Finite Element and Fatigue Analysis to Predict Limited Design Life of Separator Units
We performed a risk assessment on cracking mineral separator units. A unit had cracked after 19 months of operation, or 170,000,000 cycles of operation. After 121 additional units were found to have weld/cracking issues, we were asked to investigate the FEA models and weld procedures.
Experience shows that unexpected very high cycle fatigue cracking (>10 (7) cycles) can occur in such complex machinery operating under dynamic conditions. Even at lower applied frequencies, such thin sheet metal structures can exhibit deflections which add to effective stress amplitudes. In addition to studying client FEA models, we performed our own independent finite element models. The units were made of A36 structural steel, with a yield strength of 36 Ksi and ultimate strength of 58 Ksi.
This type of gyratory-reciprocating screener has three distinctive elements:
- Spreads material across the entire width of the screen surface
- Promotes the separation of the incoming material
- Conveys the material forward and effectively allows for precise material separation by size based on screen selection
The circular motion at the feed end helps spread material across the full width of the screen surface, stratifies the material, causing smaller particles to move down through the bed of the materials, and conveys the material forward. The elliptical motion at the middle enhances stratification of incoming material and promotes the separation of fines as the material is conveyed. Straight-line motion at the discharge end removes near-size particles improves screening efficiency with no vertical component so the material remain in contact with the screen surface.
We also performed a fatigue analysis to ASME B&PV Code. From these results, Fatigue S-N curves were generated showing Percentage of Units to Fail vs. Estimated Fatigue Life.
The image above shows a modal response of the unit at 3.1595 Hz.
In addition to performing stress, vibration and fatigue analysis – we also perform risk assessment on equipment. See Introduction to Fatigue Analysis.
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