【Major Chemical Equipment】056 – China’s largest-diameter, highest-tonnage threaded heat exchanger

Founded in the early 1960s, Daqing Petrochemical Machinery Factory has, through over 50 years of development, become a comprehensive technical enterprise specialized in the design and manufacturing of petrochemical equipment, the processing of spare parts, valve maintenance, measurement calibration, and equipment monitoring. Following the complete relocation in 2013, the mechanical factory’s production capabilities, manufacturing technologies, and service standards achieved a qualitative leap; the core competitiveness of the \"Qinghua Ji\" manufacturing brand was significantly enhanced. With over 40 different types of new, high-efficiency heat exchangers, reactors, containers, and towers, as well as more than 400 intellectual property rights and proprietary technologies related to key manufacturing services, the factory is able to supply top-quality equipment products to various petrochemical companies on a continuous basis. Over the years, the machinery factory has relied on lean management as a key approach, and through measures such as the automation of equipment manufacturing, the informatization of production processes, and the visualization of work areas, it has continuously optimized its production processes, improved production efficiency, and enhanced product quality, thereby promoting the sustainable development of the factory with high quality. Last year, the tonnage of equipment manufactured by the machinery factory increased by 233% on a year-on-year basis, enabling the company to turn losses into profits and thus opening a new chapter in its development. In August 2018, two high-pressure threaded locking ring heat exchangers made of special materials, manufactured by this factory, were successfully installed in the hydrocracking unit of an oil refinery. The successful manufacture of this heat exchanger filled the gap in CNPC’s manufacturing technology for hydrogen-facing high-pressure threaded locking ring heat exchangers made from special materials. It marked a significant breakthrough in CNPC’s capability to produce high-end refining and petrochemical equipment, giving the company a voice in the field of manufacturing equipment from special materials.

All the forces acting on the heat exchanger are borne by the threaded locking ring and the trapezoidal threads at the ends of the shell. The thread design is crucial for heat exchangers equipped with threaded locking rings; these threads bear the combined loads from both the tube side and the shell side, serving as the \"final point of force application\" for these loads. The threads of the threaded lock ring heat exchanger are short trapezoidal threads in accordance with the AISI B1.8 \"Stub Acme Screw Threads\" standard, with a size of (15/16) in (23.8125 mm). These threads feature a large intermeshing height as well as strong shear and bending resistance, making them suitable for use in threaded lock ring heat exchangers. The tolerance range for these threads is relatively wide, as there are many external factors involved in the tolerances for the fit between internal and external threads. Moreover, since the threads operate at high temperatures, the threaded forgings may experience slight deformation; if the tolerances for this fit are not chosen appropriately, it may become difficult to disassemble them. It has long been believed that the more teeth a thread has, the better. However, according to calculations, for the entire end thread, it is the few teeth located near the inside of the tube box that bear the greatest stress, while the stress on the other teeth decreases as one moves away from them. Therefore, each tooth of the entire end thread is subjected to different forces, and this is also a drawback of this type of heat exchanger, as the forces cannot be evenly distributed across all the teeth.

Selection of gaskets: Sealing performance and resilience are two important criteria for selecting gaskets. Tube sheet gaskets and inner sleeve gaskets only need to withstand the force generated by the pressure difference across the tube sheet as well as the force required for sealing at the gaskets themselves. Since the gasket coefficient m and specific pressure y of wound gaskets are low, in order to reduce the sealing force between the tube side and the shell side and to decrease the diameter and number of bolts, wound gaskets are used for tube sheet gaskets and inner sleeve gaskets; these gaskets are composed of an S30403 metal framework combined with flexible graphite material. The upper and lower surfaces of the metal frame feature concentric grooves of special shapes that are offset from each other, forming wave-like teeth; a layer of flexible expanded graphite is applied to both surfaces of the metal frame, thereby creating an integral composite gasket. The external sealing gasket is made of S30403 metal, which ensures both the strength of the gasket and its ability to meet sealing requirements. The split-ring design is an optimized feature of this heat exchanger; it addresses the issue of balanced force transmission when the pressure in the shell side and the tube side is not equal, thereby reducing the stress on the threaded pressure rings. Grooves are designed on the inner wall of the tube box, and the upper and lower coupling rings are placed within these grooves; in order to enable force transmission, compression bolts are provided on the coupling rings ； Due to assembly requirements, the clasp is designed as four pieces; although this poses difficulties in manufacturing, it enhances operational reliability and balances the forces acting on the upper and lower rings.

Should 625 alloy be used for heat exchange tubes?