Heat Exchanger Tubes
The heat exchanger tube is the main part of the high-pressure heater tube bundle. The selection of the tube material is directly related to the operating performance and reliability of the high-pressure heater. Carbon steel heat exchanger tubes are commonly used in high-pressure heaters. Low-alloy steel tubes have better erosion and corrosion resistance than carbon steel tubes but are less commonly used.
The high-pressure heater is the key equipment in the heat recovery system of thermal power units and plays an important role in improving the thermal efficiency of the unit. In the heat recovery system, the water chamber, tube sheet and heat exchanger tubes of the high-pressure heater are mainly used to withstand the high pressure of the boiler feed water. The choice of high-pressure heater material is crucial and is directly related to the safety and reliability of a high-pressure heater. At present, the material manufacturing of the high-pressure heater water chamber, tube sheet and shell has been mature, and there has been no case of the high-pressure heater being scrapped due to the failure of the shell material.
1.1 Carbon steel heat exchanger tubes
SA-556Gr.C2 carbon steel heat exchanger tubes have been widely used in high-pressure heaters, and the processing technology is relatively mature. There are no problems with the welding and expansion joints of heat exchanger tubes and high-pressure tube sheets.
1.2 Low alloy steel heat exchanger tubes
At home and abroad, some equipment also uses heat exchanger tubes made of 16Mo3. Because the tube material contains Mo element, the welding is slightly inferior to that of SA-556Gr.C2. However, the manufacturing technology of this material is mature. By improving the welding process, welding of the material has been improved. SA 213T11 are common low-alloy steel pipes and are not widely used in high-pressure heaters. In the ASME standard, materials such as SA 213T11 have lower allowable stresses than carbon steel but have better hardness and strength. In certain equipment, 15CrMo (equivalent to SA 213T11) was used for the heat exchanger tube. When the expansion test was performed on the heat exchanger tube, it was found that it was difficult to complete the expansion joint. Low alloy steel heat exchanger tubes can be used in subcritical units. If used in supercritical and ultra-supercritical units, the tube wall will be thicker, making expansion difficult, and should be selected with caution.
1.3 Austenitic stainless steel heat exchanger tubes
Austenitic stainless steel tubes are often selected as heat exchanger tubes for low-pressure heaters. Austenitic stainless steel has good processing properties. However, the heater cannot be heat treated as a whole due to the sensitization of austenitic stainless steel tubes, and the shell and welding seams of pipes need to be heat treated. When the shell thickness is thick, there are problems such as uneven heating during partial heat treatment.
1.4 Ferritic stainless steel heat exchanger tubes
SA-803TP439 is a commonly used ferritic stainless steel and is widely used in nuclear power high-pressure heaters and separators (MSR). However, the material has brittleness and its thickness is limited. It is understood that the maximum thickness of the SA-803TP439 steel pipe produced by a steel mill in the United States is only 2.2mm. Beyond this thickness, the brittleness of the material will increase significantly. When the material thickness reaches 2.5mm, its non-plastic transition temperature has reached 20°C. This material property limits the application of pipes. According to the allowable stress values given in the ASME standard, this material can only be used below 325°C. In the design of thermal power heat exchanger tubes, the design temperature and heat exchanger tube’s thickness of heat exchanger tubes exceed the allowable values of the material. Therefore, the material is not suitable for use in thermal power high-pressure heaters.
The high-pressure heater is the key equipment in the heat recovery system of thermal power units and plays an important role in improving the thermal efficiency of the unit. In the heat recovery system, the water chamber, tube sheet and heat exchanger tubes of the high-pressure heater are mainly used to withstand the high pressure of the boiler feed water. The choice of high-pressure heater material is crucial and is directly related to the safety and reliability of a high-pressure heater. At present, the material manufacturing of the high-pressure heater water chamber, tube sheet and shell has been mature, and there has been no case of the high-pressure heater being scrapped due to the failure of the shell material.
1.1 Carbon steel heat exchanger tubes
SA-556Gr.C2 carbon steel heat exchanger tubes have been widely used in high-pressure heaters, and the processing technology is relatively mature. There are no problems with the welding and expansion joints of heat exchanger tubes and high-pressure tube sheets.
1.2 Low alloy steel heat exchanger tubes
At home and abroad, some equipment also uses heat exchanger tubes made of 16Mo3. Because the tube material contains Mo element, the welding is slightly inferior to that of SA-556Gr.C2. However, the manufacturing technology of this material is mature. By improving the welding process, welding of the material has been improved. SA 213T11 are common low-alloy steel pipes and are not widely used in high-pressure heaters. In the ASME standard, materials such as SA 213T11 have lower allowable stresses than carbon steel but have better hardness and strength. In certain equipment, 15CrMo (equivalent to SA 213T11) was used for the heat exchanger tube. When the expansion test was performed on the heat exchanger tube, it was found that it was difficult to complete the expansion joint. Low alloy steel heat exchanger tubes can be used in subcritical units. If used in supercritical and ultra-supercritical units, the tube wall will be thicker, making expansion difficult, and should be selected with caution.
1.3 Austenitic stainless steel heat exchanger tubes
Austenitic stainless steel tubes are often selected as heat exchanger tubes for low-pressure heaters. Austenitic stainless steel has good processing properties. However, the heater cannot be heat treated as a whole due to the sensitization of austenitic stainless steel tubes, and the shell and welding seams of pipes need to be heat treated. When the shell thickness is thick, there are problems such as uneven heating during partial heat treatment.
1.4 Ferritic stainless steel heat exchanger tubes
SA-803TP439 is a commonly used ferritic stainless steel and is widely used in nuclear power high-pressure heaters and separators (MSR). However, the material has brittleness and its thickness is limited. It is understood that the maximum thickness of the SA-803TP439 steel pipe produced by a steel mill in the United States is only 2.2mm. Beyond this thickness, the brittleness of the material will increase significantly. When the material thickness reaches 2.5mm, its non-plastic transition temperature has reached 20°C. This material property limits the application of pipes. According to the allowable stress values given in the ASME standard, this material can only be used below 325°C. In the design of thermal power heat exchanger tubes, the design temperature and heat exchanger tube’s thickness of heat exchanger tubes exceed the allowable values of the material. Therefore, the material is not suitable for use in thermal power high-pressure heaters.
