Reformer Tube Failure Analysis
Reformer tube is used for cracking hydrocarbon with steam to liberate hydrogen and carbon dioxide. Normally, hydrogen generation is an endothermic reaction which requires a temperature range between 840 to 880 °C with catalyst. The reheater tubes are made of chromium and nickel-rich alloys such as HK40 alloys and have been evolved over the years for better life expectancy.
Often the optimization of profit by increased production rate versus effective heater tube life that can be gained - is the policy adopted. In doing so there always remains a risk of unnecessary breakdown / downtime because of tube failure. Some of the most common causes of reformer tube failure are briefly discussed below.
Creep damage is defined as the time dependent degradation of material. The damage in the reformer tubes starts from 1/3rd of wall thickness, appearing in the form of round voids randomly distributed along the grain boundaries. Creep damage generally results in multiple longitudinal cracks. Operating of tubes beyond the design limits due to temperature excursions can lead to onset of early creep damage.
Misalignment of burners, clogging of burners, and disturbance in flue gas path can lead to direct flame impingement on to the tubes. As a result, the tubes tend to damage locally by overheating. In turn an irreversible metallurgical degradation takes place which can lead to phenomenal loss of creep rupture strength.
Metallurgical degradation of tubes occurs by way of coarsening of the carbide particles seen in the microstructure of the tube material. The carbide coarsening mechanism is temperature and time dependent phenomena. Higher the temperature faster is the coarsening of carbides.
Frequent start-ups and shut-downs can affect the tube life and crack can develop which can eventually lead to failure under thermal fatigue mode.