Thread Content
In heat exchanger design, although designers use GB/T151-2014 as the basis for designing shell-and-tube heat exchangers, they often overlook certain provisions in these standards, resulting in incorrect or unreasonable designs that affect the safety and reliability of the equipment. Below are several common issues found in our drawings, to remind designers to pay attention to them during the design process. 1 Determination of the type of heat exchanger and related technical requirements: TSG 21-2016 A1.3.2 stipulates that multi-chamber pressure vessels (such as the tube side and shell side of heat exchangers, jacketed pressure vessels, etc.) should be classified separately for each pressure chamber; when making such classifications, the design pressure shall be taken from that of the respective chamber, and the volume shall be the geometric volume of that chamber ； The highest category of each pressure chamber shall be used as the category for that multi-chamber pressure vessel, and management shall be carried out according to this category; however, design and manufacturing technical requirements shall be specified separately based on the respective categories of each pressure chamber. 2 When the design pressure of the tube side of the heat exchanger is higher than that of the shell side, the selection of the test pressure for the shell side. When the design pressure of the tube side is greater than that of the shell side, in order to check the tightness of the connections between the tubes and the tube sheet, the test pressure for the shell side can be determined using one of the following methods; this should be specified in the technical specifications or requirements: (1) Increase the test pressure for the shell side to be equal to the test pressure for the tube side. When using this method, it is necessary to first calculate the stress generated in the shell during pressure testing; the calculated value of the hoop stress at any point along the shell side must not exceed 90% of the yield strength of the material at the test temperature. At the same time, the flanges must also meet the strength requirements under pressure testing conditions. (2) If, after calculation, it is not possible to use the above methods for testing, or if such methods are not reasonable from a technical and economic perspective, then after the shell side and tube side have passed the tests at their respective required test pressures, the shell side shall undergo an ammonia leak test in accordance with Appendix A of HG/T20584-2011, \"Methods for Ammonia Leak Testing of Pressure Vessels\". (3) For heat exchangers with special requirements, such as high-pressure heat exchangers, low-pressure pure ammonia can be used for leak testing, or methods such as halogen leak detection can be employed. When the tube sheet itself serves as a flange, and its diameter is large or its thickness is great, material selection becomes an issue; in such cases, it is best to manufacture the tube sheet from forged materials, avoiding the use of sheet metal as much as possible. Because thick plates tend to delaminate, it is particularly prone to problems when using such plates to manufacture tube sheets under the aforementioned conditions. 4. Calculation of the equivalent length for compressive instability of heat exchange tubes: When performing calculations related to heat exchange tubes, some designers assign a larger value to the equivalent length L for compressive instability of these tubes, rather than using the calculations specified in GB/T151. This is incorrect, as the values provided in GB/T151 are derived from the theory of pressure bars in mechanics of materials – where the connection points between the tubes and the tube sheet are considered fixed supports, while baffle plates and support plates are regarded as hinge supports. Therefore, the equivalent length for compressive instability of heat exchange tubes should be calculated based on this approach in order to ensure the accuracy of the results related to the tube sheet. 5 Multi-pass shell-and-tube heat exchangers: Issues to consider in the pass design Ⅰ. Try to keep the number of tubes in each pass roughly equal, so as to achieve approximately equal flow velocities in each pass of the heat exchanger and thus obtain good heat exchange performance. The relative error in the number of heat exchange tubes per tube bank should be kept within 10%, with a maximum of 20%. Ⅱ Ensure that the grooves for the partition plates are as simple as possible, and that the length of the sealing surfaces is as short as possible. The issue of determining the area of the baffle slots: When using calculation software to determine the parameters of tube sheets, it is necessary to enter the area of these baffle slots. Many designers simply calculate the geometric area of the baffle slots, which is incorrect. What needs to be entered here should be A as specified in GB/T151; it refers to the area within the pipe layout region that cannot be supported by the heat exchange tubes due to the need for partition grooves and tie rod structures. Additionally, the area of the partition grooves in a multi-pass heat exchanger should be taken as the sum of the areas of all the individual partition grooves. 7. Problem of baffle arrangement in horizontal heat exchangers: In horizontal condensers where gas and liquid coexist, the baffles should be arranged on the left and right sides, with a liquid inlet provided at the lowest point of the baffles. In such cases, it is inappropriate for some designers to arrange the notches on the baffle plate one above the other; when the notches are placed this way, it hinders the drainage of condensate. Especially when the level of the condensate is above the lower notch of the upper baffle, a liquid seal is formed, which hinders steam flow and affects the heat transfer and operation of the equipment. 8. Attention should be paid to ensuring that there is sufficient space for the wrenches used to tighten the tube sheet. In heat exchangers where the tube sheet extension serves as a flange, when the pressure on the shell side is very high while the pressure on the tube side is relatively low, using equipment flanges with a lower pressure rating results in a small diameter for the bolts. At the same time, the shell side cylinder is quite thick, which can lead to insufficient space for the wrenches and make it difficult to assemble the fasteners. 9. Arrangement of tube holes and tie rod holes on the tube sheet: The tube holes and tie rod holes on the tube sheet should be arranged symmetrically. With an asymmetric layout, errors are more likely to occur during the processing of the tube sheet. If an asymmetric arrangement is necessary, it is essential to ensure that the layout drawings, as well as the drawings of the tube bundles, tube sheets, and baffle plates, are consistent with each other, since their positions vary depending on the viewing angle. 10. When used as a fixed tube sheet for flanges, the issue of matching with the equipment flanges arises. During the design process, it is common for the pressure rating of the equipment flanges to change, without taking into account any changes to the tube sheet. This can result in mismatches in the contours of the two flanges, the diameter of the centers of the bolt holes, and the number of bolt holes, which can cause significant problems during the equipment manufacturing process. 11 Positions of supports, expansion joints, and tie rods for vertical fixed-tube-sheet heat exchangers: The support plane of the supports for vertical fixed-tube-sheet heat exchangers is generally above the equipment’s center of gravity and the expansion joints, in order to enhance the stability of the equipment and improve the stress conditions on the expansion joints. If equipment equipped with lugs has expansion joints, care should be taken during design to ensure that the diameter of the circle centered on the bolt holes is larger than the maximum external dimension of the expansion joints (it is advisable to first consult the structural engineering team regarding the dimensions of the beams, and then determine the appropriate diameter for the circle centered on the bolts), so as to facilitate the proper installation of the equipment. Ⅱ For vertical fixed-tube-sheet heat exchangers, the tie rods should have their fixed ends located on the upper tube sheet, provided that this allows for proper assembly; regardless of whether the shell-side inlet is located at the top or bottom, the tie rods are in their optimal stress condition under these conditions. 12 Please address the issue of joining heat exchange tubes. If joining heat exchange tubes is not permitted, it should be specified in the drawings. If it is permitted to splice the heat exchange tubes, it should also be specified in the technical requirements of the drawings that the splicing of the heat exchange tubes complies with the provisions of GB/T 151.