Second Problem
I missed in my thread before this one:
Hopper is completely excentric. Hopper dimensions are (mm)plan top 2200x2200 , height 3300, opening 400x400 located in a corner, so two sides of the hopper are vertical walls and the other two inclined. I want to calculate the wall normal loads starting with an axisimmetric silo, and applying a factor (is there any other way for a non-PhD?). It is obvious for me that most deformation energy must be applied by the inclined wall, that must act to deform and laterally push the material, but thinking in the horizontal equilibrium of a horizontal material slice, horizontal component of the wall forces resultant on each side should not be too different since there are no net horizontal external forces apllied to the disk, other than inertia. F=ma horiz could be all. From an energy reasoning; the energy required to horizontaly squeeze the material from 2200x2200 to 400x400 in a 3300 height should be about the same regardles of the plan location of the opening, so that if I calculate the wall forces for a centered opening of the same dimensions, all I have to do next is to determine how much of the force on one side should I reassign from the vertical side to the opposite inclined side of the hopper.
Any help on my reasoning sequence? ■
Wall Pressures
I’m afraid that your reasoning is a bit too simplistic. For a start, the slope of the inclined walls is just above 62 degrees to the horizontal. This means that the gully angle between the inclined walls is about 52 degrees, which is much to shallow for mass flow with virtually all bulk materials. Some regions will have static product against the wall and in other areas slip will take place, but it is very uncertain where the transition will occur in either the vertical or horizontal plane. The contact surface conditions with therefore be a very mixed bag of active and passive stresses, differing under fill conditions according to the loading procedure and also during discharge, depending on the rate of flow and the flow regime that develops within the bed.
Then there is the ambient gas to consider for normal and any exceptional circumstances. Is the hopper receiving from a pneumatic or pressurised system, what facilities are there for venting during fill and discharge. Is there explosive risk requiring containment, suppression or venting?
The subject is complex, but does not require a PhD to understand the basics. I suggest that you see www.ERPT/flowregimes and read Prof. Rotter’s book – ‘Guide to the Design of Circular, Metal Silos’, for an introduction to the subject. ■
Wall Friction
An inclined plate test can be conducted to secure a crude measurement of wall friction. The presence of oil on the pellets will generate some surface cohesion, but this can be allowed for as the contact pressures will tend to mask these and a gully test can be used to replicate the self clearing conditions if the hopper is not of mass flow design.
Likewise, simple practical tests with the materials descibed, should be quite adequate to determine whether the opening size is adequate, particularly if a much smaller size is used for the test than will apply to the full scale containe.
However, in general the cost of a friction tester and the simplicity of use in relation to the immense importance of this measurement in many aspects of bulk solids storage and handling, should make this instrument a vital part of anyone contemplating equipment design in this field. ■
Re: Typical Internal Friction Angle Ranges
www.ERPT/flowregimes
The above link appears dead, it gives a "page not found" error. ■
Untitled
Originally posted by designer
www.ERPT/flowregimes
The above link appears dead, it gives a "page not found" error.
Try this one:
http://www.erpt.org/992Q/tard-07.htm
I swear 90 percent of powders that I've measured have had effective angles of internal friction near 40°. Wall friction depends on the powder and the wall material and usually decreases with consolidation pressure. Use the Physics 101 methods for measuring friction coefficients, take the inverse tangent of the result, and then use the chart to see if you should expect mass or funnel flow. Good luck! ■
How To Measure Friction Angle
Jorge,
Maybe it is too late, but take a small sample of your product, put it in a 4" Tupperware container, a can, or any other type of, very light, very thin, wall container.
Take a piece of metal (a plate, or any other section of the vessel metal, fix the piece of metal in a drafting table. Locate the container upside down on the drafting table on top of the metal, so that the solid will touch the metalk, avoid touching the metal with the sides of the container.
Increase the slope of the drafting table 5 degrees. Wait.
Increase the slope of the drafting table another 5 degrees. When the container starts moving you have the unconfined friction coefficient.
You can start working from there.
If you have more time, attach a cable (nylong line or similar), to the container. Attach 2 pulleys to the drafting table. Install 2 buckets, one on each side of the drafting table. F
Fill one of the buckets with sand until the sample starts moving.
You will have the unconfined friciton angle. Put a weight on top of your Tupperware, repeat operation. Calculate the fricition angle.
Build up your yield locus with this data,
Youc an use Jenicke and Johansen equations from there on to calculate the arching point.
The question ere is: Do you want this silo to be mass flow, or funnel flow?
Use the same procedure to calculate the internal friction angle.
Good luck!
Marco A. Flores ■
Untitled
Nice one Marco. I use the top of a plastic drinks cup. ■
Je Je Je
I have used coca cola bottle metal caps sometimes,
whatever is available .
I learned this fom Jerry Johansen himself few persons know more on solids flow props measurement than him.
We owe him much .... ■
Typical Internal Friction Angle Ranges
Dear Friends: I am in charge of the design of three steel silos for a small process plant for chicken food. I have designed concrete silos for portland cement but no steel silos for this kind of material. I face two problems:
First Problem: Internal friction angle curve, and friction with walls. The owner is in a hurry and does not want to take the time to make the tests, since they are not available in this city. They need the design too soon for waiting. He has asked me to find an estimation of the values. Angle of repose is no big deal, but shear tests and wall friction tests are beyond my reach in this circumstances. I would need to have some typical values ranges so I can take the safe side. I dont want to make guessing without any reference, and goint too safe and overdesigning.
The materials to be handled are:
Case 1: ground corn 1mm particle size, 12% humid. Carbon steel walls.
Case 2 : corn pellets, 4mm dia x 15mm long, carbon steel walls
Case 3: same pellets but with oil added . Stainless steel walls
Would someone please hand me some values or a table from where to make an estimation? Please don refer me to books or codes, because my time span is too short.
You can email me at jorgeindacochea@gmail.com
Thanks a lot for your help ■