Full-scale Silos Research Articles

Electrostatic elimination of charged particles by DC-type bipolar electrostatic eliminator

The electrostatic eliminator is widely used to prevent silos from explosions caused by the electrostatic discharge of charged particles. However, electrostatic accidents still happen occasionally. This study investigated charge elimination characteristics of the DC-type bipolar electrostatic eliminator under different configurations of the applied voltages, air velocities, and initial charge densities of particles for helping improve the charge elimination efficiency. The results show that the unipolar corona discharge will lead to reverse charging of particles, while the bipolar corona discharge at a suitable configuration can neutralize the charged particles completely. The upper and lower ionizers in the bipolar electrostatic eliminator should be applied with positive and negative voltages, respectively, and the positive voltage should be greater than the negative one. The suitable configuration of the electrostatic eliminator is insensitive to air velocity due to the simultaneous increase of the charging capability of positive and negative corona discharges with increasing the air velocity. This electrostatic eliminator can neutralize the charged particles with a net negative charge even in the presence of positively charged particles in the mixture. Furthermore, the performance of this electrostatic eliminator on a full-scale silo was examined using a light intensifier to observe the electrostatic discharge inside the silo. The proposed electrostatic eliminator was found to be effective in eliminating the electrostatic discharge in the silo.

Evaluation of pressures in slender silos varying hopper angle and silo slenderness

In this paper, we report the results for pressures in a full-scale silo obtained from assays performed on a test station using a free-flowing product. Six different types of silo geometry (slenderness and hopper angle) were tested by filling the silo to the height of interest and observing a static phase (10 min) followed by complete discharge. Our results show that discharge flow is not influenced by silo slenderness. The lateral pressure ratio and values obtained for vertical stress in the stored material at the transition were higher than those given in the Eurocode 1, part 4. In addition, the maximum frictional pressures in the cylinder showed peaks that exceeded the standard, as did the maximum normal pressure in the cylinder with maximum slenderness and mass flow (hopper β = 15°).

Modelling of full-scale silo experiments with flow correcting inserts using material point method (MPM) based on hypoplasticity

Flow patterns in bulk solids in silos are of importance for stresses exerted on silo walls. They may be changed by using different inserts. The paper discusses numerical results on confined granular flow of cohesionless sand in a full-scale metal silo with different symmetrically placed inserts, used for correcting a flow pattern (double cone, cone-in-cone and inverted cone). The material point method (MPM) was employed to simulate the dynamic flow of sand in a silo. To describe the flow behaviour of cohesionless sand, a hypoplastic constitutive model was adopted. The emphasis was on flow patterns and pressures on both silo walls and inserts. The numerical outcomes were directly compared with the corresponding experiments in a full-scale silo. A satisfactory agreement was achieved between numerical and experimental outcomes. In MPM calculations, the effect of initial void ratio of sand, wall friction angle, insert position and grid cell size was also studied.

A continuum model of stresses in a vertical silo with a flow channel in the vicinity of the wall, using the principal stress cap surface approach for the bulk solids

Eurocode 1 (2006) gives design equations for eccentric stresses in silos, including flow channels adjacent to the wall. This has been modelled using the approach of Matchett et al. (2015, 2016).A three zone model was developed, consisting of:• The flow channel.• The transition zone.• The bulk of the solids.The flow channel and the transition zone were modelled by Janssen-type equations. The bulk was modelled by the principal stress cap approach.The transition zone is a complex region and has several purposes:1. To shelter the low stress flow channel from the high stresses around.2. To allow high principal stresses at the transition/bulk interface, within the yield locus.3. To form a transition between the dynamic flow channel and the static bulk.4. To allow transition from passive stress in the flow channel to active stress in the bulk.The model was calibrated against the data of Chen et al. (2007) for a full-scale silo, and described the data reasonably well, scaling axially and azimuthally.Large experimental data sets are required to calibrate a model. Unfortunate data points cannot be arbitrarily rejected.Further extensive, experimental data are needed to calibrate models.

Simulation of silo filling and discharge using ANSYS and comparison with experimental data

Silos are structures widely used in the industrial and agricultural sectors. Despite the advances in research during the last century, many uncertainties still remain. Some of the still unresolved questions in silos are the knowledge about the laws which control the behavior of materials stored in silos or the phenomena produced during the discharge process, when the highest thrust forces exerted by the stored materials on the walls appear.Numerical methods have been extensively employed for the understanding of these complex structures. Among them, the Finite Element Method is a suitable technique to determine the stresses and displacements appeared on the walls or the bulk solid both for filling and discharge of the silo. However, numerical models must be validated by experimental assays. Due to the high investment required, there are very few experimental installations in the world with full-scale silos for determining the actions produced by the stored materials. Therefore, very few assays can be found in the literature.In this article, it has been accomplished a comparison of the results obtained in several assays conducted using an experimental cylindrical silo with those calculated using a Finite Element Model (FEM) developed by using ANSYS software package. A mid-scale test silo was used to carry out the assays, which is equipped to measure the normal wall pressures and the friction forces. The numerical pressures predicted by the FEM are quite close of those experimentally obtained, both for filling and discharge. In addition, the mean vertical pressure obtained at transition is the same for both sets of results during the filling process. Some differences appear during the initial instants of the discharge, when the numerical model predicts higher mean vertical pressures at transition than those experimentally measured. The FEM predicts higher peak pressures than the experimental ones measured, and at a location closer to the transition than the real position of the sensor placed in the hopper to detect this peak.

Micro- and Macromechanics of Hopper Discharge of Ultrafine Cohesive Powder

Abstract Typical flow problems, like arching or bridging, of ultrafine cohesive powders are caused by undesired particle adhesion, poor flowability and large compressibility and intensified by poor permeability. Thus, the physical understanding of micromechanics of ultrafine particle flow and permeation is very essential to design properly the product quality and to improve the process performance in particle technology.A force balance at a homogenously assumed, dynamic bridge is formulated at the hopper outlet that includes the dead weight, inertia, wall and drag forces. The permeation resistance is calculated as sum of a microscopic flow-around resistance of single particles plus the macroscopic bed resistance of the moving cohesive powder bridge. The resulting differential equation is shown for turbulent flow-through conditions of air. The first integration gives analytical discharge velocity-time function, steady-state discharge velocity and the characteristic discharge time of incipient (accelerated) flow. The second integration results in analytical height-time and velocity-height functions, discharge and residence times. This method results in physically consistent, analytical models that are comfortable to handle and easy to prove. The steady-state discharge velocity is compared with measurements of full-scale silos at Coperion and Zeppelin companies. This publication describes the results of a closed collaboration between university and industry. The practical objective of the project was to solve the serious discharge problems of ultrafine, compressible, hardly permeable, cohesive powders by applying mechanical vibrations during their gravitational flow.

Design and instrumentation of a mid-size test station for measuring static and dynamic pressures in silos under different conditions – Part II: Construction and validation

Using the design described in the first of this pair of articles, a full-size silo pressure test station was constructed. In this present article, the main characteristics of this station are described, together with the results obtained from the first validation tests. In addition, these results are compared with the European standard on silos.Very few experimental installations in the world have full-scale silos, and very few assays have been conducted on them. Consequently, many uncertainties remain which require further research in order to reliably predict the behaviour of material stored in this kind of structure.The results obtained demonstrate that the theoretical design of the test station is valid for measuring silo pressures and that the station offers multiple possibilities for future research.Although the principle aim of this article is to demonstrate that the previous theoretical design is valid, some novel conclusions can also be drawn from the tests performed. For example, observation of pressure patterns under static conditions inside the silo, and of overpressure surge patterns during silo discharge, has led us to conclude that the existence of asymmetrical pressures inside the silo is not a consequence of chaotic behaviour, but rather obeys certain physical laws which have still not been sufficiently studied.

Design and instrumentation of a mid-size test station for measuring static and dynamic pressures in silos under different conditions – Part I: Description

Very few experimental installations in the world have full-scale silos, and very few assays have been conducted on them. Consequently, numerous unresolved questions remain which require further research in order to be able to reliably predict the behaviour of the material stored in these kinds of structures.In this article, we describe the design of a full-scale test station for assaying pressure in silos. The installation basically consists of a full-scale, cylindrical silo equipped with load cells to measure pressure and variable-frequency drives connected to each of the electric motors driving the filling and discharge screw conveyors, in order to study the effect on pressure of the speed at which the silo is filled or discharged. As a result of this novel design, it is possible to obtain most of the parameters which control the behaviour of stored material and to compare and validate the different theoretical models used for performing calculations and establishing current standards.The versatility of the silo design also makes it possible to adapt the geometry to the conditions which, in practice, are present most frequently in slender silos, enabling assays to be conducted for flat bottom and hopper silos, and combining both of these conditions with central and eccentric outlets. Furthermore, for each of these configurations it is also possible to study the behaviour of the material under different conditions, including static state, dynamic filling and discharge state, variations in filling and discharge speeds, pressure caused by swelling in the case of material stored when moist or saturated with aqueous solutions, and to perform simulations of an increase in silo height.

Experimental and Calculated Loads on “Cone in Cone” Installations

This study focused on the installation of small hopper inserts designed for mass flow. An FEM-“Silo-Flow-Program-System” was used for the calculations. Using full-scale silo experiments, the loads on the “cone in cone” inserts were measured as a function of the installed positions. The results obtained from all experiments and in the numerical simulation showed a rise of the vertical loads on the hopper inserts as the height position increased. The comparisons between experiment and calculation with the more complex material law variant show relatively good agreement, above all for the lower to middle height positions. The load increases as a function of the height position are reproduced well by the numerical simulations. The summarizing representation of the experimental built-in loads for the three hopper inserts shows an approximately linear dependence of the vertical load on the height position for emptying.

The influence of a cone-in-cone insert on flow pattern and wall pressure in a full-scale silo

A silo insert is sometimes placed inside the hopper to improve the flow pattern of solids in a funnel flow silo. This paper describes a recent experimental investigation of the flow pattern and wall pressures observed during filling and emptying of a cylindrical silo with and without a cone-in-cone insert. The 2.5 m diameter silo had an 8 m high cylindrical section and a 44° conical hopper, and was filled with crushed quartz sand. The wall pressures were measured at ten chosen locations using pressure cells mounted on the wall. To investigate the solids flow pattern, markers were placed at fixed positions in the silo during filling and the residence time for each marker during discharge was observed. The flow pattern can be inferred from the residence time measurements for the markers and the surface profile observations during discharge. These tests show that a cone-in-cone insert can have a strong influence on both the flow pattern during discharge and the pressure acting on the silo wall and its industrial use should be considered cautiously and implemented with care.

Correlation between the flow pattern and wall pressures in a full scale experimental silo

Although it has been known for several decades that the pattern of solid flow in a silo has a strong influence on the pressures exerted on the silo walls, no full-scale test has ever been able to measure both the flow pattern and the wall pressures together. This paper describes simultaneous measurements of both the flow pattern and the wall pressures in a full scale silo storing approximately 250 tonnes of iron ore pellets. About 300 radio frequency tags were seeded into the stored granular solid at predefined locations to measure residence times, and over 300 strain gauges were used to concurrently measure strains on the silo walls. The flow pattern was determined from the residence time measurements and the wall pressure distribution was inferred from the strain gauge measurements. The study has produced a clear definition of the pressure regime at full scale under eccentric discharge with a pipe flow against the silo wall. A strong correlation between the flow channel and the discharge pressure distribution, which was always previously assumed, is here demonstrated from the observations.

Flow pattern measurement in a full scale silo containing iron ore

Abstract The flow pattern in a silo is important because it affects both the recovery of solids and the pressures on the silo wall during discharge. Wherever mass flow is not achieved, the boundary of the flow channel has significant implications for both the functional and structural design of the silo. Many techniques have been used for the study of flow patterns in model silos, but most cannot be used at full scale, and very few quality measurements at full scale have ever been made. This paper outlines a full scale experimental study in which the patterns of solids flow and the flow channel boundaries are reliably quantified. The full scale silo was specially designed, constructed and instrumented to exhibit funnel flow and to make observations of the solids flow pattern and the silo wall pressures. It had three outlets: one concentric, one fully eccentric and one in between. Three materials were used: iron ore pellets, slag fines and crushed basalt. This paper describes experiments involving iron ore pellets. The silo was seeded with radio frequency tags whose residence times were measured by detecting them on exit during discharge. The residence time data were studied to deduce the discharge flow pattern. This paper presents the results of three different flow pattern interpretation techniques: the best of them (mass-time relationships) is shown to give a very clear identification of the solids flow pattern and the flow channel boundary.

Finite-Element Modeling of Filling Pressures in a Full-Scale Silo

A detailed nonlinear finite-element analysis is undertaken of the wall pressures exerted by iron ore pellets in a full-scale silo experiment. The ring stiffeners on the exterior of the silo wall cause local small axisymmetric geometric imperfections, and the effect of these on the pressure predictions is explored, together with its sensitivity to the chosen bulk solid constitutive model. This paper focuses on the filling pressures alone, because these are a necessary starting point if discharge pressure predictions are to be made with confidence.

Pressures from flowing granular solids in silos

Large displacement flows in granular solids occur whenever silos are discharged. Measurements of pressures during flow, combined with visual observations of flow patterns and control tests on the e...

Microbial dynamics in barley grain stored under controlled atmosphere

Barley of 2 moisture contents, 20–23% and 26–28%, was stored airtight for 2 separate seasons in 2 full-scale silos, and for 3 separate seasons in 12–16 pilot silos (1.25 m 3). The microbial dynamics were studied, using colony-forming units (CFU) of microorganisms, the proportion of internally-invaded kernels and ATP content. The ATP content was a good measure of the microbial activity, which decreased after loading and was very low during the winter period. Field-associated moulds and yeasts decreased in numbers of CFU and in the amount of invaded kernels. Fusarium species survived more satisfactorily than most field fungi. The temperature rise in spring caused an increase in ATP content, as a result of extensive growth of yeast of the genera Hansenula and Candida. This activity started earlier in the high-moisture grain (March). In May, a significant increase in CFU of Penicillium was observed and, during May to July, in many silos, all kernels were invaded, mostly by P. roqueforti. In an attempt to increase the time of safe storage, some technical improvements and gaseous and liquid chemical additives were tested. Attachment of “lungs” to the silos, and addition of organic acids or gases (CO 2, N 2, NH 3) to the grain, increased the safe storage time by one to several months. The aerobic storage stability of rolled barley taken in July from the full-scale silos was tested. The rapid deterioration following unloading limited the aerobic storage stability to a few days.

Pressures exerted by materials stored in silos: part I, coarse materials

Silos are usually designed on the basis of simple theory which is used to predict horizontal pressures on the walls. The pressures generated depend on many factors, including the pattern of flow within the contained material, the method of operating the silo and also the mechanical characteristics of the material. This Paper compares pressures measured in a number of full-scale silos containing coarse materials with the predictions of commonly accepted design theory. It shows that, although the silo arching theory gives an acceptable estimate of the average pressure on a silo wall, it does not always provide an adequate design envelope. It is preferable to use a simple straight line variation of pressures with depth for design. Les silos sont généralement calculé sur la base d'une théorie très simple utilisée pour prévoir les pressions horizontales sur les murs. Les pressions réellement exercées dépendent de plusieurs facteurs, dont la forme de l'écoulement à l'intérieur de la matière, le principe du fonctionement du silo et aussi les caractéristiques mécaniques de la matière ensilée. L'article compare les pressions mesurées dans quelques grands silos contenant des matières à gros grains avec les prévisions de la théorie classique. Bien que la théorie de 1'effet de voûte dans un silo donne une évaluation acceptable de la pression moyenne sur la paroi mur, cette théorie ne fournit pas toujours une enveloppe acceptable. II paraît préférable d'utiliser pour les calculs des pressions proportionelles à la profondeur.

A further contribution to the evaluation of the Jenike method for design of mass flow hoppers

A critical comparison has been carried out between values for the minimum outlet slot width and the minimum hopper wall slope for mass flow predicted by the Jenike method, and values obtained from experiments on a full-scale silo with symmetrical wedge-shaped hopper. The data for the minimum slot width were obtained for a fine SiC powder whilst a coarser, free-flowing powder of the same material was used to determine the critical slope of the hopper walls. It was found that the Jenike method overdesigned the critical hopper slope by 8 - 10°. It was further found that the slot width was overdesigned by from 0 to over 100% depending on the way in which the failure loci were extrapolated into the range of small normal stresses.