Safety Insight - Are You Controlling Electrostatic Ignition Hazards in Silos and Hoppers?
Why is it Important?
In silos and hoppers intended for combustible dust and pellets handling flammable atmospheres (combustible dust clouds at sufficient concentration in air) are expected to be present in the vessel headspace during normal operating conditions (unless measures such as control of oxidant, nitrogen inerting, are in place). The following electrostatic ignition sources capable of igniting combustible dust clouds could be also present:
- Cone Discharges (also known as Bulking Brush Discharges).
- Spark Discharges from ungrounded metal components, and
- Propagating Brush Discharges from the hopper wall lined by insulating layers and plastic hoses that feed the vessel.
Brush type and corona discharges could be also present inside the vessel. However these discharges are not capable of igniting combustible dust clouds in air regardless of its energy.
Cone discharges present the highest hazard. These discharges are caused by the extremely high electrical fields resulting from the high space charge density which is generated when charged insulating particles are accumulated in silos or hoppers. Model calculations have shown that conditions necessary for the appearance of cone discharges do exist when filling hoppers with highly insulating powders or granules. The discharges may already be initiated at a low level of the charge to mass ratio of the incoming product (107 C/kg in the case of a powder heap with radius 1 m).When highly charged insulating powder is filled into silos or hoppers it generates a region of very high space charge density within the heap of bulked powder. This leads to high electrical fields at the top of the heap. Under those circumstances large discharges running (radially, in the case of cylindrical containers) along the surface have been observed. The conditions necessary for this type of discharges are complex; the influencing factors are resistivity of bulked powder, charging current, volume and geometry of bulked powder and particle size. It has been reported that atmospheres of sensitive combustible powders can be ignited by this type of discharge.
As a result, combustible dust cloud explosion hazards may exist and need to be controlled.
What should be done to control such hazard?
- A hazard assessment should be carried out on the operation. The assessment needs to consider the process conditions, electrostatic properties of the vessel components including feed lines and associated dust collection equipment, and possibility of a combustible dust cloud in air being produced in the vessel headspace, bearing in mind electrostatic properties of the powder, Minimum Ignition Energy (MIE), and Minimum Explosible Concentration (MEC).
- Electrostatic measures to control explosion hazards of combustible dust clouds in air associated with silos and hoppers could include (but not be limited to):
- To avoid Electrostatic Cone Discharges
- Vessel must be made of conductive material (metal) and grounded
- In order to prevent generation of cone discharges capable of igniting dust cloud, size of the cone should be reduced by inserting internal conductive grounded cables or rods or baffles.
- To avoid Electrostatic Spark Discharges
- Grounding/bonding of the equipment metal components
- To avoid Propagating Brush Discharges
- Propagating brush discharges can be avoided by using only conductive or electrostatic dissipative (ESD) material for walls of equipment, wall liners and coatings or by ensuring that the breakdown voltage across all insulating walls and coatings is less than 4 kV
- Use of ordinary plastic hoses for conveying powders to the hopper must be avoided. Use of grounded metal piping instead of flexible hoses should be ensured where flexibility is not necessary. This is the best industry electrostatic practice for pneumatic conveying of combustible dusts.
- When flexibility is necessary, use of hoses made of antistatic plastic should be considered.
- To avoid Electrostatic Cone Discharges
- Note: It must be understood that control of ignition sources is not always reliable, or even possible. Therefore it must not be used as a Primary Basis of Safety of the silo or hopper operations. Primary basis of safety should be either preventive (control of flammable atmosphere either through exhaust ventilation, inerting, avoidance of fines), or mitigative (control of consequences of deflagration by explosion protection and isolation).