Petroleum storage tank fires due to lightning or static discharge are more common than most people think. Dozens of oil industry tank fires are reported each year worldwide, ranging from rim seal fires to multiple, simultaneous full tank fires. Many of these costly incidents are attributed to lightning or static discharge-related electrical arcing.
A key strategy to preventing lightning or static discharge-related electrical arcing is to properly bond the roof and wall of the industry’s floating roof tanks (FRTs). Traditional methods of bonding the roof and wall of FRTs face serious drawbacks including high impedance at lightning frequencies and high likelihood of arcing at seals.
Recently, production and refining companies have been adopting a new grounding method known as retractable grounding assembly (RGA) to address these concerns. RGA provides increased protection from lightning and static discharge-related FRT fires, and minimizes the need for traditional grounding maintenance.
Conditions contributing to fire risk
A lightning strike can deliver up to 10 billion volts and 510,000 amps. This can occur within about 13 milliseconds, with temperatures reaching up to 50,000° F. If lightning terminates on or near an FRT, large currents can flow across the roof-shell interface. If the impedance between the roof and shell is high, electrical arcing or sparking will occur across the seal interface, which can ignite a fire. This fire hazard can also be triggered by static discharge inducing activities such as filling a tank, when petroleum product may pick up an electrical charge passing through pipes.
To prevent this fire hazard, the floating roof must be electrically bonded to the tank shell in order to equalize electrical potential. This has traditionally been accomplished with shunts, a walkway, or a bonding cable, though these methods have significant drawbacks.
To create the roof-shell bond, metal finger-like devices called shunts are typically attached to the roof. They must be in constant contact with the tank shell. Unfortunately, shunts do not provide a certain, low-resistance bond to the shell.
Walkway ladders can also be used to create the roof-shell bond. Nearly all FRT’s have a walkway ladder with the upper end attached to the tank rim and the lower end riding on roof-mounted rails. As the floating roof rises and falls, the ladder’s lower end rolls to compensate for the roof’s changing height.
Uncertain connections at both the ladder’s upper end (it is simply a bolted hinge) and the lower end (pressure connection through two wheels riding on rails) cause the possibility of large potential differences between the roof and shell. Arcing can occur at the seal locations during lightning activity or instances of severe static build-up.
A bonding cable may also be used between the top of the tank shell and the middle of its roof. The cable is typically bonded to the shell’s rim, suspended along the bottom of a ladder, and bonded to the roof’s center. This type of bond is considered a high-impedance connection due to the relatively high impedance of the cable.
Thus, with a single roof-shell bonding cable, large potential differences will still occur between the roof and shell along with hazardous electrical arcing at the seal locations.
New retractable cable roof-shell bonding
The bond created using the RGA is considered ideal because it provides low impedance across a wide range of frequencies; it is easy to install on new tanks and retrofit onto existing tanks; and it is easy to inspect, test, and replace, if necessary.
Developed by Lightning Eliminators & Consultants Inc. (LEC) of Boulder, Colo., the RGA uses a wide, thick-braided wire cable, spring-loaded on a heavy stainless steel reel. With impedance of one ohm or less -- compared to shunts or ladders with impedance as high as 500 ohms -- the RGA offers a reliable, full-time grounding connection. •