The Development of Inherently Safe Static Protective FIBCs

September 8, 2016

10 Min Read
The Development of Inherently Safe Static Protective FIBCs
Figure 1. Corona discharge points on the trailing edge of an aircraft wing, and used as an air terminal for lightning protection

Ever since flexible intermediate bulk containers (FIBCs) first became commonly used for storing and transporting powders and granular materials, static electricity has been an ever present concern. Electrostatic charge is generated whenever materials contact, rub together and separate -- a process known as tribocharging. If one or both surfaces are electrically insulating, charge cannot dissipate and will remain on the insulating surfaces. In general, larger surface areas generate more charge. When one considers the large surface area of powders flowing over the surface of FIBCs, which are typically made from highly insulating polypropylene, it is clear that there is plenty of opportunity for tribocharging to occur.
Static electricity causes a range of different problems. Dirt and other contaminants are attracted to charged surfaces, which not only looks bad when it occurs on the outside of FIBC, but has health implications in food and pharmaceutical applications. Electrostatic attraction can also make it difficult to completely empty FIBCs of fine powders, which tend to adhere to the inside surfaces. The residue of high-value products left inside FIBCs can equate to a significant financial cost. Another problem occurs when charge on the FIBC tries to go to ground via a person. The resulting discharge will cause a shock to be felt. While such shocks are often uncomfortable and occasionally painful, they are not usually harmful in themselves. Nevertheless, shocks can be distracting, which in busy industrial environments can result in serious accidents.
Without a doubt, the most serious hazard is the ignition of explosive atmospheres. The same electrostatic discharges that cause shocks occur when any large or grounded conductor, including people, approach an electrostatically charged FIBC. These discharges can contain sufficient energy to cause ignition. Explosive atmospheres are created by dispersed clouds of dust or powder from FIBC contents, or by solvents into which FIBCs are emptied. In the case of expandable polystyrene, there may also be residual flammable gas in and around FIBCs.
In the early days of FIBC usage, there were a number of serious fires and explosions caused by static electricity. In response, the first generation of static protective FIBCs were developed. The simplest form of static control is to make things conductive and ground them. Some early FIBC designs used metalized film laminated to the fabric to create entirely conductive surfaces that were grounded by a cable connected between the FIBC and a local ground bonding point. However, these designs proved unreliable, and in some cases dangerous, because as FIBCs flexed, metalized layers would break up creating large areas of isolated conductive material. If isolated conductors become charged, the risk of a spark causing an explosion is even greater than the risk created by a discharge from a charged insulator. Later FIBC designs used wire, metal, or carbon-based fibers woven into the fabric -- either in grids or stripes -- interconnected by conductive material in the seams and grounded via a cable attached to the FIBC. Today, such designs are referred to using the standard classification Type C FIBC.
In Process Safety Progress (Vol. 12, No. 4), Dr. Laurence G. Britton reviewed a number of incidents of explosions caused by FIBCs, more than half of which involved Type C FIBCs. In all the incidents involving Type C FIBCs, the suspected cause was an electrostatic discharge (spark) emanating from ungrounded conductors in the FIBC. The conductors were ungrounded because either the operators had failed to ground the FIBC correctly, or had not noticed the ground connection had become detached before emptying the FIBC into flammable solvents. In one incident, the operator noticed the ground connection was not present, but proceeded to empty the FIBC anyway! These incidents highlight the major safety concern with using Type C FIBCs – the risk of ground system failure caused by operator error.
In his paper, Dr. Britton concluded “Antistatic and conductive FIBCs can be grounded to prevent static discharges. However, any manufacturing defect or operational error in establishing grounding could be disastrous. Ignition, fire, and operator injury have unacceptably high probabilities for a single failure, particularly when the FIBC discharges to a flammable liquid tank.” To help mitigate this risk, Dr. Britton recommended that ground indicator equipment should be used. One example of such equipment comes in the form of an interlock system that prevents valves that control flow of products into or out of FIBCs from opening unless a secure ground connection is present. While such systems do reduce the risk of operator error, they add cost and complexity to FIBC handling operations, and consequently are seldom used in smaller facilities. Even in larger facilities their use is by no means universal.
For companies supplying diverse markets, it is almost impossible to control how FIBCs will be handled by their ultimate end-users. A good example is the titanium dioxide industry. Titanium dioxide has a vast range of industrial and consumer applications. Major producers supply their product to manufacturers of all sizes, from multi-national giants, to small independent companies. Since each grade of product is made in unique production-batch runs, manufacturers often package each grade in FIBCs without specifically knowing which ultimate end-users will be receiving the product. It is impossible to communicate safe handling instructions directly to unknown end-users, and no amount of labelling can guarantee that every FIBC will be properly and securely grounded during all handling operations throughout their complex supply networks. The possibility that one badly grounded FIBC could be the cause of a catastrophic explosion was clearly a major concern, and when actual incidents of explosions caused by Type C FIBC started to be reported, industry responded with calls for safer FIBCs.
To answer industry’s concerns, FIBCs needed to be inherently safe, without any requirement for intervention by operators. In simple terms, FIBCs must dissipate static electricity even if a ground connection is not present. At first this might appear impossible; where is the static electricity to go? The answer is not so difficult. Static electricity can be dissipated into the atmosphere by a process of air ionization. When exposed to a sufficiently high electrostatic field, air will break down into positive and negative ions; in effect the air becomes conductive. Charged areas of polypropylene, such as the sides of FIBC, will create an electrostatic field. In isolation, the field is not strong enough to cause air ionization, but if small conductors are introduced, the field becomes concentrated and air ionization can occur to allow charge to be dissipated into the atmosphere. This mechanism is called corona discharge and is widely used to dissipate static electricity from things cannot be grounded. For example, it is the mechanism used to dissipate static electricity from aircraft in flight, and to dissipate charge beneath thunder clouds to prevent lightning (Figure 1), which gives some idea of just how effective corona discharge can be.
Corona is a form of electrostatic discharge, but unlike sparks that release energy in a single rapid event, corona discharges release energy more slowly and in a continuous process that is not incendiary. To create corona in a woven fabric, it is necessary to introduce small quantities of conductive material. Crohmiq fabrics were the first to perfect the use of corona for static protective FIBC applications. In Crohmiq fabrics, quasi-conductive yarns are woven into the fabric at intervals of about 0.8 in. These special yarns concentrate the field from any charge that builds up on FIBC, and drive the corona mechanism to safely dissipate charge into the atmosphere. As the atmosphere is always present, there is always somewhere for static electricity to be safely dissipated.
With the introduction of Crohmiq, a new and inherently safe type of static protective FIBC was born, which is now referred to as Type D in standard classification terminology. Standards are important to the safety of FIBCs. Standards define where it is safe to use different types of FIBCs, and define the minimum requirements that will permit safe use of FIBCs in the most demanding applications. Standards documents, including IEC/TS 60079-32-1, NFPA 77, NFPA 652, NFPA 654, and JNIOSH TR 42, provide guidance on the safe use of static protective FIBC, and refer to IEC 61340 4 4 for testing and safety qualification requirements. Testing requirements for Type C FIBCs involve comprehensive measurement of all components within FIBCs to ensure they are all electrically interconnected and can be properly grounded when FIBCs are in use. Because corona discharge is governed by both geometrical parameters and electrical properties, there is no single measurement that can be used to evaluate the safety of Type D FIBCs. The above mentioned standards state that Type D FIBCs shall only be used after proving that incendiary discharges cannot occur. Proof of safety is achieved by carrying out ignition testing as specified in the IEC 61340-4-4 standard.

Ignition testing (Figure 2) is done on full size FIBCs, including all labels, document pouches/placards, and other attachments that may be present in use. FIBCs must be completely isolated from ground during testing. The principle is to fill FIBCs under test with highly charged resin and create a localized explosive atmosphere near the FIBC using a gas probe containing a grounded electrode. Any electrostatic discharge that occurs when the gas probe approaches the FIBC under test will pass through the gas to the grounded electrode. For a safe FIBC, either no electrostatic discharge will occur, or if one does occur, it will not be energetic enough to cause ignition. An unsafe FIBC is one in which at least one ignition occurs during testing. Test parameters are controlled to ensure that conditions replicate the worst-case conditions expected in use. Charging current is negative polarity, because negative discharges are more incendiary than positive discharges, and controlled to 3 microamp, equating to the highest charging current found in use. The gas mixture is controlled to give 0.3 mJ minimum ignition energy (MIE), the lowest MIE of any solvent likely to be present in use. Testing is done at both low and high humidity, and at least 200 gas probe approaches must be made at each humidity. If any one of the gas probe approaches results in an ignition, the FIBC fails the test and cannot be considered for safe use as a Type D FIBC.

An important part of IEC 61340-4-4 for the end-user is the labelling requirement. Static protective FIBCs that meet the safety requirements of the standard are labelled to indicate the type of FIBC and the environment in which they can be safely used. For example, the label required for Type D (Figure 3) confirms to the end-user that the FIBC has been tested to IEC 61340-4-4, meets the safety requirements for Type D FIBC, and states the charging current and MIE at which the FIBC has been tested. The terminology used, including references to dust zones and gas zones, is familiar to safety engineers responsible for hazardous areas in which explosive atmospheres may be present. This lets the end-user know exactly where it is safe to use the FIBC. The label also confirms clearly that the FIBC does not need to be grounded (earthed).

The advent of Type D FIBCs and the standards that now define their use represent a major step forward in the safety of FIBCs that are used in hazardous applications.

An added advantage of using Type D technology is that it continues to operate away from FIBC filling and emptying stations. During transportation, where proper grounding is rarely possible, Type D FIBCs continue to dissipate charge, thereby eliminating the risk of igniting residual solvent vapors or gases that may be present in the packaged product. Type D technology also protects against surface contamination caused by electrostatic attraction and shocks to operators unloading FIBCs from transport vehicles. Companies that supply products in Type D FIBCs have the assurance of knowing they can ship their products to any company, for any application, anywhere in the world with proven electrostatic safety.
Dr. Paul Holdstock is technical services director, Texene LLC, Summerville, SC. For more information, visit

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