The client requested a static stress analysis for two cooling water headers, which were to be replaced with headers made from Glass Reinforced Epoxy (GRE). Additionally, an assessment of the nozzle and flange loads, as well as a supporting arrangement, was required.
Analysis
The analysis adhered to the requirements of ISO14692 (for GRE piping), ASME B31.3 (for steel piping), and the project objectives detailed in the documents provided by the client. CAESAR II was utilized to model and analyze the system for sustained, operational, occasional, and hydro test load cases. The headers had multiple connections to small-diameter branches, presenting challenges due to the differing thermal properties of GRE at nearly every intersection. Several measures were implemented to mitigate stresses at these locations.
Flanges were assessed using the equivalent pressure method, which converts axial load and bending moment into an equivalent pressure. This pressure was combined with the internal pipe pressure to form a combined pressure loading for the flange. The combined pressure load was then compared to the allowable pressure rating, which equaled twice the pipe pressure rating multiplied by a safety factor.
Results
In instances where the stress slightly exceeded the allowable limit, adding laminate to the tee, increasing the thickness, or altering the support layout reduced the stress. However, one location required rerouting to enhance flexibility. Additionally, steel was replaced by GRE to further increase flexibility. For the unknown routing of many small diameter lines, the minimum required flexible pipe length was indicated to ensure the stress did not exceed the allowable limit. The header displacement at the connection was also specified for the client to design accordingly.
The different thermal properties of GRE presented challenges at nearly every intersection. Several steel flanges were found to be overstressed according to the equivalent pressure method, a conservative flange assessment method for metal flanges. These overstressed metal flanges were further evaluated using an ASME Section VIII Div. 1 Appendix 2 calculation. All steel flanges that failed the equivalent pressure test passed the Appendix 2 calculation.
