How to explore a silo.

Dietmar Schulze

Most silo problems are a result of the way how the bulk solid flows within the silo. Especially funnel flow can be the reason for flow problems like ratholing, arching, segregation, flooding, quaking and so on. Thus, the most important information is whether mass flow or funnel flow takes place in the silo:

mass flow

funnel flow
(stagnant zones at the
hopper walls)

Funnel flow can result from :

(It is recommended to obtain more information about the flow of bulk solids in silos here.)

The solutions of these problems are manifold and depend on the flow properties of the bulk solid as well as on the individual boundary conditions. Sometimes a funnel flow silo with too rough and/or shallow walls can be converted to a mass flow silo by lining or coating the walls with an appropriate material, by installing steeper hopper walls, or by installing an appropriate insert. If the feeder is the cause of funnel flow, technical modifications of the feeder itself can be useful, e.g. for the case of a screw feeder, where the installation of a screw with increasing pitch instead of one with a constant pitch can help. However, all modifications, especially those of the hopper walls, should be checked first on the basis of shear test results. With these results it can be predicted which hopper inclination is required for mass flow, or with which kind of lining or coating, or with which other means (e.g. inserts) mass flow can be attained. This work can be done for you by an experienced consulting company. Nevertheless, often it is important to know as much as possible from a silo in order to be in a position to understand the problems arising. Here the most important information is the flow profile, i.e. the way the bulk solid flows within the silo.  

How to investigate the flow profile:

A view from the top on the silo filling can provide some information, but this information can be misleading. If the top surface of the silo filling moves downwards without changing its shape, then the flow profile can be mass flow, but it can also be funnel flow with a stagant zone in the lower part of the silo. Only if from the top stagnant regions close to the silo wall can be observed, it can be concluded that funnel flow takes place. Thus, a more reliable method for the determination of the flow profile is required. A well-established method is the use of thin rod which have to be inserted through bores in the hopper or silo wall into the bulk solid (see figure below). The rods should be 2 or 3 mm thick and 200 to 300 mm long. The diameter of the bore should be around 6 to 8 mm. Stick the rods around 30 mm into the bulk solid, whereby the rod should be somewhat inclined upwards. If  at discharge the bulk solid close to the wall moves downwards, the tip of the rod outside the silo will move upwards. Therewith it can be checked if the bulk solid adjacent to the wall flows downwards or remains at rest. If it remains at rest, it is part of a stagnant zone and, hence, the silo is a funnel flow silo. A good position for this test is the upper region of the hopper which is close to the transition from the vertical section to the hopper. Check the flow profile along the whole silo circumference, e.g. at four points uniformly distributed along the circumference. If the silo is provided with a long outlet slit, it is also recommended to check the flow on both sides of the outlet slit (i.e. in the lower part of the hopper) at various positions along the length of the slit. With this procedure it can be checked whether the bulk solid is discharged from the entire outlet opening or only from a part of the outlet opening.

How to check the flow profile
(here: mass flow)

With this simple test you should be able to find out if funnel flow takes place, and if so, from where the funnel flow is originating. Use this information to solve the problem, or discuss the results with an experienced consultant (e.g. SCHWEDES + SCHULZE).


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Last updated on July 18, 2003.