Hundreds of operations involving flammable substances are carried out in manufacturing plants every day. Failure to follow proper handling procedures and ensure proper earthing systems can lead to disaster. History shows that even a seemingly trivial activity, such as pouring solvent from one container to another, can pose a serious fire hazard.
Production processes that take place in the paint and varnish, chemical, pharmaceutical or food industries often involve handling materials in bulk (dust, granulates, chopped fibre, etc.) or liquid form (flammable liquids such as paints or solvents).
Electrostatic charges can be generated during operations related to the technology of the manufacturing process, such as mixing, transferring, dosing, sieving, spraying or pneumatic conveying. Electrification of bulk materials poses a risk of dust ignition and may contribute to serious disturbances in technological processes. The same is true of liquids – static charges can also be generated in the course of filling and emptying tanks (liquid transfer processes), mixing liquids or cleaning containers.
The degree of electrification is primarily determined by the conductive properties of the liquid, material of the tank/container, and the liquid flow rate. In production plants where electrostatic discharges occur, caking of bulk materials or contamination of product surfaces by the dust particles they attract is a common occurrence. These phenomena have a significant impact on the quality and durability of products. An electrostatic charge flashover is a frequent cause of accidents. In explosive areas, electrostatic discharges can cause ignition. In these conditions, the risk of explosion of a mixture of flammable liquid vapours and air is related to the electrification of the surface of liquid caused by processes at the plant, electrostatic charges generated on surfaces of technological equipment or electrification of personnel working in the danger zone. However, plant maintenance services often disregard the threat posed by static electricity, even though many hazards can be counteracted by using effective antistatic protection.
A fire and/or explosion hazard exists when the energy of the electrostatic discharge reaches a value at least comparable to the so-called minimum ignition energy (MIE) of flammable material present or likely to be present in the respective area. MIE determines the ignition capacity of a given material. As a rule, mixtures of flammable vapours or gases and air have a lower ignition energy (and thus a higher ignition capacity) than dust-air mixtures or settled dust (table 1).
LIQUID VAPOUR | MIE (mJ) | POWDER CLOUD | MIE (mJ) |
|---|---|---|---|
Propanol | 0,65 | Wheat flour | 50 |
Ethyl acetate | 0,46 | Sugar | 30 |
Methane | 0,28 | Aluminium | 10 |
Hexane | 0,24 | Epoxy Resin | 9 |
Methanol | 0,14 | Zircon | 5 |
Carbon disulphide | 0,01 | Certain semi-finished pharmaceutical products | 1 |
Table 1. Minimum ignition energy (MIE) of vapours and dusts. Source: UK IChemE.
An example from history – an accident caused by static electricity
At a plant operated by Barton Solvents, a company dealing in the wholesale of solvents and other industrial chemicals, large outdoor tanks were used to store materials.
On the day of the accident, July 17, 2007, at approximately 8:30 a.m., a tanker truck arrived at the plant to pump a non-conductive solvent known as white spirit into a tank. As liquids transported through pipes and valves generates an electrostatic charge that can ignite flammable substance vapours, the tank supervisor connected the tanker to an earth bonding point using an earthing wire. All equipment used in the process of transferring the liquid were also connected with wires and earthed.
Inside the 56 m3 tank was a device measuring the level of liquid – an earthed metal tape suspended from pulleys and flexibly connected to a metal float. It was this connection that proved to be a serious hazard during the tank filling operation.
The solvent pumped into the tank was stored in three compartments in the tanker. As the hose was switched from one compartment to the next, air entered the system, creating bubbles and turbulence inside the tank. As a result, an electrostatic charge was generated in the non-conductive liquid. Simultaneously, the space above the liquid filled with an explosive vapour-air mixture. The swirling, turbulent liquid caused the float to move and sway, creating slack in the metal tape. This contributed to temporary breaks in the connection, causing the float to no longer be earthed.
As the process continued, an electrostatic charge built up on the metal float. At about 9:00 a.m., a spark created on the float ignited the vapour-air mixture, causing a massive explosion. The explosion launched the tank into the air. Two more tanks then ruptured, spilling their contents onto the rapidly spreading fire.
The fire raging at the plant caused more tanks to rupture and ignite, launching heavy steel tank covers with diameters of 3 to 6 meters into the air. 75,000 litres of flammable liquid entered the spill containment area. The force of the explosion launched valves, pipes and other heavy steel objects towards a nearby housing estate. One of the covers hit a residential home about 90 meters from the site of the accident, and a pressure valve hit a nearby business 120 meters away. 6,000 residents were evacuated from the area.11 people and one firefighter required medical attention.
The accident is proof that electrostatic discharges can pose a very serious danger indeed. To avoid similar incidents, plants where flammable liquids such as kerosene, toluene, benzene or heptane are transported, transferred or stored should take extra precautions. In the case of Barton Solvents, measures such as additional guidance from suppliers of liquids with regards to static build-up, replacement of floats with excessive slack, reduction of liquid flow rates, or the addition of anti-static agents to reduce static build-up on liquid surfaces could have prevented the disaster.
Electrostatic earthing in industry
In industrial plants, electrostatic earthing reduces the risk of explosions of flammable substances caused by electrostatic sparks. Earthing systems are used in the transport and handling of flammable gases, liquids and powders.
Earthing is usually used for unloading/loading of road and rail tankers, discharging the contents of barrels, tanks, big bags, process installation components, etc. The simplest method is using earthing clamps with sharp tungsten carbide teeth capable of breaking through a layer of paint, laminate, etc., together with a hytrel sheathed earthing cable resistant to chemical and mechanical damage (Fig. 1). They can be used in two configurations. The first option is to use a set of two straight clamps, connected with a spiral wire. The second option is to connect the clamp to the container to be earthed, and the ring terminal at the end of the wire to an earth bar. The size of the clamp must be tailored to the size of the container/device to be earthed. The inability to verify the current earthing status is a disadvantage of this type of solution.
This verification is possible when using a different solution, namely a portable set of self-testing clamps, consisting of two earthing clamps. The system is used by attaching one clamp to the device to be earthed (e.g. a metal barrel or a mixer), and the other clamp to the component used to transfer charges into the earth (e.g. a hoop iron, another earthed device or a steel structure). The clamps are equipped with LEDs that will light up in green if earthed correctly. However, in some cases using this method may prove to be difficult, as the LED installed on the clamp may be obfuscated by accumulating dirt.
There are also solutions capable of both earthing devices operating in manufacturing premises and safely and easily verifying the correct transfer of charges built up on the devices into the earth by means of a monitoring unit with an indicator LED. If the device is not properly earthed, the controllers will halt the process.
Fig. 1. A metal vane earthed using a earthing clamp with a wire.
Fig. 2. An Earth-Rite RTR tank truck earthing control system enables checking for proper earthing during the emptying of tank trucks carrying e.g. petrol or resin.
Fig. 3. A big-bag earthed using an Earth-Rite FIBC earthing control system, which verifies whether the static electricity built up on the bag is properly dissipated into the earth. This system ensures safety during the filling and emptying of type C big bags
Earthing control systems are divided into several types, depending on their intended use:
- tank truck earthing control system (fig. 2) used during filling/emptying operations – a tank truck is the largest potentially insulated conductive object entering the plant, and poses a serious risk of ignition if not properly earthed,
- rail tanker earthing control system – although railcars are in contact with earthed rails, many rail tankers are equipped with non-conductive bearings and friction parts located between the railcar itself and its wheelsets,
- mobile earthing control system installed directly on tank trucks – this solution is used on vehicles parked tens or even hundreds of meters from the location of a stationary earthing control system due to the nature of operations involving the truck (e.g. cleaning or maintenance),
- c-type big-bag earthing control system, enabling the safe filling and emptying of these containers (fig. 3),
- earthing control system for process installations, designed to simultaneously monitor the earthing status of up to eight independently operating or interconnected components of the installation,
- earthing control system for movable and fixed parts of process installations,
- mains-powered (Fig. 4) or battery-powered earthing systems – the former enable the round-the-clock operation of the systems, whereas the latter are used when self-testing clamps are not expected to be connected to parts of the installation for more than 6 hours.
Fig. 4. Metal drums and IBC tanks earthed using a Bond-Rite REMOTE EP mains-powered system that discharges static electricity into the earth and checks for proper connection between the drum and the earth.
Fig. 5. The photo below shows the Sole-Mate footwear tester located in front of the entrance to a production area
ESD protection for personnel
As it turns out, apart from earthing elements of the installation itself, providing effective ESD protection to production staff is also extremely important. An electrified human body is among the major sources of fire or explosion hazard in industrial areas. Using electrostatic dissipation footwear is the most practical method of protection against such risk.
Devices that test footwear for its electrostatic dissipation before employees enter areas with flammable atmospheres are available on the market. One such solution is the Sole-Mate footwear tester (fig. 5). After both feet in ESD footwear are placed on the measuring plate, the control unit will measure the footwear’s resistance. If the LED indicator lights up green, it means that the resistance of the footwear is below the upper limit of the standard and the footwear is safe to be worn in a potentially explosive atmosphere. However, if the LED lights up red and an audible alarm sounds after a few seconds, the resistance of the footwear is above the upper limit of the standard and the footwear may not be worn in a potentially explosive area.
In addition, care should be taken to equip personnel with appropriate clothing made of conductive materials or materials with low electrical conductivity. It is extremely important for personnel to comply with prohibitions related to donning and doffing of clothing in potentially explosive areas and regulations on cleaning the clothing by wiping or sweeping it, even when the clothing is made of so-called antistatic materials.
In addition to these protective measures, it is important to ensure that the floor is properly conductive, as using protective footwear will be ineffective if workers are walking on an insulating floor or carpeting. First and foremost, however, it is essential to ensure that personnel working in a potentially explosive atmosphere is aware of the risk of ignition and observe basic safety rules.
Manufacturing plants often implement additional safety measures, which are relatively effective. Optional static electricity protection measures include:
- maintaining increased humidity,
- replacing materials with insulating properties by conductive materials,
- reducing the speed of technological processes to a minimum and modifying their conditions,
- preventing contamination of liquids and gases.
According to standards and regulations, users of equipment are responsible for preventing explosions and ensuring protection against their effects. Regulations also provide guidelines for combating hazards caused by static buildup. As a result, both leading manufacturers and smaller companies opt for using appropriate earthing control systems to ensure the safe work of personnel and smooth performance of technological processes in their plants.
