International Journal of Agriculture and Crop Sciences.
Available online at www.ijagcs.com
IJACS/2012/4-23/1752-1757
ISSN 2227-670X ©2012 IJACS Journal
Considerations in design of discharge gates of
steel silos
Ali Hasanzadeh Tolouti1, DavoodKalantari2, SeyedJafarhashemi2, SeyedRezaMousavi
Seyedi2
1.M.Sc. Student of Mechanical Engineering,Department of Agricultural Machinery, Sari University of
Agriculture & Natural Resources, sari, Iran.
2. Assistance Professor in Department of Mechanical Engineering, Sari University of Agriculture & Natural
Resources, sari, Iran.
Corresponding Author email: Ali HasanzadehTolouti, [email protected]
ABSTRACT: An analytical and experimental study is performed in this study for evaluation of
discharge rate of grain flow from different discharge gate of a silo reservoir. Three different apertures
(circular, rectangular and square shape with the same area) have been considered in this study.
Based on the experimental and theoretical results obtained in this study, it is shown that the
discharge rate of the circular aperture is the maximum. In contrast, the squared aperture has a
minimum discharge rate. It is also found that with increasing the grain height in the tank, the
discharge rate decreases. Regarding the uniform and non-oscillating discharge from a circular
aperture, it is recommended to use circular gates for discharging grain materials from a reservoir,
silos and storage tanks or grain drills.
Key words: grain, aperture, silo reservoir, discharge rate, tank
INTRODUCTION
Dynamic stresses and effects while discharging silos have been studied by many authors, see e.g.,
Levinson and Munch-Anderen.,(1994),Tejchman, J.,(1997) and Walters, J.K(1973). Initial stress in silos during
the filling and empting were measured by Walters, J.K., (1973) and compared with the calculated stresses by
Finite Element method (FEM). According to his results, initial stresses can be numerically predicted with
acceptable accuracy if density and deformation of mass of materials considered in numerical analysis.
Joftkar et al., reported that the grain frictional-induced stresses on the silo's wall occurs at grain
moisture content of 15% and temperature of 12°C. Coefficient of friction of agricultural products depends
ondifferent factors including moisture content, sliding velocity, surface characteristics of grains and
environmental conditions such as temperature, G,sitky ., (2004).
Two type of flow patterns in a vessel are illustrated in Fig.1 indicating funnel flow and mass flow. In a
funnel flow (Fig. 1a), an active flow channel forms above the outlet with non-flowing material around the vessel
periphery.In a mass flow (Fig. 1b), all the material from both the vessel center and periphery moves downward
(toward the outlet). The borders between funnel and mass flow can be obtained from the calculations of Jenike
A.W., (1970).
Figure 1.Two type of flow patterns in a storage tank.
Intl J Agri Crop Sci. Vol., 4 (23), 1752-1757, 2012
Typical problems which occur at the storage of bulk solids are:a)arching due to the adhesion forces acting
between the particles, and b) piping due to the consolidation of the remaining bulk solid in dead zone after a
funnel flow, as indicated in Fig.2.
Figure 2.A- Arching, B- Piping (ratholing).
Feeding devices of silos usually lie in the bottom of the reservoir. The flow behavior of a bulk solid is
affected by several parameters including the bulk density, the effective angle of internal friction, the unconfined
yield strength, and the wall friction angle.Martens, P., (1988).
Regarding increasing importance of mechanization of grain farming in our country and due to the exact
determination of discharge rate of the silos, it is necessary to measure and control the graindischarge rate from
silos. Therefore an analytical and experimental study is performed for evaluation ofdischargerate of wheat grain
from different discharge gate of asilo reservoir. Three different apertures (circular, rectangular and square
shape with the same area)have been considered in this study.
MATERIAL AND METHODS
Alvand wheat cultivar was used in the present experiments. Physical andfrictional characteristics of Alvand
wheat cultivar wasmeasured in 12%-13% moisture content by Razavi,et al., (2006), see table 1.
Table 1.Physical, mechanical and friction properties of Alvand wheat in the humidity 12%
true density(kg
793±6.164
)
bulk density (kg
1359±126.24
)
filling repose angle(degree)
discharge repose
angle(degree)
diameter(mm)
19.2
22.6
D
4/193
For measurement of grain discharge, a cylindrical tank with a diameter of 35cm and a conical bottom angle of
45° was used as a model of a grain silo reservoir. The tank was made by Galvanized iron.
The end of conical part was built with three different interchangeable apertures: circular, square and
rectangular. These three gates had a same cross sectional area of approximately29cm2. For each
measurement, wheat was filled inside the tank ranging from 35 to 130 cm height with seven height intervals of
about 15 cm. Discharge rate of each gateswas measured by collecting the discharged grains in another
container. Discharge rate was calculated after measuring the weight of collected grains inside the external
container and taking into account the time of discharge, using Q=V/ t.
Through this method, outlet discharge and coefficient of discharge was calculated for each height. SAS
9.1 software was used for statistical analysis. Mean comparisons were performed by using Duncan’s multiple
rage test.
Theatrical analysis of the problem
According to Fig.3, seed mass is located in a pot with a circular hole (diameter of d)located at the
bottom of it. Seeds flow out with a diameter of d1 (d1<d); thus the amount of seedspassing through the circular
aperture is equal to the volume of ABDC (see Fig. 3),while the rest of seeds are stagnated in ABEF and CDGH
areas.Slope angle of falling surfaces near the aperture is defined by ; =45 /2 ( is internal angle of seeds).
Intl J Agri Crop Sci. Vol., 4 (23), 1752-1757, 2012
Figure 3. Discharge pattern of seeds from Reservoir
To estimate the flow rate of seeds passing the aperture of the tank, a formula similar to the fluid flow
can be used, given by H, Bernatski et al ., (1998).
(1)
Where is flow index which can be obtained by measurement. The numerical value of this index for soft seeds
is considerably lessthan that of liquids.This is due to the more energy requirement in seedsto overcome the
internal friction in compare to the fluids. is the bulk density of seeds when discharging out through the
aperture. The dynamic bulk density is considerably less than the bulk density of static grain . F1 is the cross
sectional area of grain flow; which is less than the area of aperture (F), and P is the static pressure directly
abovethe aperture. For a practical use,the value of P can be estimated by(H, Bernatski et al., 1998).
(2)
which
(x). After inserting the amount of P in(1), one obtains
(3)
whered1=d-d' represents the reduction of diameter during the grain flow. According to the research
done by Bernatski et al., d'= (1.5-3)dz; dz is the seed diameter. By replacing d1 in equation (3) we obtain:
(4)
!
"
The above equation indicates that the stability of Q and therefore consistency of the grain flow depends
on the consistence of
and (d-d'). By increasing the aperture diameter (d),variation of and (d-d') reduces;
thereforethe grain flow (Q) remains consistent and more regular. With the same procedure like above, the
following expression(5) can be used to measure seed flow from a rectangular aperture.
!"
!
(5)
&
"
#$%
(
!
)
'& ( "
The amount of for a rectangular aperture is different to the amount of fora circular aperture; arching
of grains occur at the corner of rectangle’s aperture which makes some changes in the amount of
graindischarge. The frequency of Arching depends on the type of grains inside the reservoir and the size of
aperture. Circular apertures have the advantage that controlling the amount of seeds flowing downward is
easier.In this experiments, dimension of rectangular aperture were selected a=4.3 cm, and b=6.8 cm. Seed
diameter (dz) is 4.193 mm. Using an average value ofd' = 2dzfrom d' = (1.5 – 3)dz given by Bernakei et al. , we
have:
*
(6)
+,
2 32 4 5
*-./-01
Intl J Agri Crop Sci. Vol., 4 (23), 1752-1757, 2012
Considering these calculations and assuming4 5 1, the value of rectangular aperture's discharge
ratewill be obtained 0.9 of that of acircular aperture with the same cross sectional area.
After equating a=b in Eq.(5), a similar computation for the square aperture with 5.4 cm sides yields
*67,
(7)
2 89 4 5
*
Considering these calculations and assuming again4 5 1, the amount of square aperture's output discharge
ratewill be obtained0.31 of that of the circular aperture with the approximately same cross sectional area.
RESULTS AND DISCUSSION
According to the obtained results during discharging, output discharge rate and discharge coefficient of
the results of Variance analysis has been showed in table 2. The results of Variance analysis shown in table 2
indicates that there is a significant difference between various apertures and also between various heights with
1 percent possibility for each three parameter of a) discharging time, b) output discharge rate and c) discharge
coefficient. While interaction results of surface section × height for none of the investigated parameters was
significant.
Table 2. Analysis of variance table, discharge time, discharge and the discharge coefficient in three cross-sectional
area and height
Square average
Changes in resources
DF
Discharge time
Discharge
0.007ns
Discharge
coefficient
0.0003ns
Repeat
2
1.081ns
Cross section
2
42.014
0.077
0.0007
Height
2
39.329**
0.080**
0.0018**
height× Cross section
4
10.396ns
0.016ns
0.0001ns
**
**
**
Test error
16
3.572
0.006
0.0001
Coefficient of variation
10.24
9.24
13.73
ns: Non significant, *: significant at the level of 5%, **: significant at the level of 1%
Regarding the results presented in Fig.4, it is shown that the discharge rateof the circular section
ismaximum.In contrast, the squared section has minimum discharge rate. It is also indicated in this figure that
with increasing the wheat height in the tank, the discharge ratedecreases. This effect can be explained bystatic
weight of the wheat inside the tank.Based on the results shown in Fig.4, it is interesting that the discharge rate
fluctuates with decreasing the wheat height using the squared aperture. This mean that the accurate controls of
grain discharge from squared gates is difficult in silo reservoir. For the experiments performed in this study,
discharge rate from squaredaperture were highly sensitive to the height changes, see Fig. 4.
Figure 4. Comparing the output rate for the three different circle, squared and
rectangular discharge cross section
Intl J Agri Crop Sci. Vol., 4 (23), 1752-1757, 2012
Discharge coefficient of three apertures is shown in Fig.5. Discharge coefficient is a measure to show
the quality of designed aperture. This coefficient depends on output discharge rate, aperture shape, aperture
thickness, the surface area, acceleration of gravity and the height of wheat inside the tank. Discharge
coefficient has direct relation with discharge rate and has inverse relation with area, square of the acceleration
of gravity and square of the height of wheat inside the tank.
Greater discharge coefficient for a circular aperture(Fig. 5)indicates that the time duration for
discharging grains from circular gate is less than two other apertures. Square gate also needs the most
duration of time and has the least stability of grain flow, i.e, grain flow oscillates with decreasing the grain height
in reservoir during the silo empting.
Figure 5. Comparison of discharge coefficient for the three different circle, squared and
rectangular discharge cross section
According to figure 5, with increasing the height of grains inside the tank, the discharge coefficient
decreases. Perhaps the frictional forces between the grains and between the grains and the silo's wall
increases with increasing the static pressure inside the tank due to the increasing the height of materials inside
the tank.
GENERAL CONCLUSION
Considering obtained results from this research, it is understood that circular aperture is the best
choice for discharging the stored materials in silo, while the square aperture had the worst condition. Regarding
this fact that one of the purpose of an optimal silo's bottom aperture is a uniform and non-oscillating discharge;
therefore a circular aperture or at least a rectangular is recommended based on the experiments performed in
this study. Theoretical computations also indicate that the flow rate (Q) of grain material from a circular
apertures is greater than the two other gates, i.e., Qcircle>Qrect>Qsqur.
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