Explosion Protection How to Avoid Electrostatic Hazards in Bulk Materials
Electrostatic discharges create critical safety risks. This article provides examples in powder handling include pneumatic conveyors, big bag filling stations, and even silos where there is the danger of a conical pile discharge. Proper grounding techniques and the correct choice of big bags minimize the hazard.
Electrostatic charges can occur whenever two surfaces are rapidly separated and at least one of those surfaces is an insulator. Electrostatic charges and the associated discharges are one of the 13 possible ignition source types to be considered in a plant-related risk assessment. A distinction is made between different kinds of discharge, which depending on their type can be responsible for igniting gases or dusts.
Selecting a Suitable Big Bag
Big bags, also referred to as flexible intermediate bulk containers (FIBCs), are divided into types A, B, C, D according to their protective characteristics in terms of electrostatic discharge:
- Avoid brush discharges: Types C and D
- Avoid propagating brush discharges: Types B, C and D
- Avoid spark discharges: Types B, C and D.
Type C big bags contain conductive filaments to dissipate static electricity, which means that this type of big bag must be grounded. Grounding can be carried out by means of a cable and clamp. Another option is to use a grounding monitoring system such as the Farado FIBC; this provides an interlock function that allows the Type C big bag to be filled only when the grounding is correctly established (Figure 1). Type D big bags, in contrast, transfers electrostatic charges to the environment via dissipation. However, this requires grounding of all conductive equipment and objects located within reach of a type D big bag. Likewise, people nearby must wear conductive footwear, otherwise they too will become charged and could act as ignition sources themselves. The big bag must be chosen strictly in accordance with the requirements of IEC TS 60079-32-1 “Explosive Atmospheres — Electrostatic hazards, Guidance”. The selection procedure depends on the minimum ignition energy (MIE) and the zone classification. However, the appropriate division also depends on the zone defined on the outside of the big bag. If a gas zone is present at the same time, special measures must be considered.
Pneumatic Filling of Silos
Another source of electrostatic charges is in places where pneumatic transport or pneumatic filling of silos by trucks is carried out. If an explosive atmosphere develops outside the truck, and the truck acquires an electrostatic charge as a result of its movement, there will be an acute risk of explosion. For this reason it is essential to ensure that the truck is properly grounded. There are systems on the market that are able to monitor the proper grounding of the truck and, via interlocks, provide an approval system that can be used to control valves or other equipment (Figure 2). In addition to grounding the truck, care must also be taken to ensure that the conveying line is properly constructed and maintained: it must be either electrically conductive, or able to dissipate static charges, and grounded. Insulating intermediate pieces such as sight glasses should be avoided, as these may lead to propagating brush discharges.
Conical Pile Discharge in Silos
Another dangerous circumstance that is often not considered is the possibility of a so-called conical pile discharge in silos. Bulk solids are generally not to be regarded as conductive, so during pneumatic conveying they can become dangerously charged. If the silo is large and the median particle diameter of the bulk material is also large, there is a risk that the cloud of dust above the bulk material will ignite due to a conical pile discharge. If a conical pile discharge cannot be excluded as an effective ignition source, protective systems must be applied in case of doubt. Flameless explosion venting by means of a Q-Box, or explosion suppression with the Q-Bic system, offer numerous options for indoor installation. The protection of adjoining plant components must also be taken into account during the system design. In particular, the supply line must be isolated against the pressure and flame of a possible explosion. This prevents the explosion from striking back into the conveying truck or the big bag loading station. In addition, the silo discharge must be equipped with an appropriately certified rotary valve, e.g. Ex II (1) 2 D, to stop the explosion from propagating in this direction (Figure 3).