Stress Analysis of High-temperature Steam Pipeline Reducers

High-temperature steam pipeline systems are widely used in the petrochemical, electric power, metallurgy, and other industries. Reducers, as crucial components, are responsible for connecting pipes of different diameters. Under high temperatures and high pressure, reducers are subjected to complex stresses. Stress analysis not only helps ensure the safe operation of the pipeline system but also guides the optimization of design and manufacturing processes. This article discusses the stress analysis methods and results of high-temperature steam pipeline reducers in service.
 

In high-temperature steam pipeline systems, the stress sources of reducers mainly include the following:

High-temperature steam causes the thermal expansion of reducers. If there is a temperature difference between the reducer and the connected pipe or fixed bracket, thermal stress will occur. This stress mainly manifests in uneven thermal expansion and thermal cycle stress. Due to the different diameters at both ends of the reducer, the amplitude of thermal expansion also differ, resulting in stress concentration. Under the  influence of high-temperature steam, the pipeline system may undergo multiple thermal cycles, causing the reducer to repeatedly expand and contract due to heat, thereby generating thermal fatigue stress.
 

The flow of steam in the pipeline generates internal pressure, causing tensile and compressive stresses on the reducer’s wall. The flow of steam in the pipeline generates internal pressure, leading to tensile stress and compressive stress on the reducer’s wall. The steam pressure acts on the inner side of the pipe wall, causing it to expand outward, generating tensile stress. In the transition area of the reducer, the internal pressure causes the pipe wall to shrink inward, generating compressive stress. The fluctuation in steam pressure also affects the reducer. Sudden pressure increases or decreases cause stress changes in the pipe wall, leading to stress concentration and fatigue.
 

The geometry and connection of the reducer cause structural stress, especially at the point where the pipe diameter changes, where stress concentration may occur. The transition area of the reducer is prone to becoming a stress concentration point due to the change in diameter, making the stress in this area significantly higher than in other parts. The choice of welding or flange connection also affect the distribution of structural stress. Stress concentration at the welding seam is often more obvious and  likely to cause cracks.
 

Vibration, mechanical shock, and the restraint of the pipe support also cause additional stress to the reducer. The vibration generated during the operation of the pipeline system  causes dynamic stress in the reducer, leading it to work in stress fluctuations and accelerate fatigue damage. The sudden movement of the pipe support or the impact of external objects causes instantaneous high stress on the reducer, which may lead to partial stress concentration and crack propagation. The constraint force of the pipe support affects the stress state of the reducer. The fixed support limits the free expansion of the reducer, resulting in stress concentration. The sliding support allows a certain degree of thermal expansion, but if it is not designed properly, it may also produce constraint stress.
 

Commonly used methods for stress analysis of high-temperature steam pipeline reducers include finite element analysis, experimental stress analysis, and analytical methods. Finite element analysis is a numerical simulation technique that divides a complex structure into numerous small finite units and uses numerical methods to solve its physical quantities such as stress, strain, and deformation. Finite element analysis of high-temperature steam pipeline reducers can intuitively display their stress distribution and deformation in service and identify potential stress concentration areas. Under laboratory conditions, the stress and strain of the reducer under different working conditions are measured to verify the accuracy of the finite element model and to provide reference data for engineering design. Based on the theories of material mechanics and elastic mechanics, the analytical solution of the reducer under thermal stress and internal pressure is derived. Although the analytical method is relatively simple, it may have limitations when dealing with complex geometries and boundary conditions.
 

The following results can be obtained through the stress analysis of high-temperature steam pipeline reducers:
Thermal stress distribution: The thermal stress of the reducer under high-temperature conditions is mainly concentrated where the pipe diameter changes and near the weld. These areas are prone to stress concentration due to geometric discontinuities and material inhomogeneity.
Internal pressure stress distribution: The stress caused by internal pressure is gradient distributed along the wall thickness direction of the reducer, and the stress on the inner wall of the pipe is greater than that on the outer wall. The wall thickness design of the reducer needs to consider this stress gradient to prevent rupture at the weak part of the pipe wall.
Stress concentration: Stress concentration often occurs in the transition area of the reducer, especially at the fillet transition and weld. The calculation and control of the stress concentration factor are key issues in the design.
Fatigue life evaluation: The fatigue life of the reducer under high-temperature and high-pressure cyclic loads can be evaluated through the stress analysis results, providing a basis for formulating inspection and maintenance plans.
 

High-temperature steam pipeline reducers are subjected to complex stresses during service. Through finite element analysis, experimental stress analysis, and analytical methods, we can fully understand the stress distribution and stress concentration of reducers and provide a scientific basis for their design and manufacturing. In practical applications, based on the results of stress analysis, the structural design and material selection of reducers should be optimized, the manufacturing process strictly controlled, and a reasonable inspection and maintenance plan formulated to ensure the long-term safe and stable operation of the pipeline system.