Often an electrostatic ignition hazard arises when the electrostatic charge generated in a process is allowed to accumulate to levels sufficient to give rise to electrostatic discharges. Thus, the first step in an electrostatic hazard assessment is identifying where in the process electrostatic charge can be generated and accumulated. Next, if the generation and accumulation of charge cannot be controlled, the types of electrostatic discharges that can arise must be identified. Lastly, in order to verify that a hazard exists, the effective energy of these discharges must be determined and compared to the minimum ignition energy (MIE) of the prevailing flammable atmosphere or the dielectric strength of lining (the natural limitation of charge accumulation) in vessels, containers and piping.
A systematic approach for identifying electrostatic hazards is outlined in the National Fire Protection Association (NFPA) Recommended Practice # 77. This document states that an assessment of each process should be conducted to identify electrostatic hazards, including ungrounded conductors, such as metal devices and equipment, personnel, as well as items made from electrically insulating materials. The assessment should:
be conducted under actual operating conditions, to the extent practicable;
be based on actual measurements of resistance to-ground, electrical continuity, electric field strength, streaming current, and accumulated charge; and
consider prevailing environmental conditions that may affect charge generation and accumulation.
This paper discussed some of the measures that could be considered for controlling potential electrostatic hazards during liquid and powder handling operations
Electrostatic charge is most commonly generated on liquids when they flow through pipes, hoses, and filters and when they are stirred. Liquids can also become charged if they are transferred into a container that is either already charged (e.g. an electrostatically charged plastic container) or becomes charged while containing the liquid (e.g. when the outside surface of a plastic container containing a liquid is rubbed). Charge can accumulate within the liquid if it is insulating in electrostatic terms or electrostatically isolated from ground. The accumulation of electrostatic charge on the liquid's surface can give rise to electrostatic discharges from the surface. These discharges can be sufficiently energetic to ignite flammable vapor, such as that which may be evolved in the vessel headspace from a flammable liquid, or a liquid processed above its flash point.
The flow and agitation of liquids can also cause insulating (plastic or rubber) and ungrounded conductive (metal) vessels, piping, and fittings to become electrostatically charged. Charge accumulated on insulating (plastic or rubber) and ungrounded metal devices and equipment can give rise to electrostatic discharges sufficiently energetic to ignite flammable atmospheres. Electrostatic charge accumulation on the insulating linings (glass or plastic) of vessels and pipes could also result in the creation of pinholes in the lining, causing leaks, corrosion of the piping or vessel, and contamination of the liquid. It should be noted that pinholes could even occur under inert atmospheres. Consequently, it is essential to identify and eliminate or control electrostatic charge generation, accumulation, and/or discharges.