Reliability of the Operation of the Heat Exchange Tube
A common failure of high-pressure heaters is leakage. The reasons for the leakage of heat exchange tubes are complex and have a lot to do with design, manufacture and operation.
1.1 Carbon steel pipes
SA-556 Gr.C 2 pipe is a seamless cold-drawn carbon steel pipe commonly used in heaters, and is widely used in high-pressure heaters. Although the corrosion resistance of SA-556Gr.C2 steel pipes is not as good as that of stainless steel pipes, the operation effect is good due to the purified water during high-pressure operation. The metallographic structure of SA-556Gr.C2 is ferrite plus pearlite, and the crystal is body-centered cubic structure. At present, SA-556Gr.C2 is the most used in high-pressure heater heat exchange tubes. According to incomplete statistics, under normal operation and proper maintenance conditions, the first period for the leakage of the heat exchange tube in this material is 7 to 10 years after being put into operation. The reliability of the heat exchange tube in this material is relatively good.
1.2 Low alloy steel pipes
16Mo3 and 15Mo3 low-alloy steel pipes are also used in China. These pipes also leak, and the actual operation is similar to that of SA-556 Gr.C2 pipes. SA-213 T11 is rarely used in high-pressure heaters, and there is no relevant operating data yet.
1.3 Stainless steel pipes
Stainless steel heat exchange tubes are mainly made from austenitic stainless steel and ferritic stainless steel. At present, the most widely used austenitic stainless steel heat exchange tubes are SA-213TP304 tubes. Theoretically, the erosion resistance, oxidation resistance and corrosion resistance of stainless steel are better than carbon steel SA-556Gr. As a high-pressure heat exchanger tube, it should have a better operating effect, but it is not the case in actual operation. Selecting SA-213TP304 as the high-pressure heat exchange tube is immature in technology and experience. The high-pressure reactors of 660MW supercritical units abroad also use SA-213TP304 stainless steel tubes as heat exchange tubes. Some heat exchangers leak to varying degrees within one year, and the leak locations are irregular. After dismantling a high-pressure heat exchanger tube, it was found that cracks appeared near the fracture of the heat exchange tube, which was suspected to be caused by stress corrosion or intergranular corrosion. After analysis of materials and testing, the chemical composition and mechanical performance indicators of the heat exchange tubes are all qualified.
Austenitic stainless steel belongs to the face-centered cubic structure, and it is easy to produce crystals. Cracks in the heat exchange tube move after a high-pressure heater are disassembled, forming a step. Stress corrosion resistance is poor, and stress corrosion cracking is the basic form of corrosion under stress. Usually, when the stress does not exist, the corrosion is very slight. However, when there is stress and the stress exceeds a certain critical value, the brittle fracture of the material will occur in a state of no serious corrosion. Stress corrosion cracking is an important reason for failures of materials or even fracture during use.
1.1 Carbon steel pipes
SA-556 Gr.C 2 pipe is a seamless cold-drawn carbon steel pipe commonly used in heaters, and is widely used in high-pressure heaters. Although the corrosion resistance of SA-556Gr.C2 steel pipes is not as good as that of stainless steel pipes, the operation effect is good due to the purified water during high-pressure operation. The metallographic structure of SA-556Gr.C2 is ferrite plus pearlite, and the crystal is body-centered cubic structure. At present, SA-556Gr.C2 is the most used in high-pressure heater heat exchange tubes. According to incomplete statistics, under normal operation and proper maintenance conditions, the first period for the leakage of the heat exchange tube in this material is 7 to 10 years after being put into operation. The reliability of the heat exchange tube in this material is relatively good.
1.2 Low alloy steel pipes
16Mo3 and 15Mo3 low-alloy steel pipes are also used in China. These pipes also leak, and the actual operation is similar to that of SA-556 Gr.C2 pipes. SA-213 T11 is rarely used in high-pressure heaters, and there is no relevant operating data yet.
1.3 Stainless steel pipes
Stainless steel heat exchange tubes are mainly made from austenitic stainless steel and ferritic stainless steel. At present, the most widely used austenitic stainless steel heat exchange tubes are SA-213TP304 tubes. Theoretically, the erosion resistance, oxidation resistance and corrosion resistance of stainless steel are better than carbon steel SA-556Gr. As a high-pressure heat exchanger tube, it should have a better operating effect, but it is not the case in actual operation. Selecting SA-213TP304 as the high-pressure heat exchange tube is immature in technology and experience. The high-pressure reactors of 660MW supercritical units abroad also use SA-213TP304 stainless steel tubes as heat exchange tubes. Some heat exchangers leak to varying degrees within one year, and the leak locations are irregular. After dismantling a high-pressure heat exchanger tube, it was found that cracks appeared near the fracture of the heat exchange tube, which was suspected to be caused by stress corrosion or intergranular corrosion. After analysis of materials and testing, the chemical composition and mechanical performance indicators of the heat exchange tubes are all qualified.
Austenitic stainless steel belongs to the face-centered cubic structure, and it is easy to produce crystals. Cracks in the heat exchange tube move after a high-pressure heater are disassembled, forming a step. Stress corrosion resistance is poor, and stress corrosion cracking is the basic form of corrosion under stress. Usually, when the stress does not exist, the corrosion is very slight. However, when there is stress and the stress exceeds a certain critical value, the brittle fracture of the material will occur in a state of no serious corrosion. Stress corrosion cracking is an important reason for failures of materials or even fracture during use.
