Given the overwhelming number and availability of IBC options, the SIA and CBA issued “Guidance Notice 51a” which describes the key selection criteria for either stainless steel or composite IBCs with an “anti-static sheet”. The IBC selection criteria principally depends on the flash point of the solvent being used and whether the solvent can be defined as having electrically conductive or resistive characteristics. More detailed information can seen in the guidance notice, but a table listing IBC options for resistive solvents (hydrocarbons), and conductive solvents (oxygenated), are summarised in the following table:
Table 2: Parameters determining appropriate selection of stainless steel and composite IBCs manufactured with static dissipative sheaths(3).
Standards and practical advice.
The main issue with filling non-conductive plastic containers whether it is IBCs, drums or bottles is that the liquid or solid, which is charged through its own movement, will induce charges on the plastic. As the container is filled with more material, more charge will continue to build up on the inside surface of the container, which will set up opposite charges on the outer surface of the container which is exposed to the hazardous atmosphere. The charged outer surface will set up electrical potential differences with objects (e.g. tools, vessels, instruments, operator’s fingers) within the hazardous atmosphere, which could lead to incendive static brush discharges. Alternatively, if objects like metal tools become charged through close proximity to charged plastic containers, and are in themselves isolated from the earth, they could discharge static sparks more readily. The experts and standards conclude that portable containers made from non-conductive plastics should not be used in hazardous areas, unless the complete process is subjected to expert analysis (2)(4). If plastic is to be used this will most likely require inerting of the combustible atmosphere.
Although there is much guidance related to the values of resistance that should be achieved for earthing metal objects (e.g. road tankers, 205 litre drums, etc.) to 10 ohms or less there is little practical guidance addressing maximum values of resistance for static dissipative drums or IBCs. There is just one standard that specifies a maximum value of resistance for static dissipative materials and this only applies to Type C FIBCs which are used for transporting and storing powders. CLC/TR: 50404 states that the resistance through a Type C FIBC bag to its ground connection tabs should be no greater than 1 x 108 ohms(5). Currently a number of packaging manufacturers are supplying static dissipative drums and IBCs with various quoted maximum values of resistance ranging from 1 x 108 ohms, 1 x 106 ohms to 1 x 105 ohms. One example is a composite drum that has a normal PE inner lining, with the outer plastic surface made from static dissipative material. It does not necessarily mean that the liquid in the drum, even if it is conductive, will dissipate its charge quickly, but the surface of the drum exposed to the hazardous atmosphere should not generate potential differences with other earthed objects, or induce potentials on isolated metal objects, provided the correct static earthing techniques are deployed.
As these types of containers become more freely available to hazardous process supply chains, bodies and organisations that publish standards and guidelines for static control within hazardous areas should address the use of such containers. This is particularly important when it comes to the use of combustible liquids with low minimum ignition energies (MIEs). As powders normally have higher MIEs than liquids, it is important for guidance to be published that takes into account the MIEs of combustible liquids, the potential charging levels of filling / dispensing operations and the most appropriate maximum value of resistance through the container to earth.
If, for economic or material compatibility issues, manufacturers must use plastic portable containers over metal options, they should ensure their container suppliers provide them with drums or IBCs that fall within the static dissipative category if they are to be used in processes carried out within hazardous areas.
Correct Earthing techniques for static dissipative drums and IBCs.
As outlined earlier the maximum resistance value of 10 ohms at which conductive containers (metal containers) at risk of static charge accumulation should be monitored is well documented throughout many standards and process safety publications. When the values of resistance related to composite materials that provide static dissipative functionality come into question, there is little to reference other than CLC/TR: 50404 for Type C FIBCs, which recommends a maximum value of resistance of 1 x 108 ohms(5).
Should guidance be issued that addresses the use of such containers directly a different value of resistance may be deemed appropriate. However, as CLC/TR: 50404 is the only standard which provides suitable guidance for static dissipative materials in a hazardous area context it would follow that earthing of static dissipative drums and IBCs should be monitored to 1 x 108 ohms or less. Monitoring not only guarantees that the earthing system is making a secure connection to the composite container, it is also indicating that the area of the container manufactured with static dissipative material is functioning correctly. This is particularly important because for metal drums and IBCs, it can be assumed that the metal will transfer static charges very easily, provided contact inhibitors like paint coatings and rust are penetrated by the earthing system’s clamp connection. It is good practice to monitor the performance of composite containers to ensure that insulating plastic drums and IBCs that may make their way into a hazardous area are identified and removed before filling or dispensing can take place. In addition, as static dissipative drums/IBCs are “composite” in nature, it should not be assumed that the embedded static dissipative properties are correctly distributed throughout its structure. As the composite container will be subject to degradation during its lifecycle it is important to ensure that the static dissipative properties of the container are maintained for as long as it used within hazardous areas.
Static charges freely move through metals, but may be impeded and accumulate on plastic surface.
Example of Solution to safe earthing of static dissipative containers.
One example of monitoring the integrity of a static dissipative drum used on a filling line is to connect a static earth monitoring system to the drum. The static earthing system, which sends an intrinsically safe current through the drum, monitors the drum to the resistance specified in CLC/TR: 50404 which recommends 1 x 108 ohms or less. The current return path to the system is made through the drum to the surface the drum is sitting on which, for example, can be a weighing scale, conveying line or purposely made steel sheet. This surface is connected to the site’s pre-verified earthing point, which, in this example, is the copper bus-bar running along the wall. The current returns to the ground monitoring system via the earthed bus-bar. In this example the system is not only proving that the composite drum is providing the required safe level of static dissipative performance in a continuous monitored circuit to 1 x 108 ohms, it is also proving the drum has a continuous connection to earth via the bus-bar for the duration of the filling process.