Abstract :
Micro cracks have been found near some of the welds on the pipe unit. This
paper investigates the root cause of this damage and suggests
improvements to prevent such damage from occurring.
The investigation includes several simulations using Pipestress and Ansys
Mechanical, a welding and materials investigation, and a more theoretical
creep investigation using the Larson-Miller parameter. A sensitivity study of
the outlet pigtail installation and support configuration is performed as well
as a cyclic plastic analysis. A complementary analysis of the catalyst tubes,
inlet and outlet system shows that the only areas where the stress and strain
indicate an elevated risk of damage are the areas around the ends of the
outlet pigtails.
Suggested improvements include reconfiguration and load reduction of
constant hanger supports, improving welding procedures in order to lower
residual stresses and undertake more material and on site investigations to
verify causes of failure.
Keyword :
Cracks, creep, fatigue, plastic analysis, lifetime.