API 579 / ASME FFS-1 Fitness for Service Evaluation on UREA Reactor

Stresses and fracture evaluations performed on critical locations in the reactor

O’Donnell Consulting Engineers performed an API 579/ASME-FFS-1 Fitness For Service evaluation on a multi-wall UREA reactor.

Built in 1965, the reactor was designed to ASME Section VII Division 1, but the Code at that time did not have design rules for multi-walled vessels.

This evaluation was performed in 4 parts:
1) Evaluation of vessel expansion measurement data
2) Fitness for Service brittle evaluation
3) Dents, gouges and dent-gouge evaluations
4) Stress analysis and remaining life assessment

Stresses, design details, inspection results and fracture potential for critical locations in the reactor were examined to determine the locations where brittle fracture could be limiting. The outer shell of the reactor is subjected to axial pressure stresses and to high bending stresses where it is welded to the circumferential welds.

Linear indications had been found during the most recent inspection. These circumferential shell indications near the circumferential welds were long and shallow, and subject to axial tension due to pressure and bending loads due to the differential radial expansion of the circumferential welds v. the outer shell of the multi-walled vessel (which include the as-built gaps). Axial tensile stresses are relatively high at these locations, and they represent the most limiting brittle fracture risk. 

A layered 2D axisymmetric model of the vessel was constructed using ANSYS. Each individual wrap was modeled with contact elements between layers. The fineness of element details improves the representation of geometric stress discontinuities. The nominal element region is 43′ 9″ long x 6′ 9″ in diameter. 

The model takes advantage of vertical symmetry about the top and bottom. The ANSYS model extends slightly beyond symmetry but the nodes at the symmetry line have symmetric boundary conditions applied. We modeled the notch holding the zirconium liner near the vessel top. The bottom model also has a notch although physically one does not exist. (The stresses in this region are low and the simplifications applied are justified.) The forgings and covers were modeled using axisymmetric solid elements. The stud holes were represented as a band of material with “smeared” properties – a weighted combination of forging material and holes. 

Based on the FEA and engineering evaluations, we made a number of recommendations for future inspection.

We perform Fitness for Service Evaluations on various types of equipment for clients industries including manufacturing, energy and petrochemical.