API 579/ASME-1 Fitness for Service on Urea Reactor


Stresses and fracture potential evaluations on critical locations in the reactor

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

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

The 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 combination evaluation
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. The outside layer of the multiwalled segments are reduced by the presence of gas between the 24 layers.

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.


We perform API 579-1/ASME FFS-1 evaluations on various types of equipment.

Give us a call to discuss your engineering challenges.

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Introduction to API 579 /ASME FFS-1
(PDF) “Weld Defects and Failures: Quantifying Fitness For Service – How Finite Element Mathematical Simulations can Mitigate Risk, Cut Costs and Improve Overall Safety”
Publications – Heat Exchangers, Pressure Vessels, Welds and Other Applications, Fatigue, Elevated Temperature
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