Dust Safety Science: Grounding and bonding of silos storing wood chips

This interview was conducted with Jeramy Slaunwhite, Chief Technical Officer – Explosion Safety, REMBE Inc., and was originally released on December 6, 2022 on the Dust Safety Science Podcast.

Jeramy Slaunwhite is a mechanical engineering graduate from Dalhousie University in Halifax, N.S. with over 15 years of applied engineering experience. Jeramy has developed expertise in manufacturing, consulting and governmental environments in fields including material handling, industrial ventilation, mechanical building systems, energy conservation, and project management.

As the Chief Technical Officer at Rembe Inc. he applies his combustible dust knowledge and experience on explosion safety solutions and technical support for various industries and applications with a focus on NFPA compliance and risk management methodology. In addition to being an active member on several NFPA combustible dust technical committees including NFPA 664, Jeramy has extensive experience with performing combustible dust hazard assessments, applying practical hazard mitigation solutions, and dust collection and material handling assessment and design work throughout various manufacturing and process industries.

Silos play a vital role in storing and handling a wide variety of materials. From grains and powders to chemicals and aggregates, these towering structures provide a reliable and efficient solution for bulk material storage. However, when it comes to storing combustible materials like wood chips, additional precautions must be taken to ensure safety and mitigate the risk of fire or explosion.

One critical aspect of maintaining safety within a wood chip storage facility is the proper grounding and bonding of silos. Grounding refers to the process of establishing an electrical connection between equipment or structures and the earth, while bonding involves connecting conductive materials together to prevent the buildup of static electricity.

These measures are crucial for dissipating electrostatic charges and minimizing the potential for ignition. By familiarizing ourselves with them, we can take proactive steps to prevent accidents and mitigate potential damages.

Static electricity in silos storing wood chips

The evaluation and interpretation of combustible dust hazards in the wood industry starts with NFPA 652, The Standard on the Fundamentals of Combustible Dust and NFPA 664, the Standard for Combustible Dust Safety in Wood Processing Facilities.

Both of these standards acknowledge the need to manage or control static accumulations and highlight some items such as air hoses and pneumatic conveying as notable factors contributing to higher static charges. While NFPA 652 provides some details guidance on piping systems, both standards mainly suggest that equipment exposed to static charge should be controlled and taken into consideration. Nevertheless, these standards do not provide explicit guidelines on specific measures or requirements for addressing static electricity hazards.

The how-to guide to managing electrostatic ignition sources is NFPA 77, Recommended Practice on Static Electricity. NFPA 77 encompasses the best practices for managing electrostatic discharges and includes a dedicated section on dust and powders, along with a comprehensive set of controls. Notably, this standard also focuses specifically on silos, exploring the impact of static charges on these structures. By examining the guidelines provided within NFPA 77, we can gain valuable insights into effectively managing static electricity hazards in silo environments.

Controlling ignition sources

One approach to managing combustible dust explosions and deflagrations is by controlling static electricity or electrostatic discharge. To initiate this process, it is crucial to develop an understanding of the material involved and the ignitability of the resulting dust cloud.

The ignitability of various materials depends on factors such as particle size, moisture content, and specific properties. Determining the amount of energy required to ignite a dust cloud at an ignitable concentration is a fundamental consideration. Additionally, assessing the energy generated by static discharges and evaluating against the dust cloud ignition energy is essential for hazard management. It is important to note that not all static discharges possess the same energy levels. Different materials exhibit varying energies for static discharges, introducing multiple variables into the equation. Hence, a comprehensive evaluation of these factors and influences is imperative in effective static electricity control.

To accurately assess the ignition hazard, it is essential to understand the characteristics of the material involved. Typically, igniting a wood dust cloud requires thousands of millijoules of energy. However, if the material in question is dry wood flour, it is comparatively easier to ignite and requires fewer millijoules.

Imagine a belt conveyor depositing these chips into a silo. As they fall and impact each other, a cone-shaped mound of material forms, accompanied by the liberation of fine dust. This fine dust is then propelled towards a bin vent or an outlet filter. However, a significant portion of the dust tends to cling to the interior walls of the silo, forming a buildup layer. This layer serves as an ideal location for effective sampling and analysis.

If that sample is lab-processed down to a low moisture content, it can be used to represent the worst-case scenario, as it’s going to have the lowest ignition energy and be the most sensitive. In 2012, some of the worst explosions at Canadian sawmills occurred in the dead of winter, when it was cold and dry. They weren’t all initiated by electrostatic discharge, but it illustrates that the drier the dust is, the more ignition-sensitive it becomes.

Grounding and bonding for better safety

When developing explosion safety protection concepts for silos, it is crucial to be mindful of worst-case scenarios since they are often when incidents occur. Such incidents rarely happen during normal processing but tend to arise during maintenance operations, power outages, or bypasses, leading to a chain of errors and complications.

Regarding grounding, it involves establishing electrical potential with the ground, while bonding refers to achieving equal potential or connecting equipment to adjacent pieces to ensure equal potentials between them. This distinction is essential because preventing a differential in charge capacity at an item or equipment is a significant concern. If such a differential exists, it will seek an opportunity to discharge to adjacent objects, resulting in a spark jump. As the charge increases, the likelihood of a spark jump also rises.

To ensure reliable grounding, various methods can be employed, such as using grounding wire cables, braided cables, or other robust conductors that are mechanically attached. According to NFPA 77, the size of the wire is not solely determined by its current carrying capacity but primarily by its structural integrity and reliability. Since we are dealing with electrostatic current, the conductor does not need to be overly large; instead, it must be dependable. It should be visibly and securely connected either to the ground or a ground rod, such as a lightning rod or a system ground, or to another part of the building that is grounded and equally connected to the rest of the network system.

Cone discharges

Silos frequently encounter a phenomenon known as cone discharge or bulking brush discharge. It occurs when high resistivity material is introduced into the silo from the center, forming a cone shape. As the silo continues to fill, the outer shell of its wall serves as the conductive path to ground. The material inside undergoes rolling and sliding motions, resulting in electrostatic potential accumulation through self-rubbing.

By the time this charged material reaches the outer walls of the silo, which are grounded and bonded, it undergoes discharge, leading to a chain reaction. Reports indicate that this discharge can manifest as metre-long lightning sparks that traverse the side of the silo cone. The material discharges itself or accumulates and then discharges towards the side walls. This phenomenon is amplified in larger diameter silos as the rolling material has more travel time to accumulate charge.

It is important to note that the energy generated during this discharge is estimated to be approximately 20 millijoules, which is not substantial. However, certain materials within this discharge energy range may exhibit sensitivities to ignition. Therefore, the phenomenon of cone discharge and bulking brush discharge can be of concern, particularly when dealing with dry wood or other materials susceptible to ignition within this energy range.

In belt conveying and especially pneumatic conveying with higher velocity, static charge accumulation produces a lot more friction, resulting in a much higher energy output. It’s known as propagating brush discharge, and it can develop into hundreds or thousands of millijoules that could ignite a wood dust cloud. This is why bulk material handling equipment is a lot more prone to static charge accumulation and bonding and grounding becomes much more critical.

How can we effectively prevent a bulking brush cone discharge? What measures can we take to stop it? The key is to disrupt the process in which the material flows down the cone and accumulates charge as it approaches the sidewalls. By doing so, we aim to hinder the material from building up enough charge to arc and discharge when it reaches the silo sidewall.

In the past, one approach to achieving this was by vertically inserting metal rods at intermediate spots within the silo. Alternatively, rings, cones, or other conductive materials bonded to the sidewalls were utilized to establish an early path for the discharge towards the cone. Another method involved using chains suspended from the top, rods extending from the bottom, or grids stacked throughout the silo. In some cases, conductive partitions were added into the silo to effectively divide the main silo body into multiple smaller chambers above the hopper. These mechanisms allowed the material to dissipate and discharge its charge as it rolled on itself prior to reaching the sidewalls.

Implementing these strategies effectively interrupts the accumulation of charge and minimizes the risk of a bulking brush cone discharge within the silo. The reliability of conical pile discharge mitigation techniques must be evaluated against the ignition sensitivity of the material and potential for charge accumulation.

Conclusion

Grounding and bonding play an indispensable role in maintaining safety and efficiency in silos storing wood chips or other combustible materials. Electrostatic discharge is however one of many possible ignition sources of dust explosions which must be considered as part of a comprehensive prevention & protection strategy. By taking appropriate precautions, understanding the unique challenges associated with wood chip storage, and implementing best practices, we can create a secure environment that safeguards personnel, prevents fire and explosion hazards, and promotes uninterrupted industrial operations.