Solve flow problems in hoppers and silos (Bad flow, no flow)

1. What are the causes of powder flow problems in hoppers and silos ?

Bridges and ratholes can cause partial or total blockage of the flow out of a silo. It is unfortunately not uncommon that bulk solids / powder processes operators experience flow issues out of their hoppers or silos. Experiencing a poor flow out of hopper has important consequences for the whole process as it can slow it down dramatically or even stop the process completely. Industrial silo blockages, its prevention and long term fix is thus a primary concern of companies. A bad flow out of a silo can have multiple causes as the flow is dependent of the couple powder properties / silo design, however, we can list here the root causes most often encountered across the industry :

• Too small discharge angle : the slope of the discharge cone is not enough to make the powder flow (note : other geometries than cone is possible, but it follows the same logic)
• Too small discharge outlet : the diameter of the outlet of the silo is too small
• The discharging aids have not been chosen or installed properly

Those root causes usually translate in the following consequences :

• Powder bridging : a powder arch, stable, forms within the silo, the powder above the arch cannot flow
• Powder ratholing : the powder flows only within a channel, the powder around this channel, or rathole, do not flow

In clear, there is much less or not at all flow out of the hopper !

Note that other flow issues exist, showing different symptoms but are somehow less common and not detailed in this page. Among this issue, we may highlight the phenomena of flushing, which is actually the exact contrary of the problems listed above : the fluidization of the powder ends up in an uncontrollable flow outside of the silo.

2. Preventing flow issues in silos (blockage, no flow) : Silo design

How to prevent bridges and ratholes in silo ?

The very 1st action to take is actually to prevent this issue through a good design. The silo slope and outlet diameter can be calculated. The silo calculation is 1st based on measuring properly the flow properties of the bulk solids to be stored. The following page explains the calculation procedure to design a silo : link.

Note that it is sometimes possible, although costly, to modify a silo after installation to improve the flow.

If the design by itself cannot ensure a free flow, or if it is necessary to improve the flow of powder after installation, the use of discharging aids should be considered.

The following paragraph discusses the most common discharging aids used in powder handling industries.

3. How to improve the flow in silos : discharging aids

3.1 Discharging bottom - bin activator

Bin activators are a very widespread method to promote flow in silos. Such vibrating bottom allow to have a shorter silo height while keeping a good flow of product. The bottom part of the silo is mounted on dampers, fitted with a vibrating motor (usually an unbalance motor) and equipped with a reversed cone forcing the flow of product on the outside of the silo shell.

The vibration of the bottom part and the reversed cone which is fitted on it allows to break arches and create flow. In addition, the cone helps also to have a mass flow within the hopper, which is often desirable.

As the principle of the flow promotion is based on vibration, bin activators will not perform very well with powder that compact easily, which could lead to a blockage below the cone. This can be helped by including several level sensors in the outlet cone and shake only when it is necessary. Vibration as a discharging aid may also be not very efficient for powder which can absorb well the vibration, or for very fine powders for which the flow may be irregular.

Figure 1 - Bin activator / vibrating bottom

Note that in some cases, hopper designers will directly mount vibrating motors on the silos cone, without flexible parts. It has a limited effect as the hopper structure absorbs part of the vibration and can be used only for mild flow issues. The hopper and its support must also be designed to withstand the vibration which may be high especially when the hopper gets empty.

3.2 Fuidizing pads

Another way to break arches or make static area around a rathole fall is to fluidize the material. When powders get fluidized, then tend to flow almost like a liquid, a long as a degree of aeration is present. This property can be put in good use by installing pads in the silo cone which will allow to inject air at the base of the hopper and thus fluidize the powder.

The number of pads depends on the size of the cone and the compressed air pressure is typically around 2 bar g. Different designs exist, like rubber mushrooms or hard teflon pieces. Some of these design may add a certain degree of vibration to the air fluidization effect when activated.

Such pads usually give good results, they however require compressed air (which should be dry, otherwise the humidity that may be introduced with the air is likely to reduce the flowability) and are limited to small - medium size hoppers.

Figure 2 - Fluidizing pads

Other designs, based on a similar principle exist for larger silos : fluidizing bottoms of air cannons. For big silos, such discharging aids must be installed only if the design of the silos is strong enough to withstand a high stress due to the fall of part of the product stores. Indeed, those fluidizing system are efficient and could help to unblock a large quantity of powder which, when falling, could damage the silo.

3.3 Knocker

As its name indicates, knockers - also called hammers - will knock the outside of the silo cone to promote flow.

Those equipment are sometimes used as the main discharging aids for some hoppers but are usually not very efficient alone. Indeed, knocking on a full hopper is dampering a lot the shock wave, thus the force is sometimes not able to break arches.

Knockers are better to activate at the end of the discharge when only few kg are remaining on the walls of the hopper or silo. The knocking will then help to arrive to a discharge close to 100%.

3.4 Where to position discharging aids ?

Discharging aids are used when it is not possible to design a hopper that will behave as mass flow with the solids it handles, which would avoid powder bridging and ratholes. Therefore, to be efficient, such discharging aids should be positioned at the height corresponding either to the critical outlet diameter or rathole diameter, whichever is bigger (usually it is the rathole diameter). Those diameters can be calculated following shear testing of the powder.

If those diameters are unknown, the positioning of the discharging aids will be done by experience, with risks of being more or less efficient, by placing them slightly above the outlet of the silo. If using fluidizing pads are used, it may be possible to use several layers all along the height of the discharging cone (not too high, as this may be a problem to have suddenly big amount of powder flowing).

3.5 Others

Many other discharging aids are available depending on the design of the silo from which to extract the powder. A non exhaustive list is given below :

- Mechanical agitators

- Cone valves

- Vibrating screens

- Rotary extractors

- Screws / rotating screws