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|2. Examples of
metal - metal contacts
|3. Risks related to mechanical
sparks or hot spots
What is the role of metal frictions and mechanical impact in dust explosions ?
A lot of focus is put on electrostatics risks during dust explosion risks analysis, such as ATEX risks analysis or Dust Hazard Analysis DHA, which is very right, however, one should not overlook other potential causes of ignition of dust clouds, one of them being mechanical sparks or hot spots due to mechanical rubbing.
These risks of heat from friction or mechanical impact are very often related to rotating equipment within the process.
The following equipment (non exhaustive list) can be source of mechanical sparks or hot spots due to metal to metal contacts :
- Screw conveyors : excessive bending of the shaft can bring the screw flight too close to the tube or trough.
- Airlock rotary valves : the clearance in between the rotor and the stator is very narrow, making the equipment very sensitive to any mechanical issue, for example an incorrect re-assembly of the valve after maintenance.
- Mills / grinders : those equipment are typically rotating very fast and with narrow gaps in between the grinding tool and the housing or calibration grid. A mechanical failure can lead to contact in between the rotating parts or in between the rotating part and the housing.
- Mixers : the agitator is rotating fast, in case excessive strength is applied to the mixing tool, in case of over-filling from example, the agitator may bend and hit the mixer walls.
For all rotating equipment, the bearings must be a key point of focus, as any defect can lead to overheating with temperatures that can be hazardous for the powder.
The risks related to the entry of a foreign body in the process must also be assessed and controlled. Indeed, a metal foreign body could be trapped in between the rotating part and the housing, leading to metal metal contact.
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Experiments have shown that mechanical rubbing at a speed < 1 m/s is unlikely to produce enough energy for an hazardous sparks or high temperatures (note : with regular powder with MIE > 3 mJ, and a mixture with air only, the presence of a flammable gas in the mixtures steeply increasing the risk).
As a consequence, the following measures should be taken to avoid risks related to mechanical sparks or to control it :
- Design equipment with tip speed of rotating equipment < 1 m/s : if by design, the speed in between 2 metal parts susceptible to enter in contact is less than 1 m/s, the risk of dust explosion is unlikely
- Operate the equipment with a tip speed < 1 m/s : when possible, for equipment that have the possibility to operate at a quicker tip speed. The operator must however make sure that the equipment cannot accidentally rotate quicker than expected : the measures necessary should be described in the risk analysis.
- Operate the equipment at tip speed > 1 m/s but only when the powder is not in explosive concentration : this can be used for a dry mixer for example, the speed is reduced below 1 m/s tip speed during loading and discharging, while the speed is increased when the mixer is full of product for mixing
-Protect the equipment against explosion risks if the tip speed must be > 1 m/s and the dust cloud conditions cannot be guaranteed. It is the case especially for milling systems. Confinement, venting or explosion suppression can be different solutions to mitigate the effect of an explosion. It is also recommended to monitor such equipment with vibration sensors allowing to detect early that there is a mechanical problem.
If the equipment suffers bearing damages, they can overheat and lead to some explosion risks : some powder in the seals can ignite and then trigger and explosion in the equipment.
The following measures should be taken :
- For all rotating equipment : use pressurized bearing seals, it helps to keep powder away from the bearing, which will avoid damage in case of ingress of powder, and will avoid that the powder overheat and ignites
- For the most sensitive equipment : in addition, control the air flow to make sure the pressurization is on, add temperature sensor, and possibly vibration sensors. The sensors should trigger actions within the automation if an abnormality is detected
Foreign bodies can damage the equipment and lead to metal metal contact, or be trapped in between a rotor and stator and create metal metal contact as well. To avoid those issues, the process must be protected against the entry of foreign bodies by using :
- Metal Detectors in some cases
Having those equipment is not sufficient alone, procedures must be applied to check them regularly. This should be explained in the DHA / DSEAR.
Note : the values given above are valid for stainless steel, if the materials involved are different, it must be highlighted in the risk analysis and specific measures taken (oxidized steel, aluminimum...etc... can easily generate sparks, special care must be taken with those materials).