Grounding method eliminates rim fires

Oil industry engineers must be aware of bound charge and secondary arc risk to oil facilities, since secondary lightning effects such as bound charge and secondary arc can be 1,000 times more common than direct lightning strikes.

07/08/2003


Bound charge occurs when a storm cell induces an electrical charge on everything beneath it, including metal containers and the flammable product within them, for example, in floating tank storage facilities.ling or emptying a tank quickly, can cause this to occur. 

The inadequacy of shunts for oil tank grounding

Many large crude storage tanks are open without permanently attached roofs., which poses the danger of rim or other significant fire through the bound charge and secondary arc phenomena.

To overcome this problem the industry usually installs shunts, metal devices attached to the roof so as to be in constant contact with the tank wall regardless of the position of the floating roof.

History has shown that shunts require constant maintenance because:

1. Wax and other heavy, crude components tend to deposit between the tank wall and the shunt fingers, forming an insulator between them.

2. Since the roof floats, it can easily drift slightly off center and disconnect from the opposite side.

3. The gap between the tank roof and wall is so small that an arc can easily jump that space, and ignite the always present vapor when a charge is on the product.

Ultimately, if the shunts aren’t in perfect contact with the tank wall, bound charge will permit an arc to form between them and the tank wall when a storm cell is discharged by a nearby lightning strike.

Some companies have tried to use long wires that extend from the top of the tank wall down to the center of the floating roof to ground shunts.

The requirement is to make a continuous, positive connection between the floating roof and the tank wall, with an impedance low enough to eliminate bound charge risk.

The difficulties of adequately grounding

Often, typical two to four-inch wide stainless steel shunts
don’t actually touch the wall, and others only appear to make a connection.

Other shunts may in fact touch the wall but do so lightly or through a heavy hydrocarbon accumulation on the tank wall.

A new industry trend is to paint the inside of tanks with a non-conductive polymer.c.

Alternately, access ramps connecting the top of the tank wall to the roof are often assumed to satisfy grounding requirements, but rust and paint insulate critical joints.

A practical alternative: Using the storage tank itself for grounding

It is well known and easily proven that any large storage tank possesses the ability to make a solid, low impedance connection to earth without augmentation from external grounding electrodes.

Actually, the resistance or impedance to earth is not the problem.ve container that dangerously inhibits the discharge process.

Since petroleum products are nonconductors, they tend to hold any induced charge.

However, the charge near the center of the tank remains much longer, creating concentration of charge in the middle of the tank.

Obviously, the larger the tank, the more critical this function becomes.indicate.

The retractable grounding assembly (RGA

Making a good connection between the tank wall and the movable floating roof remains the only grounding problem for these storage tanks.

To satisfy this requirement, Lightning Eliminators & Consultants, Inc. of Boulder, Colorado developed the Retractable Grounding Assembly (RGA), a device that provides a direct connection to the tank roof from the tank wall using a wide, thick-braided wire cable, or strap wound on a heavy stainless steel reel, with tension held by spring loading.

The path of impedance is kept to a practical minimum by the combination of the shortest path, wide braid, and constant tension.

RGA mounted on tank rim

The RGA’s design objective was to achieve the lowest possible impedance between the roof and tank wall with minimal maintenance. In fact, the RGA works so well it eliminates the need for shunts.

The reel assembly is constructed from 316 stainless steel, with provisions for mounting on the top edge of the tank.e charge can accumulate in the center of each tank.  That charge must be carried to the closest location of the tank wall.  To assure that path is not too long for tanks with diameters of 200 feet or more, at least six or eight RGAs may be required.  This recommendation is based on the worst caseimpedance situation, where the tank is nearly empty, and the roof is resting on or near the tank floor.

Since the RGA module is designed to be bolted to the top of the tank rim, the location should be chosen so they are in perfect vertical alignment. To install the RGA, it’s best to select the anchor point on the roof structure first, and then align the reel assembly so that it’s directly above that anchor point.

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: info@LightningEliminators.com or visit www.LightningEliminators.com





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