An Underestimated (Ignition) Hazard: Electrostatic Charge
Published: June 9, 2021
By Isabelle Simonsen
In all industrial sectors in which combustible fluids or finely grained combustible bulk goods are filled, mixed or loaded, explosive atmospheres can form. Among the various measures to minimise this risk, grounding technology plays a central role – by safely dissipating or grounding high-energy electrostatic potentials, it prevents uncontrolled discharges leading to ignition. In any case, undetected damage to conductors or contact points can render grounding ineffective. Corrective measures create systems for grounding monitoring, which continuously measure the resistance of the conductive connection and do not enable any processes if there are any errors.
Even if you have never touched a door handle and received a static shock, or seen a spark jump when taking off a sweatshirt, you will have learned about the phenomenon of electrostatic charge in your science lessons at school.
Wherever materials come into contact and are then separated, electrons on the surfaces can “switch places”, forming electrostatic potential – unless there is any way to equalise the charge. If an object with a different charge approaches, the potential is suddenly discharged. When and how strongly this happens is dependent on the field strength and insulating properties of the separating medium. If the potential difference is 1000 V or higher, flashovers occur. As a general rule, humans only perceive these discharges if the flashover voltage is over 2000 V with an energy around 0.5 mJ. In contrast, discharges of 10 kV or higher that release anything above 350 mJ of energy are extremely painful and can require medical treatment. But what would merely be a small shock for one of us could, in an explosive atmosphere, act as an ignition spark and cause a catastrophe.
Plenty of potential for potentials – process technology systems and filling stations
The strength of the electrostatic potentials that build up on surfaces is determined by a number of factors. These include the conductivity of the materials or media, the size of their surfaces and the speed at which they are rubbed together and separated.
As anyone who works in electronics manufacturing will know, even wearing non-dissipative clothing generates electrostatic potentials that could destroy delicate electronic components. Process technology systems, in which conveyor belts create constant friction and bulk goods are filled and mixed, are in a completely different order of magnitude. Even fluids containing ions are not exempt, as they generate charges merely by flowing along pipe walls. These processes, which cause significant charges to build up, lead to ignition hazards. The specific type of ignition hazard that results from uncontrolled discharge in a real situation is dependent on the type of discharge and the amount of energy emitted. There are five basic types of discharge.
Different types of discharge
A “brush discharge” occurs if a charged object made of insulating material approaches a conductive object. This form of discharge is characterised by its comparatively low energy density, which can be reduced to a non-ignitable level through the targeted reduction of the insulating surface. “Corona discharges” also pose a relatively low risk of ignition; these can form on sharp edges or corners of conductive materials at field strengths of 3 MV/m or higher. In contrast, a sudden discharge between two charged objects made of conductive materials can lead to a “spark discharge”, which is considered an ignition hazard and must always be prevented by grounding all conductive objects in a system. “Propagating brush discharges” are one of the highest-energy types of discharge – these typically occur on thin insulating material surfaces such as films and coatings. Due to the extremely small distance between the charged upper and lower surfaces of the thin material layer, it is possible for large amounts of energy to accumulate. If this energy is released in the form of a sudden discharge, it is capable of igniting any explosive atmosphere based on combustible fluids, gases or dusts. The fifth type of discharge relevant to the industry is the “cone discharge”, which occurs when silos are filled with highly charged, insulating bulk goods. The strength of the discharges that occur between the upper part of the conical pile of bulk goods and the conductive silo walls is significantly dependent on the grain size, conductivity and speed at which the bulk goods are filled.
Dissipation: An absolute must-have for conductive objects
To both avoid and safely dissipate hazardous electrostatic potentials, EN IEC 60079-32-1 „Electrostatic hazards, guidance“, NFPA 77 „Recommended Practice on Static Electricity“
and local technical regulations for hazardous substances contain a range of different recommendations and regulations. These relate to different aspects such as safe surface limits for insulating materials in hazardous areas, special requirements regarding pneumatically conveying bulk goods, recommended hose types for transporting liquids, the maximum permissible dissipation resistance of work clothing, and specifications for wet cleaning of surfaces. In this respect, the regulations for all conductive equipment and objects stipulate that grounding or potential equalisation connections must always be used, in order to restrict the charge to a non-hazardous extent. Electrostatic grounding is considered to be guaranteed if the resistance to ground is less than one MΩ.
Passive grounding – restricted safety for mobile containers
In principle, the required electrostatic grounding can be implemented using simple cables and suitable clamps. However, especially in areas where mobile containers such as tankers, tank wagons and FIBCs (Flexible Intermediate Bulk Containers, also known as “big bags”) of combustible materials are filled or emptied, passive grounding is not without risks. Frequent connection and disconnection of the clamps, grinding contact with the floor or accidentally rolling over the electrical lines in a vehicle can all subject the components to high mechanical load. Even minor damage to cables or contact elements can render the safety devices ineffective. Furthermore, corrosion, dirt or coatings can impair the conductivity between the clamp and object to be grounded. In the case of passive – i.e. not actively monitored – grounding, however, there is a significant risk of interruptions in the dissipating connection going unnoticed, meaning that safe dissipation of electrostatic potentials cannot be guaranteed.
Safe grounding monitoring with process locking
Active, permanent monitoring of the connection between the object and equipotential bonding rails can be implemented as a countermeasure. The explosion protection specialist R. STAHL offers a wide range of different grounding monitoring devices and accessories. With the 8485, 8146, 8150 and 9170 series for consistent use in Ex Zones 1, 2, 21 and 22, the manufacturer covers all the corresponding specific requirements that apply when safely grounding road tankers, tank wagons, barrels, big bags and permanently installed systems. Depending on their configuration, the devices in the 8485 series are equally suitable for tankers, tank wagons or FIBCs type C, meaning that they are the all-rounders of this range.
They are designed for use in temperatures between -55 °C and +60 °C, which ensures that they can be used without any issues, even in harsh climatic conditions. Their special features include automatic object detection for trucks or big bags, which prevents operating errors caused by faulty connections. For this purpose, the connection of the clamps is checked in two stages to ensure that it is correct before the filling process is enabled. Firstly, an impedance measurement is performed in order to determine whether the device is connected to a truck or FIBC, not to part of the loading equipment. In the second stage, the resistance is measured in order to establish that sufficient grounding is provided by the grounding device. After a successful measurement, the devices indicate whether the grounding is correct or incorrect using green and red LEDs, which are clearly visible even in bright daylight and from a distance. For remote signalling and triggering a process shutdown if the grounding is insufficient, 8485 versions with up to four potential-free change-over contacts are available and can be operated with either Ex i or Ex e degree of protection. The easily accessible connection area on the water jet-protected IP65 aluminium enclosure enables fast installation and commissioning, while Bluetooth and infrared interfaces make it convenient to parameterise the device using a smartphone or corresponding PC adapter. The enclosures feature an integrated suspension point to protect the clamps from becoming worn quickly.
Monitoring drums and IBCs
In the 8146 and 8150 series, R. STAHL has specifically adapted the measurement process for grounding monitoring in tank containers, such as drums and IBCs (Intermediate Bulk Containers). What’s more, the 8146 and 8150 clamps are unique thanks to their special toothing for penetrating contact-inhibiting layers, in order to establish reliable electrical contact on corroded, coated or dirty container surfaces. The devices in an IP66 enclosure made of plastic or weather-resistant stainless steel are fitted with an insulated suspension point and are approved for use in applications with SIL 2 (according to IEC 61508). Signalling to the process control technology or to external signalling devices takes place using a potential-free change-over contact. Like the devices in the 8485 series, these product series are also suitable for grounding monitoring for tank wagons. Although it can generally be assumed that sufficient grounding is present in rail vehicles, additional monitoring is recommended when loading hazardous substances. This is also stipulated in some regions of the world.
A solution for fixed systems
Although monitoring permanently installed grounding connections has previously not required any urgent safety measures, the current trend towards modularization of plants means that this issue is becoming ever more important. The manufacturer offers the 9170 leakage monitor for installation in control cabinets for systems that comprise a large number of grounded applications such as filling and mixing stations, or machines connected via pipe systems. The two-channel module in an 18 mm-wide DIN rail modular enclosure offers users an extremely compact, easily scalable solution that eliminates the more time-consuming and expensive on-site installation of monitoring equipment. The 9170 leakage monitor is designed for installation in safe areas or in Zone 2; its monitoring circuits enable connection to objects located in Ex Zones 1, 2, 21 and 22.
In order to optimally adapt all device series to varying on-site conditions, R. STAHL offers a comprehensive selection of optional accessories. This range includes UV-, oil- and fuel-resistant spiral cables measuring five or ten metres in length, as well as high-quality stainless steel clamps, automatic retractors, visual/audio signalling devices, and multi-signal devices in explosion-protected designs. On request, R. STAHL can also design customer-specific solutions with preferred fieldbus interfaces, integrated heating systems or NEC-compliant designs for use in the USA.