I would be interested to know why you chose to use an expanded flow bin with coal - bearing in mind the need for first-in, first-out with such a material (assuming that you will not be draining the silo during its operation). If (as is commonly the case) your motivation was to obtain maximum storage capacity combined with reliable gravity discharge, would not a plane flow transition silo with dual outlets feeding towards a central outlet have been more appropriate? You would then have had no "dead regions" during operation and consequently minimise the risk of self heating.

Experience shows that many plants have expanded flow silos installed in the belief that the mass flow "feed section" bestows first-in, first out operation on the entire silo. The frequency with which problems with caking and contamination issues arise shows this assertion to be incorrect where plant operation is continuous and not batch orientated (i.e. draining down the upper silo section regularly).

With regards to the lining material for use in the upper section of your silo (bearing in mind that the material will not be flowing against the wall in any condition other than draining) this will not be as critical as that for the mass flow lower section, but it would be beneficial if a surface that will not degrade with time is used.

Regards

Richard Farnish

The Wolfson Centre for Bulk Solids Handling Technology, Univ.Greenwich, London, UK

www.bulksolids.com ■

The first interesting point is whether the 5 Meter transition was adquate to prevent ratholes forming in the non-mass flow section. The sensitivity of rathole size to product condition, particularly with something like damp fine coal, necessitates that the Jenike predictions are somewhat conservative, but practical results are the ultimate proving ground. Obviously, if the size of flow channel to prevent the formation of a stable rathole is very large, it may not be practical to incorporate this dimension in a bin of limited diameter. Nevertheless, expanded flow bins are often a sound and practical appraoch to many industrial storage duties. Taking Richards point though, it is vital that time consolidation effects be taken into account for the product that will be subjected to an indefinite storage period in an expanded flow bin.

As regards the contact surface for a non-mass flow section of bin the only requirement is that the material will self-clear to an adequate degree of cleanliness when the bin ultimately empties. Wall slip will not nrmally take place, as the repose conditions of the surface level progressively exposes the wall as the material drains away. When a large diameter transition is present, shallow residual layers of materal resting on the walls may slide down when the upper contents are neaxly exhausted, but the sliding velocity and contact pressure are low, so wear is not ofetn a consideration.

There are considerable structual advantages in round bins and conical hoppers, so it is understandable that these shapes are desirable for large storage facilities. Cones, however, have poor flow characteristics compared with Vee hoppers and the many variants now utilised in smaller commercial bins. Also, inserts of various types are used to secure mass flow at lower wall angles and can be adopted to secure wall slip in an outer shape similar to an expanded flow construction. ■

Brian Moore's useful comments highlights the need to ensure ultimate clearance of product from an expanded flow type hopper. Damp coal can form strong adhesive bond to mild steel due to the formation of crystal bridges. High density polyethene liners offer good resistance to such caking bonds and, as slip is only required for the shallow clearing layer in the non-mass flow section, it is not subjects to high contact pressure for wear.

Regarding silo quaking, this is normally only significant in relation to mass flow behaviour, but can be a serious problem due to the magnitude of the forces involved. There are various mechanisms that can give rise to this type of phenomenon, which involves intermittent movement of the main mass of the contents of a hopper, and may be caused by such as: -

1. A large difference between static and dynamic wall friction.

2. The different between static and dynamic internal friction, resulting in intermittent shear (similar to wall stick-slip) on the flow boundary of static product.

3. Cyclic switching between mass flow and non-mass flow in hoppers with wall inclinations that are marginal between the two positive conditions.

4. Backflow of gasses from pressurized delivery regions or air replacement of the displaced product that leads to systematic surging of the discharge

5. Low bulk permeability inhibiting the rate of failure of an arch by restraining atmospheric air entering the expanding voids behind the unconfined surface. Particles ‘rain’ from the arch until it exceeds the critical span and collapses to re-start the cycle. This process is termed ‘Slurping’

6. Unstable repose conditions, as may be developed under the cone of a bin activator.

7. Erratic or selective extraction by a feeder or discharge device forming voids that collapse when they exceed the critical arching span.

8. The collapse of dynamic arches. Waves of dilation progressing through the bed to reach a static stored mass at a cross section larger than the critical arching span, followed by elastic compaction as the bulk of the stored mass moves rapidly until brought suddenly to rest by the restricted rate of discharge.

9. A large velocity gradient in the flowing bed, enlarging a region of dilated bulk to an unsustainable level and/or elastically increasing stress on the boundary layers.

In all cases there is a region of void or reduced pressure that allows a rapid failure of the static supporting region. Redressing the consequences essentially requires that the flow area adequately motivates the total movement of the hopper contents. One way is to use a feeder that extracts progressively at a slow rate over a large cross section, such that the replenishment from the hopper contents is smooth and steady. Silo quaking is a problem that is difficult to predict and requires some expertise to address. ■

Dear Mr. Pjaynes

You describe well known problem. All insets and similar solution just don’t work.

In my carrier I have redesigned many of such problem-silos, many times after the people have tied to solve is with inserts or building a new come. All this solutions have not lead to success.

We use for such cases an oversized discharge equipment – THE SILETTA JUMBO-.

The biggest we have build is one for a discharge opening of 4.5 meter. Even by this oversized discharge equipment we don’t reach the mass flow. Probably it could be reached an funnel of a dimension of 5 a’ 6 meter. The silo can be emptied when the silo get completely discharged, because the ring formed column can not hold the high vertical stresses.

Look to our website WWW.IPT-ONLINE.COM. SEE EXAMPLE “REDESIGN OF AN OLD SOLO FOR CORN”, AS WELL ES THE DISCHARGE EQUIPMENT “SILETTA”.

I draw your attention to our course “Quality Control and Powder technology” which will be hold mbegin of november at University of Delft - Netherlands . Such problems will be discussed on the course.

I hope to gave you enough information for the first moment.

Please send us your full address and we will send you some additional information.

Hope hear from you soon

Dr. Ivan Peschl ■