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October 2005: Static Electricity Control in Flammable Liquid Dispensing

What is Static Electricity

Static electricity is one of the prime ignition sources in flammable liquid dispensing operations. Similar to rubbing things together, liquid flow can cause electrons to move from one atom to another. Some atoms get extra electrons. They have a negative charge. Other atoms lose electrons. They have a positive charge. When charges are separated like this, it is called static electricity. While in contact, such as a closed piping system, a redistribution of the electrons is apt to occur and equilibrium is established. When an electric charge is created on a nonconductive body it can not escape, and if it is not neutralized or eliminated as it is produced, the charge can build up and develop sufficient energy to jump to a nearby grounded object in the form of a spark which is capable of igniting flammable vapors. It should be emphasized that flammable liquids are the materials of concern. This is the Class I liquids, or Class II and III liquids heated above their flash point (Ref: NFPA 30 "Flammable and Combustible Liquids" for definitions). Static control is usually not needed for unheated Class II and III liquids.

Static Control

The generation of static electricity can be exacerbated or controlled through optimization of liquid velocity, flow rate and pipe size through delivery systems. Typically a flow rate of up to 15 ft. per second is suggested. Splashing and free fall of liquids into the receiving vessel should be eliminated as much as possible. This can be assisted by lower fill velocities, directing fluid discharge down the side of the receiving vessel and by submerging fill pipes below the liquid level in the vessel. In an empty vessel a much lower flow rate should be used until the bottom of the file pipe is submerged. Typically, a fill pipe is not considered necessary for smaller vessels such as 55 gallon drums. Where the delivery system contains filters, they should be far enough upstream to provide a "relaxation time" prior to discharge - 30 seconds is a rule of thumb. Filters can generate over 100 times the electrostatic charge as would occur in the same delivery system without filtration.

Static Dissipation

When conductive objects are isolated from ground by nonconductors such as nonconductive pipe, hose, connections, supports or gaskets they can build up a charge and should be bonded. This would include often overlooked items like metal rims on non conductive containers, spray nozzles, probes, etc. Bonding is needed to permit a path for electron flow to establish equilibrium. Bonding is an electrical connection between conducting objects with a conductor. Such bonding conductors should not exceed about 25 ohms resistance. The bonded objects should in turn be connected to a grounding system, which is the same as bonding, but conducts to a ground or "earth". The ground or "earth" can be a rod driven into the ground, a buried metal plate or an underground metal water pipe. The steel structure of a building that is grounded for lightning protection can be the grounding path. Some things to avoid in a grounding system are underground pipe that has a cathodic protection system, non conducting materials, or any electrical discontinuities such as gaskets. Where there are discontinuities, jumper cables can be permanently installed to provide a continuous electrical path. The electrical current carrying system of a facility should not be used and care must me exercised when using sprinkler system piping due to increased resistance to ground caused by joints and connectors.

The bonding and grounding system should use durable low resistance cable conductors. Connections should be direct and positive. Detachable clamps should have good strong springs and sharp points should be used to make contact with metal through paint, rust and surface dirt.

Maintenance

The bonding and grounding system should be inspected and continuity tested at the time of installation. Electrical resistance should be about one ohm for the flexible and fixed conductors and the grounding resistance can be as high as 1,000,000 ohms. Once installed, periodic inspections are important for continued effectiveness of the system. The inspection frequencies are arbitrary. The various flexible connectors are most likely to suffer wear and tear and should receive the most frequent attention. A quarterly or more frequent inspection should be made of cleanliness, mechanical damage, and general wear. Clamp points should remain sharp and the springs should still be strong. Check for broken conductor strands and tightness of cable attachments. Jumpers and the fixed conductors should be checked for damage and tightness of connections; however these may be checked less frequently since there is less possibility of damage and wear. The continuity and resistance tests should be repeated annually.

Training

A general safe practices guideline for operators is suggested. As with any flammable liquid handling, training in safe working practices is important to an understanding of the characteristics and danger of working with flammables, including static control. Make sure other protective features are up to par. This could include housekeeping, dispensing receptacles, special foot wear or clothing provided, mechanical ventilation, and hazardous area electrical equipment where appropriate.