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1 Introduction In petroleum refining and petrochemical production, exchangers are essential equipment to ensure the proper operation of the production processes. According to statistics, in the petrochemical industry, heat exchangers account for about 20% of the total weight of the process equipment, with some values reaching as high as 30%; in refineries, they even make up 40% of the total weight of the equipment. Heat exchangers are widely used in the chemical, petroleum, and many other industrial sectors to transfer heat from a fluid at a higher temperature to another fluid at a lower temperature, thereby enabling heat transfer and achieving the desired process objectives. There are also many types and structures of them. Commonly used heat exchanger types include fixed-tube-sheet heat exchangers, floating-head heat exchangers, and U-tube heat exchangers; the selection and design of such exchangers must be based on specific conditions. This article focuses on the selection, design, and testing requirements for heat exchangers under the following process conditions. This heat exchanger is a condensate oil treatment unit designed for the surface construction project of the Yingmaqi gas field complex in Xinjiang. The design parameters are as follows: Design pressure of the shell side (MPa): 2.50; Design pressure of the tube side (MPa): 12.00; Design temperature of the shell side (°C): 280.00; Design temperature of the tube side (°C): 50.00; Nominal diameter of the cylinder (mm): 500.00; Heat exchange area (m2): 40; Inlet temperature of the shell side (°C): 240; Inlet temperature of the tube side (°C): 30; Outlet temperature of the shell side (°C): 200; Outlet temperature of the tube side (°C): 58.7; Material in the shell side: Heat transfer oil; Material in the tube side: Pre-stabilized condensate oil; Flow rate of the material in the shell side (m/s): 1.51; Density of the material at the shell side inlet (Kg/m3): 737.32. Equipment selection: Given the high design pressure on the tube side and the large temperature difference between the shell and tube sides, if a fixed-tube-plate heat exchanger were used, expansion joints would be necessary to reduce the stress on the various components of the heat exchanger (tube plate, tubes, shell) due to these high temperature differences. However, the use of expansion joints would result in high stresses on the tubes and tube plate under the high pressures on the tube side, preventing them from meeting the required conditions. Therefore, a fixed-tube-plate heat exchanger is not suitable ； In floating-head heat exchangers, the high pressure in the tube side results in thick fixed and floating tube sheets, as well as thick gussets and floating flanges, making the entire device quite bulky ； From an economic perspective and to meet the process requirements, U-tube heat exchangers have fewer components such as floating tube sheets, hook rings, and floating flanges, which simplifies the equipment and saves costs; therefore, U-tube heat exchangers are chosen. 3 Equipment Design 3.1 Material Selection Based on the design requirements, 16MnR material is used for the plates of this heat exchanger; the forgings are made from the same material, namely 16Mn forgings. Due to the high pressures involved, 20G high-pressure pipes (GB5310) are used for the heat exchange tubes. 3.2 Selection of the tube box head: Due to the high design pressure in the tube side, the nominal thickness of the head is relatively large; therefore, the thickness reduction rate during manufacturing is over 12%. To ensure that the minimum thickness of the head after forming is met, the thickness of the steel plate used must be greater than the nominal thickness of the head. Based on the design parameters, the calculated thickness of the tube box cylinder is http://www.nmtech.com.cn/jishuwang/upload1/070801905447088.jpg. The nominal thickness of the tube box cylinder is 24 mm, which is the rounded value obtained by adding the thickness allowance to the calculated thickness ； If a standard elliptical head is used, the calculated thickness of the head is as shown in http://www.nmtech.com.cn/jishuwang/upload1/070801906204332.jpg. The minimum thickness for the tube box head is 22.75 mm, which is the calculated thickness plus the additional thickness allowance ； If a hemispherical head is used, the calculated thickness of the head is as shown in http://www.nmtech.com.cn/jishuwang/upload1/070801906534939.jpg. The minimum thickness for the tube box head is 12.98 mm, which is the calculated thickness plus the additional thickness allowance ； To weld with the tube sheet shell, the nominal thickness of the head is set at 24 mm. Due to thickness reduction during manufacturing, standard elliptical heads cannot meet the strength requirements; whereas if such heads do meet the strength requirements, their thickness is too great, resulting in waste. Therefore, hemispherical heads are chosen, but it is important to ensure that the thickness of the straight edges of these hemispherical heads corresponds to the nominal thickness of the tube sheet shell. 3.3 Selection of reinforcement methods for pipe box connections: Due to the high design pressure in the tube side, reinforcement rings are only suitable for reinforcing openings in medium and low-pressure vessels. Embedded reinforcement, however, is complex to manufacture and costly, so it is used only in critical equipment, such as vessels whose material yield strength is above 500 MPa. Therefore, integral reinforcement is chosen ； If the thickness of the shell is increased, calculations show that a thickness of 46 mm is required, which results in significant material waste. Therefore, thick-walled nozzles are used for reinforcement; in this type of reinforcement, all the material used for strengthening is located in the areas under the greatest stress, thus enabling more effective reduction of stress concentration. Since this design leads to an excessively large protrusion height of the nozzles, we opted for reinforced flanges made of thick-walled materials. 3.4 Selection of the sealing surface type for the tube sheet The sealing surface types for the tube sheet include full-flat sealing, raised-face sealing, male-female face sealing, tenon-and-socket face sealing, and ring joint sealing. According to JB/T 4700–4707–2000 “Flanges for Pressure Vessels,” this standard applies only to flanges with a nominal pressure of 6.4 MPa or less. Meanwhile, the Ministry of Chemical Industry’s standards HG20592–20635–97 “Steel Pipe Flanges, Gaskets, and Fasteners” permit flanges to operate at a nominal pressure of up to 42.0 MPa. These standards specify that when the nominal pressure is 10.0 MPa or greater, the allowable sealing surface types are raised-face sealing and ring joint sealing. Non-metallic flat gaskets are suitable for nominal pressures ranging from 0.25 MPa to 4.0 MPa; PTFE-lined gaskets are suitable for nominal pressures from 0.6 MPa to 4.0 MPa; flexible graphite composite gaskets are suitable for nominal pressures from 1.0 MPa to 6.3 MPa; metal-lined gaskets are suitable for nominal pressures from 2.5 MPa to 10.0 MPa; wound gaskets are suitable for nominal pressures from 1.6 MPa to 16.0 MPa; toothed composite gaskets are suitable for nominal pressures from 1.6 MPa to 25.0 MPa; and metal ring gaskets are suitable for nominal pressures from 6.3 MPa to 25.0 MPa. The design pressure for the tube side of this heat exchanger is 12.0 MPa, so wound gaskets, toothed composite gaskets, and metal ring gaskets can be used. Since this heat exchanger is a pressure vessel that is frequently disassembled, and wound gaskets as well as toothed composite gaskets are disposable items, metal ring gaskets are preferred because they are both wear-resistant and easy to manufacture. Therefore, metal ring gaskets are chosen for this heat exchanger, with ring joint sealing as the corresponding sealing surface type. Since the metal ring gasket does not come with a partition seal, an elastic and flexible aluminum tube is chosen based on experience as the sealing gasket for this partition. 3.5 Selection of tube arrangement pattern: There are two ways to arrange the tubes: in a triangular pattern or a square pattern. The triangular arrangement is used when it is necessary to keep the fluid on the shell side clean, to prevent scaling, or when the fouling on the shell side can be removed through chemical treatment ； The square arrangement allows for mechanical cleaning of the outside of the tubes, and it is suitable for fluids prone to scaling; therefore, the square arrangement is chosen. The bending radius of the bent section of the U-tube must be at least twice the outer diameter of the heat exchange tubes. The outer diameter of the heat exchange tubes used in this heater is 25, so the bending radius of the U-tube’s bent section must be at least 50. According to GB151, the distance between the centers of adjacent tubes on either side of the partition slot is 44, while the distance between the centers of the heat exchange tubes themselves is 32. Due to the requirements of the process conditions and structural constraints, the first set of bent tubes can only be arranged at an angle. If the second set of bent tubes were arranged vertically, the distance between adjacent tubes would be 89, which still does not meet the minimum bending radius requirement for the U-tube’s bent section. Therefore, the second set of bent tubes also has to be arranged at an angle. It is important to ensure that the tubes do not collide with each other during arrangement, and they must remain within the limits defined by the tube layout, as shown in Figure 1. http://www.nmtech.com.cn/jishuwang/upload1/070801907336234.jpg Figure 1: Tilted arrangement of the bent tubes. Since the product of the square of the fluid velocity at the inlet of the shell side of this heat exchanger and the fluid density is 737.32×1.512=1681, which is less than 2230 kg/(m·s2), it is not necessary to install anti-erosion plates or guide cylinders at the inlet pipe of the shell side ； Based on the above analysis, the design of this heat exchanger is shown in Figure 2. http://www.nmtech.com.cn/jishuwang/upload1/070801908036517.jpg Figure 2: Design diagram of the high-pressure heat exchanger. Figure 4: Equipment inspection. Heat exchangers must be inspected and accepted in accordance with relevant standard specifications during and after manufacturing; this is also an aspect that must be taken into consideration in heat exchanger design. 4.1 Inspection and acceptance of materials: In addition to following the regulations applicable to materials used in general pressure vessels, for this equipment it is necessary to note that since the design pressure on the tube side is 12 MPa, the steel plates used for the shell of the tube box must be subjected to ultrasonic testing one by one; they must meet the Grade III requirements as specified in JB4730 \"Non-destructive Testing of Pressure Vessels\" ； Furthermore, since this equipment is a Class III pressure vessel, the tube sheet section made of steel plates and forgings must be re-inspected in accordance with Article 25 of the Pressure Vessel Regulations. 4.2 Inspection and acceptance during equipment manufacturing The inspection and acceptance during equipment manufacturing shall be carried out in strict accordance with Article 6 of GB151. 4.3 Pressure Test The pressure test, also known as a pressure resistance test or strength test, is a final comprehensive inspection carried out in accordance with standard requirements after the manufacture of pressure vessels before they are released for use; it serves as an important basis for the acceptance of such vessels. This high-pressure heat exchanger is subjected to pressure testing in accordance with the requirements of GB151. The sequence of tests is as follows: ① Perform pressure testing on the shell side using a test pressure ring, while simultaneously checking the joints between the heat exchange tubes and the tube sheet ； ②Piping pressure test ； The difficulty in pressure testing this equipment lies in testing the joints between the heat exchange tubes and the tube sheet. GB151 stipulates that \"when the test pressure on the tube side is higher than that on the shell side, the joint testing shall be carried out in accordance with the specifications in the drawings, or using a method agreed upon by both the supplier and the buyer.\" This provision specifies that the designer should consider solving this issue before manufacturing. The welding of tubes to the tube sheet is crucial in the design, manufacturing, and inspection of heat exchangers; it directly determines the quality and service life of these devices. Yet, it represents a relatively weak point in heat exchangers. In practice, while the equipment’s casing and flanges may still be usable, the heat exchanger itself often becomes unusable due to corrosion of the heat exchange tubes or leaks at the tube ends. Therefore, selecting the appropriate pressure testing method for the tube ends and accurately identifying the leakage points are key factors in producing high-quality heat exchangers. For heat exchangers with the same design pressure for the tube side and shell side, a hydrostatic test can be carried out directly to check for leaks at the tube ends. In this particular heat exchanger, since the hydrostatic test pressure for the tube side is higher than that for the shell side, the test pressure on the shell side can be increased to match that of the tube side in order to verify the tightness of the connection between the tubes and the tube sheet. However, it is necessary to calculate the stresses generated in the shell during the test; the calculated value of the tensile stress at any point in the shell must not exceed 90% of the material’s yield limit at the test temperature (or the yield limit corresponding to a residual deformation of 0.2%). Additionally, the connections such as nozzles and flanges must also meet the strength requirements under pressure testing. Due to the excessive difference in design pressures between the tube and shell sides of this heat exchanger, this method is clearly not applicable; therefore, we opted for another method—the ammonia leakage test. In this method, after the tube side and shell side are tested at their respective test pressures, the shell side is subjected to an ammonia leakage test using compressed air containing about 1% ammonia at a pressure of 1.05 times the design pressure for the shell side, or using low-pressure pure ammonia. In this way, without increasing the test pressure on the shell side, it is still possible to determine whether there are leaks at the tube ends, thereby improving the sealing performance, reliability, and safety of the container. 5 Conclusion In the design of high-pressure heat exchangers, it is essential to take all relevant factors into consideration, select the appropriate type of heat exchanger equipment as well as the suitable manufacturing and inspection methods, in order to eliminate potential safety hazards. At the same time, factors such as ease of processing and cost-effectiveness must also be taken into account in order to design high-quality pressure vessel products.