Dr. Rivkin,

Two possibilities come immediately to mind.

[1] Use air cannons, which can be activated manually or automatically whenever a bridge situation occurs. This should work, if bridging always occurs in the same place.

[2] Cut access ports in the each storage unit wall (requires a good seal system) to allow direct access to clear bridges.

There are other possibilities, including Rema's bladder system; the use of water; mechanical pushers and borers; and interior configuration alterations, but these are all more expensive and, in the case of water, potentially damaging to the product.

Regards, ■

Dear Dr. Rivkin,

For similar system for fertilizer storage, the hoppers mouth are often covered by steel sheet resting on grid. This prevents solidification of material in hopper, with passage of time.

The feed into hopper is done by partially clearing the material, and removing sheets from hopper mouth. Or storage is done such that hopper mouth portion is kept clear of material storage, and steel sheets are removed, to facilitate material flow by gravity or by pay loader. ■

Dear Dr. M Rivkin,

Referring to your question, I clarify as below. The method / procedure mentioned by me is often recommended / used in fertilizer plant parabolic silo of say 125 to 250 m length.

The reclaim tunnel conveyor has 5 / 10 reclaim hoppers. The material is stored such that two hoppers mouth are not buried under material (even partial opening to mouth is acceptable). It is designed such that feed from 1 or 2 hoppers meet the conveyor capacity.

During stacking operation, when stockpile extension comes near hopper mouth, it is covered by removable sheets. The feeding to open hopper can be by pay loader or by positioning the tripper discharge on stockpile sloping face, such that gravity flow goes into hopper.

It is to be noted that material is not expected to flow properly / reliably into deeply buried hopper.

This method may allow you to have actual storage as nearly 70% of gross storage.

Please think about revision in style (management) of stacking / reclaiming, and possibly it may be of real help for you. Further suggestion needs stacking / reclaiming arrangement drawing.

Try to keep material relatively live throughout the silo by shortest time interval, in a planned manner. ■

Dr M.Rivkin,

Further to my earlier messages, the other expensive (mostly not possible at this stage) solution is to have reclaim conveyor app 3.5m above floor, supported on shelf from one side of silo. The reclaiming is by bi-rail scraper-reclaimer with swiveling – luffing boom. The scraper reclaimer discharge boom always remains attached to reclaim conveyor. The centraly positioned bi-rail remains burred under material, but scraper reclaimer itself clears off material as it advances into stockpile face. The machine reclaims from exposed cross section face and hence it is free from difficulty you are facing.

The present day urea longitudinal silo uses such system instead of hopper arrangement used previously, in view of similar difficulty . The manufacturers budgetary price for above said system may be around USD one million. ■

Mr. Rivlin,

I have not found a full description of the hopper slot configuration( height, dimesions of upper and lower(1x2m) openings with respect to wall angles).

What is the slot lined with? A UHMW plastic like Tivar-88 or glass/ceramic lining is substantially better than any steel surface.

Surface morphology and chemistry of the product can give clues to its behavior and control.

These points would be helpful understanding the nature of the material consolidation and in defining a best option to relieve the arch.

Have you tried suspending the hopper and vibrating its long wall surfaces, resembling an activated bin bottom? It would also seem prudent to articulate the grate above the hopper to break the arch within the small openings.

Lawrence Nordell

Conveyor Dynamics, Inc.

nordell@conveyor-dynamics.com

www.conveyor-dynamics.com ■

Michael

Why do you say that a low friction lining is not required?

Part of the reason for bridging is the anchoring of the product on the hopper sides, forming the arch or bridge, on the hopper walls due to friction. Lowering the surface friction of the hopper side walls may reduce the arch strength to a level that inhibits its formation.

The suggestion of using Tivar or porclein or glass like ceramic should lower the friction by 50% as measured in many tests.

You can apply the Jenike Johanson formula, for hopper design, to gain understanding of the likely arch formation and modifications suggested that may yield sufficient improvement to solve your problem.

Or you could give the concept to CDI. We can analyze the behavior of the existing and the proposed change, using DEM, and simulate the problem and benefit I note above.

The 48 deg. long wall angle may be telling but its position is not clear. What is the depth of the hopper? Is the bottom opening symmetrically centered with the top opening?

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Michael,

I reread the post and appologize for missing your point that the arch or bridge is formed above the grate opening.

The bridge anchor needs to fail. Have you that about trying to break the bridge by sliding the ends of the grate across the narrow slot opening. This would require cutting the concrete floor to facilitate the articulation.

Before any such effort is attempted, you need to characterize the product strength as a function of the consolidation pressure, such as by the Jenike shear cell testing procedure.

This data can then determine the potential arch dimension that may form for the height you noted.

LK Nordell ■

Dear Mr. Rivkin,

So small a hopper opening for so great stock - pile height signifies a seriously faulty concept of reclaim-system. Hopper mouth opening certain minimum size is essential in relation to stock - pile height to avoid such problems.

My judgement suggest a ratio around 1:3, steel hopper with UHMW liners and hopper wall vibrators (electro-magnetic or mechanical unbalanced) would have averted the problem. But now it is fait accompli. I suggest two things:

1. Please email me the sketch showing silo, stock - pile, hopper, conveyor and tunnel. I will see if I can be of help to you.

2. I remember one company in Australia who makes mechanical agitator / shaker for material around hopper mouth. It is heavy / sturdy device and they say it works under coal pile of 15m height, to ensure proper flow. Now, whether existing arrangement / civil work can accommodate and take load is a doubtful issue. In the past I had obtain their quotation, but at the moment I do not remember name. However, concerned people may respond from Australia, reading this message. ■

Dear Dr. Rivkin,

It is not clear from the description whether the material is forming an arch on the 250 x 250mm openings of the grating or forming a deep span arch 1 x 2 meter above the grating. The former would appear to be most likely if the hopper beneath empties without a problem. A possible improvement may be given by changing the grid construction to a ‘stepped level’ grill arrangement for the cross rails and dressing the top edges of the bars to a sharp edge to offer less support for the material.

The ‘Stepped level’ grill system is to removed the square opening grid and fit two levels of cross rail grills that are at 90 degrees to each other, and has clearance between the two grills in the vertical direction. Sticking to the same plan size of open spacing, this means that the top layer offers longitudinal slots 250mm wide x 1000mm long. The lower layer will be 2000mm long, but is shielded from direct overpressures by the upper layer. The immediate benefit of such an arrangement is that the flow characteristics of the top slotted openings would be equivalent to square openings almost 500mm across. However, the lower layer is dealing with product that is subjected to less compacting pressure and is looser by virtue of flowing through the upper grill. It may therefore be a better option to increase the spacing of the top grill, to say 400mm, and reduce the spacing of the lower grill, to say 200mm, to give a somewhat similar protection as the 250 square grid against the passage of lumps. Together with the sharp top edges, this type of grid will offer far less support for arches to form on the grill bars.

If the material clears from immediately above the grill, but forms a deep arch over the 1 x 2 Metre opening, there is a more difficult problem. Even if the arch can be collapsed, it is probable that a ‘pipe’ would develop to the surface of the pipe. The divergence of a pipe from the outlet hole may be expected to be negligible with a condition of product that has formed an arch. It would therefore still be difficult to clear the remaining bulk by means of power shovels because of the deep and nom. 8 metre wide bed of material left between the outside of the pile and the openings. Before attempting to address this problem I suggest the above situation be considered first, to establish whether it offers a viable solution.

with b est regards,

Lyn Bates ■

Just a thought, and this may not be relevant, but is it at all possible to stretch a steel cable across the main diameter of the floor of the silo in such a manner that it can rotate through 180 degrees. This movement should undercut the footings of the bridges from my understanding of the description.

Implementation might require some thought but it might be a cheap option if this addresses the root of the problem.

Regards

Steve Davis

Hatch Associates ■

Dear Mr. Michael Rivkin,

In my earlier message, I had mentioned that some companies manufacture vibrating device on the feed inlet / hopper mouth.

Please refer the message by Silexport inter in the thread - Bulk Coal Discharge - in the New Projects, Tenders, Inquiries Forum.

Possibly, this may be of interest & help to you.

Regards,

Ishwar G Mulani

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Email : parimul@pn2.vsnl.net.in ■

Hi,

Send me some of your sketches explaining the problem. May be i may contribute.

Chhabra HN ■

Bridging Problem

Potash being of similar characteristics to that of salt (which I am more familiar with) is very hygrosopic.

Do you notice the problem more during high humidity periods?

The bridging is most likely caused by the level of moisture in your product causing binding of the crystals to eventually form a bridge.

We experience similar problems with salt and found that the ventilation of the buildings by means of simple agricultural style fans to reduce humidity in the dome buildings works quite well. You would need to size the fans to move enough air to change the air in the building over a 10-12 hour period.

Hope this helps.

Gary Blenkhorn ■

Dear Dr. Rivkin,

Hope you have found a solution. In case, yes fine otherwise i think i have got a solution. in the meantime, i just want to be sure of your problem and the amount of trouble it is creating for you.

Apart from the above, I just want to know how much money you have budgeted in mind to solve this problem. As the solution I have in mind is simple, practical. But one need to know that is your silo roof is strong enought to withstand a load of 30 kN approximate at each point where the bridging formation is taking place.

In my opinion, the air canons / blasters function very nice in case these are installed at the periphery of the silo or the hopper. In such case, the effectiveness of the aircannon is most as it minimises the adhessive forces acting between the material and the walls of the silo or hopper or chute.

In a case like yours, you have to use mechanical means.

I am sure i have a solution in my mind.

What Reward do you offer, in case the solution is a real solution. LOL.

In the mean time please send me a sketch showing the overall height of the bridge chimney. You have mentioned that the base of the bridge is 1 m x 2 m am I right ? Yes, then let me know the maximum and the minimum height of the chimney ( what ever you call this space under this bridge.) . On the basis of this information, I will send you a sketch showing the arrangement to break in this problem.

It is in a very preliminar stage, but I am sure it will meet your purpose.

In the mean time, please be informed that in the storage silos of Cement, Rawmeal for Cement Plants, Fly ash and Grould Slag this kind of phenomenon do take place and we take care of the same.

Apartt from the above, in the above menetioned silos, the material cakes formation take place on the silo walls and we have mechanism to clean those.

On the same pricipal, i will devise a mechanism for breaking the bridging formation in your dome type silo.

In case, you are interested and the reward which you are gonna offer me are attractive, the 100% effective solution is yours.

Please donot hesitate to contact me over phone.

You may find me a little blunt but "me is what I is." a straight person.

Looking forward to hear from your end.

Wishing you Good Luck.

Regards.

HN CHhabra ■

Dear Dr. Rivkin,

It is sad to note the way, you intend to close the matter.

You are not the Operating Manager !!!!

You don't want to use the trial and error method, Fine.

Now give 2 minutes to the thought mentioned below. Analyse it. In case it appeals to you please let me know. I may refine the solution.

You and all our colleagues who posted their advice or soltions on the net are capable and smart engineers. But the fact remains, no solution is found so far.

My proposal is a little crude but I am sure it will work.

The solution is as hereunder:

Please install a wire rope of 60 / 63 mm diameter ( twisted Steel Wire rope from the top of the silo roof). The length of the rope shall be such that it always remain in the area where the cavity forms after the bridging formation. The one end of the rope is connected to the output shaft of a vertically down output shaft type gearbox. The gearbox is connected to a motor of may be 11 kw variable speed type AC motor having maximum rpm as 3000.

At the other end of the shaft connect a cross or star. The length of each arm from the center of star or cross is 500 mm approximately. Connect a rope of 22 mm ( twisted Wire rope) of length approximate 1000 mm. At the free end of this rope attach 10-15 kgs steel ball or any jarred edge ball / box size 300 mm dia or face width. Whenever the bridging formation is likely to take place or have taken place, Run the 11 kW motor, increase the speed of motor from 300 rpm to 3000 rpm slowly say in 3 minutes time.

Because of the high speed of the star, the wire ropes attached at the end of the star or cross will try to straighten due to the centrifugal forces.

Now, the starightened ropes and the ball will hit the material and break the material.

Of course, the solution suggested is not based on a study. But I am sure it will work.

In case, you can let me know the plant address, I don't mind contacting the plant operating manager and would like to have the feed back from him on this issue.

If it appeals to you also wonderful.

With Kind Regards.

Chhabra HN

PS : Youcan send me a reply on my personal email chhabrahn@yahoo.com.

My Telephone numbers are 00 91 9810390819 or 01 91 124 2306370 after 20.00 hours and before 08.00 as per Indian Standard Time. ■

Hi all,

I wasn't going to add more to this, but after Mr Chhabra's interesting suggestion and Mr Rivkin's note that the silo is in fact rectangular, I note that to drag a rope, cable or beam attached to rails at the toes of the long sides of the silo along the length of the silo to undercut the bridges would be relatively easy. This feature would of course be parked at one end to allow access for payloaders during normal operation, or could be removed or lifted clear.

Again, basic engineering, no proof available, but it could not take much force to undercut a pile of potash that has compacted under it's own weight. ■

Dear Michael,

So, One year or so on, how have U gone and solved the sever bridging problem.??

We would be interested to learn

Cheers

James Morrish ■

eager to know if you could implement the idea and what have been your observations.

Regards ■

Dear Sir your problem has an easy fix, in this miners opinion.

what you need to do is purchase a small slusher hoist from a mining equipment supplier and purchase a large slusher bucket to go along with it.

The beauty of it is you could set up the small slusher in one or more spots inside the dome and then have a huge radius of points to anchor the sheave wheel which allows the cable to run back and forth with forward and reverse motion of the wire rope alternately riding over the piles of material and scooping them up and dragging them toward the draw point.

A very small double drum slusher with over-under cable set up with fairlead rollers will solve your problem very nicely with little set up time and no major rebuilding of systems. Also the slusher/tugger hoists are designed to be portable as well so you could use one slusher for your entire operation/ I would buy an electric powered unit rather than an air powered unit due to the corrosive atmospere in the dome itself. these machines work very efficiently and quickly moving material as well.

The big thing you must always keep in mind is that you do not want people near those draw points as they will fail without any notice-I will testify to this because I lost a very close friend due to this same type of situation 22 years ago in a salt mine where I used to work.

Putting your problem in algebraic terms using the X-Y plane

the slusher itself could be set up on 0, -12 and the sheave for the cable could be mounted any where from -12,-5 to 12,-5

with almost complete coverage inside each storage dome.

lzaharis

"Semper Excellsior" ■