FAQ - Silo and Hopper
Q. We are about to change the product being stored in our silos. Will the design of the silo be suitable for the new product?
A. The new product will need to be tested for its characteristics, and the silo will be subject to different load conditions for structural requirement as well as geometry for flow requirements and needs to be reviewed. The wall friction will also need to be tested to ascertain suitability of the existing hopper angles. The new product will most certainly have different characteristics, and may not be able to be stored under the same head height of material.
Q. We are about to manufacture a hopper, and have not had the product tested for flow characteristics. Is it possible to use hopper wall angles that are slightly steeper than the angle of repose?
A. The angle of repose is mostly useful only for the calculation of volume/capacity in silos and hoppers. The hopper wall angles need to ensure mass flow of product. Wall friction needs to be checked for the various consolidation pressures. The product has an angle of internal friction plays an important role.
Q. We are having flow problems in our silo, and want to line the hopper with a low friction material. Is this a good idea?
A. Depending on the product, the installation of low friction liners is a quick fix to minimise the effects of adhesion of the product to the hopper walls. In general, and depending on the feeder or discharger type in use, the installation of liners will assist
The reduction of wall friction (and maintaining the same hopper wall angle), will increase the consolidation pressure, which in turn will result in higher feeder or discharger loading.
Q. We have a silo that has reliable flow during our production periods, however, upon start up after the weekend, we find it difficult to get the material to flow. What is causing the problem?
A. There are a number of reasons for this, and mostly product related. The probable main cause is that there is consolidation of the product. Depending on the nature of the bulk solid or powder, air that is entrapped within the product escapes, and the product increases its mechanical strength, and supports the load in the silo. If the product is hygroscopic, it will absorb moisture from atmosphere, making it more cohesive. Some products also form a mechanical bond due to chemical reaction, and will become solid, taking the shape of the silo or hopper.
It is also important that the system be cleared of product. Depending on the discharger type, if there is adhesion of the product to the feeder or discharger, a funnel flow condition may occur, and cause problems.
Q.We have a silo design that has a 1200mm diameter opening to suit mass flow conditions. Our feeder vendor has specified a 300 wide feeder as a suitable design for the required discharge rates. We used a transition to match the silo to the feeder. We are finding that a flow channel is forming and experiencing poor discharge.
A. No doubt that the silo is of suitable design, and your reputable vendor has designed the feeder of good design to feed at the required discharge rate. However, the flow problems experienced are due to the throttling down of the silo to the feeder, effectively reducing the opening area well below the critical arching dimensions. Basically the silo and feeder selection has not been properly interfaced.
The use of flow promotion devices such as external vibration or high pressure air blasting to break the bridges (if placed in the appropriate positions) may assist in promoting flow. However, it leads to operator safety issues and potential structural damage to the silo.
The resolution to this problem is a rework of the feeder system.
Q. We have a ladder on the inside of the silo wall for access purposes. Does the ladder affect product flow?
A. The inside of the silo must be kept clear of obstructions, and must not contain any ledges (including level sensing instrumentation). The ladder will cause build up of product over time, and eventually, will result in loss of silo capacity, and ratholing of product. This leads to safety issues for operators, having to clear blockages and potential mechanical failure of the silo due to uneven loading or unfavourable flow patterns.
Q. We have supplied standard silo and feeder arrangements in the past all to the same dimensions, capacity and the same discharge rates. On the last job, we increased the silo height by a few metres to get a more storage capacity out the system. We are now finding that we are having flow problems.
A. This is understandable. Once again, it is related to the product properties. When designing silos, the silo to product pressure relations must be considered. Simply by exceeding these relationships, bridging and ratholing can occur.
Q. Is bin venting important?
A. Bin venting is very important. Pressure venting needs to be sized for sufficient capacity to suit the volumetric inloading rates, and design pressures of any inloading system, as well as a safety margin.
It is also important to consider vacuum relief. If the silo is subject to poor product flow, ratholing or bridge collapse is experienced, the product within the silo or bin will collapse at an extremely high rate which is subjected to by gravity. A relative small drop down of product can generate a large amount of vacuum generated under these conditions can be extremely high, and require large opening areas.
Where there are multiple silos linked via an enclosed conveyor system, or an upper gallery, the transfer of positive or negative pressures is possible to other silos, unless there is an effective seal system in place.
If there is explosion risk due to dust or chemical reaction of the product, then explosion venting and suppression systems are required. Explosion venting must be directed to a safe area away from operators and any areas that may increase risk of additional explosion propagation.
Q. What sort of maintenance is required on a silo or hopper?
A. Silos require periodic inspections for wear, and any structural fatigue of the silo and support steelwork. If the silo has surface treatment, either external or internally, the condition of the surface treatment needs to be inspected as well to prevent premature failure due to corrosion.
The condition of any liners or friction reduction surface must be checked for uneven wear or missing components. Uneven flow down of product can cause flow reliability issues and undue structural loading.
Should there be any build up of product on silo walls, the build up needs to be removed.
Check for any ingress of moisture in silos that are located outside. Any manhole covers must be closed at all times. If the silo has insulation and cladding, deterioration of insulation, or dislodgement of poorly fitted insulation during transport or installation may cause localised condensation and affect the product.
All dust filters, vents and relief valves need to be checked for correct operation. Any pneumatic truck loading transfer lines must clear of product at all times.
Any systems in place such as fluidisation or conditioning air need to be checked for correct operation and even pressures are maintained.
