Static electricity, often dismissed as a benign force, can lead to devastating consequences in industrial settings. Despite its association with fires and explosions, static electricity remains poorly understood and underestimated. This article delves into real-life case histories, revealing incidents where static discharge resulted in explosions, injuries, and significant financial losses.
Mystery of Static
Despite its frequent implication as a cause of fires and explosions, static electricity remains one of the most misunderstood and underestimated hazards within industrial workplaces. Even in environments where static is believed to be adequately controlled, the occurrence of ignitions is often a result of insufficient awareness, lack of understanding, or outright disregard for its potential risks. Unmanaged static electricity poses a significant threat to various industrial processing operations. While the generation of electrostatic discharge is commonly linked to the movement of products in flammable or combustible environments, static electricity serves as a subtle ignition source in routine operations, such as the cleaning of storage tanks.
Case Histories
A road tanker driver was on top of his vehicle, inspecting compartments and preparing for material transfer, when vapours inside ignited, causing a subsequent explosion. The force of the blast threw the driver off the truck, resulting in extensive damage to the tanker and nearby vehicles. The estimated cost of the destruction amounted to $200,000. The driver sustained severe injuries, including second and third-degree burns to his hands, arms, legs, and a broken left knee, necessitating a three-month hospital stay.
In another incident, an employee was engaged in cleaning a storage tank partially filled with sludge when an ignition event led to an explosion. Positioned near the tank opening, the hose operator witnessed a sudden flash and quickly took cover. Despite efforts to avoid the combustion, the operator suffered minor burns to the face.
A process operator tasked with manually transferring approximately 20 kg of combustible powder from a polyethylene plastic drum into a metal process vessel. The powder, with a minimum ignition energy of 12 millijoules, ignited into a dust cloud as he removed the drum from the vessel after completing the transfer.
In a pneumatic conveying system, a process operator investigating a crackling noise during the transportation of powdered material between the classifier and loading hopper inadvertently received a significant static shock upon contact with a section of the duct. Pneumatic conveying systems possess the capability to generate substantial amounts of electrostatic charge during the movement of products through plant equipment. Tribo-electrification, involving the contact and separation of powder with processing equipment walls, the powder itself, or other factors like surface contaminants, stands as the most prevalent method of electrostatic charging in such process operations.
Causes
Static electricity can be generated in a chemical plant due to various factors. When materials come into contact and then separate, friction can occur, leading to the transfer of electrons between the materials. This transfer of electrons can result in one material becoming positively charged and the other negatively charged, creating static electricity. The flow of liquids or gases through pipes and hoses can lead to the generation of static electricity. This is especially true when the fluids pass through non-conductive materials, creating a separation of charges. Mixing or agitating liquids, powders, or granular materials can cause friction and separation of charges, leading to the generation of static electricity. Handling and transferring powders and solids can cause friction between particles, resulting in the build-up of static charges on the surfaces of the materials. When dissimilar materials come into contact and then separate, such as during loading and unloading processes, static charges can be generated. The presence of insulating materials, such as certain plastics, rubber, or dry non-conductive powders, can contribute to the accumulation of static charges. Low humidity conditions increase the likelihood of static electricity generation. Moisture in the air helps to dissipate static charges, so dry conditions can exacerbate the problem. The design of process equipment and piping and materials used can influence the generation of static electricity. Insulating materials in these structures can contribute to charge build-up.
Tanker Operations
The transfer of flammable and combustible products during the loading and unloading of road tankers poses a significant and severe fire and explosion risk in industrial site operations. As far back as 1967, a study conducted by the American Petroleum Institute (API) highlighted static discharges as the cause of more than 60 incidents in road tanker loading operations, underscoring the longstanding recognition of this potential threat. The vulnerability of road tanker operations to electrostatic discharge as a potential ignition source is aggravated by the absence of a grounding system or the use of an inadequate one, coupled with the movement of products within a flammable and combustible environment. In the absence of effective static grounding protection to prevent discharges, the movement of products through the vehicle or connected plant equipment leads to an escalation of charge levels. Once the accumulated charge reaches a critical threshold, the likelihood of a spark discharge becomes significantly elevated. In processes involving relative movement, static electricity is generated through the contact and separation of materials. Most plant equipment prone to static charge accumulation is constructed from metal. If the metal body of a tanker lacks proper grounding, it swiftly transforms into an isolated metal conductor, emerging as the primary source of a static spark ignition hazard.
Characteristics and Mechanisms
Unlike other forms of electricity like current or lightning, static charges are often invisible and build up silently, making it difficult to predict when or where a discharge might occur. This unpredictability adds to the element of surprise and can be unsettling. The exact mechanisms behind static charge generation at the atomic and molecular level are still not fully understood. While theories like triboelectric effect (friction-induced charge transfer) and flexoelectric effect (bending-induced charge separation) provide explanations, the specific roles of various factors and their complex interactions remain an area of ongoing research. Sometimes, identical materials rubbed together under similar conditions may produce different charging patterns, and the reason behind this variability is not always clear. This “stochastic” or random behaviour makes it challenging to consistently predict and control static electricity.
New Insight
Scientists have challenged conventional views on static electricity noting that identical materials can transfer charges, contrary to the belief that only different materials can generate static. By employing high-resolution scans using Kelvin force microscopy, researchers discovered that surfaces exhibit mosaic patterns of positive and negative charges on a micrometre scale. The study suggests that the traditional explanation of charge transfer through the exchange of electrons between surfaces is incomplete. Instead, the research points to chemical reactions and material transfer as crucial factors. The origin and dynamics of these mosaics are still being investigated, adding another layer of complexity to the phenomenon.
Epilogue
Static electricity manifests in diverse ways, from harmless hair-raising experiences to potentially dangerous sparks and explosions. This versatility underscores the need for deeper understanding and control measures across various fields, from everyday life to high-tech industries.
