Discharge Bins & Silos

 

 

Bulk materials stored in bins and silos frequently do run out of the open flap extremely slowly or not at all.

For this are two reasons responsible:
• It’s a feature of the material to stick to the walls of the bins resp. silos and is possibly sticking together itself.

• The material flows particulary easy with the same speed along the walls of a symetric designed bin resp. silo in direction to the flowing-out opening. The various material flows do block each other and form a material bridge.

 

Proposels for improvements of the flowing out of the materials without vibrators:
• Execution of walls of the bins and silos with different inclinings
• Smooth coating of two walls of the bin which are located side by side
• Welding on of restraining guide rails of two walls which are located side by side.

The purpose of the above described measures is to create a different speed for the materials which flow in direction to the flowing-out hole of a silo. The most efficient solution is to fix an external vibrator to one of the walls of a silo resp. bin.

The vibrator does create an oscillating movement to the silo wall which does transfer the vibration into the bulk material. The forced vibration has the effect that the adhesion of the particles of the bulk material is temporarily reduced to a minimum. Thus the lack of static friction is forcing the material which is located next to the vibrated wall with an increased speed in direction to the flowing-out opening.

 

 

Hints for selection and installation of vibrators:

If the vibrator is in operation the transfer of vibration via the wall of the bin will create a temporarily deformation of the wall. The bulk material will be in an oscillating movement.

Bins with a quadratic or rectangular cross-section have close to the flowing-out opening trapezoidal areas which are easy to vibrate (temporary deformation of the sheet metal walls).

The following sketches are meant to help with the decision which type of vibrator either a rotational or a pendulum vibrator should be installed.

This sketch does show the deforming effect of a rotational vibrator on the sheet metal wall of a bin resp. silo.

This sketch shows the deforming effect of a pendulum vibrator on the wall of a bin resp. silo.

The comparison of the deforming effect of a rotational and a pendulum vibrator on a binwall made out of sheet metal shows, that the rotational vibrator is more efficient concerning the deforming movement of the bin wall.

In case of the use of expensive sheet metal f.i. stainless steel sheet a pendulum vibrator is recommended because of the more gentle impact on the sheet metal wall.

For the vibration of bins and silos vibrators with the following feature should be used exclusively:

• approxm. 3000 rpm corresponding to a vibration frequency of 50 Hz at a power frequency of 50 Hz
or
• approxm. 3600 rpm corresponding to a vibration frequency of 60 Hz at a power frequency of 60 Hz

In case that the required centrifugal force should be achieved by means of a lower vibration frequency a bigger rotating unbalanced mass is necessary. But this method will lead to a bigger amplitude and deformation of the binwall and is therefore to be used with caution.

 

 

Mounting of vibrators on bins with a rectangular cross section

A KNAUER ENGINEERING external vibrator will be mounted as shown in the opposite sketch at the lower part of the sheet metal wall of the bin.
In order to achieve a reliable fastening of the vibrator a round reinforcement sheet should be welded on the bin wall.

The opposite sketch does show, that a squared reinforcement plate is not suitable for mounting to the wall of the bin, because the forces created by the vibrator which do raise the edges of the plate and could tear up the bin wall.

 

In contrast to that a round reinforcement plate as shown by the sketch does distribute the stress evenly on the weldment joints.

 

It could happen that the vibrator and bin resp. silo have an identical frequency. Bin and vibrator reach the resonance state which will result in fatique fractures at the bin. The vibrator shows an extreme drawing of current which is not allowable.

An extreme drawing of current is a reliable hint that the vibrator and bin resonance.

Normally the alteration of the vibrator is not possible. Therefore the resonate frequency of the bin resp. silo has to be adjusted by alteration of the construction of the bin. This can be done by welding on flat steel bars above and below the vibrator as shown by the opposite sketch. 

The thickness of the sheet metal of the binwall is an important criteria for the decision which type of vibrator is suitable - either a rotational or a pendulum vibrator.
The following table shows the application of suitable vibrators depending from the thickness of the sheet metal of various sizes of bins resp. silos. 

   

Height of Bin
h in mm
from - to

Thickness of
Metal Sheet
in mm

Pendulum
Vibrator
Type

Rotational
Vibrator
Type

a mm

g  M

l mm

d mm

D mm

300 - 700

3

A 80/200

 

80

M10

80

12

210

 

RZ 15 K

65

M12

140

700 - 1200

4 - 6

A 200 / 600

 

110

M16

110

20

245

 

RZ 30 K

90

M12

125

12

1200 - 2000

7 - 8

A 200 / 600

 

110

M16

110

20

 

RZ 65 K

90

M12

154

12

2500 - 4000

9 - 12

A 700 / 1700

 

160

M24

160

20

350

 

RZ 120 K

100

M16

180

4000 und größer

12

A 700 / 1700

 

160

M24

160

 

RZ 165 K

100

M16

200

  

 

 

Mounting of vibrators on bins with circular cross-section

The lower part of a circular bin resp. silo does form a cone.

Circular object like cylinder and cones are hard to deform, especially objects with a small diameter.

Application of a pendulum vibrator is in this case not advantageous, circular cross-sections request the application of a rotational vibrator.

The vibrator for circular bins will be mounted on a U-shaped steel profile which is welded to the bin wall.

Thus an even mounting base for the vibrator will be achieved.

 

The height of U-shaped steel (h) profile is depending from the length of the footing of the vibrator (b) resp. the distance (e) of the fastening holes of the vibrator.

The length of the U-profile (z) should be 3 times bigger than the width of the footing of the vibrator.

 

The size of the vibrators is depending from the thickness of the sheet metal of the silo wall and is listed in the following table. 

Silo Ø mm
from - to

Sheet metal thickness
in mm

External vibrator
type

a mm

g M

l mm

m mm

- 1200

3 - 4

RZ 15 K

65

M12

140

160

1200 - 1500

5

RZ 30 K

90

M12

125

162

1500 - 2000

6

RZ 65 K

90

M12

154

186

2000 - 2500

8 - 10

RZ 120 K

100

M16

180

220

2500 - 4000

10

RZ 165 K

100

M16

200

240

The sizes of the vibrators are related to the recommended wall thickness of the silos considering the allowed maximum centrifugal forces which do not destroy or demage the silos.

The necessary centrifugal forces are also determined by the ability of the bulk material to flow slowly or faster. In order to reduce the stress applied by the centrifugal forces to the silo resp. bins it is necessary to operate with the lowest possible centrifugal force, which just does create a flow of the material.

Caution:
In case that a silo is closed by a cover vibration does compact the bulk material and the material flow is more difficult or impossible.

In order to optimise the discharge of bulk material out of bins and silos it's possible to synchronise the start of the driving motors of transport- and discharge devices like:  

• Vibroconveyors
• Screwconveyors
• Cellular wheel conveyors
• Conveying belts with the switch - on/off vibrators

It is advantageous to operate the vibrator which is connected to the discharge and transport device (f.i. a belt conveyor) only if the material flow is interrupted. A limit switch will be activated by a seesaw which is located in the middle of the flowing-out hole (see sketch). 

The distribution box for the vibrator and the transport device should be located outside of the area which is influenced by the vibration. The vibrator has to be connected to the distribution box by a flexible feed wire.

Application of vibrators for emptying of big silos and bins made out of concrete, masonry walls or extremely solid sheet metal, which do resist any deformation.

 

 

Application of vibrators for emptying of big silos and bins

made out of concrete, masonry or extremely solid sheet metal, which does resist any deformation.

At bins with trapezoid wall surface near to the flowing-out hole a steel plate acting as a membrane will be fixed via rubber buffers paralell to the binwall.
From the centre of steelplate a steel pipe goes through a hole in the binwall to the outer wall of the bin. A pendulum vibrator located at the end of the pipe does vibrate the steelplate.
Thus vibration will be transferred into the bulk material.

The effect is the same as for bins made out of sheet metal which can be deformed. The selection of the size of the vibrators is determined by the thickness of the sheet metal of the steel plate acting as membrane. For selection of the appropriate vibrator size see also the above table.

At silos where the application of a steelplate acting as membrane is not possible (f.i. at circular silos) the use of a pyramidal displacer which is located above the flowing-out hole is recommended.

A pendulum vibrator does activate the displacer which is moving within the bulk material by means of a slide shaft activated from above.

• This method is applicable for a siloheight of approxm. 2,5 meter.

• For silos higher than 2,5 meter the operation of the slide shaft from the side as shown in the opposite sketch is recommended.

In this case the displacer is hanging at a steelrope fixed to the upper part of the silo. 

 

Displacer are normally only used for silos with bigger dimensions. The approximate measurements of those displacers see opposite sketch:

  • The recommended thickness of sheetmetal for the displacer is 12 mm

  • Type of pendulum vibrator: A 700/1700

 

Displacer are normally only used for silos with bigger dimensions.

  

 

 

Mounting of vibrators on transportable bins (bin pallets)

Especially in the chemical industry for bulk materials transportable bins are employed for the internal transport.

In order to use the available capacity of bins as good as possible the bulk material should be compacted by means of vibrators at the filling station.

At the discharge station the material-flow shall be accelerated and the compacted material shall at the same time become loose by means of vibration.

Due to the relatively big number of transportable bin in a chemical plant it is not feasible to attach a vibrator to every bin. The solution is the use of a vibratory station as shown by the opposite sketch. 

Such a station is consisting of steel profiles executed as spring suspension equipped with a vibrator on one side or also two sides of the vibratory station if necessary.

The bins will be placed on the vibratory station which is located at the filling resp. discharge station. This station is acting like a vibratory plank.