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Thread: Problems with Emptying a Silo

  1. #1
    Join Date
    Jan 04

    Help a dummy

    First of all, hello everyone very nice board you got here.

    I'm really new to that bulk material handling thing, I have a formation in industrial engineering but I was shipped to finding solutions in an area I barely know because I'm the «new guy» and this is pretty messy work.

    My problem is :

    I have a 40 tons silo filled with cement dust, rock powder, small rocks (very small) pretty much everything sucked by the cyclone in the dryer is sent to that silo.

    At the bottom of the silo there is an inversed «Y» shaped division. One side goes straigth down to a hopper (in case of conveyor failure) and the other side dumps on a 15 degrees inclined 13 ft long belt conveyor (depending on which one is selected) The belt is 18 inches wide but the «flat» area is about 14 inches wide. That conveyor travels at 275 ft/min. A slide gate covers an 8''X8'' opening just over the conveyor.

    With this setup, only 29 metric tons/hour of material is taken out of the silo which is waaaaayyyyyyyy too slow since it is supposed to dump in a truck (90 tons /hour)via another conveyor which capacity is correct. (120 metric tons/h).

    My options are the following:

    - get rid of the 15 deg. angle and place the conveyor horizontally right under the bottom of the silo. That would also get rid of the Y separator hopping for a better volume per hour. (management idea)

    -instal a screw conveyor of appropriate diameter and keep the Y separator.

    - keep it as it is and find a magical way to make it spit more.

    Somehow i have a feeling that it might be the 8X8 opening that doesnt give enough material flow. How am I suppose to calculate the theorical capacity of the set up? i tought of an old formula i ve learned in school long ago which is Flow=speed X section, however after a quick math session i discovered that it might not be appropriate for anything other than a liquid.

    Anyone can point me in some direction or recommend me a book on that subject it would be greatly appreciated.


  2. I'm going to risk making a fool of myself...
    The 8x8 opening should be good for the 18" wide conveyor.
    Witout knowing the density of the material is hard to say if you can get 90 tons per hour.
    I would make a small manifold with four or five nozzles from 1/4 pipe, able to apply a pressure of 5-10 PSI.
    By inserting the nozzle arrangement inside about 1 foot from the opening I would try to fluidize the system somewhat.
    If it improves the flow than you can find a supplier of proper gadgets for that purpose.

    Good luck,

    Antonio Reis
    Vitrom Mfg Consultants
    Your Process and Manufacturing Solutions
    Phone: 209.834.1900
    Fax: 209.834.1039

  3. #3

    Firstly could you clarify. It sounds like the problem is restricted flow from the silo.

    Are you getting consistant flow from the silo?
    Does the flow stop at anytime while discharging to the conveyor?

    Does the flow increase on the by-pass chute going to the hopper?

    Your 18" conveyor has more than enough capacity for 90 MTPH as well as the 8x8 opening.

    The problem may be in the angle of the chute to the conveyor. Improperly designed chutework can greatly restrict flow from a storage bin or silo. The minimum angle should be 45 degrees but the steeper it is the better for flow. Further complications can occur if the direction of flow changes as it loads onto the conveyor.

    Look for any material build up in the chute or foregn material which may be restricting flow.

    Managements idea of moving the conveyor directly under the silo may warrant further investigation.

    You could try less expensive approaches first before you get into major expense by installing a vibrator on the chute to the conveyor to assist the flow.

    A formula for designing the chute can be found at the following link.

    Good luck,

    Gary Blenkhorn

  4. #4
    Join Date
    Jan 04
    Thank you very much for your time and replies.

    First the material is really dry and has a density of a 100 lbs/Cf.

    To answer your question Gary:

    -The flow coming from the silo looks consistant, i didnt notice major variations while I was watching it.

    -I did not check the flow going to the hopper. The hopper is about 15 feet under the Y separator and is connected to it using a rubber tubing which dimension I have to verify.

    It is possible the angle of the chute is not steep enough, i will have to take measurement on it since no drawings are available and I will look at potential build up or restriction.

    So adding a vibrator or an air output to it should «fluidized» my material and makes it easier to go through the chute onto the conveyor?

    Sorry for the bad spelling, I'm home sick and it feels my head is going to explode.

    have a nice day


  5. #5

    Pictures say a thousand words. If you can get your hands on a digital camera take a picture of the chute and attach it to your next reply. This would give us a better understanding of your layout.



  6. #6
    Join Date
    Jan 04
    Good day,

    I followed Gary's advice and took some pictures of the current set up. I hope you will not find the pictures too brutal.

    conveyor and chute (close up)

    Conveyor is on the right

  7. Looking at the pics, you may have to fluidize at the top before the valve.
    Be careful so that you don't loose control of the flow.

    Antonio Reis
    Vitrom Mfg Consultants
    Your Process and Manufacturing Solutions
    Phone: 209.834.1900
    Fax: 209.834.1039

  8. #8
    Join Date
    Mar 03
    Looking at the photo's I wonder -
    Is that a diverter valve under the hopper cutoff slide valve?

    If it is and the inlet to it is 8" x 8", then my gut feel is that the flow of your material under gravity conditions is likely to be sluggish at best.

    If you try to add air to improve the flow be cautious. People have been known to end up with most of a bins contents in a big heap on the floor very quickly by adding too much!

    IMO a better bet would be to enlarge the bin outlet and stick with gravity flow, adding a feeder to control the rate.

  9. #9

    Sorry I didn't get back to you sooner. (Good pics)
    It also appears that there is a cut off gate with a cylinder on the chute to the belt. This may be causing part of the flow problem.

    The closer that you can get this diverter valve to the silo the less chances of material buildup.

    It appears that you have enough room to extend the tail pulley to allow the chute to drop straight onto the belt. Replace the diverter chute below the cut off gate with a new straight down and 45 off chute for bypass. (see photo)

    There are many manufacturers of this style of diverter gate in Canada and the USA.

    Attached Images Attached Images  



    Looking at your problem, U have a nice one. The issue is what can U do on a sliding scale of minor mods vs costs vs effect.

    The transfer to the belt does not look good.....U say cement dust, the plant looks clean...

    U will need to be carefull about overfilling the belt.

    My suggestions are:

    Observe the flow behaviour in the chute feeding the belt...look at the belt and also open the hatches....see what is happening and then try to assess/determine why it is behaving like it is.

    Is is surging out, is is smooth and even, is it full chute flow at the top of the Y chute etc...

    If U get material coming out thru the upper door then the lower part of the chute is the issue.

    Be carefull with being able to stop the flow quikcly if U need to.

    Look at how the material is loading onto the belt,

    What is the rock size...chutes should be at least 5 times the largest length...

    From first observation, I think that your bin outlet is not big enough to sustain adeqaute free flowing rates...

    The system as it is built looks to be designed to act as a choke feeder - that is the material fills up the chute and loading skirts 100% and the belt draws out the material from the skirts. The rate of transfer is then dependent upon the belt speed and the cross sectional area on the belt.

    Hope that this helps U.

    James Morrish

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