There are several reasons why your material does not flow reliably. I hope this article helps to shed light on two of the major influences on material flowability.

Typically, there are four environmental conditions the can have an effect on your particular solid’s flow properties. They are:

Moisture Content
Typically, if a material’s moisture content increases, its cohesive strength also increases. Hygroscopic materials can experience significant increases in moisture content when exposed to humid air.

Particle Size
As particle size decreases (becomes finer) usually, your material becomes more difficult flowing.

Temperature
A bulk solid’s cohesive strength typically increases with increases in temperature. This will be discussed in detail later in this article.

Time of Storage at Rest
When material remains at rest without any movement, it can become more cohesive and difficult flowing. As with temperature above, this will be addressed later in this article.

The basis of the science of bulk solids handling developed by Dr. Andrew Jenike, is testing to determine material flow properties. By using a Jenike Shear Tester, we can simulate storage of solids in bins and hoppers. This test procedure allows us to simulate the several conditions effecting material flowability, as described above. We can adjust the sample's moisture content and particle size while the direct shear tester allows us to simulate the effects of time of storage at rest and temperature.

Our discussion here will concentrate on specifically on temperature and time of storage.

Temperature
Some materials are sensitive to the temperature at which they are handled or stored. A solid's temperature environment can affect its cohesiveness (see photo 1). For example, many chemicals and plastic powders become more difficult to handle as their temperature rises. These type materials soften or form stronger cohesive bonds as their temperature increases. Usually increases in temperature are troublesome. However, freezing temperatures can cause the individual particles of materials like coal, sand, etc. to freeze together forming very strong bonds.

Some materials exhibit more strength at constant temperature, while others gain cohesive strength as the temperature changes during heating or cooling. An example of this phenomenon is soybean meal. If soybean meal is stored at temperatures of 90° or less, it is usually not difficult flowing material. If, however, the soybean meal is allowed to heat to 120° or greater (such as during summertime conditions), it behaves completely different. At the elevated temperature, soybean meal becomes extremely cohesive and capable of bridging or arching over very large openings.

A Jenike shear tester (see photo 2) is used to measure a material’s cohesive strength. This test device has a heated base that allows the sample being tested to remain at the representative temperature.

Time of Storage at Rest
Storage at rest is responsible for many of our industries flow problems. Many materials are free flowing if handled in a continuous fashion. In other words, as the material is placed in a storage vessel, it is immediately discharged and not allowed to remain stagnant for extended periods of time. Unfortunately, it is not practical to design storage vessels for only continuous flow conditions.

Most solids are required to be stored at rest for some period of time. Large silos are expected to store quantities of material at rest for some time, to be supplied to trucks, processes, etc. Whether they are stored just overnight or for a weekend, most solids are sensitive to time of storage at rest. Cohesive bonds become stronger as the materials remain stagnant. In time, some solids can gain tremendous strength which leads to bridging or ratholing over even very large outlets.

Case Histories
While we cannot reveal who the client was in the following case histories due to confidentiality, we can discuss generalities.

Temperature
We were asked to test soybean meal at two different temperatures as the client experienced severe bridging problems after a particularly hot summer. As you will notice, the soybean has reasonable opening size requirement at 115°F. However, after a hot summer day where the temperatures in a silo can reach 130°F, major problems ensued.

The following is some of the date generated showing the significant effect of temperature increase has on the soybean’s flowability.

Material                Temperature       Time at Rest       Conical Opening to      Slot Opening to
                                                             Hours            Prevent Arching, ft     Prevent Arching, ft

 Soybean meal            115F                      0                             0.0                              0.0
                                                                72                            0.4                              0.2
                                   130F                     72                          >300’                        >`150’

These values are ridiculous, but definitely confirm temperature effects on cohesiveness.

Time of Storage at Rest
We were given the task to determine if cement could be stored in a silo for up to five years without movement. Yes, I said five years! We obviously did not run a five-year time test, but we did address the problem in this way: We ran tests to determine the effect of storage at rest on the cement after one, three, seven, and 14 days. We assumed the cement would stop gaining strength somewhere within this time frame, and we were right. Here are the test results:

Material                                     Time at Rest                      Conical Opening to                   Slot Opening to
                                                      Hours                           Prevent Arching, ft                Prevent Arching, ft

Cement                                              0                                        1.6                                            0.7
                                                         24                                       1.8                                            0.8
                                                         72                                       2.1                                            1.2  
                                                       168                                       2.6                                            1.5  
                                                       336 (14 days)                      2.6                                            1.5

Somewhere between seven and 14 days the cement stopped gaining strength and as such, we designed the silo based on this information.

In summary, both temperature and time of storage at rest can significantly affect your materials flow properties and testing is required.   

Joseph Marinelli is a consulting engineer and president of Solids Handling Technologies. He has been providing testing and consulting services since 1972. As a former consultant with Jenike & Johanson Inc., he has years of experience testing powders and designing bins and feeders for reliable flow. He lectures frequently on the topic of powder handling, and has published several papers, including an article in a chemical encyclopedia and two in a food powder book. For more information, contact Joe at 803-802-5527 or [email protected].

Joe will be presenting a number of sessions at the upcoming Powder & Bulk Solilds Conference, April 24-26 in Rosemont, IL. Click here for more information.