Frequently Asked Questions
by Joseph Marinelli

Having been a consultant for many years I have obviously been asked some of the same questions concerning solids flow, time and time again. Here are some of the most frequently asked questions and typical answers:

1)  When I modify my bin to ensure mass flow, say by lining the hopper section with 2B stainless steel, should the cylinder section be lined also? 

The answer is no! In fact, the cylinder section should have a rougher surface. The reason is that the rough surface will take up some of the solids load in shear along the walls. This will reduce the loads on the mass flow hopper below.

2)  I need the benefits of 2B stainless steel to promote mass flow; however, my hopper is large and requires plate thickness steel to withstand the solids loading. Can I use 2B stainless steel plate for the hopper surface?

The answer is no! 2B stainless steel is only available in sheet thickness. The are vendors who supply 2B stainless steel sheet in 7 gauge thickness. You may have to fabricate your hopper, say out of carbon steel and then line it with 2B stainless steel sheet.

3)  I want to handle gypsum in my bin. 

All gypsum is the same so that my consultant can rely on his experience with gypsum to design my bin. Unfortunately, although we have worked with gypsum many times, not all gypsum is the same. There are product differences from vendor to vendor. Solids are also effected by many variables such as: moisture content, particle size, temperature, time of storage at rest, bin geometry, etc. It would be much easier to use previously generated flow data to design from; however, we highly recommend running flow properties tests to evaluate just how your gypsum will flow. Keep in mind also, that your gypsum may be easier flowing than others, and may not require as conservative a design. You will not know this until you test your product.

4)  How much sample is required for testing? 

Obviously, this is dependent on the amount of the testing that is required to simulate your process, environmental conditions, etc. Typically, a volume equivalent to about five (5) gallons of product is required. This is so that we can use fresh material for each test. Reusing product may effect test accuracy. Sometimes, there is only a small amount of material available or perhaps the material is extremely expensive. In this case, you should check with your testing laboratory for the minimum amount of product required for testing. For instance, tests can be run using small test cells, which reduces the amount of material required to conduct a full range of tests.

5)  My feeder design is not critical, so I will attach any type of feeder to my mass flow bin, and it should still flow in mass flow, right?

 WRONG! The feeder design is as important as the bin design. An improperly designed feeder may draw your product preferentially from the bin, converting it from mass flow to funnel flow. You have wasted your money modifying your bin. The feeder must be designed to discharger your product over the entire cross-sectional area of your outlet. If you take advantage of the benefits of a wedge shaped hopper configuration, you will likely use a screw or belt to control discharge rate. This feeder must increase in capacity in the discharge direction, thereby pulling from the entire outlet cross-section.

If you have questions that you would like answered, please email me and I will try to answer them or at the very least get an answer for you.

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.