Static Control

Static electricity, or the build up of electrostatic charge, is present all around us. In everyday life, a static spark is seen as a nuisance; in a combustible atmosphere, its effect can be catastrophic. Many plant fires and personnel injuries can be directly linked to an electrostatic spark igniting a vapor, gas or dust atmosphere. There are, however, various protective measures that can be adopted across industry to control this ever-present threat to people, plant and processes.

When implementing safety measures in potentially explosive atmospheres, there are many issues to consider. Eliminating potential ignition sources is the best starting point, both in terms of good engineering design and general operating procedures. However, in any type of combustible atmosphere, there may be hidden dangers present in the form of "isolated conductors." These are conductive objects that are either inherently or accidentally insulated from ground, preventing any static electricity generated from safely dissipating, resulting in accumulation of charge on the object. These isolated conductors include metal flanges, fittings or valves in pipework systems; portable drums, containers or vessels; road tankers, rail cars and even people. Isolated conductors are probably the most likely source of static-ignition incidents in industry.

To understand the extent of the danger and how it may be controlled, the fundamentals of static electricity, and how it is manifested, must be considered. In any industrial process where there is movement, the coming together and separation of materials will generate static. This could be liquid flowing through a pipe, powder dropping down a chute, a mixing process, or a person walking across a floor. While the potential differences (voltages) induced on objects can be very high, the extent of the streaming current is usually very low, typically no greater than 0.1 mA. If the object or piece of plant is in good enough contact with ground, this charge will be dissipated as it is generated. However, if the object is insulated from ground, the charge will start to accumulate, leading to an increase in voltage.

Tires on vehicles, paints, coatings, gaskets, seals and other non-conductive materials can all be sufficiently insulating to prevent safe static dissipation. Static charge can quickly build up to a very high potential, with voltages ranging from 5 kV to in excess of 30kV. Depending on the capacitance of the object, this may result in significant levels of energy available for discharge, well above the minimum ignition energy (MIE) of the surrounding flammable atmosphere.

Potential energy

The voltage of objects rise quickly when the resistance of the path from the object being charged, to ground, impedes the dissipation of charges. When another object that is at ground potential (or lower potential), comes in to close proximity to the charged object an electrical field is immediately set up between both objects. Spark discharges occur when the electric field strength exceeds the breakdown voltage of the atmosphere between the two bodies. The average breakdown voltage of air is approximately 3 kV per millimeter. However, because of many variables including charging mechanisms, charge generation rates, the breakdown strength of the air, gas or vapor mixture, the resistance to ground of objects and even the geometry of objects, it cannot be assumed that lower potentials will not discharge incendive electrostatic sparks.

The potential energy of static spark discharge can be calculated from the formula:

W = ½ CV2

where:

W = The potential energy of a spark discharge (mJ).

C = The capacitance of object subjected to charge accumulation.

V = The voltage of object, caused by charge accumulation.

Typical MIEs vary according to whether the flammable atmosphere comprises vapor, dust or gas, but many commonly used solvents have MIEs of well below 1 millijoule. If the isolated conductor comes into proximity with another object at a lower potential, much of this energy could be released in the form of an incendive electrostatic spark. Of course, in order for an ignition of the combustible atmosphere to occur, there would also need to be a suitable concentration of fuel (vapor, dust or gas) in the air; but for the purposes of safe plant design, the very fact that there is an identified combustible atmosphere should suggest that ignition is possible or likely. The problems associated with isolated conductors can be remedied by effective grounding (also known as earthing) and bonding.

As the speed and scale of modern manufacturing techniques increase, and the range of materials used and processed grows, this basic approach to safety will be even more important.

Grounding may be defined as linking the conductive object to a known grounding point via a mechanically strong and electrically conducting cable, thereby ensuring the object is at 0V, or otherwise known as ground potential. "Bonding" (or equipotential bonding) may be described as linking together adjacent conductive objects so as to equalize the potential difference between them. At some point these linked items should be grounded, ensuring that all conductive objects are at ground potential. In the case of fixed installationssuch as pipework or storage tanks, this is relatively simple to implement. However, it is more difficult in the case of mobile/portable objects such as tank trucks, vacuum trucks, drums and intermediate bulk containers (IBCs). In these instances, purpose-designed temporary grounding and bonding devices should be used, with strict procedures in place, to ensure they are always connected prior to starting the process. This will prevent any static charge accumulation.

In the case of people, static dissipative (S.D.) footwear and gloves may be worn to ensure that the person is continually grounded. Testing devices are available to ensure that footwear conforms to the relevant standard (eg. EN ISO 20345, the Cenelec 50404 level in Europe or ASTM F2413-05 in the U.S.). When a working area is designed, it is important to ensure that the floor has a suitable level of conductivity, as static dissipative footwear will be rendered ineffective if the wearer is walking on an insulating floor or floor covering. If the combustible atmosphere has a very low MIE, static dissipative clothing may also be required.

Even when the appropriate static safety equipment has been specified, there are some further concerns that must be addressed by all those responsible for operations within potentially explosive atmospheres. In operational terms, attaching a grounding clamp to a plant item is always a physical action. Even if the operators diligently carry out their duties as detailed in company safety procedures, they can never be sure that the clamp has made a low enough resistance connection with the conductive object to enable any static generated to be safely dissipated to ground.

Insulating layers

The fact remains that many conductive objects that are capable of accumulating high levels of static charge also have insulating layers on their surfaces that can prevent the necessary low resistance contact. This may be caused by the paint or coating on drums, tank trucks, vacuum trucks and other mobile plant, or may be the result of product build-up caused by normal working conditions (for instance, where insulating liquids, powders and other materials are part of the process). Many grounding and bonding clamps show very high resistance readings when clamped onto conductive objects with insulating surfaces. Worse still, if a company tries to reduce costs by using standard welding clamps or lightweight alligator clips for static grounding in place of purpose designed and approved clamps, these devices have an even higher failure rate.

To solve this problem, intrinsically safe (IS), self-testing grounding clamps may be specified. From an operator's point of view, these devices are used in exactly the same way as conventional grounding clamps. Where they differ is in the way that they reassure the operator that the clamp has not only been physically attached, but is also performing its intended function of safely dissipating any static electricity that is generated. These clamps employ active electronic monitoring circuits that are powered from an internal low energy battery or a certified, externally mounted, line power supply and IS interface. The circuit is only completed when the clamp achieves a low resistance contact onto the object to be grounded, and the operator receives visual confirmation of this via an indicator (usually a flashing LED). The self-testing grounding clamp also monitors cable condition back to the designated grounding point and will also fail to register a permissive signal if the cable has worked loose or is broken.

To move to an even higher level of security, ground monitoring systems can also be used that not only give visual verification to the operator, but also provide interlock switching contacts that can be linked to process pumps, valves, alarms or control systems. This means that the process cannot be started until the conductive object has been safely grounded, and, if at any time during the operation the condition changes (due to a clamp being accidentally removed), the system automatically switches to non-permissive and shuts down the process.

These systems are generally powered by a line fed power supply, and employ approved IS circuits to limit the monitoring energy to safe levels. Systems can also be fitted to tank trucks or vacuum trucks and can be powered by the vehicle battery. Static ground monitoring and interlock systems are typically used in ultra safety-critical applications such as loading or unloading tank trucks, vacuum trucks, IBCs, mixing processes, fluid bed drying operations and wherever there is a high likelihood of static-charge accumulation in very low MIE combustible atmospheres.

Static ground monitoring clamps and grounding systems with equipment interlock capability tend to have an important beneficial effect on the operators using them. As their use builds an additional check into the operation, they help reinforce the static safety procedures of the company. In short, the operator is more likely to observe the correct procedures as he or she is kept aware of the need to control static electricity properly on a daily basis.

Pipes, drums

In all situations, it is important to make regular, periodic tests of the control measures used, checking clamp and cable condition and the all-important connection back to the grounding point (bus bar). Resistance testers or multi-meters may be used to perform this function but, of course, these will need to be approved IS instruments if working when a potentially combustible atmosphere may be present. Recording of test results is a positive way of ensuring that standards are maintained. The frequency of testing will depend on the nature of the operation and the type of control measures in place. Generally, non-monitored devices will need to be tested more frequently than self-testing clamps or interlocked equipment.

In addition to these engineered static safety controls, due consideration should also be given to all plant and packaging materials used within the hazardous area. Today, specialized non-metallic static dissipative materials are increasingly being used for making drums, flexible containers, linings, and hoses, in applications not suited to traditional materials such as steel. These materials are safe to use in combustible atmospheres, provided that they are treated in the same way as conductive items and appropriately grounded during static-generating operations. It is important to note that insulating plastics, such as those used in certain IBCs and bags, may pose a serious static-ignition risk. These materials cannot be safely grounded, and it is not recommended to use them where a combustible atmosphere is likely to be present.

It should also be noted that charge can build up on the actual materials being processed (liquids, powders, gases), so it is necessary to make sure that these are in sufficient contact with grounded, conductive piping, vessels and plant, thus providing a safe discharge path. Conductive materials in good contact with a ground path will not retain significant levels of charge. However, as many of these materials are highly resistive, it is imperative to ensure that any conductive equipment (e.g. pipes, drums, containers, tank trucks, vacuum trucks) with which the charged material comes into contact, are grounded or bonded to grounded objects.

In conclusion, the dangers of static electricity in hazardous areas demand a holistic approach to plant, process and personnel safety, as any control measures are only as good as the weakest link in the chain. As the speed and scale of modern manufacturing techniques increase, and the range of materials used and processed grows, this basic approach to safety will be even more important.

Michael O'Brien is product manager at Newson Gale Ltd. This article was reprinted with permission from Newson Gale Ltd.