Key points for modeling and analysis of the tower in CAESAR II

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Towers and cylindrical vessels used for distilling feedstock (crude oil) are very important equipment in any process industry. Every process piping industry must have several vertical cylindrical equipment units. Pipelines of various diameters and properties (process parameters) are connected at different heights of the tower. Stress analysis is required for all large-diameter pipelines connected to the tower to properly assess pipe support and nozzle loads. A close inspection of the tower’s structure reveals that it has several trays at different heights. The temperature at each tray varies depending on the process conditions. In the following, we will detail the method of performing stress analysis on cylindrical structures such as towers using CAESAR II. The stress analysis of cylindrical equipment will be discussed mainly from the following aspects: 1. Input parameters required for analysis 2. Creation of temperature profiles for the cylinders/towers 3. Modeling in CAESAR II 4. Support for the piping systems connected to the towers 5. Verification of nozzle loads. 1. Input parameters required for analysis: Simulating and analyzing the pipelines connected to the tower requires the following data: a) The overall layout of the tower’s equipment, including all dimensions, nozzle positions, and material specifications. .b) Segmentation temperature of the tower. c) Pipeline critical table/pipeline table/pipeline parameters and P&ID. d) ISO diagram of the connecting pipeline. e) Table of allowable loads on pipe nozzles as defined in the project specifications. 2. Sectional temperature of the cylinder/tower: Different organizations use various methods to determine the sectional temperature of the cylinder. Here, two of the most commonly used methods will be mainly introduced. The temperature curve of the tower is generally created based on the connected outlet pipeline. Therefore, in the P&ID, the outlet nozzles of the columns with large sizes should be marked (large size means that the size of the nozzles has a significant impact on the temperature changes inside the column). Then, note the operating and design temperatures next to these pipelines in the pipeline table. Assume that there are three large-sized outlet nozzles in a typical tower (N1, N2, and N3 as shown in Figure 1). Then, the segment temperatures for this tower can be created, as shown in Figure 1. This method is the most widely used among EPC companies at present. http://bbs.intergraphbbs.com/data/attachment/forum/201701/17/103737p7czm88ya87oo0fe.jpg Figure 1: Creation of typical column section temperatures — Method 1. Once again, the method mentioned in Figure 2 can also be used to create the section temperatures for such columns, and many companies employ this approach as well. http://bbs.intergraphbbs.com/data/attachment/forum/201701/17/103741v69qeoet4q6emoph.jpg Figure 2: Creation of typical column section temperatures — Method 2. Few organizations use the operating and design temperatures specified in the equipment’s overall diagram as such for equipment modeling. The thermal elongation of the device obtained using the above two methods is closer to reality. 3. Modeling the tower in CAESAR II: All equipment modeling methods are similar. Based on your own choice, you can decide to model from the skirt upward or start modeling from the nozzle of interest downward until reaching the skirt. It is also a good practice to assign separate numbers to device nodes, as this makes it easier to distinguish them from pipeline nodes. When I model things personally, the device models start from node 5000. Here, let’s start with a typical nozzle flange. Model 5000-5020 as a nozzle flange, with the nozzle diameter and thickness as specified in Equipment GA diagram. Sometimes, detailed drawings from the equipment manufacturer may not be available (in the initial stage of a project); in such cases, engineering design drawings can be used as a basis for modeling. Typically, the piping discipline also provides charts for the minimum nozzle thickness based on flange class and corrosion allowance. Obtain the nozzle thickness from this diagram, or assume that the nozzle thickness is two grades higher than that of the connected pipe. As indicated in the reference equipment diagram, moving forward from the flange, temperatures such as pressure, corrosion allowance, material, and insulation thickness and density use those specified in the above two diagrams (Figure 1 or Figure 2). Then, 5020 to 10 are modeled as pipe units, with their lengths obtained from the reference drawings (generally, the distance from the pipe end to the equipment centerline is provided; in this case, the length of the nozzle is calculated by subtracting the radius of the equipment and the flange thickness, which have already been modeled). Add ANC constraints at node 10 and connect the Cnode to point 5040. Numbering the nozzles as 10, 20, etc., will place these nodes at the top of the restraint summary report, allowing us to quickly view the nozzle loads. If you wish, you can also use separate nodes. The nozzle model is now complete. Now we will model the device. Treat 5040 to 5060 as zero-weight rigid components, with a length equal to half of the external diameter of the equipment; the material used is the one specified in the reference diagram, the temperature is as indicated in that diagram, and the pressure and other parameters are taken from the diagram of the reference equipment. This unit will take you to the heart of the device. Starting from this section, it is possible to simply model the device as a pipe element with a temperature curve as shown in the figure above. Check the diameter and thickness in the reference diagram, as these values may sometimes vary as one moves from the top to the skirt. Finally, the skirt is modeled as a pipe element; its temperature is as described in the last paragraph of this paper, while its pressure, fluid density, and corrosion margin are set to 0, and a fixed point is placed at the bottom of the skirt. As shown in Figure 3, it is a simple simulated column model, with different colors representing different temperatures. http://bbs.intergraphbbs.com/data/attachment/forum/201701/17/103743ak8fbpnigeaa2j8a.jpg Figure 3: A simple tower model in CAESAR II. 4. Simulation of supports for nozzles attached to the tower: Pipelines are typically supported by the tower shell. This type of support is called a sheet metal support. The first support near the nozzle of the cylinder is the load-bearing frame that carries the total vertical load of the pipeline. Try to place this load-bearing frame as close as possible to the nozzle. All the remaining brackets are guide brackets. Since the patch plate is welded to the tower shell, it is necessary to simulate the patch plate as well as the brackets connected to its Cnode in order to obtain the thermal expansion effect at that location. The load-bearing capacity of the panel bracket is usually specified in the bracket manual. Therefore, sometimes the load on the first support frame may exceed the bearing capacity of the backing plate (this can happen if large-sized wires are connected to the top of the cylinder and are suspended). In this case, we must build a second load-bearing frame. The bracket must be a spring bracket, as it will share part of the load with the first load-bearing bracket. From this point onward, guide supports can be used for the span according to the standards specified in the project specifications. The CAESAR II model is shown in Figure 4 and can serve as a reference for the tower-attached support approach. The tower-mounted support is modeled as a weightless rigid member; the properties of the interior part of the device are the same as those of the device itself, while the exterior part is set to the ambient temperature. http://bbs.intergraphbbs.com/data/attachment/forum/201701/17/103745w03jso66kceciobw.jpg Figure 4: Support arrangement for the piping in the tower in CAESAR II. 5. Check of nozzle loads: The allowable load for nozzles is usually provided by the equipment manufacturer or mentioned in the overall equipment diagram. Some manufacturers provide standard load tables based on the pipe mouth diameter and flange class. Therefore, the load calculated at the fixed point is compared with these allowable values to determine whether the calculated value meets the requirements. If the allowable load value is exceeded, the nozzle load can be reduced by modifying the support or pipe routing. When the routing cannot be adjusted, WRC or FEA (NozzlePro) can be used to further verify the nozzle loads. Alternatively, send the load to the equipment supplier for confirmation. Calculation of the skirt temperature: The skirt temperature can be calculated using the following formula: Average skirt temperature = (T – Ta) * F + Ta; in degrees Celsius, where Ta represents the ambient temperature℃ ； T=Temperature at the top of the skirt base ； F= ； K=thermal insulation constant=1.0 (refractory bricks for insulation)=1.6 (without insulation) ； h and t represent the height and thickness of the skirt, respectively.