Static electricity is the accumulation of electric charges on the surface of a material, usually caused by friction, contact, or separation of materials. You've probably seen and experienced simple ways of creating static electricity by dragging your feet across a carpet or rubbing a balloon on your head. These are harmless ways to demonstrate this natural phenomenon (not to mention give a jolt to an innocent bystander or show off a new hair style!). Static electricity generated by industrial equipment, however, can pose serious explosion hazards if it ignites flammable dusts, gases, or vapors in the presence of oxygen. Bulk solids or powders, such as flour, sugar, coal, or plastic, can generate static electricity when they are processed, transported, or stored. To prevent explosions and other hazards during material handling, it is important to understand and control the factors that influence static electricity generation and discharge. Here are four key points to consider:
1. The Type and Properties of the Materials Involved
Some materials are more prone to generate static electricity than others, depending on their conductivity, resistivity, polarity, and moisture content. For example, synthetic materials, such as plastic, tend to have higher resistivity and lower conductivity, which means they can accumulate more electric charges and hold them for longer. Moisture can affect the conductivity and resistivity of materials, as well as the humidity of the environment. Generally, lower humidity increases the risk of static electricity generation and discharge.
2. The Magnitude and Duration of the Electric Charge
The amount of electric charge generated by a material depends on the surface area, speed, and force of contact or separation with another material. The larger the surface area, the faster the speed, and the greater the force, the more charge is generated. The duration of the electric charge depends on how long the materials remain in contact or separated, and how quickly they can dissipate or transfer the charge to another material or to the ground. The longer the charge lasts, the more likely it is to cause a spark or shock.
3. The Energy and Frequency of the Electric Discharge
The electric discharge is the release of the accumulated electric charge from a material. The discharge can occur in different forms, such as sparks, shocks, corona, or brush discharges. The energy and frequency of the discharge depend on the voltage, capacitance, and resistance of the materials and the circuit. The higher the voltage and capacitance, and the lower the resistance, the more energy and frequency the discharge has. The energy and frequency of the discharge determine the severity of the ignition risk. Generally, sparks and shocks have higher energy and lower frequency, and can ignite flammable dusts, gases, or vapors more easily than corona or brush discharges, which have lower energy and higher frequency.
4. The Presence and Concentration of Flammable Dusts, Gases, or Vapors
The flammability of dusts, gases, or vapors depends on their chemical composition, particle size, shape, and distribution, as well as the oxygen level and temperature of the environment. The flammability of dusts, gases, or vapors is expressed by their lower and upper explosive limits (LEL and UEL), which indicate the minimum and maximum concentrations of the substance in air that can be ignited by a source of ignition. The LEL and UEL vary for different substances and conditions. The closer the concentration is to the LEL or UEL, the more sensitive the substance is to ignition.
For more information, we highly recommend investigating the resources provided by the National Fire Protection Association (NFPA). The NFPA is a non-profit organization that develops and publishes codes and standards for fire and electrical safety. The NFPA has several standards and guidelines that address the hazards of static electricity in bulk solids or powder handling, such as:
- NFPA 77: Recommended Practice on Static Electricity
- NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids
- NFPA 499: Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas
There are also several commodity specific standards which may provide information more directly related to your specific process. These standards and guidelines provide recommendations and best practices for the identification, assessment, and control of static electricity hazards in various industries and applications. They cover topics such as material properties, equipment design, grounding and bonding, charge dissipation and neutralization, ignition prevention and protection, personnel training and awareness, and hazard analysis and risk management.
De Dietrich Process Systems has experience in creating customized powder handling solutions to safely and efficiently convey your material to meet the requirements of your product and application, taking into account static electricity and other hazard concerns. Contact us today so we can discuss the needs of your business.
