Search Past Articles
Explore Past Articles
Haz Mat "Specialist Course"
« TFE-14 | Main | TFE-12 »
Thursday
Jul072011

TFE-13

Bonding and grounding for Tank Transfers

     Bonding and grounding has had a tumultuous history insofar as it relates to different vessels. The concern has always been with, “HOW” to bond and ground, not getting the electricity to flow to ground. It should not be about which method is right or wrong, rather it should be about which method is “better”. Basic NEC {National Electric Code} safety guidelines dictate ample size conductors for the amount of electricity you may have to discharge into the ground. The larger and more conductive the transfer tissue {wire} the less resistance buildup you generate. The less resistance the less heat generated. In a hazardous atmosphere, this is important. Each safety organization has their own guidelines but the concept remains the same. Tank Farms also have their own guidelines. First, lets’ focus on the concept, and then outline an emergency procedure you can implement during a tank farm emergency. If you understand the electrical concept, you will be able to recognize a proper technique from an improper one.

     Cathodic protection in flammable bulk storage tanks is primarily engineered for the purpose of preventing long term corrosion of the tank bottom. By previous definition, this may also ground the tank in the advent of a lightening strike. This is accomplished by sacrificing an electrically conductive anode material to not only ground the tank, but more importantly corrode “AT” the anode rather than the tank bottom. This practice deters corrosion in real time for years. If the storage tanks in question contain and frequently transfer volatile flammables, permanent grounding systems will be installed. Electrically speaking, any device or material has the ability to hold “static” or {not moving} electricity and can be considered an insulator. Since there are no “perfect insulators”, all objects will sooner or later trickle off “static” electricity as “kinetic” or {moving}, electricity by flowing across its surface moving towards a ground. It is this “kinetic” electricity that builds into a point, called a “corona” and eventually has the strength to discharge across any surface as a “spark”. Through bonding and grounding, and the elimination of these “sparks”, flammable liquid bulk storage tank ignition can be prevented.

     Since the FLBSS {Flammable Liquid Bulk Storage Specialist} may have to improvise proper techniques due to grounding equipment damage, we will focus on portable units. These can be made of a variety of materials as long as they are electrically conductive. When constructing a portable unit, the focus must be on ground resistance. The lower the resistance, the easier the flow to ground and the less chance of generating a “spark”. To achieve this, safety authorities advise driving at least a ½” conductive rod 20 feet into the earth, serving as a ground connection point. However, resistance is a function of temperature and humidity and in extreme cold and low humidity this technique may not function the way intended. Also, if the site is concrete pavement, this will be a challenge. Since a construction procedure like this will challenge most emergency operations, there is another solution. To satisfy these requirements, lower resistance at your ground with the addition of hot water and salt. This addition affords you control over temperature and humidity.

     The KEY to electrical dissipation is in ground resistance. By “decreasing” the overall ground resistance we increase the effortless flow of unwanted electrical charges while decreasing the potential for coronal “sparks”. To do this, arrange conductive webbing, such as a piece of 4’ X 6’ fencing in an area of depression nearby your connection point. I have used sewer grate covers in place of fencing. This serves as your grounding rod. Before connecting the grounding lines to this “fencing” saturate it with warm Engine pump water and salt. Whether you connect the lines to ground first, or the lines to your pump first, should be based on the explosive limits of the product. If the product limits dictate that the explosive “range” is closer to the tank contents, make the pump connections first, and carry a potential “corona” out of explosive range before connecting to the ground. If the product limits explosive “range” is farther from the tank and its contents, such as in the case of propane, connect to the ground first, then to the pump. The concept is to connect the system, {where an initial “stored load” spark is possible} to an area below the explosive limit. It is true that a coronal spark is possible at every connection point, but remember as the “static” becomes “kinetic” and

flows through your “bonding” line; the greater spark potential will be at the second connection point, outside of the explosive “range”. The amount of energy we are speaking about is small, and our goal is to keep it that way, minimizing the chance of a “spark”. As the salt dissolves, maintain a slush-like mixture covering as much of the fencing section that is possible. This will require dedicated personnel to monitor this salt “slush” throughout the emergency transfer operation. It has been documented that adding 20% salt by weight of the total moisture in the existing soil, an electrical decrease in resistance of 100 times is possible.

     Here is the bottom line situation for the FLBSS. A full surface tank fire is determined to not have enough space remaining inside the tank for a Foam attack to begin. The option is to remove the product by pump-off to give your attack team the Foam space inside the tank walls. The problem; the pump transfer station for the tank was destroyed in the fires ignition. A portable unit is brought in but no permanent bonding or grounding hook-ups are intact. Since most pumps constructed for this type of use are grounded for safety, your connection points for your electrodes, {cable grips} shall be at the pump intake and pump discharge. Remember, frictional flow generates increasing electrical current. The opposite cable ends are attached to your fencing material {ground} which lies in the small depression filled with salt water slush. If you have no natural depression, dig one out just deep enough to hold your ground and salt water mixture. Many times you will be faced with the inability of compliance with NEC or other codes, so it becomes a matter of how close you can come to a safely grounded, pump-transfer. Much of the accepted data indicates that for a “best” ground, your grounding rod should be driven down to reach the local water table. The above technique will yield a safe ground and bond for emergency operations. But, if you can drive a grounding rod far enough into the earth to reach the local water table, good for you.

                                     Haz Mat Mike

 

 

 

PrintView Printer Friendly Version

EmailEmail Article to Friend

Reader Comments

There are no comments for this journal entry. To create a new comment, use the form below.
Editor Permission Required
You must have editing permission for this entry in order to post comments.