TUNRA Bulk Solids’ experts discuss how the inserts in hoppers affect gate loads.
Hopper flow patterns significantly impact the performance of bulk material handling systems. Funnel flow or stagnant regions within a hopper may result in flow blockages and induce high dynamic wall loads. These issues not only compromise operational efficiency but also pose potential structural integrity concerns.
To mitigate these problems, inserts have been introduced as retrofittable devices to modify flow patterns and alleviate flow blockages in silos [1].
Although the application of inserts in hopper systems has shown promising results, their impact on gate loads, which directly influence the design and operation of slide gates, has received limited attention in the current design standards, e.g. AS3774 [2].
Traditional approaches for estimating gate loads often rely on simplified assumptions and empirical correlations, overlooking the relationship between insert and resulting gate loads.
To address this, the present study employs the Discrete Element Method (DEM) modelling technique, which is capable of capturing the complex behaviours of granular materials.
DEM has been used as a research tool for the analysis of silo wall loads and gate loads [3,4]. By simulating the interaction between particles within the hopper system, DEM modelling provides insights into the mechanics of bulk material flow and allows for the prediction of gate loads under the influence of hopper inserts.
The primary objective of this project was to investigate how hopper inserts affect gate loads, specifically focusing on their quantity, dimensions, shapes, and positions relative to the outlet.
The potential influence of bulk material compaction in the hopper and the impact on the gate was also explored, shedding light on the underlying mechanisms responsible for load reduction. The findings can help develop improved design guidelines for gates.
DEM simulation for a hopper without inserts
The DEM software Rocky has been employed for gate load modelling in this study. The accurate calibration and selection of DEM parameters represent crucial steps in the simulation procedure.
In this project, the DEM software’s modelling parameters were chosen based on an interpretation of the measured flow properties of the bulk material, such as bulk density, wall friction, and internal friction.
The simulated conical hopper had a circular outlet with a diameter of 2 m and a hopper half angle of 15 degrees. The simulation commenced with particles progressively filling into the empty hopper with the outlet/gate closed. The equivalent fill level was approximately 4.5 m.
Figure 1 depicts the empty hopper alongside a snapshot obtained from a DEM simulation, demonstrating symmetric filling. The particles were systematically generated on a circular surface concentric to the hopper axis and located 8 m above the gate. The generated particles were subjected to gravitational forces (settling) until the desired fill level was attained.
The vertical pressure exerted on the gate during the filling process is presented in Figure 2 based on the DEM results. The analysis reveals a rapid increase in vertical pressure upon the initial impact of the burden with the gate. Subsequently, the vertical pressure demonstrates a relatively linear relationship with respect to the fill level.
Once the equivalent fill level of 4.7 m was attained, the vertical pressure on the gate stabilised at 33.2 kPa and remained nearly constant during the short period of settling of the material.
The determination of vertical pressure on the gate during the initial fill can be obtained through the application of AS3774 standards. The selection of the j factor significantly impacts the determination of gate load. As the j value increases, the predicted gate load decreases due to the stress field approaching a passive state.
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Figure 3 illustrates the gate loads associated with j values of 0.1, 0.45, and 0.9 based on the AS3774 for reference purposes. Under hydrostatic conditions, when the j value equals zero, the gate load peaks with an estimated magnitude of 44.3 kPa. The DEM simulation yielded a gate load of 33.2 kPa, which corresponds to a j value of 0.6 as per the AS3774 standard.
Gate loads with hopper inserts
This study examined various hopper insert options to investigate their impact on gate loads. Figure 4 illustrates the configurations of the insert options employed in this study, focusing on factors such as quantity, dimensions, shapes, and positions relative to the outlet.
Configuration 1 featured a triangular insert with an apex angle of 30 degrees, positioned 1.2 m above the gate.
In Configuration 2, the insert retained the same triangular shape but was positioned 2.4 m above the gate. Configuration 3 consisted of two triangle inserts, with the first insert identical to Configuration 1 and a second insert located 3.2 m above the gate and offset 90 degrees about the hopper axis.
The insert in Configuration 4 was also triangular, but with an apex angle of 40 degrees. Configuration 5 utilised a conical insert with an apex angle of 30 degrees. Additionally, Figure 4 presents snapshots of the filling profiles obtained from the DEM simulations.
The bulk material was introduced gradually through a concentric inlet located 8 meters above the outlet. In the absence of any inserts, the vertical impact velocity of the material was measured at 12.5 m/s. The impact pressure exerted on the gate during the initial filling stage can be quantified using Equation 1.
pvi=ρv
where ρ is bulk density, ρ is the vertical component of the impact velocity. When inserts were introduced into the hopper, a portion of the bulk material came into contact with the insert surfaces, resulting in a reduction of momentum.
The impact energy on the gate was influenced by factors such as the area, impact position, and impact angle of the inserts. Figure 5 provides a comparison of the impact energy for hopper configurations with and without inserts.
Overall, the presence of inserts led to a reduction in impact energy. The vertical component of the impact velocity decreased with an increase in the insert area, a decrease in the spacing to the gate, and an increase in the number of inserts.
Among the insert options, Configuration 5, which utilised a cone-shaped insert, exhibited the highest impact energy due to its smaller impact area, which was less effective in buffering the bulk materials.
The presence of inserts in the hopper also affected the compaction of the bulk material. Figure 6 presents a comparison of the bulk density near the gate for the different insert options. Configuration 4 exhibited the lowest bulk density due to the larger area of the insert located near the gate.
In Configuration 3, the presence of the second insert acted as a further buffer, resulting in reduced compaction of the bulk material above the first insert. The trend observed for the variation in bulk density followed a similar pattern to that of the impact energy among the different insert options.
The variation of gate pressure with stress switch heights is depicted in Figure 7. Configurations 1, 3, 4, and 5 exhibited pressure switches at a filling height of approximately 1.6 m, as these options had inserts positioned 1.2-2.0 m above the gate.
In Configuration 3, an additional pressure switch was observed at a filling height of approximately 3.6 m due to the presence of a second insert positioned 3.2-4.0 m above the gate.
Configuration 2 had a switch position at approximately 2.8 m since the insert was located further up. Above the inserts, the gate pressure exhibited a relatively linear relationship, gradually increasing with the filling level.
Figure 8 presents a comparison of the vertical pressure on the gates for hopper configurations with and without inserts. The DEM results highlight the significant reduction in loads acting on the bin gate.
Among the investigated insert options, configurations 3 and 4 exhibited the best performance in terms of gate load reduction, attributed to factors such as larger area, shallower angle, or increased number of inserts.
Compared to configuration 1, higher gate loads were observed for configuration 2 when the insert was positioned further up the hopper. Configuration 5, featuring a cone-shaped insert with the smallest impact area, was found to be less effective in reducing gate loads.
Conclusions
This study investigated the influence of hopper inserts on gate loads using DEM modelling. During initial filling, the vertical component of the impact velocity decreased with an increase in insert area, a decrease in spacing to the gate, and an increase in the number of inserts.
The presence of inserts also influenced the compaction of the bulk material near the gate. Comparing hopper configurations with and without inserts, the DEM results demonstrated a significant reduction in gate loads when inserts were present.
Configurations 3 and 4 performed best in terms of gate load reduction, attributed to factors such as larger insert area, shallower insert angle, or increased number of inserts.
Configuration 2, with a higher insert position, exhibited higher gate loads compared to Configuration 1. Configuration 5 with a cone-shaped insert showed less effectiveness in reducing gate loads due to its smaller impact area.
Overall, this study provides insights into the impact of hopper inserts on gate loads, contributing to a better understanding of slide gate design in relation to insert design
This article is an excerpt of the paper originally published in the proceedings of the 14th International Conference on Bulk Materials Storage, Handling & Transportation in Wollongong, 2023, and has been re-published with the permission of the authors Bin Chen, Alan Roberts, Timothy Donohue and Shaun Reid.
REFERENCES
[1]
Roberts, Basic Principles of Bulk Solids, Storage, Flow and Handling, The University of Newcastle Research Associates (TUNRA), 1998
[2]
Loads on bulk solids containers, AS 3774-1996, Australian Standard
[3]
B. Chen, A. Roberts, and T, Donohue, DEM Modelling of silo loads asymmetry induced by eccentric discharge, Proceedings of the 7th International Conference on Discrete Element Methods, Dalian, China, 2017
[4]
J. Shen, A. Roberts, and C. Wheeler, DEM simulations on gate loads and bin storage characteristics before discharge, Powder Technology, Vol. 383, 2021, pp. 280-291.