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Silo Content Levelling
Primasonics Acoustic Cleaners can be used during discharge and filling both to aid mass flow like behaviour and to significantly reduce the angle of repose. This can remove much of the uncertainty currently experienced by stock controllers and engineers as to the real levels in the silos. Stock control relies on accurate figures to keep the plant operating. Engineers entering silos have perhaps even more pressing heath and safety reasons to ensure that a silo that is showing as empty on a level sensor does not have areas of wall build up that may dislodge suddenly.
Determining the correct amount of material stored within a silo can be critical to stock control, production planning, purchasing and delivery and logistics. Without an accurate figure for stock currently held safety stock and stock to replace lost production during a shutdown can not be guaranteed.
If we consider a silo where the material inside is level as shown in Fig.1 below then as long as the level in the silo can be measured (even by lowering a rope) the following formulas can be used to quickly calculate the volume of material held.
Volume of a Cylinder pi * radius2 * height
Volume of a Cone (1/3) * pi * radius2 * height
Fig. 1 (right) Silo Cutaway drawing showing level material.If the silo in Fig.1 were to be partially discharged and did not exhibit mass flow characteristics it is likely that the level top of the material would be lost as shown in Fig. 2 below.
If we were now to try to measure the level using a rope the fill level measured would depend almost entirely on the position of the top hatch used. Even using more sophisticated measures than simply a rope lowered (by hand or automatically) such as radar or ultrasonic sensing with steep angles on the material accurate results are hard to obtain.
One method to overcome this would be to mount the silo on load cells, as long as the density of the material is known then an accurate volume can be calculated.
Load cells also have the advantage that they produce the result in mass terms which avoids problems with level sensing that can occur. With even the best level sensors errors can creep in due to bridged material containing a void space of unknown size or density changes due to fluidisation or impact packing during filling.
Of course retrofitting load cells to a silo structure is an expensive procedure and for most concrete silos not supported on metal legs they are not suitable.Fig. 2 (left) Silo Cutaway drawing showing ratholing material after an attempted complete discharge.
If the same Silo shown in Figs. 1 and 2 is now refilled it is unlikely that the material will level itself within the silo and depending on the filling position significant sloping or peaks of material can be formed as seen in Fig. 3 Below.
Fig. 3 (right) Silo with a peak below the filling point.
From a point of stock control and rotation the silo shown in Fig. 3 will give the process engineers three significant problems that are listed below.
1. When the silo discharges again they can not be sure how much material is from this batch and how much is the stuck material from previous batches, this can lead to cross contamination and poor product in the case of perishable materials.
2. The high level trip of any installed level sensing equipment may have been tripped which will prevent further filling even though there remains a large void area within the top of the silo.
3. With level sensing equipment even if no trips have been activated the same problem exists as with the rat holed silo in Fig. 2 in that it is very hard for a single level sensing device to accurately measure the all the material levels on the other side of a peak to produce an accurate average.
These problems can all be reduced or eliminated with a single acoustic cleaner easily installed through an existing hatch on the top of the silo.