Floating Roof Tank (FRT) shunts are far from the safe conductors of stray electrical energy they're intended to be and can, in fact, trigger tank fires. Recent tests conducted for the American Petroleum Institute (API) RP 545 "Lightning Protection for Above Ground Storage Tanks" task group have shown that "shunts can generate showers of sparks during lighting strikes," according to a bulletin of the task group.
"If there is a gap between the seal and the tank wall during a lightning strike and if a flammable mixture is present, a tank fire may result," continues the bulletin.
Despite the widespread use of shunts, hundreds of tank fires occur each year worldwide. Recently in Minatitlan, Veracruz, Mexico, a lightning strike ignited a tank containing 55,000 barrels of gasoline. Fearing the fire would spread to other tanks in the facility led authorities to evacuate an entire neighborhood until the fire was extinguished. In New Orleans, a lightning strike ignited a tank holding 200,000 barrels of gas recently. Fire officials stated the blaze began in the seal of the tank, requiring local responders and company fire crews to bring the fire under control.
Independent third-party testing, performed in cooperation with the API and the Energy Institute in England, has shown that arcing will occur at the shunt-shell interface under all lightning conditions, whether or not the shunts are above the roof or submerged. If the shunts are above the roof, however, arcing occurs in the worst possible place: in a Class I Division I hazardous area, which may have a highly ignitable concentration of fuel-air vapor.
Floating installations are more susceptible to unintended sparking and fire than other oil and gas facilities due to their flash points being reached more easily. The severity of fires and explosions in FRTs, the difficulty of fighting those fires, and the risk of the loss of the entire facility have upped the urgency of establishing a reliable means of bonding the tank roof and shell.
The API RP 545 task group plans additional testing to evaluate alternative methods of providing a conductive path between the tank roof and tank wall to meet the intent of NFPA 780, the Lightning Protection Code.
Recently, companies as varied as B.P., Shell, ExxonMobil, ChevronTexaco, Conocco Phillips, Bahamas Oil Refining, and Petroleos de Venezuela (PDVSA) have been adopting a patented grounding technology known as Retractable Grounding Assembly (RGA). The grounding method, which provides increased protection from lightning and static discharge-related FRT fires while minimizing the need for traditional grounding maintenance, addresses areas of concern regarding shunts.
Currently, to create the roof-shell bond, NFPA 780 requires shunts to be spaced no more than every ten feet around the tank roof perimeter. Shunts are attached to the roof and must be in constant contact with the tank shell. Unfortunately, shunts do not provide a certain, low resistance bond to the tank shell for a number of reasons.
First, since heavy crude oil components such as tar and paraffin tend to coat the inside of the tank shell, this forms an insulative barrier between the shell and shunts. If the inside of the tank is painted, the paint will also insulate the shell from the shunts.
To address this, the RGA, developed by Lightning Eliminators & Consultants, Inc. (LEC) of Boulder, Colorado, 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.
With impedance of one ohm or less - compared to shunts with impedance as high as 500 ohms - this innovative grounding method offers a reliable, full-time grounding connection that can help prevent lightning or static discharge-related petroleum fires. It is easy to install on new and existing tanks, as well as easy to inspect, test, and maintain.
A second concern for shunts is that any corrosion (rust) on the inside of the shell creates a high resistance connection between the shell and shunts.
However, the RGA cable, constructed from 864 strands of #30 AWG copper wire is tinned for extra corrosion protection and braided together to form a strap 1.625" wide by 0.11" thick. Its 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 cable'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.
A third concern for shunts is that large tanks are typically out-of-round by several inches. This means shunts tend to pull away from the tank shell, eliminating the grounding connection that prevents the electrical arcing fire hazard.
Since the RGA functions independently of the condition of the tank shell however, it can serve as a more effective primary safety system for preventing lightning or static discharge-related fire hazard. Employing multiple units is the most effective approach to ensure multiple positive bonds between the tank shell and roof, and the lowest likelihood of arcing at a seal.
The developer of RGA technology, LEC, will present two technical papers titled "Lightning Protection for Flammable Storage Facilities" and "Retractable Grounding Assembly Improves Lightning Protection of Floating Roof Tanks" at a dual event in Maceio, Brazil, Nov. 26-29, 2006. The event combines the 2nd LPE (International Conference on Lightning Physics and Effects) with Ground'2006 (International Conference on Grounding and Earthing).
Lightning Eliminators & Consultants, Inc. designs, manufactures, and installs integrated, engineered systems to prevent and eliminate grounding and other lightning related problems. For more information, write to 6687 Arapahoe Road, Boulder, Colorado 80303; call 303-447-2828; fax 303-447-8122 Email: firstname.lastname@example.org or visit www.lecglobal.com/rga-001.