Screening Assessment (level 1)
A Level 1 assessment is based on a screening assessment that relies on published creep material behaviour and is limited in its applicability.
An applicable example is that of a localized hotspot that was prevalent for 1 month (744 hours) on a processing unit for which locally the internal refractory lining had deteriorated. The plant had the unit shutdown and the lining repaired and wanted to know whether the shell was still fit-for-service given the creep degradation that could have been incurred.
The thermography survey showed maximum temperature of 536°C on the outer surface of the unit during the excursion period.
The material of construction was reportedly carbon manganese steel and the unit had no prior temperature excursion.
An Finite Elemental Analyses (FEA) based model indicated stress at the affected region to be approximately 40 MPa.
Using the screening data published in API 579-1 (2007), it was possible to conclude that the conditions (i.e. the combination of endured temperature and stress) was within the allowable range for carbon steel. The screening curve indicates that for carbon steels operating at 536°C and exposed to a stress of 40 MPa, a minimum creep service life of 2500 hours can be expected. Due to the excursion being shorter than the permissible hours the unit was considered fit for continued use. No repair of the shell was required, and the unit was returned to service.
Creep FFS and RLA assessment (level 2)
For a level 2 FFS assessment reference stress solutions can be applied. Occasionally, FEA models are made, specifically for components exposed in the creep range where discontinuities exist close to the regions of interest. When FEA models are used, the extraction of the peak stress values can be considered as a conservative approach, however, due to creep strain stress redistribution that may follow, membrane stresses are normally preferred and provide more realistic results.
When the creep life consumed to date, and the expected creep life to be consumed is calculated provision is made for the tri-axiality of stresses. Conversely, the Omega properties determined via uni-axial testing is converted to multi-axial values for this reason. Should the actual material condition have been characterised or Omega properties ascertained for service exposed material, representing the worst-case damage, then the past operation can be ignored during a desktop review of the creep remaining life study. It is important to incorporate a future corrosion allowance and/or the prevailing corrosion rate as the stress is likely affected by the latter and this, in turn, drives additional creep degradation.
The creep fitness-for-service and remnant life calculations considers creep damage fractions over various load or temperature periods as indicated by the schematic timeline.
The plant data received from the client indicated that some temperature excursions above the design temperature were sporadically experienced as detailed by the metal skin temperature history.
By incorporating the data above, with the reported long-term corrosion rate and operating pressure over the service life of the component, it was possible to calculate the creep life fraction deemed consumed till date and to predict the remnant life in accordance with paragraph 10.5.2 of API 579-1/ASME FFS-1. It is customary to include a safety margin with all remnant life studies and therefore a creep life fraction of 0.8 is typically considered end of life (albeit that actual creep failure may not yet occur).
Various scenarios were considered for the client during the desktop review, since the original material condition was not known, and since in-situ replication results indicated that the material condition was more favourable than initially anticipated from scenarios 1 to 3, which incorporated conservative assumptions. The outcome of the various scenarios as contemplated by the equipment owner is given in the figure below. There were a couple of scenarios that rendered the component safe in terms of creep degradation till 2022, provided the scenario operating conditions were met.