Static Electricity Produced in Combustible Liquid Storage Tanks

There is no generation of static electricity in the quiescent storage of liquids because static charges are produced only by some form of motion, for example the flow of liquids into or from tanks or other containers through pipes, hose, or even from one container into another. All storage tanks, blowcases, stills, sulfonators, autoclaves, and other equipment used in the storage and handling of flammable or combustible liquids and which are not inherently grounded by contact (electrically connected through metallic contact) with the grounded steel structure or piping shall be grounded.

Portable Drums, Containers, and Safety Cans

When drawing off flammable or combustible liquids from drums, or tanks into metal containers, both vessels shall be bonded together and grounded in accordance with Figures 1, 2, and 3.

Open-Ended Hose

Open-ended hose carrying flammable or combustible liquids, vapors, or solids, and having a metal outlet shall either be conductive hose or have the metal outlet bonded to the supplying vessel which, in turn, shall be grounded. The receiving vessel shall be grounded, and the metal hose outlet shall be in contact with the receiving vessel.

Closed-Loading Hose

Transfer hoses for flammable or combustible liquids or vapors, having metal couplings, may be of either conductive or nonconductive material. The receiving vessel or pipeline shall be grounded, and each metal hose outlet shall be in contact with the grounded receiving vessel or pipeline. No exterior hose-bonding wires are required. Ungrounded internal reinforcing wires in liquid transfer hoses do not create a static ignition hazard as long as liquid flow (velocity is lower than 0.91 m (3 ft/s)), or if hose length is 9.14 m (d30 ft), and no filters are installed in hose lines.

Storage Tanks (Non-portable)

Liquids that are reasonably good conductors of electricity (see Hazardous Area Classification) greatly aid in the dissipation of static and, generally, no special precautions beyond adequate tank grounding are necessary, except as outlined in ‘inlets to tanks’. However, dielectric liquids have the capacity of separating and holding electrical charges, particularly on large free liquid surfaces exposed to air or vapor. Here the potential gradient may become steep enough to cause spark-over along the liquid surface to the tank shell, and with the coincidence of a proper vapor-air mixture, and cause explosion. Tanks shall be grounded for protection from lightning and static ignition.

Inlets to Tanks

When flammable or combustible liquids are allowed a free fall to the bottom part of a tank, a static charge can be developed which can cause an explosion. All tanks which contain ignitable mixtures of air and vapor during filling operations shall be equipped with dip pipes, of such size and construction as to minimize turbulence in the liquid contents and minimize the development of static charges. It may also be advantageous to arrange the inlet piping for discharge near the bottom of tanks even though they are equipped with an inert-gas breather system and contain a low flash-point liquid, inasmuch as there are bound to be instances when the breather system may be inoperable. Where this is impractical for some reason, a suitable open spiral chute welded to the top inlet nozzle and carried all the way to the bottom of the tank to provide a safe discharge is recommended, see mixing operations. Dip pipes, as described above, are not required for loading tanks used exclusively for transportation of flammable or combustible liquids that have positively been determined to have conductivity of 10-3 P:/cm or greater.

Tank Agitation

Circulation or agitation in tanks containing flammable or combustible liquids never shall be started until there is sufficient depth of liquid over the agitator to prevent violent disruption of surface films. When using either propeller or jet mixer, care shall be taken to avoid agitating a possible layer of water at the bottom of tanks containing flammable liquids. The propeller or jet stream shall be directed so as not to break the liquid surface. Where flammable vapors are encountered above the liquid surface, inert-gas blanketing is recommended.

Tank-Car Loading and Unloading Spots (Flammable or Combustible Liquids)

Some tank cars may be adequately grounded by their construction and be satisfactorily grounded by their contact with the rails. However, tests show that, in many instances, grease in the car wheel journals and rust on the rails may interpose a high resistance between the car tank and ground. Since there always is present the possibility of stray currents at car spots, provisions shall be made for grounding tank cars during filling and discharge operations.

Tank-Truck Loading and Unloading Spots (Flammable or Combustible Liquids)

These shall be grounded in accordance with Bonding and Grounding & Installation of Grounding and Bonding Conductors, Figure 6. The mechanics involved in loading tank trucks or any other rubber-tired vehicle introduce hazards from electrostatic fields. When rubber-tired vehicles move over roads, static charges will accumulate on the vehicle during periods of low humidity. For this reason, it is imperative that the truck body be grounded directly at the loading and unloading spots before the tank hatch is open, in accordance with Bonding and Grounding & Installation of Grounding and Bonding Conductors, Figure 6. All bonding cables shall be connected before hatches are opened or piping and hoses attached, and they shall not be removed until all piping and hoses have been disconnected. The use of drag chains to drain off static charges from the truck during transit is unreliable, especially during dry weather, because of the unavoidable high resistance of the chain contact with the ground.

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Storage in Cylinders

Locations used for the storage of hazardous liquids, or liquefied or compressed gases when contained in approved sealed cylinders, normally are not considered hazardous if provisions are made for racks and fasteners to protect cylinders from accidental damage or dislocation, and the building construction affords reasonable protection against lightning hazards. The increased protection offered by the installation of lightning rods may be desirable for some types of buildings, particularly those of wood frame construction in outlying areas.

Mixing Operations

In some operations it may be desirable to ‘turn over’ the flammable dielectric contents of a tank or vessel so as to minimize chances of polymerization, obtain uniformity in the mixture. In these cases, inlet piping arranged for discharge near the bottom of tanks would defeat the purpose of ‘turning over’ the contents. A suitable open spiral chute welded to the top inlet nozzle and carried all the way to the bottom of the tank will provide a safe discharge of flammable or combustible dielectric liquids onto the top of the contents (whatever the liquid level may be) for the purpose of ‘turning over’ or mixing. It is important that good metallic connections be made between the chute and tank, and between sections of the chute in order that any static which may be generated will be conducted away.

Spray Painting

Conventional spray equipment does not require grounding of the spray gun and paint supply system or the object being sprayed since only small charges of static electricity are generated during operation. Airless spray systems generate sufficient static electricity to require bonding or grounding of the object being sprayed (if conductive) and the spray gun and paint supply system.

Tanks and Barges

Loading and unloading of flammable or combustible liquids from or into steel hull tank ships and barges presents the potential for fire and explosion hazards due to static electricity and stray currents. While the principles in this standard apply to marine operations, ‘ ISGOTT, International Safety Guide for Oil Tankers and Terminals’ and KSA Ports Authority Safety & Pollution Booklet” provide specific guidance related to static hazards in all aspects of marine operations, including loading, unloading, sampling, gauging, tank cleaning, etc., and shall be followed.
Insulating joints (or flanges) in the pipe lines shall be required to prevent the pipelines from being alternate paths for stray currents.
It is recommended not to start transfer operations or, if started to discontinue them during severe electrical storms.
Pumping of flammable or combustible liquids into storage tanks having a vapor space applies to the flow of such liquids into the ships’ tanks. It is recommended that transfer operations shall not start or, if started, shall be discontinued during severe electrical storms.