Introduction
Following the replacement of an older tail gas heater, issues with leakage from the top tubesheet flange connection have emerged during production. Despite attempts over two years to mitigate this through different bolt types, materials, and configurations, the problem persists particularly during the heating of the tail gas heater.
In response, Dynaflow Research Group conducted a transient thermal analysis to investigate the underlying mechanisms and propose viable solutions. This analysis encompassed the flange connection, bolts, nuts, tubesheet, and adjacent parts of the shell.
Analysis
The analysis revealed a significant thermal expansion mismatch between the flange body and the bolts, resulting in excessive strain on the bolts. This strain sometimes exceeded the yield point of the bolt material, leading to plastic deformation. Although the temperature difference between the flange body and bolts decreased as the heating process progressed, the damage had already occurred. This compromised the gasket pressure, causing subsequent leakage.
Results
The study found that external mechanical loads exerted a pressure significantly lower than the design threshold of 10 Barg, suggesting they were not primary contributors to the flange leakage.
Key findings from the transient thermal analysis indicated that the most critical temperature difference between the flange and bolts occurred towards the end of the heating process. This temperature differential was identified as exceeding the yield stress of the bolts by more than 15%.
Proposed Solutions
To address the thermal stresses causing flange leakage, several solutions were proposed. The most promising solution involved the application of metal bushes around the bolts. These bushes, with a recommended length of 66 mm and adequate thickness, would be placed between the washers under the nuts and the flange body. This approach would effectively accommodate differential thermal expansion under conservative conditions, potentially allowing for the use of longer bolts without increasing the flange thickness.
