09-03-6 Topic — Discuss the materials and structure of high-pressure heat exchangers (prizes for participants)

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Gaoga products: the water chamber cylinder and tube sheet are made of 20MnMo, the shell is made of Q345R, and the heat exchange tubes are made of 20G. The structure comes in two types: U-tube and coiled tube.

Urea stripping tower: titanium alloy or zirconium material, tube-and-sheet falling film heat exchanger.

The cylinder is made of Q345R, while the heat exchange tubes are made of 20G

High-pressure heat exchangers generally use shell-and-tube heat exchangers, and this is related to the thickness of the shell. Generally, it has a shell of Q345R and heat exchange tubes of 20G.

Firstly, the main factor affecting the pressure resistance of heat exchangers should be the structure. On the other hand, it is also unclear what level is used as a reference for the high pressure tolerance mentioned by the original poster. The material has a significant impact on temperature resistance. I will introduce the Ω-ring high-pressure heat exchanger, which can withstand a maximum pressure of 32 MPa. It consists of components such as tube boxes, tube bundles, and Omega rings, and its structure is basically the same as that of a conventional U-tube heat exchanger. The difference is that the gasket is replaced by an Ω ring. The principle is as follows: Under operation, the medium enters the Ω ring; since the small tubes can withstand high pressures, they are capable of withstanding pressures of up to 32 MPa ； Due to the presence of medium in the Ω ring during operation, the allowable gasket specific pressure can be set to zero, reducing the bolt pre-tightening load and thereby decreasing the diameter of the studs.

Shell Q345R, heat exchange tubes 20G.

I brought up this topic; I have summarized it for your reference. The main materials used in high-pressure heat exchangers include 12Cr2Mo1 forged material, 15CrMo(H) forged material, 12Cr2Mo1R(H) steel plates, 15CrMoR(H), as well as heat exchange tube materials such as 0Cr18Ni10Ti, 00Cr19Ni10, T22, and 15CrMo. High-pressure hydrogenation heat exchangers are generally used at the outlet of reactors, where heat exchange takes place between the reactor outputs and the hydrogenated feedstock. Therefore, high temperature and pressure are required. The high-pressure heat exchangers currently in use basically come in two forms: one is the flange type, and the other is the threaded locking ring type. 1). Structural forms, advantages, and disadvantages of flanged heat exchangers: Flat gasket structure: Metal flat gaskets are used on both sides of the tube sheet between the tube box flange and the side flanges of the tube box. Advantages: Simple structure, easy to manufacture. It is more advantageous when the thermal expansion coefficients of the flange and the gasket differ, especially under large-diameter conditions. Disadvantage: The gasket provides very little compensation for axial rebound; once there is a temperature difference between the main bolt and the flange, and if the thermal expansion of the flange exceeds that of the main bolt, such that the elongation of the main bolt exceeds its own elastic deformation limit, and this occurs during the cooling process, leakage will take place. The adopted measure: Shortening the length of the main bolts alters the sealing effect; in other words, threaded bolts should be used as much as possible, but this makes maintenance and disassembly more difficult. 2. Octagonal gasket structure (see figure below): Due to the low rebound of flat gaskets, the octagonal gasket design was adopted to address this issue; it is widely used today. However, leakage problems have been observed when the diameter of the heat exchanger is large. The material of the octagonal gasket often has a different linear expansion coefficient from that of the flange; when the temperature is high and the diameter is large, the significant difference in expansion can lead to leakage. Furthermore, wrinkles and deformations are often found on the sealing surface of the octagonal gasket, as well as indentations in the grooves of the flange sealing surface. This is why the sealing surface must be repaired again and the octagonal gasket replaced during maintenance. 3. Ω-shaped structure (as shown in the figure below). In factory production, leaks can cause significant damage and pose serious risks to the environment and personnel; therefore, the Ω-ring structure is used. Advantages: It provides reliable sealing and is less prone to leakage. This design offers significant rebound compensation; when the main bolts are stretched too much beyond the range that elastic deformation can cover, the Omega shape can open and close, thus providing substantial compensation capacity. It is safe and reliable for use in food applications. Disadvantage: It is difficult to disassemble; during maintenance, the O-ring must first be cut along the center line, and upon reinstallation, the cut must be repaired and then welded shut. An Ω ring cannot be used multiple times, as this will reduce its reliability. 2) Threaded locking ring heat exchanger: 1. Structure and interrelationships between components. It has a complete housing, with both the tube side and shell side sharing the same cylinder. One end of the cylinder is fitted with a head, while the other end features a threaded pressure ring and a gland that are pressed against the sealing disc. Metal gaskets surrounding the sealing disc ensure isolation from the outside environment, preventing leaks. The tube bundle is placed inside the cylinder; gaskets are used between the tube sheet on the tube bundle and the inner step of the cylinder to separate the tube side from the shell side. The part of the tube sheet adjacent to the tube box side is in close contact with the inner sleeve; the other end of the inner sleeve is pressed by a compression bolt, which is mounted on a collar. The collar is placed in a groove on the inner wall of the tube box, and it is designed as several separate sections to facilitate installation. By tightening the compression bolt on the collar, force is transmitted through the inner sleeve to the gasket at the tube sheet, thereby compressing it. A compression ring is placed on one side of the collar, with its other side resting against the sealing disc. The compression ring serves two functions: firstly, it limits the position of the gland to prevent damage to the sealing disc during transportation; secondly, it transmits the compressive force exerted by the bolts on the inner ring of the threaded compression ring. If a leak is detected in the gasket on the tube sheet side during operation, the inner ring bolts can be tightened directly to eliminate the internal leakage. When tightening the inner and outer ring bolts on the threaded ring, the internal pressure rod is pressed first; since this rod is relatively long, it acts like a spring after being compressed, thereby compensating for any insufficient rebound of the gasket. The inner sleeve is divided into two semi-cylinders by a partition; one of these semi-cylinders is covered with a semi-circular lid, thereby completely separating the inlet and outlet of the tube box. To prevent short circuits, a sealing mechanism is installed in the other semi-cylinder and fixed to the inlet of the tube box, so as to avoid short circuits occurring in the annular gap between the inner sleeve and the cylinder body. This post was last edited by zgd1237 on 2009-3-6 11:40]

Hydrogenation high-pressure heat exchanger; 225Cr-1Mo+0Cr18Ni11Ti composite steel plate. The mixed hydrogen feed oil heat exchanger is a key device in hydroprocessing units; it operates under conditions of high temperature, high pressure, exposure to hydrogen, and hydrogen sulfide over extended periods of time. The main material used for this device is 225Cr-1Mo+0Cr18Ni11Ti composite steel plate. Through analysis and testing, reasonable forming and assembly procedures were developed, enabling solutions to the challenges associated with rolling thick-walled composite steel plates, forming end caps, and assembling the components. After final inspection, all specifications of the device met the required standards, satisfying the design requirements. The mixed hydrogen feed oil heat exchanger is composed of two high-pressure U-tube heat exchangers assembled one on top of the other; it belongs to category III pressure vessels. Its structure is shown in Figure 1, and its main design parameters are listed in Table 1. 　　 The equipment casing, as well as the cylinders and elliptical end caps of the tube banks, are made from 225Cr-1Mo+Cr18Ni11Ti stainless steel composite plates. The pipe flanges, as well as the flanges on the casing and tube banks, are forged as single units; a double layer of stainless steel is welded onto their inner surfaces – specifically, 225Cr-1Mo+ is welded over E309+E347. The sealing surfaces of the pipe flanges, as well as those of the casing and tube bank flanges, are made of metal elliptical gaskets and octagonal gaskets. 2 Head pressing: Since the thickness of the base steel plate for the head is 50 mm and its diameter is only 1000 mm, hot pressing is required for head formation. However, the base layer of this steel is chrome-molybdenum steel, while the overlay layer is austenitic stainless steel; as a result, the heat forming temperatures, heat treatment temperatures, and control methods required for the base layer and the overlay layer differ from each other. During heat forming or heat treatment, it is necessary to take into account both the corrosion resistance of the stainless steel overlay layer and the mechanical properties of the chrome-molybdenum steel base layer. Through analysis, it is believed that the mechanical properties of chromium-molybdenum steel should be primarily ensured. Although, when cooled slowly at 550–850°C, chromium carbide (Cr23C6) may precipitate at the grain boundaries in stainless steel claddings, leading to grain boundary corrosion, these materials have a low carbon content and contain elements that enhance stability. By taking appropriate measures during processing to minimize this effect—such as, when heating, adhering to the heating guidelines for Cr-Mo steels while minimizing the time the stainless steel spends in the sensitization zone, reducing the number of heating cycles, and controlling the heating temperature and final pressing temperature—to the greatest extent possible, the mechanical properties of the base material can be preserved. To this end, two test plates were used for the experiments. First, the heating process during hot pressing was simulated according to Figure 2 (with particular attention paid to controlling the heating and final pressing temperatures); one plate underwent a thermal cycle in the N+T+PWHT mode, while the other was subjected directly to PWHT at 690℃±10℃ for 3 hours. Subsequently, samples were taken for testing of mechanical properties, corrosion resistance, and grain size. Strict control of the heating temperature and final pressing temperature during head hot pressing, as well as whether N+T treatment is carried out after hot pressing, has no significant effect on the mechanical properties of the base material. It has little impact on the corrosion resistance of the clad layer. To prevent a decline in the bonding quality between the clad layer and the base layer of the composite steel plate, as well as in the corrosion resistance of the protective layer at the joints between the head and the shell, it was decided to implement strict control over the heating and final pressing temperatures for the head; no further normalizing and tempering treatments will be carried out after hot pressing. After head forming, the geometric dimensions, wall thickness reduction, and adhesion between the base layer and the coating all met the required standards. The test specimens of the heads treated in conjunction with the furnace underwent various performance tests after the overall heat treatment of the products, and all results met the specified requirements. 3 Unloading and rolling of the cylinder shell: Due to the high requirements regarding the misalignment between the composite steel plate segments and the end caps during assembly, only a misalignment of 2 mm is allowed. To ensure that this misalignment does not exceed the specified limit, the perimeter of the cylinder segments must be calculated based on the inner diameter of the end caps, with the values converted to the diameter of the cylinder segments themselves. Moreover, stricter requirements are imposed on the length of these segments than those specified in the standards; the allowable tolerance for the roundness of the cylinder segments is 4 mm. The center radius Rf of the tube box joints and the shell cylinder is 525 mm. The total thickness of the base layer and overlay combined is 46 + 4 = 50 mm. The deformation rate after rolling is given by δ/2Rf(1 – Rf/R0) × 100% = 4,76% (where R0 is the center radius before bending). This value falls within the plasticity limits of SA387Gr22CL2 material, allowing for cold rolling; the cold-worked hardened structure resulting from cold rolling can be eliminated during the subsequent welding and overall heat treatment processes. During rolling, to prevent iron ion contamination caused by direct contact between the clad surface and the iron rollers, a thin stainless steel plate with a thickness of δ=0.5 mm is placed on the stainless steel clad side before rolling it directly on the rolling machine. On the rolling machine, the tube sections are first pre-bent to have straight edges; after passing inspection using templates, those straight edges are removed, followed by rolling to give the desired shape, ensuring that the roundness of the tube body meets the requirements. 4 Tube section alignment: To ensure proper alignment and weld quality, tube sections are aligned using positioning blocks, as shown in Figure 3. These positioning blocks are welded to the base layer, and their welding requirements are the same as those for the main welds. 5 Alignment of the nozzles and the cylinder: To prevent excessive welding deformation after welding the nozzles to the cylinder, thereby failing to meet the assembly requirements, a deformation prevention fixture as shown in Figure 4 was used. First, spot-weld the pipe connections to ensure their dimensions and orientation, and then fix the pipes using connection plates, fixing reinforcement ribs, and adjustment shims. Due to its high rigidity, large tensile stresses exist in the welds. During welding, a low wire feed rate and narrow weld beads are used, along with multiple layers of welds; hammering is employed between layers to eliminate the welding stresses. The anti-deformation fixtures were removed after the overall heat treatment to prevent deformation, and actual measurements showed that the dimensions of the pipe flanges met the installation requirements. 6 Overlapping Assembly: To ensure reliable sealing between the nozzles and flanges of the upper and lower heat exchangers, as well as between the tube boxes, the shell, and the tube sheets after the two heat exchangers are overlapped, it is necessary to maintain high assembly precision. Since a metal elliptical gasket is used for hard sealing between the connection surfaces (see Figure 5), high precision is required in aligning the upper and lower flanges. The design specifications require that the deviation between the center of the connection pipe and the sealing surface of the shell flange be within ±3 mm, while the deviation between the center of the tube box connection pipe and the shell connection pipe should be within ±6 mm (see Figure 1). This precision makes it difficult to meet the sealing requirements between the upper and lower flanges when the two units are overlapped. Furthermore, the seals between the shell, tube box, and tube sheet are all metal octagonal gasket seals; it is difficult to make adjustments once the centerlines of the tubes in the upper and lower shells are not aligned. Therefore, a secondary pre-assembly scheme was developed to ensure assembly accuracy. First pre-assembly: First, assemble the shell by overlapping it according to the drawings (the upper connections and upper supports of the lower heat exchanger shell are not welded together). Install the upper and lower tube boxes on the shell respectively; at the same time, fit the octagonal gaskets between each tube box and the shell flanges, and tighten the flange bolts and nuts. Next, insert the connection flanges and elliptical gaskets for the two heat exchanger tube boxes, and tighten the bolts and nuts (the connections for the upper unit have already been welded). Using the upper unit as a reference, determine the assembly of the upper connections for the lower unit’s tube box, and weld the flanges of those lower tube box connections. After the tube box inlet flanges and partition plates are welded together and stress-relief heat treatment is carried out, a second pre-assembly is performed. Second pre-assembly: First, install the upper and lower tube bundles into the shell respectively, attach the corresponding tube boxes to them, and also insert octagonal gaskets; then tighten the bolts and nuts. Assemble the two heat exchangers on top of each other in accordance with the diagram requirements. Next, fit sealing gaskets onto the pipe flanges of the two heat exchangers and tighten the bolts and nuts (all pipe flanges except those of the lower shell have been welded). Use the tube box pipe flanges as a reference to weld the pipe flanges of the lower shell. Through two pre-assemblies, the errors caused by machining, welding, and heat treatment distortions can be eliminated, ensuring dimensional accuracy and enabling a successful final assembly and hydrostatic test on the first attempt. Since its installation, the equipment has been operating well and meets the process requirements for production.

Let me talk about the structure. Generally, they have thicker walls, and the material is usually in the normalized state ； For reinforcement of the takeover opening, integral reinforcement is used, with forged takeover fittings being common. Unilateral (shell or tube side) high pressure generally results in a larger thickness of the tube sheet, which should be a focus of manufacturing inspections; the forging grade should be at least grade III. To meet the sealing requirements, the type B connection is generally adopted. When both the tube side and shell side are at high pressures, the tube sheet is usually designed using a pressure difference approach.

How widely is the threaded lock ring type used? ? ? How can it be set high enough?

Shell-and-tube heat exchangers using Q345R for their shells are quite common; to improve their heat transfer efficiency, finned tubes welded electrically in the tube side are being used more and more often.

Commonly used high-pressure heat exchangers mainly include thread-locking ring heat exchangers, Ω-ring heat exchangers, shell-and-tube heat exchangers, and U-tube heat exchangers. The threaded lock ring heat exchanger has a complex structure, requires large amounts of metal, involves many machined components, results in high costs, and entails substantial maintenance work ； The Ω ring of Ω-ring heat exchangers is difficult to manufacture, and maintenance is inconvenient ； Shell-and-tube heat exchangers require more space for the same heat transfer area, and their cost per unit of heat transfer area is high; they are generally suitable only for applications with relatively small heat transfer areas ； U-tube heat exchangers feature a simple and compact structure, as well as minimal metal consumption under high-pressure conditions; therefore, they are used far more frequently in such conditions compared to thread-locking ring heat exchangers, Omega-ring heat exchangers, and shell-and-tube heat exchangers

High-pressure heat exchangers mostly use chromium-molybdenum steel, or forged-welded shells