Risk assessment
When assessing the risk of electrostatic discharge-initiated explosion in a hazardous area containing an explosive atmosphere, there are two main parameters to consider: the sensitivity to electrostatic ignition of the flammable substance creating the explosive atmosphere, and the level of electrostatic charge being generated. The sensitivity of a flammable substance to electrostatic ignition is defined by the minimum ignition energy (MIE), which is the least amount of energy in an electrostatic discharge that will ignite the substance. MIE is measured by standardized laboratory methods, and for most common gases and vapors, MIE data is readily available in safety data sheets and other literature. The level of electrostatic charge being generated is process and environment dependent. Although some guidance can be found in literature, accurate values can be determined by measurements of the process of concern. In the case of product flowing into and out of bulk bags, the level of electrostatic charge can be quantified by measuring the charging current.
The most convenient method of measuring charging current of a flowing product is to allow the product to fall into a Faraday pail, which is essentially just a conductive container. The principle of a Faraday pail is that any charge inside the container is induced on the conductive sides of the container. If the Faraday pail is connected to ground, the induced charge will flow to ground creating a current that is equal to the current flowing into the Faraday pail. Charging current can be measured by inserting an ammeter in the ground line.
On-site measurements at an EPS manufacturing facility
When an EPS manufacturer was considering changing packaging from gaylord boxes to CROHMIQ® Static Protective Type D FIBC, Texene visited the production factory to measure charging current using the Faraday pail principle. The measuring arrangement is shown in Figure 1. In this case, the conductive container forming the Faraday pail was a Type C FIBC. As no support frame was available for the bulk bag, it was placed inside a gaylord box lined with a nylon/EVA laminate liner, which isolated the bulk bag to prevent current leaking to ground. The network of conductive tapes in the bag was connected to ground via a Keithley 6½-digit multimeter operating as an ammeter to measure the charging current.
With cup grade EPS flowing into the Faraday pail at 2 to 2.5 kg/s, the measured charging current varied between 0.3 microamp and 1.0 microamp, with an average of 0.6 microamp.
The measured charging current is well within the safety limits for CROHMIQ FIBC, which are qualified to IEC 61340-4-4 for charging currents of up to 3.0 microamp. The ignition testing specified in IEC 61340-4-4 requires the MIE of the gas mixture used to be 0.14 mJ. The MIE of pentane, the blowing agent commonly used in EPS, is 0.22 mJ. Therefore, safety testing to IEC 61340-4-4 qualifies CROHMIQ Static Protective Type D FIBC as safe for use in EPS operations where pentane gas is used as the blowing agent.
Figure 1. Arrangement for measuring the charging current in an EPS manufacturing operation.
The dangers of using the wrong type of bulk bag
Texene is aware that some EPS manufacturers are using Type B FIBC to package EPS containing pentane. In the Type D FIBC Technology Monthly Newsletter for February 2024, we explained why Type B FIBC can never be a safe replacement for Type D FIBC. All safety standards and recommended practices for the control of electrostatic risks in hazardous areas prohibit the use of Type B FIBC when flammable gases can be present. Type B FIBC are intended only for use when packaging combustible powders or granular materials that form combustible dust, such as corn starch, milk powder, sugar, etc. It has been argued that nitrogen inerting when filling bulk bags prevents the formation of an explosive atmosphere. Whilst this may be true during filling operations, by the time it comes to emptying EPS from bulk bags into pre-expanders, nitrogen will have dissipated and been replaced by a potentially explosive mixture of pentane and air.
When a bulk bag is opened, there is a rapid flow of EPS into the receiving hopper. The flow of EPS will slow down as the hopper starts to fill, but in the first few seconds when the flow is rapid, significant electrostatic charging can occur. The duration of an electrostatic discharge is less than a millisecond, so in the first few seconds after a bulk bag is opened, multiple electrostatic discharges can occur. It only takes one of these discharges with sufficient energy to cause ignition. When a bulk bag containing EPS is first opened, there will be an outrush of pentane, which being heavier than air accumulates near the bottom of the bag. This creates an explosive atmosphere in the same space where electrostatic discharges are most likely to occur. Therefore, there is a real risk of explosion.
Another argument we have heard is that EPS charging is less than the 3 microamp at which Type D FIBC are qualified, and the MIE of pentane is higher than that required for Type D FIBC testing, making it less sensitive to ignition. There is no denying that this is true, but there is no validity in this argument to support the use of Type B FIBC. There is no mechanism in Type B FIBC to dissipate electrostatic charge. Consequently, even low charging currents can result in incendiary electrostatic discharges. To demonstrate this, Texene has conducted ignition testing on a production Type B FIBC that is currently being used to package EPS. The testing was done in accordance with IEC 61340-4-4, but with the charging current reduced to levels previously measured at the EPS manufacturing facility, and with the MIE increased to a level similar to that of pentane. The video in Figure 2 shows that multiple ignitions occurred, clearly proving that the Type B FIBC is not safe for use in EPS operations where pentane is present.
Charging Current
-0.5 μA
(half the maximum current measured at the EPS factory)
MIE
0.24 mJ
(equivalent to pentane)
Figure 2. Ignition testing a Type B FIBC using parameters typical of EPS operations.
Using the wrong type of FIBC can result in explosion, destruction of factory assets, and serious or fatal injuries to workers. The price difference between a CROHMIQ Static Protective Type D FIBC and a Type B FIBC amounts to about 0.3 cent per pound of packaged EPS. Risking explosions that can severely damage or destroy factories to save such a tiny cost makes no sense. Risking the lives of workers to save costs of any size is never acceptable. Companies receiving EPS in bulk bags should insist on, and only accept safe bulk bags with full static protection. CROHMIQ Static Protective Type D FIBC are the safe bulk bags for all EPS operations.