Featured below are some Fitness-for-Service (FFS) application examples that we have performed. Some of these were completed by staff members prior to their career at Stress Engineering. This is only a brief list of examples we have performed, but they illustrate the unique problems encountered and the solutions we provide.
1. We analyzed an existing chemical plant piping system and found the thermal stresses to be excessive. We recommended that the piping be redesigned for improved flexibility and Code compliance. We completed the redesign including pressure, thermal, and wind under various configurations.
2. A caustic quench pressure vessel with circumferential cracks was reviewed for a chemical plant. We recommended temporary structural repairs and reinforcement to maintain pressure integrity while a new replacement vessel was under construction.
3. Numerous pressure vessel re-rates have been done for heat exchangers and vessels in ethylene service which had not corroded in 15 years. The re-rates took advantage of the validated zero corrosion on vessels that had up to 1/8" corrosion allowance.
4. A carbon-steel vessel with a monel liner had corrosion-under-insulation damage. Extensive corrosion was found, which reduced wall thickness below acceptable limits. In fact, some areas were so severely corroded that the carbon steel was completely missing, leaving only the monel liner to contain pressure. This vessel was temporarily repaired by using an overlay patch as permitted by API-510. Temper bead welding was used to post-weld heat treat in place of full circumference conventional heat treat. This repair scenario resulted in a minimal shutdown of the unit. Since all the procedures used were non-intrusive, there was no cleanup, purging, or contamination of the process.
5. An evaluation was performed for an internal catalyst grid support ring using elastic/plastic finite-element analysis. The objective was to determine the limiting load that could be supported by the ring. This sophisticated means was needed to avoid rebuilding the support structure inside the vessel. This was a classic miscommunication between the support grid manufacturer and the vessel fabricator. Each placed responsibility of the structural adequacy on "others." We assisted in the redesign of internal support clips, which were themselves over-stressed, and did the calculations to justify the stresses imposed on the vessel wall. This work was done wholly with conventional calculations.
6. Cracking of a vessel shell under an internal head was evaluated using finite-element formulation and fracture mechanics techniques to determine the critical flaw size and expected life. This project included a 3D shell/plate element model to determine the gross loadings, a 2D axisymmetric model to determine peak loading, and crack initiation and growth in the area of concern.
7. Two fracture related FFS tasks were carried out related to pressure containment structures in a 1500-MW pressurized water nuclear plant. The first was initiated by a non-conformance in welding procedure during the welding of the vertical seams in the steel lining of the secondary containment. These welds were made by a proprietary process similar to electroslag welding, which produced large grain sizes with consequent low Charpy impact values. The question was whether any thermal or mechanical loadings experienced during the remainder of construction (since the lining was the inner staging for casting the concrete walls) or during an in-service transient, could cause a brittle fracture and release of contaminated steam to the environment. The work entailed developing correlations between subsize Charpy impact test results and conventional fracture mechanics parameters, then using these to investigate behavior of the lining under a wide range of loadings, including shrinkage of the concrete, causing inward buckling of the liner, thermal transients due to general and local steam impingement, and mechanical damage due to pipe whip or missiles.