Acoustic cleaners keep hoppers flowing freely

Audiosonic acoustic cleaners are extremely simple in their operation, requiring only normal plant compressed air for their initial energy source.

Primasonics International has now employed its Audiosonic acoustic cleaner technologies to solve the bulk storage problems of cement powder in more than 30 countries worldwide.

The acoustic cleaners have been of tremendous help is solving the two main problem areas within silos, which are: rat-holing - where the material builds up on the sidewalls causing severely reduced "funnel flow", which also restricts the silo being used to its full storage capacity; and bridging - where the material bridges over the outlet area either severely restricting flow or stopping it completely.

Irrespective of whether the bulk storage facility is under 1000kg or over 30,000kg capacity Primasonics Audiosonic acoustic cleaners have been proved to eliminate both these problems.

Audiosonic acoustic cleaners are extremely simple in their operation, requiring only normal plant compressed air for their initial energy source.

Compressed air enters the wave generator and forces the only moving part, the ultra-high-grade titanium diaphragm, to reverberate very rapidly within its specially designed housing.

These rapid reverberations create the base tone and the different bell sections convert, amplify and distribute this base tone into a range of selected key fundamental frequencies.

Primasonics has selected six such key fundamental frequencies for the Audiosonic acoustic cleaner range from 60 to 420Hz.

There are three key problems that occur in silos that the Audiosonic acoustic cleaners can eliminate.

The first of these is bridging, which is caused when the design of the hopper section has insufficient angle of repose, side walls with excessive frictional properties or an insufficiently sised outlet for the material being contained.

Consequently these problems can become severe if a silo is used for materials different from those for which it was designed, either through process or plant layout changes over time.

To combat bridging, an acoustic cleaner is mounted just above the outlet: when this sounds, single particles and clusters of particles move at different speeds therefore bridges are collapsed and material flow is restored.

Once the acoustic cleaner has ceased to sound new bridges and arches will begin to form.

To counter this the sounding may be controlled in one of two ways: either simply on a timer device, typically set for a few seconds every 10 minutes, or alternatively it can be controlled automatically via a flow sensor downstream of the outlet or even from load cells.

The next problem is rat-holing (or funnel flow), which 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 rat-holing, acoustic cleaners with a long wavelength 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 Gyro Whip system.

If an acoustic cleaner were to be sounded in a silo with these very thick hardened buildups 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 rat-holing 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.

The third area in silos and hoppers where acoustic cleaners can help is in preventing material crosscontamination.

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 occurs when material hits or contacts with colder sidewalls and moisture precipitates from the bulk material onto the sidewall.

This moisture forms a meniscus between the sidewall 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 alternating compressive and rarefactional forces break the surface tension of the moisture and allow this material to flow out leaving the vessel completely empty.

Such has been the demand to prevent cement building up and hardening on the walls of larger 20,000-plus tonne silos that Primasonics developed the Model PAS-60, the most powerful Audiosonic acoustic cleaner available.

This single acoustic cleaner has now been installed on numerous very large cement silos, over extremes of climate conditions throughout the world - and has proved successful in preventing cement powder build up on side walls on 100% of the installations.

Indeed nowhere is this success more evident than in India, which experiences extremely dry and wet climatic conditions.

With the assistance of local agent Protos Engineering, Primasonics International has gained an unsurpassed reputation for solving such silo flow problems and eliminating cement powder build up throughout the Indian cement industry.

Primasonics Audiosonic acoustic cleaners, especially the new Model PAS-60 have now been successfully employed on a wide variety of silo types for cement groups such as: Vasavadatta, Shree, Grasim, Ultratech, Dalmia Bharat and Ambuja.

PAS-60 acoustic cleaners are easy to install, for example on existing silo top hatches; they only require standard plant compressed air and are virtually maintenance free.

In the case of the Rajashree plant as with all other similar installations, the very powerful Model PAS-60 was an instant success, eliminating the severe material build-up on the silo sidewalls and preventing any recurrence.

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