The life of reformer tubes is controlled by creep. However, the rate at which creep damage
accumulates is not linear and is dependent on a number of complex factors including the large changes
in creep properties as the material ages, the variable stress distribution through the wall of the tube and
the uncertainties associated with actual metal operating temperature. This paper describes the
development of a life assessment methodology to calculate level of damage from tube diameter
inspection data.
At the heart of the methodology is a specifically developed model that describes the rate at which creep
damage accumulates and relates that damage to strain and strain rate.
A comprehensive database of tube material creep properties has been developed for input to the
model. This data base has been created from reformer tube material that has been pre-aged to a range
of metallurgical conditions.
The methodology is completed by detailed finite element analysis that takes into account the influence
of plant cycling caused by start up- shut down and trips. Of particular note is the fact that the model has
little dependence on measured tube metal temperature. In fact, temperature is an output from the
model calculated from the knowledge of strain (inspection data) and creep behavior.
Key Words: Reformer tubes, creep, inspection, life assessment.