During the implementation, the research team studied and determined the general structure and operating principle of the expansion joint. Accordingly, the expansion joint consists of three basic parts: A clamping and sealing part that seals the pipe within the piping system; A liner tube part for guiding the medium through: it is welded fixed to one end of the expansion joint and can slide along the pipe when the outer bellows expands or contracts; A stainless steel bellows part: having a spring-like cross-sectional structure similar to a flexible hose. Under the effect of temperature, the localized metal of the expansion joint elongates and undergoes thermal deformation, but due to its shape, the overall length of the expansion joint does not change.

The main purpose of the expansion joint is to reduce noise, vibration, and accommodate all directional thermal movements. Noise and vibration are reduced by creating a discontinuity between the pipe materials; the expansion joint acts to dampen these disturbing frequencies and absorb energy, minimizing the transmission of noise, vibration, and stress to the piping or adjacent equipment.

Based on the studies, the project team designed a corrugated expansion joint with the following basic parameters: dimensions 4008 mm long, 1008 mm wide, 300 mm high, material SUS304, distance from the pipe fixed point to the expansion joint is 12 meters. The temperature of the internal duct is insulated with ceramic wool. According to actual statistics at Cao Ngan Thermal Power Company, there have been instances of overheating due to temperature control system errors, with the temperature at the expansion joint reaching up to 300ºC. The design parameters all meet the expansion and contraction requirements of the piping system.

Due to the relatively complex structure of the expansion joint, an accurate 3-D model is necessary. Therefore, the expansion joint model was built using the specialized 3D drawing software Solidworks, and Ansys Workbench software linked with Solidworks was used to simulate and evaluate the loads, stresses, and fatigue life of the expansion joint. The maximum simulated working cycle value of the expansion joint reached approximately 1.106 at the rib edges of the bellows, and the minimum value at the weld locations – where stress is concentrated. The expansion joint has the highest risk of failure at the corner positions of the joint.

According to Engineer Do Thanh Trung, each manufacturer of rectangular expansion joints has different steps for their forming and bending process. However, they all share a set of basic steps used by all manufacturers. The process begins with a large coil of sheet metal that can range in width from 25mm to 1200mm with a thickness from 0.3mm to 6 mm. Before a rolling process for the expansion joint is carried out, it must be prepared for this process.

“The expansion joint product is formed from SUS304 sheet metal, consisting of 2 layers, each layer 1mm thick, total thickness 2mm. The two-layer corrugated sheet is rolled so that the layers are pressed tightly together (no gap between the layers). When calculating the profile, the interface between the two layers is taken as the basis for calculating deformation during the operations. The product is formed from a sheet with a straight generatrix, the curvature radius of the profile ρi is ∞; the profile angle γ i = 0˚ through the profiling steps until the final operation where the profile radius ρi = 20; the profile angle γ i = 90˚,” the project lead stated.

To manufacture the expansion joint, the research team implemented a manufacturing process for the stainless steel bellows using the roll forming method with basic dimensions of the two-layer stainless steel corrugation 1mm thick, 70mm high. After being roll-formed, the corrugation is cut to size, beveled at a 45-degree angle, and then the sheets are clamped and welded on a jig to ensure the corrugations are perpendicular and aligned with each other. Finally, ceramic wool is wrapped, the components fixing the corrugation are welded, and the quality of the expansion joint is inspected.

After more than 1 year of implementation (from April 2021 to December 2022), the project was completed and put into operation: 01 expansion joint with dimensions 4008x1008x300 applied to the secondary air fan system of Cao Ngan Thermal Power Plant; and 04 expansion joints with dimensions 5200x5200x600 applied to the boiler flue gas duct system of Uong Bi Thermal Power Plant. At the same time, a technological process for manufacturing corrugated expansion joints with a rectangular cross-section for the fitting part and side dimensions from 2÷5 (m) was established.

 

The project's products all meet Vietnamese standards and practical working conditions, achieving technical specifications equivalent to imported products currently used in thermal power plants. A high localization rate was achieved, suitable for domestic manufacturing capabilities. Specifically, the localization rate: design part: 80%; equipment manufacturing: 100%; installation, commissioning, and warranty: 100%. 

The successful commissioning of 05 application products demonstrates the significant economic efficiency and applicability of the project, which has been highly appreciated by the plants for its progress as well as cost (only about 75% of the imported price). The project has also enabled domestic manufacturing facilities to master the design and manufacture of large-sized expansion joints serving industrial plants.