Carbon Steel, Stainless Steel and Alloy Steel Heat Exchanger Tubes
1.1 Carbon steel heat exchanger tubes
The SA-556Gr.C2 tube is a seamless cold-drawn carbon steel tube commonly used in heaters and is widely used in high-pressure heaters. Although the corrosion resistance of SA-556Gr.C2 steel tubes is not as good as stainless steel tubes, the operating effect is good. The medium during high-pressure operation is purified water. The metallographic structure of SA-556Gr.C2 is ferrite and pearlite, and the crystal has a body-centered cubic structure. Currently, the most commonly used material in high-pressure heater heat exchanger tubes is SA-556Gr.C2. According to incomplete statistics, under normal operation and proper maintenance, the first leakage period of heat exchanger tubes made of this material is 7 to 10 years after being put into operation. It can be seen that the reliability of heat exchanger tubes made of this material is relatively good.
1.2 Low alloy steel heat exchanger tubes
16Mo3 and 15Mo3 low alloy steel pipes are also used in China. These pipes also leak, and their actual operating conditions are similar to those of SA-556Gr.C2 pipes. Cr-Mo such as SA-213T11 is rarely used in high-pressure heaters, and there is currently no relevant operating data.
1.3 Stainless steel heat exchanger tubes
The materials of stainless steel heat exchanger tubes are mainly austenitic stainless steel and ferritic stainless steel. At present, the austenitic stainless steel heat exchanger tube used more in high-pressure heaters is SA-213TP304 pipe. Theoretically, the erosion resistance, oxidation resistance and corrosion resistance of stainless steel are better than those of carbon steel SA-556Gr.C2. As a high-heat exchanger tube, it should have better operating effects, but in actual operation, it's not true. The selection of SA-213TP304 as a high-pressure heat exchanger tube is not yet mature in terms of technology and experience. SA-213TP304 stainless steel pipes are also used as heat exchanger tubes in heat exchanger tubes in a foreign 660 MW supercritical unit. Some heat exchangers have leaked to varying degrees within one year, and the leakage locations are irregular. After a certain high-pressure heater was disassembled, cracks were found near the fracture of the heat exchanger tube, which was suspected to be caused by stress corrosion or intergranular corrosion. After material analysis and testing, the chemical composition and mechanical performance indicators of the heat exchanger tube are qualified. Austenitic stainless steel has a face-centered cubic structure, which is prone to crystal slip and the formation of thick slip steps. It has poor resistance to stress corrosion, and stress corrosion cracking is the basic form of corrosion under stress. Usually, when stress does not exist, the corrosion that occurs is very slight. However, when stress exists and the stress exceeds a certain critical value, brittle fracture of the material will occur without serious corrosion. Stress corrosion cracking is an important reason for failure and even fracture of materials during use.
2. Comparison and calculation of materials
Heat exchanger tubes are the core parts of high-pressure heaters, and the selection of tube materials is directly related to the subsequent operation of high-pressure heaters. It can be seen from the analysis that the pressure of subcritical units is low. There is not much difference in the thickness of carbon steel tubes, alloy steel or stainless steel heat exchanger tubes. No matter which material is used, it is feasible. However, the pressure of supercritical and ultra-supercritical units is different. The higher the pressure is, the greater the difference in the wall thickness selection of the heat exchanger tube becomes. Therefore, it will bring different results to the manufacturing and operation of the heater. 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 it requires a thicker tube wall and has problems such as difficulty in expansion joints. Therefore, when selecting alloy steel pipes, the pipe plate processing should be considered so that the heat exchanger tubes and the tube plate pipes are connected to ensure the reliability of the connection between the pipes and the tube plate to ensure the quality of the butt welding seams of the heat exchange pipes. In supercritical units, if stainless steel pipes are used as high-pressure heat exchanger tubes, certain issues need to be considered in the calculation.
(1) The wall thickness of stainless steel heat exchanger tubes is approximately 30% greater than that of carbon steel tubes.
(2) The heat transfer performance of austenitic stainless steel is lower than that of carbon steel and low alloy steel, and because the wall thickness selected is 30% thicker than that of carbon steel, the heat exchange area of the high-pressure heater increases. According to statistics, it approximately 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 three stainless steel pipe high-pressure heaters is about 420t, and the total weight of three carbon steel pipe high-pressure heaters is about 305t.
Austenitic stainless steel tubes are very sensitive to chloride ions. A very low chloride ion content can cause serious corrosion to the heat exchanger tubes in a short time. This is a fatal shortcoming of austenitic stainless steel. During shutdown, the evaporation of water causes the accumulation of chloride ions, which is difficult to avoid. It is precisely the major factor in the corrosion of austenitic stainless steel pipes caused by chloride ions. 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 used as heat exchanger tubes for high-pressure heaters, and stainless steel heat exchanger tubes should be used with caution.
The SA-556Gr.C2 tube is a seamless cold-drawn carbon steel tube commonly used in heaters and is widely used in high-pressure heaters. Although the corrosion resistance of SA-556Gr.C2 steel tubes is not as good as stainless steel tubes, the operating effect is good. The medium during high-pressure operation is purified water. The metallographic structure of SA-556Gr.C2 is ferrite and pearlite, and the crystal has a body-centered cubic structure. Currently, the most commonly used material in high-pressure heater heat exchanger tubes is SA-556Gr.C2. According to incomplete statistics, under normal operation and proper maintenance, the first leakage period of heat exchanger tubes made of this material is 7 to 10 years after being put into operation. It can be seen that the reliability of heat exchanger tubes made of this material is relatively good.
1.2 Low alloy steel heat exchanger tubes
16Mo3 and 15Mo3 low alloy steel pipes are also used in China. These pipes also leak, and their actual operating conditions are similar to those of SA-556Gr.C2 pipes. Cr-Mo such as SA-213T11 is rarely used in high-pressure heaters, and there is currently no relevant operating data.
1.3 Stainless steel heat exchanger tubes
The materials of stainless steel heat exchanger tubes are mainly austenitic stainless steel and ferritic stainless steel. At present, the austenitic stainless steel heat exchanger tube used more in high-pressure heaters is SA-213TP304 pipe. Theoretically, the erosion resistance, oxidation resistance and corrosion resistance of stainless steel are better than those of carbon steel SA-556Gr.C2. As a high-heat exchanger tube, it should have better operating effects, but in actual operation, it's not true. The selection of SA-213TP304 as a high-pressure heat exchanger tube is not yet mature in terms of technology and experience. SA-213TP304 stainless steel pipes are also used as heat exchanger tubes in heat exchanger tubes in a foreign 660 MW supercritical unit. Some heat exchangers have leaked to varying degrees within one year, and the leakage locations are irregular. After a certain high-pressure heater was disassembled, cracks were found near the fracture of the heat exchanger tube, which was suspected to be caused by stress corrosion or intergranular corrosion. After material analysis and testing, the chemical composition and mechanical performance indicators of the heat exchanger tube are qualified. Austenitic stainless steel has a face-centered cubic structure, which is prone to crystal slip and the formation of thick slip steps. It has poor resistance to stress corrosion, and stress corrosion cracking is the basic form of corrosion under stress. Usually, when stress does not exist, the corrosion that occurs is very slight. However, when stress exists and the stress exceeds a certain critical value, brittle fracture of the material will occur without serious corrosion. Stress corrosion cracking is an important reason for failure and even fracture of materials during use.
2. Comparison and calculation of materials
Heat exchanger tubes are the core parts of high-pressure heaters, and the selection of tube materials is directly related to the subsequent operation of high-pressure heaters. It can be seen from the analysis that the pressure of subcritical units is low. There is not much difference in the thickness of carbon steel tubes, alloy steel or stainless steel heat exchanger tubes. No matter which material is used, it is feasible. However, the pressure of supercritical and ultra-supercritical units is different. The higher the pressure is, the greater the difference in the wall thickness selection of the heat exchanger tube becomes. Therefore, it will bring different results to the manufacturing and operation of the heater. 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 it requires a thicker tube wall and has problems such as difficulty in expansion joints. Therefore, when selecting alloy steel pipes, the pipe plate processing should be considered so that the heat exchanger tubes and the tube plate pipes are connected to ensure the reliability of the connection between the pipes and the tube plate to ensure the quality of the butt welding seams of the heat exchange pipes. In supercritical units, if stainless steel pipes are used as high-pressure heat exchanger tubes, certain issues need to be considered in the calculation.
(1) The wall thickness of stainless steel heat exchanger tubes is approximately 30% greater than that of carbon steel tubes.
(2) The heat transfer performance of austenitic stainless steel is lower than that of carbon steel and low alloy steel, and because the wall thickness selected is 30% thicker than that of carbon steel, the heat exchange area of the high-pressure heater increases. According to statistics, it approximately 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 three stainless steel pipe high-pressure heaters is about 420t, and the total weight of three carbon steel pipe high-pressure heaters is about 305t.
Austenitic stainless steel tubes are very sensitive to chloride ions. A very low chloride ion content can cause serious corrosion to the heat exchanger tubes in a short time. This is a fatal shortcoming of austenitic stainless steel. During shutdown, the evaporation of water causes the accumulation of chloride ions, which is difficult to avoid. It is precisely the major factor in the corrosion of austenitic stainless steel pipes caused by chloride ions. 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 used as heat exchanger tubes for high-pressure heaters, and stainless steel heat exchanger tubes should be used with caution.
