News
Security elevator
24-03-2022
According to statistics from the food and feed industry, tray elevators are often at the top of the list of installations where most dust explosions occur. Reason enough to consider the explosion risk and necessary measures in these types of elevators.
Security elevator is customised
Explosive mixtures
For a dust explosion in an elevator it is necessary that there is an explosive dust/air mixture in the elevator.
In an elevator conveying powders, one would expect that the risk of an explosive mixture would be high. In practice, however, the dust concentration in this elevator is often so high that it is above the upper explosion limit. This became apparent (among other things) during a test programme in England. An attempt was made to cause an explosion in a number of elevators that transported starch or flour. It appeared that, during operation, no ignition was possible or only a mild explosion occurred. Also in Germany, an attempt was made to create an explosion in a powder elevator. It appeared that the most violent explosion could be created shortly after the start-up of an elevator, without feeding product. Vibrations occur at the start-up of an empty elevator. These appear to be sufficient to drop dust deposits and to give approximately an optimal concentration!
Yet explosions do not occur only at start-up. Mostly, it is not about elevators that transport powder, but granular materials, such as grain or wood pellets. There is only a small amount of dust in these, but still enough to cause dust deposits and therefore, especially in the case of vibrations, dangerous dust-air mixtures.
In Germany, dust concentration measurements were carried out in an elevator for granulated sugar during operation. If the elevator was not extracted, the concentration was close to the upper explosion limit, with good extraction the concentration was much lower: about optimal (for an explosion).
Consequently, elevators conveying coarse products are often the most dangerous. If the granules are really dust-free, or the dust is sticky, then there is much less of a risk of explosion.
Tip for the doubtful: Take a look through the elevator's viewing window. Or, if there is no window, dismantle a plate. And give the elevator a good slap. Do you see dust swirling down? Then there is a risk of explosion!
Ignition sources
For an overview of all ignition sources in an elevator and the measures to avoid them, see NPR-CEN/TR 16829:2016.
In practice, when explosions occur in elevators, most ignition sources are related to mechanical problems:
- Tyre slippage.
- Tightening of the tyre.
- A detached tray.
- Breakage of the tyre (or chain).
- Broken bearing.
This may cause mechanical sparks, but also especially hot surfaces. At the same time, such problems will cause the elevator to vibrate, which may also result in an explosive mixture.
Preventive measures
Dust extraction on an elevator does not always reduce the risk of explosive mixtures, but it is recommended to prevent leaks to the outside.
Grain is often thoroughly cleaned as part of the process. Ideally, this cleaning should be carried out first, after which an almost dust-free product enters the elevator. In practice, this is usually not feasible. It is then recommended to use one specific elevator for non-cleaned grain, in the other elevators the risk is already smaller.
Return of extracted dust to the product is of course strongly discouraged.
Besides all preventive measures in NPR-CEN/TR 16829, good preventive maintenance of elevators is of utmost importance. Raising awareness among operators is also not to be underestimated.
Provided that preventive measures are in place, NPR-CEN/TR 16829 states that explosion protection is only necessary for powders or dusty products. Thoroughly cleaned grain is no longer dusty, so that preventive measures may suffice here. It is very difficult to set a clear limit for this rule, this has to be evaluated case by case.
Protection against the effects of explosions
Explosion venting with rupture discs is often used to protect elevators. Detailed information is given in NPR-CEN/TR 16829 for the dimensioning of these venting systems.
It is important that explosion decoupling is not forgotten. Because many elevators dump directly (or via a chain conveyor) into a silo, without a good disconnection at the outlet, there is a big chance that the explosion will spread to the silo.
Especially with large-capacity elevators, it is difficult to secure the outlet with a mechanical system such as a rotary valve or possibly a quick-close valve. A chemical barrier, locally blowing in a dense cloud of extinguishing powder, is then the only available alternative. But, in combination with pressure relief on the elevator, a chemical barrier is usually not reliable: certainly if the explosion starts in the foot, there is a big chance that the barrier is already blown away when the explosion reaches the top. In that case it is better to protect the elevator as a whole with explosion suppression.