Strength analysis of a vertical water boiler using FEM

In pressure vessels operating under demanding conditions, even local stress concentrations can lead to accelerated wear of components, assembly difficulties, or damage that becomes apparent after years of operation. In such cases, FEM (Finite Element Method) numerical analyses allow for determining the stress state throughout the entire structure and identifying areas requiring design modifications—before problems escalate.

One of the projects carried out was a strength analysis of a 12.8 MW vertical water-tube boiler, in which hot flue gases flow through the firetube and the tube bundle. The main objective of the calculations was to verify whether the operational issues observed by the client and the difficulties encountered during the lifting and installation of the equipment were confirmed by the FEM analysis results.

Scope and Methodology

The calculations were performed in accordance with EN 12953-3 and PN-EN 13445-3 standards. The scope of the analysis covered boiler operation under working pressure as well as lifting and installation operations—two load scenarios that generate fundamentally different stress distributions.

The numerical model was prepared in ANSYS Mechanical and SpaceClaim. Due to the device’s geometry, a shell model was chosen for half of the boiler with a symmetry condition, which reduced computation time without compromising the accuracy of the results. This required a partial reconstruction of the geometry for the model. The model accounted for operating pressure, hydrostatic pressure, the effect of temperature on material properties and the stiffness of selected components, as well as accelerations during lifting.

 Results of the analysis under operating conditions

In the analysis of operating conditions, the resulting stress maps identified stress concentrations in the firetubes, flat anchors, round anchors securing the firetube sheet, heads, and at the joints of selected components. These locations coincided with areas that the client had previously identified as potentially problematic.

The calculations also demonstrated the influence of stiffening elements on local stress concentrations—their geometry and arrangement affect the load distribution in a way that a classical computational approach does not capture with sufficient accuracy.

Lifting and Assembly Analysis

The second stage focused on the boiler’s behavior during assembly operations. Accelerations resulting from the lifting process, support conditions, and the structure’s self-weight were taken into account. The calculations identified the areas most vulnerable to overloads and compared their behavior with the results of the operational analysis. Based on this, it was determined which components require additional stiffening.

Results and Recommendations

The analysis confirmed stress concentrations in the areas indicated by the client, provided a numerical basis for assessing the condition of the structure, and identified directions for modifying the stiffening elements. The client received data that allows them to plan the scope of any potential modernization without having to rely on guesswork.

In pressure vessels—which are subject to material fatigue, deformation during transport, and local overloads resulting from their geometry—classical analytical calculations often do not accurately reflect the actual stress distribution. FEM models cover the entire structure and allow for the detection of critical areas before they lead to costly failures or the need to take the equipment out of service.