Design and selection principles for silos

Thread Content

Shape: When selecting and determining the shape of a silo, the principle is that the chosen shape should be suitable for the flow properties of the loose material, thereby enabling an optimal flow pattern of the material. Silos with a smaller nominal volume are easier to design; the larger the nominal volume, the more factors need to be taken into account. Whether it is square, circular, or a composite polygon, the first thing to consider is the taper of its discharge cone; the inclination of this cone must be such that the material flowing through the silo can move smoothly without any dead zones. Moreover, the steeper the inclination of the cone, the greater the flow rate ； On the other hand, from the perspective of the cross-section of the silo, in the straight cylindrical section, no particular shape is decisive; whereas in the lower discharging section, silos with circular hoppers exhibit much better fluidity than those with square hoppers or flat bottom surfaces. The latter two types of discharging sections tend to develop stagnant areas and transition zones where flow is restricted ； Furthermore, in terms of the discharge port, its size affects the flow pattern of the material. When the discharge port is large, the material flows more smoothly; whereas when it is small, the flow is poor and material arches are likely to form. By installing an inverted cone above the discharge port, the material is given a larger zone of positive flow, which helps to eliminate arching of the material inside the silo as well as tube formation in the outlet pipe. Strength and Structure: When determining the strength and structure of a silo, designers have to take many more factors into consideration. With the development of solid material conveying equipment in recent years, its conveying capacity has improved significantly. In terms of pneumatic conveying, there are various methods such as dilute phase, slurry phase, and dense phase conveying. During feeding and discharging, the high-volume airflow and the material itself exert greater impact forces on the walls of the silo, and these forces act under dynamic conditions; static calculation methods cannot be used to analyze such situations. Therefore, dynamic calculation methods should be employed when designing silos to address such issues. The theory behind power calculation states that during feeding, both the airflow and the material exert forces on the warehouse wall; the magnitude and frequency of these forces change dynamically ； During discharge, the pressure in the silo changes, and the pressure acting on a particular area of the silo wall varies dynamically; this can lead to increased local stresses. Extensive data provided by experts in their research show that the peak values of silo wall pressure can be several times higher than those calculated under static conditions. Some foreign standards have taken this situation into account in recent years. A simpler approach is to use dynamic influence coefficients added to the static calculations in order to correct the errors in those static calculations; these coefficients are obtained through experiments or by summarizing practical experience. In the past, static analysis methods were used in China for calculating silos; the wall thickness of these silos was determined solely based on the pressure exerted by the materials on the silo walls. For the silos designed today, the flow velocity of the materials is relatively low, and it is still safe to use the existing design principles and calculation methods for analysis. Therefore, steel and aluminum silos can still be designed using static analysis. In engineering design examples for medium and small silos, when the feeding and discharging rates are high, the methods provided in JIS B 8511 and DIN 1055 can be used; by adding certain dynamic influence factors to the static calculations, the errors in those calculations can be corrected. If it is possible to obtain data on these coefficients through experiments, then the problems related to dynamic design can be resolved.