Bellows compensator Why did the leak occur?
 

　　Leakage in corrugated expansion joints is mostly caused by improper construction and installation, which results in the pipeline shifting from its designed position during operation. Due to inconsistent pressure-bearing capacity of the pipeline foundation, when uneven dynamic or static loads are applied to the ground above the pipeline, the pipeline undergoes uneven settlement, leading to leakage in the sleeve-type expansion joint. Therefore, deformation of the corrugated expansion joint is the root cause of leakage in such joints.
 

　　The compressive strength of the native soil or backfill beneath the fixed support foundation differs from that of the side backfill. If dynamic or static loads are applied to the ground above the fixed support, the support will experience deflection. The torque generated by this deflection will be transmitted through the pipeline to the corrugated expansion joint, potentially causing leakage in the pipeline compensator. Typically, steel pipes have a length of 12 meters. To meet the requirement for extended installation lengths, multiple directly buried insulated pipes need to be welded together. In reality, this results in a pipe that is effectively a broken line.

　　The sleeve expansion joint consists of a main pipe, an expansion pipe, a pressure flange, and a sealing ring. It is used in water supply and drainage projects, providing convenient installation and replacement for water towers, pump house meters, and valves in urban areas. Moreover, it effectively compensates for thermal expansion and contraction caused by temperature differences in long-distance pipelines. The sleeve expansion joint comprises an inner sleeve and an outer sleeve that can undergo relative axial movement; the space between the inner and outer sleeves is sealed by a stuffing box. During operation, the pipes at both ends can move along the same axis. Guiding supports are installed at both ends of the expansion joint. Its advantages include low frictional resistance to fluid flow and a compact structure; its disadvantages, however, are poor sealing performance and significant thrust on fixed supports.

　　According to the specifications, for internal-pressure-type installations, pre-stretching is required based on the ambient temperature; for external-pressure-type installations, pre-compression is required based on the ambient temperature. Since bellows compensators are pre-stretched or pre-compressed before leaving the factory—or come with a substantial margin—on-site adjustments by the construction unit are not necessary. For detailed instructions, please refer to the manufacturer’s requirements. However, prior to use, it is essential to determine whether the compensator is of internal-pressure or external-pressure type, as the two types are fundamentally different and have distinct application requirements. The internal-pressure type can be cut open to expose the corrugated section and allow visual inspection; in contrast, the external-pressure type cannot be cut open—it effectively exposes the working steel pipe, which could lead to medium leakage.

　　The rectangular bellows expansion joint is one of the auxiliary devices used in power plant flue gas ducts. The rectangular bellows expansion joint features a rectangular cross-section and rounded wave profiles, enabling a single expansion joint in the pipeline to accommodate two-dimensional displacement. A bend assembly consisting of two expansion joints can handle three-dimensional displacement. Round-cornered rectangular metal bellows expansion joints are available in both full-height and half-height versions. Depending on the dimensions of the flue gas duct, users can select from a variety of connectors based on their stress and strain requirements.

　　After selecting a suitable rectangular expansion joint based on the thermal displacement of the user’s piping system, the universal bellows expansion joint must at least provide the following parameters: the circulating medium within the pipeline, the design pressure of the flue gas duct, the high operating temperature, the selected waveform for the flue gas duct cross-section (with a full height of 216 mm and a half height of 108 mm), and the number of corrugations (with fewer than 6 corrugations per wave). These specifications will enable us to design and manufacture the expansion joint structure.