Gas 2024 Issue

100000

10 years

Maximum operating temperature of 1750 ˚F = 10 years life time

10000

1 year

1000

1 month

100

1 day

Never operate above

2050

1750

1850

1950

Metal temperature (˚F)

Figure 6 Lifetime vs metal temperature chart and illustration explaining creep damage progression through a tube wall

tube damage. As the 3D image highlights, plant engineers can see the beginning of damage patterns and address what is needed. It is essential that NDT inspection methods applied produce repeatable results, starting from the baseline to ongoing routine inspections (see Figure 4 ) and from one inspection period to the next. Repeatable results allow data to be overlaid, enabling quantification of damage level increases and highlighting shifting of damaged area. Remaining tube life complexities Provided the NDT inspection technique(s) applied can generate adequate discrimination, the inspection test results can also be used to determine the remaining tube life and, therefore, decide the time at which the tubes need replacing. Some inspection techniques simply package test results into A, B, C, and D type categories, which do not provide sufficient detail nor consider the wide scatter of damage throughout a reformer furnace with respect to tube damage. Even ignoring the variables in the tube operating environ- ment, some tubes will fail before the expected design life (see Figure 5 ). This is primarily because the design meth- odology utilises a probabilistic approach based on analysis of controlled short-term destructive testing of tube sam- ples. In most cases, 95% of tubes will exceed the design life – this means that 5% of the tubes will not reach design life.

Unfortunately, when it comes to predicting which tube may fail first in a reformer, variation in tube life is a fact of life due to the following factors: • Wall thickness variations within manufacturing specifica - tions and design tolerances. • Material property variation in chemistry and macrostruc- ture between each tube section. • Temperature variation due to reformer design, flue gas flow plus uncertainty, and errors in temperature measurement. The latter factor is particularly important as relatively minor differences in temperature have a significant impact on tube life. Typically, a 68ºF (20ºC) increase in tube tem- perature will reduce the life by as much as 50% (see Figure 6 ). This continues to stress the importance of focusing on accurate temperature measurements while online by an owner/operator or trusted contractor. Tube harvesting Applying a tube harvesting approach allows plant operators to focus replacement efforts towards those tubes where operating life has been consumed beyond acceptable tol- erances. In many steam reformer designs, there are cooler regions within the firebox where tube temperatures rarely reach material creep strain thresholds. Therefore, operating life will exceed other tubes operated in regions of higher temperatures. Applying a tube harvesting approach allows plant operators to spread capital expenditures over time

Gas 2024