Published January, 2012 in Tank Storage Magazine
A new way to dissipate electrical charge
One common solution to lightning risks may actually be making the problem worse
On a fairly regular basis, certain types of tanks explode or catch fire, sometimes, but not always, during electrical storms. This is a recurring industry problem without a reliably effective solution.
There are two possible causes of this type of incident: a direct or proximate lightning strike, or static discharge.
Direct attachment may be the cause of some incidents, but it is not the likely culprit in most cases. There have been many incidents that occurred when lightning was not present in the area. Additionally, there have been many incidents involving steel tanks. It is unlikely that lightning attachment caused burn-through or heating ignition of vapor in these tanks. Therefore, the most likely cause is static discharge.
The source of static may be the result of normal operations, such as filling or draining, or it may be secondary effect from a direct or nearby lightning strike. Secondary effect arcing is also static discharge, albeit high energy and occurring over a very short time frame. This arcing is produced by the inrush of ambient ground charge toward the point of a lightning strike. The inrushing charge can arc across gaps in its path, thus providing both a static charge and a static discharge
Therefore, the ideal protection system would address both causes. Fortunately, the solution is the same. A system that protects against lightning strikes, both direct and nearby, will also protect against static discharge ignition.
Probability versus consequences
The probability of this type of incident is unpredictable. It could be years between incidents or years without incidents, followed by a single or series of catastrophic events. On the other hand, one north Texas operator lost two batteries in one day.
The consequences of this type of incident include lost production, the cost of replacing the damaged facility, environmental impact and clean up, and bad press, especially if the subject tanks are located in a populated area or a local fire company responds.
Conditions leading to ignition
According to API 2003, A.7, in order for an electrostatic charge to become an ignition source, four conditions must be met:
1. a static charge must be generated,
2. the charge must be accumulated to the level at which it is capable of producing a incendive spark (A.6.2), that is, a spark with adequate energy to ignite,
3. an appropriate gap across which the accumulated charge may arc (source of ignition),
4. an ignitable gas mixture must be present around the source of ignition.
Sources of static charge (Rub two molecules together)
The primary source of static charge appears to be turbulence from mixing fluids, either from pumping, particularly through non-metallic pipe or from filling, especially splash filling, with the falling fluid penetrating standing fluid. Air/foam injection to increase flow rates may also be a primary source. The tiny droplets of fluid above the product are separated by the dielectric gas or air, allowing the charge to build in the vapor space.
A secondary source may be bubbling of the air/gas mixture. This leads to a suspicion that the boundary layer between the liquid and gas may play an expanded role in this problem. There are also miscellaneous sources, such as clothing on people. This factor is humidity sensitive similar to toughing a doorknob on a dry day, and the charge does not usually build to the level where it becomes incendive.
Accumulation of static charge
Charge dissipates from a fluid into points and sharp edges, not flat surfaces. That is why charge does not readily dissipate into the shell of a metal tank – it is flat. This allows the charge to accumulate at a rate faster than it dissipates. The presence of a carbon veil in a fibreglass tank does not accelerate charge dissipation. It still presents a flat surface to the bound charge on the liquid. An epoxy-lined steel tank is similar to a fibreglass tank regarding static charge dissipation.
Because the static charge eventually relaxes, an incendive spark is most likely while the charging mechanism is active.
Source of ignition (Sparking)
When the static charge exceeds the dielectric of the intervening medium, the medium breaks down and a potential equalizing arc occurs. The arc may occur between masses of inductance, such as piping, fittings, the thief hatch and its collar (if it is loose enough to rattle, it is loose enough to arc), electronic sensors on the tank, and vacuum trucks, or between the bound charge on the stored protect and any of the above.
The likely source of gas is the ‘Coca Cola’ effect. Gas is suspended in the fluid underground. When it reaches the wellhead, the reduction in pressure allows the gas to escape, much like carbon dioxide escapes from Coca Cola when the can is first opened.
The turbulence involved with further handing allows more gas to escape, much like drinking Coke through a straw, then blowing it back into the can and drawing it out again. Splash filling, while helping to accelerate molecular breakdown and speeding the separation process, also allows additional gas to escape. Air/foam injection to increase flow rates also generates gas.
To allow combustion, oxygen must be available in sufficient concentration. Oxygen may enter the tank from atmospheric vents or from a thief hatch left open, a common practice. Oxygen may be introduced to prevent a vacuum in the tank during the process of emptying. Therefore, the conditions for combustion may be high when filling an empty tank, as static has been generated by the flowing liquids and oxygen is present in the tank.
Lightning caused ignition
Ignition due to lightning is caused by the ground charge induced by the cloud base charge on the surface of the earth beneath the storm. The storm cloud generates charges within the cloud, and a charge on the base of the cloud. This charge induces an opposite charge on the surface of the earth beneath it. The attraction of opposite charges attempts to pull this ground charge off the surface of the earth, so it is dragged along the surface of the earth beneath the cloud.
When lightning strikes the surface of the earth, it relatively vacates the ground charge at the point of the strike. The surrounding area remains highly charged, so the remaining ground charge flows toward the point of the strike. If this inrush of charge crosses a gap, it may arc. This all happens very quickly, with the storm cloud providing the source of the charge and a sufficient accumulation of charge to form an incendive spark. The tank structure provides the source of ignition and the ignitable mixture.
The most common lightning fix is a catenary (overhead wire) system. This system consists of grounded masts or poles supporting a wire or wires over the site. Based upon the above description of the problem, this system is far from ideal. The catenary wire is intended to get in the way of a lightning strike, and convey it to ground. When used to protect tanks and similar structures, this system cannot mitigate secondary effect arcing, the primary cause of ignition. In fact, if a catenary performs exactly as designed, it brings the lightning energy to ground near the base of the tank, thereby maximizing the likelihood of secondary effect arcing across the tank and appurtenances.
The catenary system has no effect on the bound charge on the stored product, does not provide bonding to miscellaneous masses of inductance on the tank, and does not affect the likelihood of a direct strike by influencing streamer formation.
Additionally, several options have been employed to control the conditions necessary for an electrostatic charge to become an ignition source.
Condition 1: the generation of a static charge. Among the solutions suggested and tried are employing piping of successively increasing diameter to reduce the flow rate, and locating fill pipe discharge near the bottom of tank. Although they do reduce the rate of charge accumulation, they are only partially effective, as turbulence still creates static.
Condition 2: the charge accumulation to the level at which it is capable of producing a incendive spark. Among the solutions to this is inserting a metallic rod or similar conductor into the tank to increase the rate of discharge, as recommended by API 2219, 188.8.131.52. However, this is not optimally effective, as the inserted item still constitutes a flat or curved surface.
Condition 3: an appropriate source of ignition. This is commonly accomplished by bonding all masses of inductance. In addition to tank battery appurtenances, vacuum trucks must be bonded to the system. This bonding conductor may be a simple flexible cable, which is just as effective and not as expensive as retractable reel system, or as complex as a conductive hose, which may be difficult to test on a regular basis and to maintain. Any system used should provide strain relief for the driver who forgets to unhook truck before driving away.
API 2219 allows grounding of a truck to a ground rod separate from the site ground. As this ground rod may be at a potential substantially different from that of the remainder of the site, particularly that of the stored product, this technique may be inadequate.
These steps are moderately effective, but still do not address the bound charge on the stored product.
Condition 4: the presence of an ignitable gas mixture. Normal maintenance should include the dump valve, and operating practices include close thief hatch and controlling the rate of filling and emptying a tank.
The wild card in tank protection has always been equalizing the bound charge on the stored product. Charge dissipates from a liquid onto points and edges. Even in a steel tank, there are none to help dissipate the bound charge on the stored product. The liquid simply lies against the side if the tank, and the charge must inductively couple onto the flat surface. It takes time for the potential to relax, allowing the static charge to accumulate faster than it dissipates.
Lightning Master has designed an in-tank static drain consisting of a stainless steel cable with stainless steel electrodes inserted into the wind of the cable. This drain, installed through the thief hatch and secured to the top of the tank, introduces thousands of electrically sharp points into the stored product, offering a low-resistance path for bound charge to leave the liquid and vapor space. It sucks the charge out of the product, allowing it to relax much more quickly. This allows the charge to dissipate faster than it accumulates.
On a steel tank, the only additional bonding required is a jumper between the thief hatch and collar (if it is loose enough to rattle, it is loose enough to arc).
On a fibreglass tank, a conductor system is installed, bonding the top vent pipe or manifold, the in-tank static drain, thief hatch collar, walkway handrail system, and tank conductive elements such as a carbon veil, and the drain pipe at the base of the tank.
The bonded mass of the tank system is then electrically bonded (grounded) through existing electrically continuous metallic piping or with dedicated conductors on non-conductive piping to the injection well, truck load-out, and site electrical service ground. This brings all site components and structures to the same potential and to ground potential, thus reducing the possibility of arcing. Truck drivers should be trained to bond their trucks to the site bonding system without exception. The truck bonding system may consist of a retractable reel grounding wire, or may be as simple as a flexible cable with a spring pressure clamp attached to its end. In either case, a means of strain relief must be provided to compensate for the driver who drives away with the grounding clip still attached to the truck.
To provide direct lightning strike protection, streamer-delaying air terminals (lightning rods) are installed atop the tank, vent or vent manifold piping and handrail structure. These air terminals are UL Listed lightning rods, and also retard the formation of lightning completing streamers from structures, thereby reducing the likelihood of a direct lightning attachment.
These systems are simple, lightweight, and easy to install, and have been employed successfully on hundreds of steel and fibreglass tanks throughout the US and overseas.
This system is designed in accordance with the standards contained in NFPA 780, and is approved by the city of Ft. Worth and many other municipalities for installation on tanks and batteries within city limits.
It costs less than commonly used systems, such as the catenary system, and is more effective at controlling the conditions that constitute the proximate cause of tank incidents.
For more information: This article was written by Alan Roachell, Rosewood Resources and Bruce Kaiser, Lightning Master Corporation, http://www.lightningmaster.com/