Knowledge of converter tubes

Thread Content

The hydrogen production converter is an important hydrogen-generation device in refineries, fertilizer plants, methanol plants, and other facilities. The converter tubes are exposed to high temperatures and pressures for extended periods, operating in process media rich in H2, S, C, etc.; simultaneously, the feed gas inside the tubes is converted into CO and H2 under the action of catalysts. Harsh process conditions can lead to a reduction in the service life of furnace tubes through phenomena such as high-temperature creep, degradation of materials at high temperatures, stress corrosion, and carburization. In particular, overheating and overburning during operation can cause carbides within the furnace tubes to precipitate rapidly and grow in size, or the precipitation of σ-phase, thereby reducing the tube’s high-temperature performance. Eventually, cracks are formed as a result of creep to varying degrees, leading to the rupture of the furnace tubes. Furthermore, uneven temperature differences within the furnace, frequent start-up and shutdown operations, or rapid temperature changes during such operations can also lead to excessive thermal stress, resulting in uneven deformation of the furnace tubes, severe bending of the same, and ultimately damage to the furnace tubes. Therefore, non-destructive testing of the furnace tubes must be carried out regularly to assess their actual condition in use. The current tube inspection techniques mainly include ultrasonic testing of furnace tubes, creep measurement, and phase analysis. There are no ready-made **standards** for furnace tube evaluation. Some testing agencies have developed internal evaluation standards by accumulating experience through practice. The furnace tubes of hydrogen production converters are generally austenitic centrifugally cast tubes. Such furnace tubes are typically made of high-carbon austenitic precipitation-type materials with a fine microstructure and well-developed columnar grains. At high temperatures, supersaturated carbon precipitates in the form of fine M23C3 particles throughout the matrix, distributed within the austenite grains; this dispersion strengthening provides high-temperature creep resistance. Moreover, these materials exhibit good high-temperature oxidation resistance and permeation resistance, allowing them to operate for extended periods at temperatures of 800–1200°C under pressures of 3–4 Mpa. Furnace tube manufacturing technology: Due to the extremely high operating temperatures, the materials used for furnace tubes in reforming and cracking furnaces operate in an austenitic microstructure state. Currently, traditional centrifugal casting methods are employed in the manufacturing of furnace tubes; the resulting microstructure consists of columnar crystals and equiaxed crystals combined in certain proportions. According to the manufacturing standards of relevant enterprises, in terms of the thickness of the dense layer, the equiaxed crystal layer accounts for 15–55% of this thickness, while the columnar crystal layer accounts for 45–85%. The use of the electromagnetic centrifugal casting process can effectively promote the transformation of columnar crystals into equiaxed crystals, enable the modification and control of the solidification structure of centrifugally cast parts, and improve their mechanical properties. Common defects in centrifugally cast pipes: Original manufacturing defects of centrifugally cast pipes: During the centrifugal casting process, these pipes are prone to defects such as pores, shrinkage cavities, sand holes, and cracks. Defects that occur during the use of conversion furnace tubes: During the long-term high-temperature aging process of centrifugally cast tubes, carbides gradually aggregate and grow larger, until pores form at the interface between the carbides and the matrix. Eventually, these pores connect to form chains of holes that develop into cracks; this entire process accelerates as the aging temperature (the temperature of the furnace tube) increases. Furthermore, in the HK-40 material, the σ phase may also form in the carbide-depleted regions at temperatures between 600 and 900°C, leading to embrittlement of the material. Hydrogen production and city gas manufacturing units operate at low temperatures (850°C, 1.85 MPa); after prolonged use, tiny amounts of carbon dioxide can be observed throughout the entire heated length of the furnace tubes, along with the formation of large chunks of σ-phase. Ammonia synthesis plants operate at medium temperatures (930°C, 3.0 MPa), which facilitates the precipitation of the σ phase in the upper and lower parts of the tubes as well as in the welds. Methanol and steel-making reducing gas production units operate at high temperatures (950°C, 0.93 MPa); the σ phase hardly precipitates in the pipes, but coarse carbides can form in the parts of the pipes where the temperature is highest, and creep voids can appear at the grain boundaries, ultimately leading to crack failure. For transformation tubes, what concerns us the most is the formation of creep cracks and creep phenomena. Furnace tube inspection method: The purpose of furnace tube inspection is to understand the changes in the furnace tubes after operation at each stage, analyze their operating conditions, identify and eliminate any local damage, and determine the remaining service life of the furnace tubes. The radiation furnace tubes are mainly inspected for their creep damage. 1. Visual inspection: Check for corrosion on the surface of the furnace tubes and the color of the scale; conduct chemical analysis if necessary ； Carefully inspect for any macroscopic cracks and surface defects ； Check for any changes in the position of the furnace tubes, as well as for any deformation such as bending or twisting. By combining the results of visual inspections and abnormalities observed during operation, an effective inspection plan for furnace tubes can be formulated. 2. Outer diameter measurement: The purpose is to determine the degree of creep of the furnace tubes in the radiation section under long-term high-temperature operation, as well as the rate of creep expansion and creep velocity of the tubes per cycle. The cost of replacing furnace tubes is too high; each tube costs tens of thousands of dollars, and reckless replacement leads to significant waste. The best time for replacement is when the remaining lifespan of the furnace tube is less than one maintenance cycle. This ensures production while maximizing the efficiency of the furnace tubes. A comprehensive evaluation of the furnace tubes is carried out through testing, in order to understand their condition of use and achieve a win-win outcome.