical 0.5” diameter or 250 MCM roof-shell bonding cable, for instance, the RGA offers significantly better conductivity.
Since the RGA functions independently of the condition of the tank shell, it can eliminate the need for more traditional roof-shell bonding methods like shunts, or serve as a more effective primary safety system for preventing lightning or static discharge-related fire hazard.
Unlike a walkway ladder or single roof-shell bonding cable, multiple RGAs can be used to ensure multiple positive bonds between the tank shell and roof, and the lowest likelihood of arcing at a seal.
Because of the numerous advantages of RGAs, they are being used worldwide by companies such as ExxonMobil, ChevronTexaco, Shell, B.P, SINOPEC, Saudi Aramco, Petroleos de Venezuela (PDVSA), and Bahamas Oil Refining, often alongside other lightning protection equipment such as LEC’s Dissipation Array System (DAS®).Download