Measuring the level of solids material can be difficult, if the level being measured is changing or the powder or other bulk-solid material being measured doesn’t flow easily through a bin. When dealing with powder and bulk solids in bins, some materials won’t flow very well, producing a “rat hole” or “bridge” that can be problematic for level-control or inventory-monitoring when using level-measuring devices.
A “rat hole” forms when during vessel discharge material flows through the center of the material in the vessel. A “bridge” exists when flow suddenly stops as material forms a bridge, usually just above the discharge opening. In these cases, solve the root cause first and then apply the most appropriate level-sensing device for the material and application.
Why are some bulk-solid materials difficult to discharge from their container? According to the well-known expert Dietmar Schulze, in his paper titled “Flow Properties of Powder Bulk Solids,” the flow characteristics of a bulk solid material is dependent upon several factors, including particle shape, particle-size distribution within the vessel, moisture, temperature and chemical composition. These factors impact adhesion between the material particles and other materials, such as the vessel walls, as well as cohesion, the attraction between the material particles themselves.
Fully grasping the factors involved for any specific bulk-solid material is difficult, if not impossible. Even the awareness of a potential problem prior to vessel selection or design is not common. And that’s really why these problems persist, the flow characteristics of a bulk-solid material are not adequately known prior to the selection or design of the vessel to be used with it.
Why this is a problem
Level measurement and detection devices for solids fall into two categories, those for point-level measurement and those for measuring vessel contents on a continuous or regular periodic basis. Point-level sensors monitor and detect the presence or absence of a bulk-solid material at a predetermined point in the vessel. Examples are high- and low-level detection usually for purposes of controlling vessel filling. With flow problems that produce a “rat hole,” the level detection sensor is often still detecting material because flow continues in the center of the mass of bulk-solid material. For any level sensor mounted from the vessel side or a short distance in from the side wall on the vessel roof it correctly detects and indicates material presence, even though flow has stopped because of the “rat hole.” A continuous level sensor used for contents monitoring also will likely indicate a quantity of material even though flow has stopped and the vessel is apparently empty. Most often the level sensor is accused of faulty operation until an investigation is done and the flow problem revealed. For acceptable level detection and measurement, the material flow problem must be resolved.
There are at least three ways to solve the kind of flow problems that result in a “rat hole” or “bridge.” These are vessel design, vibration and aeration. See my book, titled “Solids Level Measurement and Detection Handbook”, copyright © 2012 by Momentum Press®, LLC all rights reserved, for further and complete information. The section below is taken from the books appendix.
Vessel Design: prevents the flow problem by converting the material flow from funnel flow to mass flow. One such example is illustrated in an October 2003 article appearing in POWER magazine that describes a particularly difficult and dangerous flow problem that resulted in an explosion at the facility in June 1991 (Dantoin et al., 2003). Subsequently the company made several changes including to its coal hoppers so that the material flowed as mass flow. To achieve this, the hopper walls of the cone section were changed to a much steeper angle, the discharge size was increased, and a new lining was installed in place of the existing Gunite lining. These changes were made on two coalbunkers at a very high price, but the results have been good: no fires or explosions since, as reported by the article.
Vibration: The second method, vibration, can also be an effective means of dealing with flow problems. However, unlike vessel design, the use of industrial vibrators does not treat the problem at its root, it only deals with the symptom and, in some regards, defies logic. When you vibrate a powder in a container you actually pack the material, the exact opposite of what you would want to do to resolve the flow problem, right? But the use of high levels of vibration energy transmitted through the vessel wall into the material serves to break loose the material particles from the walls of the vessel and each other. Many flow problems are a result of adhesion (material adhering to vessel walls) and cohesion (particles packing together, clinging to one another and forming lumps, clumps and a solid mass that is difficult to flow).Vibration energy, enough of it, can dislodge many clogs of this type and thereby promote material flow. Sizing and selecting the correct industrial vibrator is important (Ruggio, 2003).
Aeration: The third method, aeration, like vibration, also does not deliver a true solution. Aeration uses air pressure to separate bulk-solid particles from each other and from vessel walls. As we said before, adhesion and cohesion are the culprits in flow problems. The air injected into the material at the location of the flow problems serves to lift and separate material particles from each other and the vessel walls, thereby promoting flow. Where vibration may typically, but not always, use a single industrial vibrator to resolve the flow problem, use of bin aerators almost always requires a number of aerators throughout the problematic vessel section. An example of the common use of aerators to resolve material flow problems would be silo cone sections of silos that contain cement powder or fly ash within a concrete batching plant.”
Solving any material flow problems in the vessel first, before applying any level measurement and detection device, will produce more reliable, accurate and complete level control and material contents measurement.