Evaluating Vibrating Screen Efficiency by using the Discrete Element Method

Vibrating Screen Equipment
Vibrating screens are equipment used to separate and transport granulated materials in various processes throughout the mining, agriculture, pharmaceutical, food and chemical industries. Although vibrating screens have many applications, problems such as adhesion, clogging, corrosion, wear, and uneven feed distribution are still quite common. These problems are strongly related to the productivity of the process, and minimizing those problems usually results in financial, productivity and time benefits.
In applications where increasing productivity is desired, maximizing throughput is the typical focus. In this sense, it is essential that the equipment yields not only a high throughput (i.e., a substantial amount of graded granular material), but also a high capacity to separate particles into different sizes, that is, a high efficiency. The analysis of vibrating screen efficiency is therefore very important when designing or choosing the proper equipment for certain processes. Nevertheless, investigating these types of equipment for ways to increase efficiency can be challenging as the mechanisms of particle motion are not fully comprehended.

Experiments vs. Simulations
Experiments can be expensive and may take too long to guarantee their repeatability. For example, adhesion and clogging might represent problems if it becomes necessary to clean the screen before a new test. Material that breaks easily due to the screen’s vibratory motion might pose problems if the broken material cannot be used for the next experiment. Screen wear and corrosion can also be issues during testing since geometries that differ among tests make true comparisons difficult.
Rocky DEM software has a successful approach to analyzing the mechanical behavior of particulate material: the Discrete Element Method (DEM). Parameters can easily be changed in simulations, eliminating the necessity of preparation, excessive time and resources for the experiments. Also, several tests can be done at the same time. Another great advantage of a computational analysis is the possibility of simulating equipment that does not yet exist, allowing engineers and technicians to decide which equipment is better before it is constructed.

Parametric Analysis
An important tool when choosing the proper design criteria for a vibrating screen is a parametric analysis. High frequencies may cause particles to overshoot the screen and otherwise miss making it into the screen holes. On the other hand, low frequencies may result in accumulation of the particles on the screen surface, excessive wear, and reduced throughput along the process.
If the inclination angle of the screen is too high, the particles could move too fast over the screen and miss opportunities to pass through the screen aperture. If the angle is too low, the particles could accumulate on the screen, which can cause excessive wear and reduced throughput.
Low particle mass flow rate in the feeder will result in reduced throughput, which is usually the opposite goal of the process. But if the mass flow rate is too high, it may cause deformation of the equipment due to higher stresses, particle accumulation on the screen and premature wear of the screen.
Rocky DEM can be used as a tool to better optimize the process, assisting ion the proper selection of these and other design considerations. Each particle must have a sufficient number of opportunities to pass through the screen aperture to achieve efficient size separation. Obtaining a correlation between vibration screen parameters and efficiency may result in valuable insight for industrial equipments.

Particle Shapes
Because the function of a vibrating screen is to sort various size and shaped materials, it is highly desirable to model these attributes. The shape and the orientation of particles determine whether or not the solids will pass through the screen aperture. Rocky DEM is able to simulate accurate, polyhedral convex and concave particle shapes.
Many standard DEM codes use the glued spheres representation to model shapes, which is a lot less realistic when compared to polyhedral shapes, as observed in Figure 1. This is because with the sharp edges and corners, not only will the particles appear more like they are in reality, but they will also produce more realistic void fractions, settling behavior and friction results, all of which have a positive impact upon the accuracy of the simulation results.


Figure 1 – Glued sphere shape used in standard DEM software (left); Polyhedral shape used in Rocky DEM (right)

Additionally, because each polyhedral-shaped particle is modeled as a single meshed shape, the calculation takes fewer resources to process than many individual spheres glued together, frequently reducing significantly the simulation time for the same amount of particles.
An example of a vibrating screen where the particle shape is taken into account can be observed in Figure 2.


Figure 2 – Vibrating screen equipment simulated in Rocky DEM with realistically-shaped particles

Cohesive Particles
Depending upon the application, particles may have adhesive characteristics and in vibrating screen operations this usually represents a problem. Particles can become attached to the screen aperture, reducing the size of it or even blocking it completely, which restricts the passage of particles that should be screened. Rocky DEM is able to simulate these cohesive/adhesive properties, allowing for the simulation of particles that form agglomerates, as shown in Figure 3, and also stick to the walls.


Figure 3 – Simulation of particle clusters formed due to adhesion properties

Structural Analysis
Rocky DEM enables the coupling with ANSYS for structural analysis, which is a convenient way to investigate the influence of particle mechanics upon equipment. The video below is an example of a vibrating screen simulated in Rocky DEM that was also coupled with ANSYS mechanical. In this type of analysis, Rocky DEM predicts the particle load, which is then transferred to the mechanical software to calculate the structural response. Therefore, the stresses and forces caused by the particles upon the screen structure can be analyzed.

Want to know more about vibrating screen? 

Several industries including mining, agriculture, pharmaceutical, food and chemical use vibrating screens to separate and transport granulated material. Although vibrating screens have many applications, problems such as adhesion, clogging, corrosion, wear, and uneven feed distribution are still quite common. However, analyzing these types of equipment for ways to increase efficiency can be challenging as the mechanisms of particle motion are not fully comprehended. To find out more, watch this free webinar.




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