Model Input
The filter was modeled with a linear elastic shell and solid elements. It consists of the following main structural components:
- Floor slab
- Filter bottom
- Container wall with torispherical head and ring flange applied at bottom
- Brackets on container wall
- Support plate for agitator
- Pipe connection and lifting eyes on torispherical head
- Circulating serpentine pipes
The connection of the container and floor slab was especially interesting for the modeling process. The ring flange was applied in the form of a solid element to the bottom of the tank plate. It was fixed to the floor slab with 53 clamping bolts (M 27).
They were uniformly distributed around the circumference to ensure a constant prestressing force. In order to anchor the ring flange against the floor slab with defined prestress forces, a circulating counter element was modeled as a solid element on the tank wall. Thus, it was possible to clamp the container flange against the conical floor slab.
To reproduce this conical fastening in RFEM, it was necessary to define the contact property between these structural components in the analysis model. Contact solid elements defining an elastic spring were distributed uniformly around the circumference.
Loading
The following loads were applied to the structure:
- Design pressure for container -0.01 psi, heating for flat bottom -0.006 psi, heating coil -0.006 psi
- Vertical loads due to self-weight and equipment
- Design temperature -4/392°F
- Internal positive and negative pressure with design of pressure fluctuations from 0.0 to 43.5 psi for 28,000 cycles of load according to AD‑S1 and S2
- Agitator loads for 2 million Stress cycles
Calculation in RFEM
Five load groups were created from the single load cases and the FE mesh was generated. Then the internal forces, stresses, and deformations were determined in RFEM.
In addition to the general stress analysis, fatigue designs due to pressure fluctuations and agitator loading were performed.
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