Tubes used in hydrogen, methanol, and ammonia reformer furnaces are subject to creep degradation during long-term elevated-temperature service. These tubes are centrifugally cast from heat-resistant alloys such as HK-40, HP-50, Nb-modified HP, and micro-alloyed HP. This paper reviews the major issues that must be addressed in assessing the service life of reformer tubes. These include modeling operating conditions, characterizing creep behavior, modeling the evolution of creep damage in tubes, accounting for metallurgical aging, including the influence of material defects, and evaluating the effect of high-temperature corrosion.
Reformer furnaces are widely used in the petroleum and chemical industries. Primary reformers are an essential component of traditional ammonia and methanol plants. Reformers are also used to produce hydrogen in refineries. Reformer furnaces need to operate reliably and without unplanned shutdowns to keep plants online and running effectively. Unplanned shutdowns can lead to significant economic losses. Such unplanned shutdowns are often caused by catalyst tube failures. It is important to avoid such failures by replacing tubes in a timely manner during planned maintenance shutdowns. In order to replace catalyst tubes before they fail but not when they still have significant useful life, an accurate, reliable method of assessing tube life is required.
Reformer tubes are made of centrifugally cast heat-resistant alloys. In the 1960s and 1970s, most tubes were made of alloy HK-40 (25Cr-20Ni-Fe). In the 1980s and early 1990s, many reformers started using Nb-modified HP alloy (25Cr-35Ni-Nb-Fe) because it has been creep strength than the HK-40 alloy. From the mid-1990s to the present, micro-alloyed HP (25Cr-35Ni-Nb-Ti-Fe) tubes are becoming widely used because their creep strength is even better than that of Nb-modified HP alloy tubes. Thus, reformer tube materials have been improved over the last 40 years to increase their creep strength. The improved creep strength allows tubes to operate at higher temperatures or higher stresses (thinner tubes) or gives improved tube life at comparable operating conditions. Reformer tubes typically operate at metal temperatures in the range of 820 to 950ºC (1510 to 1740ºF) with internal pressures in the range of 2,000 to 3,500 kPa (300 to 500.psig). The long-term operating life of reformer tubes under such operating conditions is typically governed by the creep strength of the tube material. Figure 1 shows a typical creep-rupture failure of a reformer tube. As shown, the final failure consists of a longitudinal split in the tube because the largest primary stress in a tube under internal pressure is the circumferential or hoop stress. The creep cracking develops perpendicular to this stress. Accurate assessment of reformer tube life requires the application of a model that realistically predicts the development of such creep damage and cracking under tube operating conditions.