Silo Cleaning Applications
Ratholing or funnel flow occurs when an insufficient angle of repose is used
in the silo for the contained material this usually occurs with material
that displays a mechanical interlocking properties or very quickly bonds
without compaction for example by means of solvent evaporation. To defeat
Ratholing Acoustic Cleaners with a long wave length are placed on the top
of the silo usually through an existing manhole or inspection hatch. If
there is a large amount of hardened material that has build up over a long
period the silo is then cleaned using traditional offline cleaning methods
such as the Prima
Whip™ system. This is because
if an Acoustic Cleaner were to be sounded in a silo which these very thick
hardened build ups and the adhesion forces between particles were greater
than those between the particles and the walls of the silo there is a risk
that large lumps of material may dislodge and block the outlet because of
the power of the low frequency Acoustic Cleaners to continue there sound
transmission through the bulk material until the solid and reflective surface
of the silo wall is reached.
If we consider a full silo that is prone to Ratholing being slowly emptied we can imagine the central core of the material lowering small amount while an outer ring close to the side walls remains at the same height. Without acoustic cleaning this process would continue until eventually the central core had been discharged and the outer ring is left adhering to the side walls. The process control engineer must now decide whether to remove the silo from active process lines while offline cleaning takes place or refill the silo and risk lower control over the quality of product as older adhered material will mix with the new material coming in.
Using Acoustic Cleaners once the central core has lowered slightly the Acoustic Cleaner will sound the bonds holding the outer ring will collapse and this will form a flat surface along the top of the material in the silo. This pattern is repeated as the silo empties even if the silo is refilled before complete discharge has taken place. This now allows a first in first out mass flow pattern. Again the Acoustic Cleaner can be controlled by a simple timer or a PLC or central control system.
Another area in silos and hoppers where Acoustic Cleaners can help is in preventing material cross contamination, if we take the example of a weigh hopper sometimes not all the material is discharged before the next batch enters the weigh hopper, this can lead to product contamination which can lead to quality control problems.
The reasons for material failing to discharge from hoppers are similar to those already mentioned but in weigh hoppers or similar batch filled vessels an additional problem when material hitting or contact with colder side walls and moisture precipitates from the bulk material onto the side wall. This moisture forms a meniscus between the side wall and the particles which holds the powder in place so that even after discharge a thin layer or powder remains in this instance the Acoustic Cleaner is activated when the bin is being emptied the alternate compressive and rare factional forces break the surface tension of the moisture and allow this material to flow out leaving the vessel completely empty.
