Failure Analysis: Design, Fabrication, and Elevated Temperature Operation
Project Overview
Tube-to-tubesheet weld failures occurred on a superheater operating at a copper smelter's sulfuric acid plant. Failure to isolate the superheater in time resulted in extensive damage to downstream process equipment. O'Donnell Consulting Engineers was retained to perform a comprehensive failure analysis covering design adequacy, fabrication quality, and the elevated-temperature operating history of the unit.
Root Cause: Cyclic Thermal Loading
Plant startup required introducing saturated steam — which contains moisture — into hot tubes. The sudden temperature drop caused the tubes to contract sharply, while the tubesheet lagged behind in thermal response. The mechanical constraint imposed at the weld root produced severe thermally induced stresses with each startup cycle.
Operating records showed the superheater had experienced more than a thousand severe thermal cycles between initial startup and the incident — far exceeding what would be expected under normal operating conditions.
Failure Progression
Physical examination established the following failure sequence:
- A fatigue crack initiated at the tube-to-tubesheet weld root due to repeated cyclic thermal loading.
- The crack propagated through the tube wall.
- Steam leaked through the crack and migrated through the interface between the tube OD and the tubesheet hole ID to the gas side.
- Steam mixing with SO₃ on the gas side created an aggressive corrosive environment, accelerating crack growth through the tube wall.
- Increased steam leakage jetted under pressure through the gap, causing progressive damage to adjacent tubes.
Analysis Methods
Finite element analysis (FEA) was performed to determine temperature distributions and the resulting thermal stresses and strains generated by transient operating conditions.
Cyclic gas and steam temperatures were used as boundary conditions for the tube-to-tubesheet FEA model. The analysis captured the complex thermal interaction between the tubes and tubesheet, including a key nonlinear behavior: at uniform temperatures, the rolling process creates an interference fit between tube and tubesheet. When cooler steam is introduced, a gap opens at the interface. Nonlinear FEA confirmed that this gap opened and closed with each thermal cycle — a condition directly driving fatigue crack initiation.
FEA results were applied to a low cycle fatigue analysis to evaluate cumulative damage from plant cycling.
Findings
| Area | Finding |
|---|---|
| Design | The superheater was designed in conformance with ASME Code requirements and accepted engineering practice. |
| Materials & Fabrication | Physical examination and material testing confirmed materials matched drawings and specifications; the tube-to-tubesheet joint was properly fabricated. |
| Cause of Failure | Weld cracking resulted from cyclic thermal loads imposed during plant startup. Each startup generates thermally induced stresses at the weld joint. The operating history far exceeded the permissible number of thermal cycles. |
