Dust Explosion Hazards

Dust Explosion Hazards
Dust explosions represent a serious danger when handling or processing bulk solids. Unfortunately, due to the nature of handling or production technology, the formation of explosive atmospheres can often not be ruled out. Explosion protection measures must be taken to prevent ignition sources. A very dangerous source of ignition is electrostatic discharges. They cannot be seen in advance and when they happen, it’s maybe already too late for countermeasures.

When is the risk of explosion greatest?

The most obvious hazard is dust commonly known as combustible dust from coal or wood. Often underestimated is the explosion danger of dusts made from metals, plastics, sugar or flour. Technological processes such as grinding of light metals, spray drying of organic products, suction, conveying or processing of metal powders can lead to an explosive dust atmosphere and strong electrostatic charges at the same time – a very critical situation.

Only grounding protects against electrostatic charge

Electrostatic charges can accumulate on isolated objects or non-conductive substances. To avoid dangerous sparks, the charges must be dissipated to earth safely. Therefore, all involved plant equipment, movable containers, and even the personnel in the work environment must be appropriately grounded. The path to ground must be below 10^5 to 10^6 Ohm. For operational practice, this means: All personnel must wear dissipative shoes and clothing. All plant parts must be connected in a conductive manner. All movable containers must be grounded by grounding clamps and cables.

Bad grounding connections

The main source of errors related to grounding is that the required connections are not made or not made properly. This is especially the case when trucks or mobile containers (IBC, FIBC, barrels) are loading or unloaded. Operators forget to attach the grounding cable, place the grounding clamp at non-conductive parts, attach the grounding too late or detach it preliminary, don’t pay attention to damaged grounding lines. Unfortunately, these are everyday situation at filling stations. Under Best Practice considerations, there is only one safe way to avoid these errors and related hazards: the consistent usage of electronic Grounding Control Devices at all filling stations.

Three features of safe grounding control devices

Grounding Control Devices monitor the conductivity of the grounding connecting. Only if the connection is good they issue permissive with optical indication (green LED light) and electronic switching (control outputs for PLC interlock). But what distinguishes state-of-the-art grounding devices from outdated or basic ones? First, state-of-the-art systems provide an intelligent explosion protection approval for hazardous areas (ATEX, IECEx). This is characterized by the ability to easily open the main housing for mounting, commissioning, and replacement of wear parts, partially under power in the Ex zone. Secondly, state-of-the-art means an object detection function is provided. This function ensures the grounding clamp gets attached the right place and has good connection to the object to be grounded. Misuse or manipulation by attaching the clamp elsewhere can be detected and avoided. Thirdly, state-of-the-art devices are electronically controlled and monitor themselves continuously. This includes monitored safety relays, analysis, and diagnostic functions, a SIL2 approval with software tests, as well as the detection and compensation of external interference. This ensures the device is always in a safe state – 24 hours a day, 365 days a year.

Conclusion

Electrostatics are a serious danger in explosive atmospheres, but can be kept under control with appropriate measures. A risk assessment of all dust-generating and releasing processes is the basis for this. State-of-the-art grounding control devices can reduce the risk of dust explosions at the filling stations effectively and help to maintain a safe work environment

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