We detected, that your browser supports another language than the called one. This page is also available in your language! Would you switch to this page in your language?
Finite element method (FEM)
Just like the numerical flow simulations (CFD) provide important insights into the flow processes of the entire system vessel agitator and the associated components (e.g. heat exchangers, current interrupters, or pipelines), the finite element method (FEM) is also used to gain a deeper insight into the mechanical design of agitators, vessels, and their components. It allows a more reliable design, thereby preventing damage and loss of production.
The load data obtained by measurements as well as the flow simulations and the geometry information obtained from the CAD system serve as important input parameters or general data for subsequent FE analysis.
The FE method has now found its way into almost all conceivable physical disciplines. The most important fields of application in agitation and mixing technology are:
- Structural–mechanical calculations (operational strength, deformations)
- Modal/vibration analyses for the assessment of resonance
- Thermal calculations
- Even the resolution of coupled field problems via “multi-physical applications” in which the individual disciplines can be linked is possible. An example of this is fluid–structure coupling
In addition to the generally static loads resulting from the process pressure and the temperature, agitator vessels and their components are subject to a high dynamic load emanating from the agitator.
For example, alternating bending moments and transverse forces as well as torsional moments and axial forces as reaction forces are passed from the agitator top to the vessel. Vessel components (e.g. baffles, gassing devices, heat exchanger tubes, and supply/drain pipes) are also subjected to a fluctuating load because of their turbulent inflow.
These dynamically loaded vessel components must also be subjected to an operational strength analysis in order to demonstrate their fatigue strength as part of their strength calculation (e.g. as described in the international pressure equipment regulations like ASME 2013, Section VIII, Div. 2 or AD2000, S2).
Because the vessel is also the “machine foundation” for the agitator, it must also meet certain stiffness requirements to ensure safe agitator operation. For this purpose, a deflection calculation must be performed for the agitator nozzle – for open tanks – the agitator bridge. This proves a maximum permissible angular or axial deformation.
However, the most important is the vibration-proof design of the entire vessel agitator as well as the vessel components. With a finite element modal analysis, their natural frequencies can be reliably calculated so that resonance can be reliably excluded with the excitation frequencies generated by the agitator.
In general, an FE simulation is recommended for:
- Process changes associated with higher power input. As a rule, this involves a larger agitator, which exerts higher forces on the vessel and its components.
- Vessels that are retrofitted with agitators because they were not originally designed for such loads.
- Large storage vessels (e.g. with agitator bridges with large span).
- Thin-walled vessels (e.g. fermenter with typically low operating pressure).
- Vessels for large agitators with power inputs in the megawatt range or when high specific agitation performance is associated with high operating pressures and temperature changes.