A Comparison and Calculation of Heat Exchanger Tube Materials
As the core part of the high-pressure heater, the heat exchanger tube is directly related to the subsequent operation of the high-pressure heater. It can be seen from the analysis that the pressure of the subcritical unit is low, and the difference in the thickness of the carbon steel pipe, alloy steel or stainless steel heat exchange tube is not big. No matter which material is used, it is feasible, while the pressure of the supercritical and ultra-supercritical units is higher. There is a big difference in the wall thickness of the heat exchanger tube, so it will bring different results to the manufacture and operation of the heater. Both SA -556Gr.C2 and 16Mo3 pipes have been used in ultra-supercritical units, and there are no problems in design, manufacturing and processing.
In supercritical and ultra-supercritical units, alloy steel heat exchanger tubes are often used. This material has good corrosion resistance and erosion resistance in theory, but thicker tube walls are required; there are problems such as difficulty in expansion joints. Therefore, when selecting alloy steel pipes, the connection processing of the tube sheet should be considered, so that the heat exchanger tube and the tube sheet form a butt joint, and the quality of the butt welding of the heat exchange tube ensures the reliability of the connection between the tube and the tube sheet.
In the supercritical unit, if stainless steel tube is used as the high-pressure heat exchanger tube, certain issues need to be considered in the calculation.
(1) The wall thickness of stainless steel heat exchanger tubes is about 30% higher than that of carbon steel tubes.
(2) The heat transfer of austenitic stainless steel is lower than that of carbon steel and low-alloy steel, and because the selected wall thickness is 30% thicker than that of carbon steel, the heat transfer area of the high-pressure heater increases. According to statistics, it increases by 50%.
(3) The length and weight of the equipment have increased, which increases the difficulty of transportation and installation (the total weight of 3 sets of stainless steel tubes is about 420t, and the total weight of 3 sets of carbon steel tubes is about 305t).
Austenitic stainless steel tubes are very sensitive to chloride ions, and very low chloride ion content can cause severe corrosion to heat exchanger tubes in a short time, which is a fatal shortcoming of austenitic stainless steel. During the shutdown period, it is difficult to avoid the enrichment of chloride ions due to the evaporation of water, which is precisely the major factor that causes the corrosion of austenitic stainless steel pipes, especially for units that use seawater as the water-cooling medium. When the condenser leaks, a large amount of seawater often enters the water supply system, and a large amount of chloride ions will cause irreparable damage to the stainless steel heat exchanger tubes. Therefore, from the perspective of economy and reliability, carbon steel pipes or low alloy steel pipes should be selected for the heat exchanger tubes for high-pressure heaters, and think twice when using stainless steel heat exchanger tubes.
Conclusion
Through the comparative analysis, SA-556 Gr.C2 and 16 Mo3 materials are used as heat exchanger tubes, which are the preferred materials for high-pressure heat exchanger tubes, and a large amount of manufacturing and operational experience has been accumulated. Although Cr-Mo alloy steel pipes are more resistant to erosion and corrosion than carbon steel pipes, they lack the support of operating experience. If they are used in units above supercritity, the problem of pipe expansion needs to be considered. When using stainless steel heat exchange tubes, ferritic stainless steel should be preferred, and think twice when choosing austenitic stainless steel.
In supercritical and ultra-supercritical units, alloy steel heat exchanger tubes are often used. This material has good corrosion resistance and erosion resistance in theory, but thicker tube walls are required; there are problems such as difficulty in expansion joints. Therefore, when selecting alloy steel pipes, the connection processing of the tube sheet should be considered, so that the heat exchanger tube and the tube sheet form a butt joint, and the quality of the butt welding of the heat exchange tube ensures the reliability of the connection between the tube and the tube sheet.
In the supercritical unit, if stainless steel tube is used as the high-pressure heat exchanger tube, certain issues need to be considered in the calculation.
(1) The wall thickness of stainless steel heat exchanger tubes is about 30% higher than that of carbon steel tubes.
(2) The heat transfer of austenitic stainless steel is lower than that of carbon steel and low-alloy steel, and because the selected wall thickness is 30% thicker than that of carbon steel, the heat transfer area of the high-pressure heater increases. According to statistics, it increases by 50%.
(3) The length and weight of the equipment have increased, which increases the difficulty of transportation and installation (the total weight of 3 sets of stainless steel tubes is about 420t, and the total weight of 3 sets of carbon steel tubes is about 305t).
Austenitic stainless steel tubes are very sensitive to chloride ions, and very low chloride ion content can cause severe corrosion to heat exchanger tubes in a short time, which is a fatal shortcoming of austenitic stainless steel. During the shutdown period, it is difficult to avoid the enrichment of chloride ions due to the evaporation of water, which is precisely the major factor that causes the corrosion of austenitic stainless steel pipes, especially for units that use seawater as the water-cooling medium. When the condenser leaks, a large amount of seawater often enters the water supply system, and a large amount of chloride ions will cause irreparable damage to the stainless steel heat exchanger tubes. Therefore, from the perspective of economy and reliability, carbon steel pipes or low alloy steel pipes should be selected for the heat exchanger tubes for high-pressure heaters, and think twice when using stainless steel heat exchanger tubes.
Conclusion
Through the comparative analysis, SA-556 Gr.C2 and 16 Mo3 materials are used as heat exchanger tubes, which are the preferred materials for high-pressure heat exchanger tubes, and a large amount of manufacturing and operational experience has been accumulated. Although Cr-Mo alloy steel pipes are more resistant to erosion and corrosion than carbon steel pipes, they lack the support of operating experience. If they are used in units above supercritity, the problem of pipe expansion needs to be considered. When using stainless steel heat exchange tubes, ferritic stainless steel should be preferred, and think twice when choosing austenitic stainless steel.
