When flammable or combustible atmospheres are present, uncontrolled discharges of static electricity are potentially dangerous or even catastrophic. A significant portion of industrial explosions and fires are attributable to static electricity each year. In theory, controlling static electricity by grounding potential sources is simple, but in practice, doing so effectively requires thorough knowledge of processes and operations, sound engineering controls, properly specified safety equipment and properly trained operational staff. This column provides information on potential sources of static electrical discharge.
Static risk
Static electricity discharges are possible almost continuously in the chemical process industries (CPI), because static electricity is generated whenever surfaces come into contact and then separate. In most cases, the charging currents generated over time in industrial processes are small — typically no greater than 1 × 10–4 Amps. However, in hazardous areas, even small charges can be a problem when the allowed to accumulate on objects that are not at ground (earth) potential. If no ground is present, voltages in excess of 30 kV can develop. Depending on the capacitance of the object, this may result in significant levels of energy being available for discharge. If the energy equals or exceeds the minimum ignition energy (MIE) of the surrounding flammable atmosphere, the potential for an explosion and fire exist. Many commonly used solvents and other flammable chemicals have MIEs that are relatively low — on the order of 1 mJ or less (Table 1).
Table 1. Typical MIE values |
|
| Material (gas/vapor or powder/dust) | Minimum Ignition Energy (MIE), mJ |
| Carbon disulfide | 0.009 |
| Methanol | 0.14 |
| Xylene | 0.20 |
| Toluene | 0.24 |
| Propane | 0.25 |
| Ethyl acetate | 0.46 |
| Zirconium | 5.00 |
| Epoxy resin | 9.00 |
| Aluminum | 10.00 |
| Sugar | 30.00 |
| Wheat flour | 50.00 |
| Note 1: Minimum ignition energy (MIE) is defined as the minimum energy that can ignite a mixture of a specified flammable material with air or oxygen, measured by a standard procedure. Note 2: MIE values are provided for guidance only — specific MIE data for any material should be verified. Source: NFPA, IChemE | |
Isolated conductors
Isolated conductors are electrically conductive objects that are either inherently or accidentally insulated from earth. During day-to-day operations at industrial facilities, isolated conductors are probably the most likely source of static ignition incidents.
The insulation effectively keeps any static electricity buildup from safely discharging, thereby resulting in accumulation of charge on the object. If the isolated conductor then comes into proximity with another object at a lower potential, energy could be released in the form of an incendive spark.
Isolated conductors may arise from metal flanges, fittings or valves in pipework systems; portable drums, containers or vessels; tanker trucks, railcars and intermediate bulk containers (IBCs); and even people.
Many modern industrial paints, coatings, gaskets, seals and other non-conductive materials are sufficiently insulating, so as to to possibly prevent the proper dissipation of static charge.
Static discharge sources
Static discharges come in several forms, the most important for CPI interests being spark and brush discharges. A spark is a discharge from a charged isolated conductor to another conductor at lower potential. A brush discharge occurs from an electrostatically charged insulator to a grounded conductor.
Typical possible sources of static electrical discharges include the following:
• Spark discharges from any conductive, but not earthed (grounded), bag, bin, drum, container and so on, from which a powder is transferred into a reactor
• Brush discharges from any non-conductive bag, bin, drum, container, and so on, from which a powder is transferred into a reactor
• Spark discharges from any conductive, but not earthed, auxiliary device used in the transfer procedure, including, but not limited to, shovels, funnels, chutes and pipes
• Spark discharges from the operator, if he or she is not adequately earthed
• Brush discharges from any non-conductive auxiliary devices, such as shovels, funnels, chutes and pipes
• Brush discharge from the dust cloud formed within a reactor during powder transfer
• Spark discharges from any conductive, but not earthed, fixtures and fittings within a reactor
• Brush discharges from the charged solvent, suspension or emulsion preloaded in a reactor
• Brush discharges from the powder heap formed on top of the liquid phase within a reactor
• Cone discharges from the powder heap formed on top of the liquid phase
• Liquids flowing through pipelines or filling into drums and tanks
• Persons walking across an insulating floor
Static discharge prevention
Where recommendations tend to converge is in the recommendation to always use conductive or static dissipative materials, and to ensure effective bonding and grounding. For information on grounding best practices and examples of preventing static electrical discharge, consult National Fire Protection Association (NFPA; Quincy, Mass.; www.nfpa.org) standards 77 and 30.
In this context, the term “conductive” would apply to metal materials, such as stainless or carbon steel, aluminum and others; and “static-dissipative” may indicate rubber or plastics that have been formulated with some added semi-conductive additives. “Bonding” means linking these objects together by means of a suitably strong conductor (wire), and “grounding” refers to a true “ground/earth” connection that is applied to one or more of the bonded objects.
When one or both of these techniques is applied, and while a low resistance connection between the objects and ground is maintained, operators are able to prevent dangerous levels of static charge from accumulating. In the case of fixed installations such as pipe work, storage tanks and so forth, grounding is relatively simple to implement.
However, these preventive measures are more difficult to implement with portable objects, such as drums, IBCs and tankers. In these instances, purpose-designed temporary grounding and bonding devices must be used, with strict procedures to ensure that they are always in place prior to starting the process. For instance, specific types of clamps and devices for grounding and bonding portable or mobile plant equipment, drums and containers are recommended in NFPA 77, and such grounding clamps and devices generally should employ sharp contact points. These contact points should be made of a wear-resistant material, have positive spring pressure, and be universally adaptable to a wide range of plant objects.
References
1. Tyers, G., Avoiding Static Sparks in Hazardous Atmospheres, Chem. Eng., June 2009, pp. 44–49.
2. Glor, M., Preventing Explosions During the Transfer of Solids into Flammable Solvents, Chem. Eng., October 2007, pp. 88–95.
Editor’s note. This edition of “Facts at your Fingertips” was adapted from information in the two articles referenced above.