A boiler was opened during a recent turnaround for inspection and minor repairs. What was found was a significant failure of the refractory used to protect the integral tube sheet from hot process gases. When the tube sheet material is exposed to high temperature, metallurgical deterioration occurs which significantly reduces its strength. Replacing the tube sheet in the boiler, which was a critical path item for the turnaround, was undesirable for cost and schedule reasons. If the temperature profile through the tube sheet thickness could be determined, the depth of metallurgical concern would be identified.
Situation
An Unpleasant Surprise
Engineering Specialties Used
Turnaround Support
Fitness-For-Service
Boiling Heat Transfer
Finite Element Analysis
Solution
Predictive Modeling Affirmed by Empirical Testing
A three-dimensional thermal finite element model (FEM) was built for a portion of the tube sheet with several tubes. Convection boundary conditions for both gas heating and boiling water cooling were calculated and applied to the model. It was found that most of the tube sheet thickness was below the critical temperature. Because the potential for metallurgical deterioration of the tube sheet was predicted by the FEM to be so shallow, it was decided to confirm this by drilling into the tube sheet a short distance. This procedure confirmed the analysis findings and the tube sheet remained in service.
Finite Element Model of the Waste Heat Boiler Area of Interest
Summary
Your Turnaround Partner
Argos Engineers is ready to partner with your turnaround team to evaluate unexpected findings. Let us provide the analysis input needed for quality decisions. Make safe choices, but avoid costly ones that are overly conservative.
