Q&A on Pressure Vessel Design (III)

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Question 1: How should the supports of an overlapping heat exchanger be arranged? Answer: The support design for overlapping heat exchangers can take the following special considerations into account: (1) Shims for adjusting height should be installed at the supports between overlapping heat exchangers. (2) The distance from the support base plate to the equipment centerline should be at least 5 mm less than the distance from the sealing surface of the connection flange to the equipment centerline. (3) If necessary, the supports and casing of the lower heat exchanger shall be checked; in addition to the load from the heat exchanger itself, the weight of the upper heat exchanger must also be taken into account. (4) When the mass of the overlapping heat exchanger is large, an additional set of supports can be added. Question 2: What are the requirements for the width of the tube sheet hole bridges? Answer: The width B of the hole bridge between two adjacent tube holes in the tube sheet, as well as the minimum width Bmin of such hole bridges, must be determined based on measurements taken on the side where drilling ends, in order to ensure the quality of the connection between the tube sheet and the heat exchanger. Question 3: What is the difference between design pressure and calculated pressure? Answer: (1) The definitions are different. (2) They have different properties. (3) The basis for value assignment is different. (4) Different uses. (5) For pressure vessels with non-circular cross-sections such as rectangles, the calculation pressure should be the pressure during the hydrostatic test, that is, the maximum load-bearing pressure during inspection and use. (It has no direct relation to the design pressure. ) (6) The design pressure is indicated in the technical data sheet of the pressure vessel’s general layout or assembly drawing, as well as on the pressure vessel’s nameplate ； Calculation pressure only appears in the calculation documents for pressure vessels. Question 4: What is the maximum allowable operating pressure? Answer: The maximum allowable operating pressure is defined as the maximum gauge pressure that the top of the vessel is permitted to withstand at the design temperature. This pressure is calculated based on the effective thickness of the shell sections of the container, and the minimum value is taken. Using the maximum allowable operating pressure of the vessel as the starting pressure for overpressure can maximize the potential of pressure vessels and ensure smoother operation. However, the designer must strictly calculate the maximum allowable operating pressure in accordance with the relevant definitions. The container strength and pressure test pressure are calculated using the maximum allowable operating pressure of the container instead of the original design pressure, and this is indicated on the drawings and nameplate. Question 5: What is the design temperature? Answer: (1) The design temperature refers to the metal temperature of the pressure-bearing components under normal operating conditions (the average temperature along the metal cross-section of the components). It, together with the design pressure, serves as the design load condition. However, unlike design pressure, it represents the loading condition of the entire container ； The design temperature refers to the loading conditions of a specific pressure-bearing component. (2) The design temperature indicated on the technical data sheet of the general layout or assembly drawing and on the product nameplate is the maximum or minimum design temperature of the shell, and can be regarded as the design load condition for the entire container corresponding to the design pressure. Question 6: What is the difference between a pressure test and a leak test? Answer: Pressure testing is a mandatory test for pressure vessels, and it also serves as an important basis for their acceptance. For pressure vessels, the main purpose of pressure testing is to evaluate the overall strength of the vessel, check the integrity of the welded joints, and assess the sealing performance of the sealing mechanisms. For pressure vessels, including vacuum vessels, the main purpose of pressure testing is to verify the integrity of the weld joints and the sealing performance of the sealing structures, rather than to test the vessel’s stability under external pressure. The purpose of the airtightness test is to take advantage of the higher permeability of gases compared to liquids, in order to rigorously assess the tightness of welded joints and the sealing performance of sealing structures, and to determine whether there are any unacceptable leaks in the container. Airtightness testing is not a mandatory test for pressure vessels. This is because most pressure vessels do not have strict requirements regarding leakage, and pressure testing also serves a certain purpose in detecting leaks. The airtightness test is only permitted after the pressure test is successful. It should be clearly specified in the design documents whether a leak test is required for containers that have undergone a pressure test. Question 7: Regarding the installation of the air barrier? Answer: When the design temperature of the tower or its bottom section is equal to or greater than 400°C, an air barrier should be installed at the upper part of the skirt cylinder, near the head, in order to reduce the thermal stress at the joint weld between the lower head and the cylinder. Question 8: What are the considerations regarding the installation of manholes in plate towers? Answer: (1) For plate towers, a manhole should be provided every 10 layers of plates; in other words, the distance between adjacent manholes should be around 5 meters, and the manholes should be located between adjacent plates. The distance between trays at the location of the manhole should be determined based on the diameter of the manhole; it must be greater than the sum of the nominal diameter of the manhole, the height of the tray support beam, and 50 mm, and shall not be less than 600 mm. (2) The distance from the centerline of the manhole to the upper surface of the platform should be between 600 mm and 1000 mm, and shall not exceed 1200 mm. (3) The manholes of plate towers should be positioned away from internal components such as liquid distributors ； (4) When the nominal diameter of the tower is less than 1 m and the spacing between the manholes is small or their number is large, it is not advisable to locate the manholes in the same orientation, in order to avoid significant bending deformation of the tower shell after welding. (5) If necessary, handles and ladder rods for accessing the manholes inside the tower should be provided. Question 9: What should be the minimum thickness for head forming? Answer: The minimum thickness of the head formation shall not be less than the calculated thickness plus the corrosion allowance. This thickness shall be indicated in both the schedule and the drawings. If there are openings in the head, the reinforcement calculation shall be performed using the minimum thickness of the formed section. Pay special attention to the potential step change in the allowable stress for nominal thickness.