This article is for controlling the generation of static electricity when handling liquids and gases in hazardous areas and in normal areas. Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.
Static Electricity References
American Petroleum Institute (API)
Recommended Practice 2003
International Safety Guide for Oil Tankers & Terminals (ISGOTT)
Safety & Pollution Booklet
National Fire Protection Association (NFPA)
70 National Electrical Code (NEC)
77 Recommended Practice on Static Electricity
How Does Static Electricity Work
Following are considered to be the main hazards and problems:
a. An incendive spark in a hazardous area which could produce an explosion or fire in the area itself
b. An incendive spark where a low-conductivity flammable or combustible liquid is discharging into a container and has a chance to mix with air producing an explosive mixture. An explosion within the container can result.
c. Personnel contact with a charged section of system resulting in injury due to reflex action from the static shock
d. A problem in manufacturing processes where charged materials create operating difficulties
See OSHA (for example 1910.106, 1910.108, 1910.109, 1910.219) for potential hazardous effects from static electricity.
Electrical installations shall be in accordance with NFPA 70. Following video is showing how does static electricity work.
https://www.youtube.com/watch?v=gXOajw2HDHk
{Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.}
How Electricity Charge is Produced in Plant?
Contact electrification or volta effect is produced between clean, dry metallic surfaces or surfaces of metallic conductors and semiconductors. When two dissimilar uncharged metals are placed in contact with each other, one becomes positively charged and the other negatively charged. Depending upon the nature of the metals, a difference of potential is set up between them. Materials differ widely in their ability to acquire electrons from other materials during contact. See Table I.
Ionization in air may be produced by:
a. Intense electrical fields which develop near fine wire, sharp points, or fibers
b. High temperatures, as in a flame or possibly near a heat wire
c. The invisible radiations and particles emanating from radioactive materials
d. The action of some wave lengths of ultraviolet light
e. X-rays
To avoid discharges or sparks, the static voltage on metal-to-metal objects shall be maintained below a minimum sparking voltage which has been determined to be about 350 volts. Electrons in metal are quite mobile and easily transferred and, for this reason, there will be no appreciable static buildup on the metal if the metal part is grounded. Acquired charges will drain off rapidly.
See Figure 2, Detail A for capacity effect produced on non-conducting material. When static electrical charges are produced on non-conducting materials, for example paper, webbing, rubber, plastics, and leather, it is not possible to draw off the static or neutralize it by ordinary grounding methods. Such systems of stored electrical charges in the form of energy can be considered condensers or capacitors. An accumulation of electrical charges on two contacting substances, for example a coated webbing with a steel roll, can lead to a spark discharge. Low-resistance materials, for example two metallic plates, permit almost instantaneous discharge of all electrostatic energy accumulation after a spark has been initiated. If the discharge occurs through a very high resistance path, it will be slowed appreciably. In such instances, discharge will occur harmlessly as current leakage rather than as a spark. High voltages occur after the separation of two insulating-type materials, or one insulating and one conducting type material. Correlating charge intensity (in volts) with typical coated webbings plastic materials moving through processing equipment indicates the following:
a. Dirt and dust pickups begin at charged intensities of about 3 kV. (All potentials measured 2 inches from charged surface).
b. Explosion and fire hazards exist in flammable atmospheres at about 4 kV
c. Handling problems become noticeable on thin stationary films and laminates between 4 and 5kV
d. Packaging problems present difficulties at 6 to 7 kV
e. Controlling and regulating problems become serious in the 12 to 15 kV range
{Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.}
The presence of flammable vapors in coating and spreading operations dissipates static charges without producing static sparks and to provide adequate ventilation. If it is impractical to increase the conductivity of the material so that the static will be conducted to a grounded metal roll or other machine part, the static shall be removed through air. The following safeguards are recommended for effective static dissipation:
a. Electrically connect all metal parts of equipment including metal rolls and connect machine frames to ground
b. Provide static collectors (see Figure 3) or static neutralizers at or near the points where tests show that static is generated. This will include all points where the fabric or webbing is unrolled, where the compound is applied, and where the fabric or webbing leaves idler rolls and pull rolls.
c. Provide a relative humidity of 60 percent or more if the static charge is not adequately eliminated by the other safeguards listed and humidity will not harm the product. Metallic powders and some of the pyrotechnic mixtures, cannot be exposed to air with 60 percent relative humidity because of the possibility of spontaneous ignition.
The development of electrical charges may, or may not, be in itself a potential fire or explosion hazard. The following are sources of ignition, fire and explosive hazards:
a. There shall be an effective means of static generation
b. There shall be a means of accumulating the separate charges and maintaining a suitable difference of potential
c. There shall be an ignitable mixture present
d. There shall be a spark discharge of sufficient energy to ignite the above mixture
The generation of static electricity can be minimized. Static generation is caused by spark discharges or sudden recombination of separate positive and negative charges. The ability of sparks to produce ignition is governed largely by their energy.
For static generating materials that have high flash points. Such a material presents no hazard of static self ignition since an ignitable mixture cannot form. An introduction of air bubbles, or water in turbulent loading operations may produce a static ignitable mist even though the material temperature is below the flash point. However, it can still generate static which can present a hazard because of other ignitable mixtures in the area or personnel exposure and, in such cases, shall be grounded in accordance with this standard. Occasionally, such materials are heated above their flash point and are then capable of forming an ignitable mixture.
When hazardous static conditions are unavoidable in certain operations, assure that there are no ignitable mixtures at the points where sparks may occur.
Reducing Hazards of Static Electricity in Plant
An electrically charged material can be discharged by (1) making the material such a good conductor of electricity that electricity can flow along the object and onto some other grounded object in contact with it, and (2) making the air around and adjacent to the material sufficiently conductive of electricity that electrons can flow through it. All methods of static control depend upon one or the other of these two mechanisms.
There is no other way of correcting the inequalities of electrical charge, once they have been created. The following reduce static electricity:
a. Grounding and bonding of machinery and equipment including all moving parts, connecting apparatus, and pipeline personnel discomfort
b. Maintaining a relatively high humidity in areas where static may be a hazard
c. Increasing the electrical conductivity of air by ionization
d. Application of conducting surfaces or finishes
e. Use of nonmetallic shields or guards to prevent contact with metal parts where problem is one of
For bonding and grounding. Grounding system shall be designed, installed, and maintained as other parts of an electrical installation. Humidity (60 percent or greater) shall be maintained for preventing the accumulation of static charges. For surfaces heated above ambient air and for static on the surface of oils and some other liquids, high relative humidity does not help drain off the static charge.
https://youtube.com/watch?v=zryEPJm2pXU
{Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.}
Controlling Static Electricity in Hazardous Locations
All electrical equipment, circuit wiring, building structure, and non-electrical equipment exposed to lightning effects shall be grounded in accordance with Bonding and Grounding – Installation of Grounding and Bonding Conductors.
It is assumed that the equipment is immune from lightning effects, and is not part of a fault-current return path for electrical circuits. If the equipment, piping, or ductwork if entirely within the confines of a structural steel building, or if (in the case of piping) it leaves such a building but is adequately grounded at the building perimeter, it may be considered as immune from lightning effects.
If the above conditions apply and the conductivity of the material being handled is less than 10-4 P:/cm, indicating possible static electrification, the resistance to ground of the equipment or to any section of the piping or ductwork involved shall be 100 ohms or less. Generally speaking, the worst static generators are those whose conductivity is around 10-6 P:/cm. There are no clean-cut rules on relationship between charging tendency and conductivity.
The 100 ohm maximum shall be checked initially from every section of pipe and every fitting to the plant ground system. An additional test after 3 months to check on the permanence of ground continuity is in order and, after that, at periods determined by plant experience shall test at least annually. Corrosive areas shall be tested more frequently than non-corrosive areas.
Bonding or jumpering of a joint in piping or duct shall be done only when the construction of the joint is such that sufficiently low resistance cannot be obtained by its normal mechanical bolt-up, and then not if all sections satisfy the 100 ohm requirement due to support or connection to grounded steel. Current-carrying capacity for static is so small that it is never a problem and pipe-joint jumpers need be only large enough for mechanical strength with 6 mm (No.10 AWG) being the smallest recommended.
Bonded and non-bonded systems shall be checked for resistance to ground.
{Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.}
If a system is dismantled for cleaning and reassembled:
a. If in a Class I, Division 1 area, every section and fitting shall be checked to ground every time
b. If in a non-hazardous or Division 2 area and dismantling occurs often, then good judgment shall prevail and no hard and fast rules shall be established. For example; if the cleaned pipe-joint surfaces and bolts appear in good condition and the system is being reassembled without changes or additions, no retesting is required. Frequent disassembly, cleaning, and reassembly will tend (in most cases) to keep the ground continuity in good shape.
The checking of systems for compliance with the 100 ohm maximum presents no problem in non-hazardous areas. However, in Division 1 or 2 areas, a safe tester shall be used. Such a tester is available. This meter has a sealed battery capable of delivering 0.002 watt maximum under short circuit with internal resistor, neon light, and push button for checking wiring in hazardous areas for voltage before taking a resistance reading. In checking a piping or duct system, one lead shall be connected to the plant ground system and the other to a long-handled sharp probe to facilitate checking individual sections.
If the building is not as described above, then grounding and lightning protection shall be provided for the building in accordance with Bonding and Grounding & Installation of Grounding and Bonding Conductors. Static protection shall be required as above but lightning grounding will require resistance of 10 ohms or less.
where mixing or grinding equipment is ventilated, the ductwork shall be grounded and electrically bonded.
All main drive and jack shafts associated with mixing and grinding operations involving flammable materials shall be grounded by means of a static elimination brush. Selfgenerated currents sometimes are present in shafting, especially when the shaft is directly connected to grinding or crushing machinery, or when an impeller is immersed in a water solution. Such shafts may be kept discharged by the use of brushes or wipers on the shaft.
Static electricity can be generated by rapid currents of dust particles or liquid droplets through pipes of various materials and by steam under high pressure. Under general conditions, the discharge of this energy may not be a matter of much importance. However, where any low flash-point solvents, or flammable vapors or materials are present, all metal in the vicinity of rapid currents of air or steam shall be grounded.
{Main keywords for this article are How Does Static Electricity Work, Static Electricity in Hazardous Locations, Reducing Hazards of Static Electricity, How Electricity Charge is Produced.}