Risk Assessment on Cracking Mineral Separator Units

A Risk Assessment was Performed on Cracking Mineral Separator units. A Unit had Cracked after 19 Months of Operation – Equivalent to 170,000,000 Cycles. Subsequently, 121 additional units were inspected and found to have weld / cracking issues. We were asked to investigate – and make recommendations.

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.

Equipment Dynamics

The gyratory-reciprocating screening system employs three motion profiles:

Circular Motion (Feed End):

• Distributes material across screen width
• Initiates particle stratification

Elliptical Motion (Middle Section):

• Enhances fine particle separation
• Maintains material conveyance

Linear Motion (Discharge End):

• Optimizes near-size particle removal
• Eliminates vertical displacement for consistent screening

 Failure Analysis

1. Material Properties: Units constructed from A36 structural steel (Yield Strength: 36 ksi, Ultimate Strength: 58 ksi) exhibited vulnerabilities under cyclic loading.
2. FEA Validation: Both client-provided and independent FEA models were evaluated to assess stress distributions and vibration patterns.
3. Fatigue Assessment: Analysis using ASME Boiler & Pressure Vessel Code standards revealed:
   • Progressive crack initiation at stress concentrations
   • Reduced fatigue life due to dynamic deflection effects in thin sheet metal components
   • S-N curve projections showing failure probability vs. operational cycles

This investigation highlights the importance of combining FEA and fatigue analysis for predicting failure modes in complex screening systems. By correlating computational models with field performance data, engineers developed risk-based maintenance strategies and design modifications to address the failure mechanisms.

As a result, both computational models and physical failures revealed critical insights into high-cycle fatigue behavior, particularly in equipment subject to multi-mode dynamic loading. Subsequent recommendations included weld procedure improvements, operational monitoring, and fatigue life extension strategies based on the stress redistribution patterns identified through finite element analysis.

In Addition to Performing Stress, Vibration and Fatigue Analysis, O’Donnell Consulting Performs Risk Assessments on Equipment.