Shunts can trigger tank fires
Companies from BP and Shell to ExxonMobil and ConocoPhilips are adopting a new grounding technology known as Retractable Grounding Assembly.
Floating Roof Tank (FRT) shunts are far from the safe conductors of stray electrical energy they are 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 petrol. Fearing the fire would spread to other tanks in the facility led authorities to evacuate an entire neighbourhood 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 the UK, 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 vapour. 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.
