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BACKGROUND

Introduction to Silos

A silo is a vessel used for storing large quantities of materials and they are used in many industries throughout the world. There are many other terms which are used to describe silos in many different industries such as bins, bunkers, hoppers and tanks. In our project we will use the term silo which will either be flat at the bottom or have a hopper at the bottom part. The Hopper can have either a conical or a wedge shape.

Silos can vary greatly in size from a small capacity of less than 1 tonne to much larger capacities of up to 100,000 tonnes. It is important to note that a silo is designed to contain a very limited range of solids. To store a solid which the silo has not been designed for could lead to structural damage and/or even complete failure.

Below is a diagram of a silo showing the terminology for each part of it. It starts at the roof and continues down to the columns which support the silo itself. 

Elemental Geometry of silo


Flow Patterns and Problems

It is important to introduce the different flow patterns which occur inside the silo when discussing silo wall pressures as these flow patterns play a big part on test results.

Flow Patterns

The terminology used to describe the flow patterns we use in this project corresponds to that used in Eurocode 1 Part 4 and are described below.

Mass Flow: In this flow pattern all the solids in the silo are in a state of motion during the discharge period i.e. from the moment the outlet is opened to complete discharge of the solid.

Funnel Flow: This is the distinction between "mass flow" and "all other flows".  Although there may be a need to distinguish between "internal flows" and flows with an "effective transition" against the silo wall we will not distinguish between them as it has been found to be very difficult to predict if transition will occur. We will therefore refer to all other flows which are not mass flow as funnel flow. Funnel flow is divided into two categories: "mixed flow" and "pipe flow".

          Mixed Flow: This is a mixture of mass flow and pipe flow. The mass flow occurs in the upper part of the silo and pipe flow occurs in the lower part between the stationary solids. During discharge the solids against the wall in the lower part of the silo are motionless whilst the solids in the upper part of the silo move throughout the entire cross-section in the direction of the outlet.

          Pipe Flow: This is where the flow of solids above the outlet is in motion in a vertical or near vertical channel whilst the surrounding solids are motionless. The flow of solids against the silo wall may arise in pipe flow if the outlet is eccentric or if specific factors cause the location of the channel to move from above the outlet. Internal pipe flow is a term used to define when the vertical channel above the outlet does not reach the silo wall within the height of the stored material. Therefore pipe flow can be divided into two different categories “internal pipe flow” and “eccentric pipe flow”.

Flow Problems

There are many different problems associated with flows in silos. the main cause of flow problems are as follows:

·          Arching
·          Rathole
·          Incomplete Clearout
·          Segregation
·          Vibration, Quaking and Honking
·          Collapse

Some of these problems are briefly described below and the figure below shows a cross-section view of arching, rathole and incomplete clearout.

Arching: This occurs when a stable arch forms over the silo outlet with an empty space beneath it, stopping flow of the solids in the silo.

Rathole: This occurs when a vertical or near vertical channel forms above the silo outlet and flow of the solids becomes clogged either side of the channel.

Incomplete Clearout: This occurs when the solids in the bottom part of the silo remain stationary against the silo wall after a gravity discharge. This problem mainly occurs in flat bottomed silos but also occurs in hoppers.

Segregation: This is where the solids of varying sizes and densities become slighty separated and occurs during filling and discharge.

Shaking Quaking and Honking: This is a lot let understood than the above flow problems. These vibration and noises occur during the discharge and are often unexpected.


Silo Wall Pressure Measurements

A normal stress against a silo wall is represented by the term “pressure”. This pressure is the calculated average effect of a large number of individual particle contacts. The most important aspect of pressures in a silo is there effect on the structure designed to contain the particular solid. Considering that the properties of industrial bulk solids vary from year to year and from source to source and the pressures vary very much due to the different particular solids stored in the silo. It is therefore important to note that a silo is designed to hold a limited range of solids and that if this silo is used to store a different kind of particular solid it may easily lead to damage or complete failure of the silo.

Many experiments have been conducted over the past hundred years on the pressures exerted on the walls of silos by many different types of solids. There are numerous researchers who have preformed simple experiments and developed simple theories from their findings but the most important of these researchers was Janssen. He carried out experiments on a tall square model silo and then developed a theory which is used universally.

These experiments lead the researchers to believe that the highest pressure observed during discharge at any one location on the silo wall was the most important. However it has been found that in most cases the highest pressure is not the most important. In most silos the worst case is not one of high pressure at every point on the silo wall but in fact the case of the greatest asymmetry of pressure on the silo wall.

The pressures applied by bulk solids on silo walls vary between filling and discharge. In particular the pressures during discharge of a silo are still badly understood and there is a great uncertainty in there prediction. During the discharge of a silo the pressures vary between high and low within small periods of time making the interpretation difficult. A graph of such varying pressures recorded on a vertical line of cells in the test silo is shown below. This graph is the 5th experiment done in Edinburgh using a rough walled silo with a flat bottom which was concentrically filled and concentrically discharged. The material chosen for this test was sand.

Flow Pattern Effect on Pressure

Earlier we discussed the different types of flow patterns. The effects that these flow patterns have on the pressures in the silo wall is discussed in some detail below.

Under Mass Flow: A high pressure develops at the top of the hopper which is caused by a high hopper pressure, F. This is associated with solid beneath this point being in a passive state. Despite much being made of this high local pressure, structural research studies have shown that it is not critical to the strength of metal silos. 

Under Pipe Flow: The pressures against the wall under pipe flow are largely unaffected by the flow. Lower design pressures are possible if the designer can be sure that no flow of solids against the wall will occur except at the surface. 

Under Mixed Flow: The boundary of the flow channel strikes the wall and a local high pressure often can develop against the wall which is comparable to that in the hopper. However, this pressure is somewhat unpredictable. It should also be noted that very few silos have actually failed by bursting at an effective transition.

The figure below shows the average pressure patterns for each flow pattern after filling and during discharge.

Classical Image of Silo Pressures

- Assumes single pressure at any level (symmetrical pressures). We know that it is rare to obtain symmetrical pressures within a silo

- Focus on vertical walls of slender silos as most research is carried out on slender silos. For a silo to be regarded as slender the height divided by the diameter of the silo is greater than 2.


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