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Development and Selection of Pipe Material for Converter

Attention:     Issuing time:2019-09-26 16:29

Development and Selection of Pipe Material for Converter
The piping system of the converter is the core of the converter, including the upper gas collecting pipe, upper tail pipe, conversion pipe, (lower tail pipe), lower gas collecting branch pipe and lower gas collecting main pipe. According to the scale of hydrogen production, hot-wall collecting pipe and cold-wall collecting pipe are generally used in lower collecting pipe. Converter equipment with smaller scale of hydrogen production is often used because the expansion of outlet collecting pipe is not too large, which will not cause difficulties in the design of lower tail pipe. For large-scale reformer, the expansion of the collector tube at the outlet of the converter is more than 300 mm. Because of the large displacement caused by thermal stress, it is impossible to absorb the hydrogen by the lower tail tube alone. The fracture of the tail tube joint often occurs in the production process, and the cold-wall collector tube can alleviate these contradictions. Because the lower collecting branch pipe and the lower collecting main pipe adopt the full cold wall structure with lining, the outer wall temperature drops below 200 C, which greatly reduces the expansion of the lower collecting branch pipe and itself, thus eliminating the need to use the lower tail pipe.
Owing to the high temperature of the tube wall, the material of the hydrogenation furnace tube is generally HP-40Nb. At present, the whole cold wall collecting tube structure has been adopted in the design of converter tubes. Converter tube adopts the structure form of lower support expanding upward, and its weight depends on the spring suspension to reduce the stress of pipeline system in hot state. There are two upper gas collecting pipes on the upper part of the converter pipe, which are connected with the converter pipe through the upper tail pipe. The upper tail pipe is used to compensate for the displacement in different directions between the upper gas collecting pipe and the converter pipe. The upper gas collector and the upper tail pipe are made of chromium-molybdenum steel because of their low working temperature (450 540 C). The lower part of the converter tube is directly located on the lower gas collector branch.
Development and Selection of Pipe Material for Converter
The development of tube material for converter has gone through several stages. The earliest material used is 18Cr/8Ni series. The temperature and pressure used are not high (below 600 C to ensure heat-resistant strength). Later, the thermal strength of 25Cr20Ni is better. In order to further improve its strength, the mass fraction of C is increased from 0.1 to 0.1. % Up to 0.4%, HK-40 high temperature centrifugal pound furnace tube was developed as reforming tube. In the 1960s and 1970s, almost all hydrocarbon steam reforming tubes used HK-40. The original converter tube did not process the inner hole, the wall thickness was as high as 22mm, and the thickness was not uniform. This not only made the inner stress of the furnace tube big, the weight of the material big, and the cost high, the thermal efficiency low, resulting in the medium conversion efficiency low. After processing the inner hole, although improved, but after using for a period of time, it was found that HK-40 temperature was about 800 C. The harmful phase A is easily produced by range operation. In order to reduce the generation of phase a, an improved HK-40mod was subsequently produced.
In the 1980s, HP series Superalloys for ethylene cracking furnace tubes were used in hydrocarbon steam reformer. HP internal heat alloys are developed on the basis of HK series, in which the mass fraction of Ni is increased by 15%. Ni is an element that strongly forms and stabilizes austenite and enlarges the austenitic phase zone. In austenitic stainless steel, the tendency of phase a formation decreases significantly with the increase of nickel content. Under the same carbon content, HP has more eutectic carbides than HK, so the creep fracture strength increases. The maximum service temperature of the steel can reach 1100 C. In order to further improve the high temperature creep fracture strength and high temperature willfulness of furnace tubes, Nb, W, Mo and Ti alloying elements were added on the basis of HP to form a series of steel grades of alloy furnace tubes. HP-40Nb is the most widely used material in hydrogen production converter.

HP-40Nb contains Nb and W, but HK-40 does not contain these two elements. Nb and W elements can improve the carburization and crack resistance of the material when the temperature changes dramatically, thus improving the high temperature toughness of the material. The linear expansion coefficient of HP-40Nb is smaller than that of HK-40, while the thermal conductivity is higher than that of HK-40, which improves the thermal shock resistance of the material. In addition, the element Nb in HP-40Nb can produce fine and uniformly distributed Nb (C and N) compounds in the alloy matrix, which greatly improves the creep strength of the alloy at high temperature. The high temperature strength at 1000 (?) C is about 80% higher than that at HK-40. Composition and properties of HP-40Nb and HK-40 converter tubes will be listed in Table 1 and Table 2.
The wall temperature distribution of reformer tube is shown in Fig. 2. The high temperature section is at the lower part and the low temperature section is at the upper part. It is generally believed that the furnace tube is easy to be damaged because of the high temperature. In fact, the damage of the furnace tube is in the middle and upper part. The temperature of 600 800 C in the tube is the most vulnerable to be damaged. This is due to the high temperature of the outer wall, the low temperature of the inner wall and the uneven expansion of the inner and outer wall of the furnace tube, resulting in the thermal stress of the furnace tube, which is easy to cause the damage of the furnace tube. Although the lower part of the pipe wall has a high temperature, it is not damaged due to the small temperature difference between the inner and outer walls. Thereby, the temperature stress is the main factor in the failure of high temperature furnace tube, and the temperature difference stress is greater than the stress produced by internal pressure. Therefore, at present, the design of high temperature furnace tube should reduce the temperature difference stress as much as possible. At the same pressure and temperature level, in order to reduce the temperature difference stress of the furnace tube, the best way is to reduce the wall thickness of the furnace tube. At present, the wall thickness design calculation of furnace tube is based on the high temperature endurance strength of material obtained from Larson/Miller curve. Therefore, in order to reduce the wall thickness of Shanghai tube, only to improve the high temperature endurance strength of furnace tube materials. Therefore, this is also the reason why HP-40Nb is chosen for hydrogen production converter.

Figure 2. Wall Temperature Distribution of Converter Tube


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