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Ever had a problem getting powders or other bulk solids to flow out of a bin or silo?  Sure, it's a common enough problem.  Material flow problems can create major headaches and cost your company in lost production, increased labor and maintenance as well as opportunity losses associated with diminished throughput.  In addition, you can have lost product due to caking and even vessel failures as a result of flow problems.  This all means lost money, profit, etc.  That's why it's important.

The Cause
When powders are placed in a bin or storage silo it enters the vessel in a 2-phase form as a mixture of particulate and air.  Often powders are conveyed pneumatically, introducing additional air.  Flow problems will occur as the air entrained in the 2-phase mixture is expelled and the particulate packs together.  The degree of flow problem can be dependent on many factors including the material agglomeration factors and coefficients of friction.

Once entrained air escapes and the powder packs, a number of conditions can be created.  These include ratholing, bridging and flooding problems.  The illustrations here show what a rathole or bridge may look like.  Flooding is uncontrollable material flow, usually resulting from a collapsing rathole.

Solutions
Like many other things, there is "more than one way to skin the cat" (where did that expression come from?..... click here to find out ).  First, and most obvious to me is to eliminate the cause with something you can control, and do it before you have a problem.  What I am getting at is the bin or silo design.  Jennike & Johansen are experts in this field so I'll let them do the talk and just provide a link to a PDF document for more information on mass flow bin/silo design.  But don't wait until you have a problem to solve.  Consider this before you decide on what silo to buy and use.

However, most problems are existing which means the bin/silo is already in place and you can't get the powder out properly.  You need a solution quickly.  The next alternative, and not necessarily the best, is to use an external source of vibration to shake everything up so the material flows.  Does this work?  Yes, but....... it can create problems too.  Vibrators (Cougar, NAVCO, VIBCO and others) can actually pack the material by shaking it and removing remaining air.  Vibrators are typically only used when the flow of material is required.  The selection and application of an appropriate vibrator should be carefully considered and reputable companies should be consulted.

Another solution is the use of aeration devices to fluidize the powder (adding air) creating a flowable state, thereby initiating material flow.  There is a wide variety of aerating devices that come in all shapes and sizes.  The proper selection, quantity and location must be done based on experience in order to produce the desired results.  Aerating devices add entrained air back into the material creating the natural 2-phase mixture allowing it to flow.  This is successfully used on many types of powders and the aerating device itself is relatively inexpensive ($15 and up per each).  Multiple aerators will be required.  The aerators are typically in operation full time, unlike vibrators.  One example is a cement silo where 8-12 aerators are used in the cone section to promote material flow.  Each of these aerators cost about $20.  In addition, dry plant air is required along with all of the piping.  For more information on using bin aeration devices to solve flow problems, click on the image to the left.


Joe Lewis
Vice President - Marketing & Sales
Monitor Technologies LLC
jlewis@monitortech.com
www.monitortech.com
www.flexar.info

Comments

Posted by: Navneeth at October 16, 2008 09:57 PM

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