Providing adequate and effective lightning protection for storage tanks constitutes a beneficial and cost-effective step in assuring both personnel safety and reliability. Fortunately, securing such protection is not difficult or complicated, and guidance is readily available. It helps to become familiar with some basic recommended practices and standards for reference. We will be referring to the National Fire Protection Association NFPA 780, Standard for the Installation of Lightning Protection Systems; the American Petroleum Institute API 545, Recommended Practice for Lightning Protection of Aboveground Storage Tanks for Flammable or Combustible Liquids; and the American Petroleum Institute API 2003, Recommended Practice for Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents.
Whenever considering lightning protection, it helps to fall back upon the three basic steps: bonding and grounding, surge suppression, and structural lightning protection.
BONDING AND GROUNDING. The first consideration is bonding and grounding. According to API 545, flat-bottom tanks are inherently self-grounding for lightning protection purposes. The mass of the tank and surface area of its bottom in contact with whatever material it occupies provides a sufficiently low-impedance path to conduct lightning currents without increasing the risk of ignition. This applies whether or not a non-conductive containment membrane is in place under the tank.
It should be noted that, although adequate for lightning grounding, the path to ground may be high resistance, rendering it unsuitable for AC power grounding. In the event of an AC power ground fault, the lack of a low-resistance return path may leave the tank energized. Therefore, we recommend at least one, and preferable one each 100′ of tank perimeter, “solid” connection to ground. This usually consists of a conductor attached to a grounding tab at the base of the tank shell running to a ground rod or to the grounding grid.
Bonding is simply a matter of electrically connecting different masses of inductance (metallic masses) together to maintain them at the same potential, to equalize changing potential, and to provide a path for lightning current between them.
The major area of concern is obviously the floating roof. On an external floating roof tank, there are three lightning events that can cause arcing between the roof and tank shell. The first is a direct strike to the roof itself or its appurtenances. In this case, all of the lightning energy must flow across the seals to the tank shell and to ground. The second is a direct strike to the top of the tank shell. In this case, the lightning energy flows down the shell to ground, and the roof potential must be equalized to that of the tank shell. In the third case, a nearby strike changes the potential of the tank shell, and much less difference in potential must be equalized between the roof and tank shell.
Lighting energy consists of two components with an intervening transition component. The first is a high-energy, short-duration surge of energy. The second is a lower-energy, longer-duration event. The first segment, although conveying high amperage, is so short that it does not normally cause ignition. Think of passing your finger quickly through the flame of a candle. However, the second segment consists of a few hundred amps (about equivalent to the electrical service into your home) over half to three-quarters of a second. When faced with resistance between the floating roof and tank shell, it can easily produce sufficient heat to cause the ignition of any flammable gasses present. Think of your kitchen stove on steroids.
Therefore, two types of conductors are required between the floating roof and tank shell. The first is a sliding contact between the roof and shell, and is intended to handle the short-duration, high-energy pulse. This has historically been addressed by the use of shunts between the roof and tank shell. These were developed to overcome the shortcomings on non-conductive seals. However, most modern tanks employ metallic shoes as the primary seal between the roof and shell. These shoes have many times the surface area of shunts. According to wording which will presumable be adopted in the next revision of both NFPA 780 and API 545, the presence of primary metallic shoe seals will negate the requirement for shunts.
Shunt Primary Metallic Shoe Seals
However, contacts sliding on contaminants produce arcing and sparking, raising the need for a second type of conductor, the bypass conductor. This is a hard electrical connection between the floating roof and tank shell. Because the bypass conductor must be of sufficient length to allow full range of motion of the tank roof, it requires time to become conductive. When it becomes conductive, it quenches any arcing at the sliding contacts, and conducts the long-duration, lower energy second segment of the lightning strike.
Another area of concern is thief hatches. The hatch itself rests on a rubber seal and is connected to its collar by a pin-type hinge. In the field, we have measured a high resistance between the thief hatch and its collar. Lightning current flow across that resistance is capable of producing sufficient heat and arcing to cause ignition. Therefore, a flexible jumper between the hatch and collar should be added to each.
SURGE SUPPRESSION. The second step in securing adequate protection is surge suppression. Any conductor running to or from a tank is perfectly capable of introducing all types of mischief. A surge suppressor is simply a device that keeps that from happening. Typical conductors found on a tank include AC power for site lights, pumps, valves, etc., and for data collection including levels, temperatures, flow rates, etc. Surge suppressors should be installed at the tank end of such conductors and also at their origin. This will limit the transient gremlins in their mischief.
STRUCTURAL LIGHTNING PROTECTION. The third step in securing protection is structural lightning protection. When we think of structural lightning protection we normally think of lightning rods on the roof of a building. It is important to remember that the purpose of a lightning rod system is to convey lightning energy around a non-conductive structure, such as a house or barn, thereby keeping that structure from burning down.
Note that there is absolutely no benefit to installing lightning rods on a tank. According to NFPA 780, the tank itself is inherently self-protecting. There are three components that make up a lightning rod system: the lightning rods, conductor system and grounding system. On a tank, the tank itself is of sufficient thickness to be substituted for the lightning rods, the shell is of adequate cross section to be substituted as the conductor system, and the site ground is more than adequate for lightning protection purposes. Therefore, the tank is self-protecting without the need to install additional components. Lightning rods would only tend to attract lightning to the tank.
There is, however, a technology alternative to conventional lightning rods. These are streamer-delaying air terminals. These air terminals, colloquially known as “fuzzy ball™” lightning rods, are designed to interrupt the lightning completion process by delaying the formation of lightning-completing streamers from objects on the surface of the earth.
A lightning strike begins with the formation of stepped leaders from the base of the storm cloud. These leaders jump in steps of around 150′, working their way downward towards the surface of the earth. When they reach to within 500′ or so of the surface, they begin pulling streamers of ground charge off of objects on the surface. Whichever streamer meets a stepped leader first determines what gets hit. As the ground charge builds on a streamer-delaying air terminal, the sharp points break down into corona under a low potential. When it comes time for a streamer to form from a protected object, the ground charge that would constitute the streamer has been partly dissipated into the atmosphere, thereby reducing the likelihood of a direct strike.
We use NFPA 780 as the design standard for protecting a tank. As the tank contains flammable material, we reduce the diameter of the rolling sphere to 100′, reducing the spacing between air terminals to jus over 12′. We install them around the perimeter of the tank shell on the foam injection plates and rim, and on the gauging platform. We also install them on the walkway handrail, if one is installed.
Streamer delaying air terminals on storage tanks
API 2003, Annex C, Direct Stroke Lightning Protection, C.1 notes that conventional lightning protection systems do not protect against indirect lightning currents or induced voltages. These are both major causes of ignition, particularly in production tanks. It further notes in C.2.1 that, according to vendor claims, streamer-delaying systems may have some benefit in protecting against indirect lightning currents of induced voltages. This type of performance is obviously preferable.
In the real world, we have seen a very high success rate with operators installing this type of system. Indeed, the cost has been justified many times over in both actual savings associated with extinguishing a fire and reducing lost time in service.