Design and structure of the silo weighing system

Thread Content

1. Introduction: When manufacturing tanks equipped with sensors, it is necessary to pay attention to some important basic rules. For example, storage tanks usually have to withstand climatic conditions or production-related influences. When constructing a new open-air silo (storage bin, coal bin) that is related to a building, it is necessary to comply with building regulations. For building codes, the retrofitting of weighing equipment can also be considered an important change. In this case, it is advisable to consult a building engineer, as building regulations define the \"technical condition\" in terms of safety. For example, wind load is assumed in the German standard DIN 1055, Part 4, on structural loads. The designers and manufacturers of tank structures should also be aware of the specific, often internal to the company, regulations; if hazardous materials are stored and forklifts are to be used for handling, tanks located on the roof should also generally be protected against lifting forces. 2. Weight distribution: The optimal sensor configuration for determining the weight of a tank is achieved when the tank is supported at three load-bearing points, with a sensor installed at each of these points. This situation is known as static determination. To this end, the entire load should be distributed as evenly as possible among the three sensors. For vertical or suspended cylindrical tanks, the optimal distribution is achieved when the three sensors are equidistant from the tank’s vertical axis and are spaced 120° apart from each other in a plane. For the arrangement of load-bearing points on horizontal or lying-type material tanks, refer to Figure 1. If sensors are not installed on all supports in a given piece of equipment, then a non-uniform distribution of support loads is more appropriate. A support with sensors should be able to bear more load than one without sensors; this measure helps improve the accuracy of weighing equipment. When designing the equipment and selecting sensors, it is important to ensure that as similar a load as possible is applied to the sensors. Figure 1 shows the load-bearing points A, B, and C of a horizontal tank. If the tank is supported by four or more points, there is an excess of static degrees of freedom. For this application, sensors must be considered at all load-bearing points. The uniform distribution of the load on a single sensor can only be achieved through installation. To achieve this, first measure the load on each sensor separately; if a significant difference is observed, the height of the corresponding sensor should be adjusted (for example, by inserting a metal sheet). In principle, sensors with too low a load should be installed facing each other. 3. The center of gravity of a fully loaded material tank should be as low as possible below the support points of the tank. This requirement is often difficult to meet in practice. It is advantageous for the stability of the mechanism when the center of gravity is below the load-bearing point. The position of the center of gravity changes with the filling level, and this position has a crucial impact on the number of sensors required. Under symmetric filling, the layout of sensors allows it to be possible to create weighing equipment using a single sensor, as the center of gravity moves along a vertical line. If the center of gravity shifts laterally along the centerline due to changes in the filler, sensors should be installed at all support points. Figure 2 explains the necessity of using sensors at all support points when the center of gravity (laterally) changes. Figure 2: A tank with an inclined outlet plate, whose center of gravity varies depending on the level of filling. 4. Interface tanks such as those used for loading and unloading materials often require connections to other interfaces as well, such as for the insertion and removal of contents from the tank, as well as for electronic, hydraulic, or pneumatic auxiliary devices attached to the tank. The loading and unloading interface of the material tank can generate additional forces, resulting in errors in the weighing accuracy of the scale. Therefore, the interface must be flexible in the vertical direction. Figures 3 to 7 show several reasonable interface formats, which should be considered during the conceptual and project phases. When connecting inflexible rigid pipes, it is reasonable to connect the tank to a horizontal pipe that is as long as possible. Horizontal pipes are flexible in the vertical direction, and the longer they are, the more flexible they become. The stress it generates on the sensor is correspondingly small, and it has no significant impact on measurement accuracy. (Fig. 3). Several sections of easily bendable connecting pipes can replace long horizontal pipes (Fig. 4) by being connected with deformable elastic material hoses, thereby avoiding additional forces. Here, it is necessary to examine the acceptability of elastic materials for the contents filled in the container as well as for cleaning agents (such as in food and pharmaceutical technologies). Other possibilities for reducing additional forces involve the use of curved pipes (Figure 5). In cases where pipes are to be installed vertically, when measurement in the direction of gravity is required, or when hoses cannot be used, a reliable pipe connection can be achieved through the use of bellows compensation (Figure 6). However, when assembling such compensators, it is necessary to keep the amount of deformation very small. If a second bellows is used, connecting it to the first one through a tube allows for a greater amount of deformation. Tip: In some areas that require special cleaning (such as the food industry), bellows are not allowed. The open-ended connection for short cargo pipes shown in Figure 7 is indicated as the best option for reducing additional forces, as the open joint prevents contact between the pipe and the tank; however, this design cannot be used in closed systems such as pressure vessels. Tip: It is essential to keep in mind that the material in the connecting pipes also contributes to the weight measurement. The inlet and outlet pipes that are directly connected to the storage tank should maintain a constant weight during the weighing process; in other words, the pipes are either empty or always full. Figure 3: Long horizontal pipe connections; Figure 4: Flexible tubular connections; Figure 5: Curved pipes; Figure 6: Expansion joint connections using bellows; Figure 7: Open short pipes for material loading. 5. Pressure loads: In closed systems, system pressure has an effect on weighing results. This is particularly true in the chemical industry, where reaction processes require very high pressures. Conversely, dust collection equipment generates a negative pressure of 100 to 300 millibars on powdered materials being weighed. When the pipes are connected vertically within a container, as shown in Figures 5 and 6, there are additional forces that act directly on the measurement. Its effect corresponds to the product of pressure and the cross-sectional area of the pipe; when the pressure remains stable during measurement, this factor can be taken into account or calculated at that time. Horizontal connecting pipes are advantageous; therefore, they are preferred over vertical pipes. In this case, the corresponding additional forces are absorbed by the mounting accessories. 6.1 Several Typical Sensor Installation Methods: This section uses illustrations to show several typical tank structures; the structural details related to specific issues will be discussed in the relevant chapters. 6.1 For vertical packing tanks handling homogeneous materials such as liquids and bulk goods, a configuration with two fixed bases and one sensor is feasible. The condition is that the capacity configuration is symmetrical, and the center of gravity line of the filler remains approximately on a vertical line throughout the filling process. In other cases, especially at higher precision, three or more sensors must be installed. 6.1.1 Rigid mounting of sensors – A simple rigid mounting of sensors on the load-bearing structure is generally not recommended, as such a setup does not allow the sensors to avoid measurement errors caused by deformation resulting from changes in loading conditions, vibrations, or temperature variations. 6.1.2 Vertical tank containers equipped with two supports and one sensor: This measurement system utilizes a pendular sensor along with two fixed supports, which also serve to secure the vertical tank container. This cost-effective design helps to minimize certain measurement errors. Figure 8: Vertical tank with a rigid mounting structure for a sensor. Figure 9: Vertical tank with two fixed supports and one sensor. 6.1.3 Vertical tank containers supported by three or four sensors. For accurate measurement of the filling level, three sensors are usually used as measuring elements; however, a four-point configuration can also be seen in structures with right-angle symmetry. Although this arrangement is not ideal in principle due to its excessive number of degrees of freedom and higher cost, it is still easily considered by designers. Although double-rope elastic supports do not require guide rods, most of them need to be combined with fixed limiters. For particularly tall vertical tanks, it is required to install additional guide rods in the upper portion; in the illustration, the round rods are pre-tightened with a relatively low preload. At such a position, the fixed stop rod can come into contact with the vertical tank even with only a slight, inevitable misalignment; this results in additional forces due to contact friction. The use of rolling stops or rope guides is generally not recommended here. Figure 10: Vertical tank with greater height. Figure 11: Vertical tank on a base. 6.1.4 Use of vertical tanks with three different weighing modules. The guide rods of the three weighing modules are tangent to the outer edge of the structure, allowing the tank to remain horizontally stable without any additional measures. It also has anti-lift protection to prevent the vertical tank from toppling over. This allows for minimal structural details in outdoor installations; this legend shows weighing modules for low, medium, and high loads. Using standard weighing modules can simplify the structure and significantly reduce costs, but details such as the parallelism of the loading surfaces and height alignment must be taken into account. 6.1.5 In practice, when using weighing modules with flanged vertical tanks, it is common to find that the casing of such circular vertical tanks extends down to the bottom in order to ensure the stability of the entire structure; in such cases, a special installation method is required to properly install the sensors. Figure 12 shows the principle of this structure as measured by a sensor; it is through this mechanism that weight is applied to the sensor. Of course, the weighing module also includes lift protection (it is not shown in Figure 12 for clarity). Even a slight elevation of the structure is sufficient to transfer the entire weight to the sensor. Therefore, a circular sealing ring is needed in this structure for sealing; it is too soft to generate any additional force. Figure 12: Weighing module configuration for vertical tanks with rims 6.1.6 The rectangular bin at the loading station, equipped with four sensors. At the loading station, vibrations from the conveying mechanisms, loading, and unloading equipment can cause instability in the rectangular bin; moreover, when the weighing bin is part of a movable device, acceleration further contributes to this instability. This does not take into account the significant lateral forces generated when large masses of bulk material are poured into the bin, due to the impact on its inclined sides. In this case, it is particularly necessary to consider in advance the stable fixation of the guide rod using pre-tensioning force. Sometimes, when weighing is not necessary, the scale container remains fixed, being free only during weighing. A right-angled symmetrical structure is highly advantageous for stability, which is why it is also adopted in most sensor installations. At the same time, sensors generally have an elastic support, such as in pendulum sensors. 6.2 Suspended Weighing Tanks: Suspended weighing tanks often use a simpler, flexible rod-based mechanism to eliminate or reduce issues related to self-centering and height adjustment. In addition to the frequently required fall protection, guide rods to prevent swinging and rotation are necessary. 6.2.1 Suspended weighing tanks equipped with two or three sensors generally require more tangentially arranged guide rods for their simple structure; in cases where stresses are low, these can be provided by the tubes located below on the lateral side. Figure 14: Tank suspended from two or three sensors 6.2.2 Structure with suspension at the center of a single sensor In this structure, protection against swinging and rotation is necessary. Figure 15: Weighing tank attached to a sensor 6.3 Horizontal storage tanks – In most cases, the movement of the liquid’s center of gravity is linearly related to the loading height. Therefore, installing a sensor under one saddle and fixed bases under the other two saddles can meet the measurement requirements for relatively simple loading conditions. An ideal horizontal storage tank distributes its weight equally between a double-rocker pivot sensor and two fixed supports; under normal conditions, other fixations are not necessary. For very long horizontal liquid tanks, additional protection should be provided to prevent the sensors from being knocked over by lateral forces; for example, mechanical barriers can be used to restrict the two endpoints on the tank saddle. Tip: In practice, the symmetry of the distribution is often disrupted by a slight tilt on the side of the liquid outlet; for precision weighing tasks, a configuration with three double-rocker column-type sensors is the best choice, with fixed baffles being the most suitable for ensuring horizontal stability. Figure 16: Horizontal storage tank with C16 sensor (sketch)