Commonly Used Heat Exchangers in Different Materials
The high-pressure heater is the key device in the heat recovery system of the thermal power unit, and plays an important role in improving the thermal efficiency of the unit. In the heat recovery system, the high-pressure heater mainly bears the high pressure of the boiler feed water by the water chamber, the tube sheet and the heat exchanger tube. The selection of high-pressure heater material is very important, which is directly related to the safety and reliability of high-pressure heaters. At present, the material manufacturing of the high-pressure water chamber, tube sheet and shell is mature, and there is no case of scrap for high-pressure heaters due to the failure of the shell material.
The high-pressure heat exchanger tube is responsible for the heat exchange between the heating steam and the boiler feed water, and bears high pressure of the boiler feed water. If the material of the heat exchanger tube is not selected properly, the leakage of the heat exchanger tube is more likely to occur. Once the high-pressure heat exchanger tube leaks, it must be shut down immediately. Otherwise, the heat exchanger tubes around the leaking tube will be continuously damaged, resulting in more and more heat exchanger tubes leaking. After the high-pressure heat exchanger is out of service, the power generation load of the unit will drop by 10% to 15%, which seriously affects the power generation efficiency of the unit. Therefore, the selection of the material for the heat exchanger tube is a very important issue.
1. Commonly used heat exchanger tube materials
According to the domestic and foreign requirements for the material of high-temperature heat exchanger tubes, the common materials of heat exchanger tubes are carbon steel, low alloy steel, and stainless steel. Copper tubes were used in the early days, but they have low strength and poor high-temperature performance. They are no longer used in high-pressure heaters.
1.1 Carbon steel pipes
The common carbon steel heat exchanger tube mainly uses SA-556 Gr.C2 and 20G. SA-556 Gr.C2 and 20G are equivalent materials. SA-556 GrC2 is widely used in various heat exchangers and is also the recommended material in ASME SA-556 or SA- 556M standard. The material is carbon-manganese steel pearlitic steel, which has good plasticity and strength, and also has good stress corrosion resistance. The chemical composition of SA-556 Gr.C2 is shown in Table 1.
Table 1 Chemical composition of SA-556 Gr.C2
The high-pressure heat exchanger tube is responsible for the heat exchange between the heating steam and the boiler feed water, and bears high pressure of the boiler feed water. If the material of the heat exchanger tube is not selected properly, the leakage of the heat exchanger tube is more likely to occur. Once the high-pressure heat exchanger tube leaks, it must be shut down immediately. Otherwise, the heat exchanger tubes around the leaking tube will be continuously damaged, resulting in more and more heat exchanger tubes leaking. After the high-pressure heat exchanger is out of service, the power generation load of the unit will drop by 10% to 15%, which seriously affects the power generation efficiency of the unit. Therefore, the selection of the material for the heat exchanger tube is a very important issue.
1. Commonly used heat exchanger tube materials
According to the domestic and foreign requirements for the material of high-temperature heat exchanger tubes, the common materials of heat exchanger tubes are carbon steel, low alloy steel, and stainless steel. Copper tubes were used in the early days, but they have low strength and poor high-temperature performance. They are no longer used in high-pressure heaters.
1.1 Carbon steel pipes
The common carbon steel heat exchanger tube mainly uses SA-556 Gr.C2 and 20G. SA-556 Gr.C2 and 20G are equivalent materials. SA-556 GrC2 is widely used in various heat exchangers and is also the recommended material in ASME SA-556 or SA- 556M standard. The material is carbon-manganese steel pearlitic steel, which has good plasticity and strength, and also has good stress corrosion resistance. The chemical composition of SA-556 Gr.C2 is shown in Table 1.
Table 1 Chemical composition of SA-556 Gr.C2
| Items | Composition/% |
| C | Less than and equal to 0.3 |
| Mn | 0.29 to 1.06 |
| Cr | / |
| Mo | / |
| P | Less than and equal to 0.035 |
| S | Less than and equal to 0.035 |
| Si | Si being greater than and equal to 0.1 |
20G is also a commonly used material grade. It is a high-pressure carbon steel pipe used for boilers, which has good plasticity, toughness and weldability and is often used in heating surface tubes of high-pressure or high-parameter boilers, such as low-temperature superheaters, front screens of panel superheaters, economizers and water-cooled walls. It was also used as heat exchanger tubes for high-pressure heaters for units below 200MW in the early days. At present, with the wide application of SA-556 Gr.C2 in large units, 20G has been gradually replaced by SA-556 Gr.C2.
1.2 Alloy steel pipes
Alloy steel pipes are seldom used for high-temperature heat exchanger tubes, and the material grades and chemical compositions of alloy steel pipes are mainly used, as shown in Table 2.
Table 2 Grades and chemical composition of commonly used alloy steel pipes
| Items | The composition of the element/% | Standards | |||||
| C | Mn | Cr | Mo | Si | |||
| Grades | SA-213T11 | 0.05 to 0.15 | 0.3 to 0.6 | 1.0 to 1.5 | 0.44 to 0.65 | 0.5 to 0.1 | ASME II |
| SA-213T12 | 0.05 to 0.15 | 0.3 to 0.61 | 0.8 to 1.25 | 0.44 to 0.65 | 0.5 to 0.1 | ASME II | |
| SA-213T22 | 0.05 to 0.15 | 0.3 to 0.6 | 1.9 to 2.6 | 0.87 to 0.113 | 0.5 to 0.1 | ASME II | |
| 16Mo3 | 0.12 to 0.2 | 0.4 to 0.9 | / | 0.25 to 0.35 | / | EN 10216 | |
| 15Mo3 | 0.12 to 0.2 | 0.4 to 0.8 | / | 0.25 to 0.35 | 0.15 to 0.35 | DIN 17175 | |
Table 3 Grade and chemical composition of stainless steel heat exchanger tubes
| Items | The composition of the element/% | Standards | |||||||
| C1 | Mn | Cr | Mo | Si | Ni | N | |||
| Grades | TP304 | Less than and equal to 0.08 | Less than and equal to 2.0 | 18 to 20 | / | Less than and equal to 0.75 | 8.0 to 11.0 | / | ASME II |
| TP304L | Less than and equal to 0.035 | Less than and equal to 2.0 | 18 to 20 | / | Less than and equal to 0.75 | 8.0 to 11.0 | / | ASME II | |
| TP304N | Less than and equal to 0.080 | Less than and equal to 2.0 | 18 to 20 | / | Less than and equal to 0.75 | 8.0 to 11.0 | ASME II | ||
| TP316 | Less than and equal to 0.080 | Less than and equal to 2.0 | 16 to 18 | Less than and equal to 0.75 | 11.0 to 14.0 | / | ASME II | ||
| 316L | Less than and equal to 0.035 | Less than and equal to 2.0 | 16 to 18 | Less than and equal to 0.75 | 10.0 to 15.0 | / | ASME II | ||
| TP439 | Less than and equal to 0.070 | Less than and equal to 1.0 | 17 to 19 | / | Less than and equal to 1.0 | Less than and equal to 0.5 | / | ASME II | |
Cr and Mo are added to SA-213T11, SA-213T12, SA-213T22 and other grades, and Cr can increase the hardenability of steel and have the effect of secondary strengthening. After adding Cr, the material has good high-temperature oxidation resistance and oxidation resistance, which increases the thermal strength of the steel. After adding Mo, the oxidation resistance of the material can be improved. Therefore, SA-213T11, SA-213T12, and SA-213T22 have better anti-erosion and anti-oxidation properties.
16 Mo3 and 15 Mo 3 are selected for different standards, but the chemical composition is similar, and Mo is added based on carbon steel to improve the oxidation resistance of the material. Cr is a non-controlling element, so its scouring resistance and corrosion resistance are not substantially improved compared with carbon steel pipes.
1.3 Stainless steel heat exchanger tubes
Commonly used stainless steel heat exchanger tubes are mainly austenitic stainless steel tubes and ferritic stainless steel tubes. The main grades and chemical composition of stainless steel heat exchanger tubes are shown in Table 3.
Stainless steel 304 and 316 pipes are divided into seamed pipes and seamless pipes. The more common ones are mainly SA-688 and SA-213. At present, high-pressure heat exchanger mostly uses heat exchanger tubes made from SA-803TP439.
According to the chemical composition of stainless steel, it can be divided into two kinds: chromium stainless steel and chromium-nickel stainless steel, represented by Cr13 and Cr18Ni8.
SA-803TP439 is ferritic stainless steel, which has good performance, low sensitivity to corrosion cracking under great stress, excellent resistance to pitting and crevice corrosion, and resistance to intergranular corrosion. However, the notch sensitivity of the material is high. As the thickness of the material increases, the no-plastic transition temperature increases significantly; the brittleness of the material increases, and the plasticity decreases. SA-803TP439 ferritic stainless steel has weaker chloride ion resistance than austenitic stainless steel, but this type of material is widely used in nuclear power heaters.
TP304, TP304L, TP304N, TP316 and TP316L are all austenitic stainless steel, with good corrosion resistance, erosion resistance and oxidation resistance. When the carbon content of TP304L and TP316L is less than or equal to 0.03%, the single austenite structure is very stable, which can be used in various temperature ranges, and has good corrosion resistance. When the carbon content of stainless steel TP304, TP304N and TP316 is greater than 0.03%, it exceeds the amount of dissolved carbon in austenite, which reduces the corrosion resistance to a certain extent, and easily leads to intergranular corrosion, pitting corrosion, corrosion and cracking due to stress along the intergranular pattern. Therefore, austenitic stainless steel is treated with solution heat treatment to obtain a single austenite structure. The carbon content of 304L and 316L is low; the remaining carbon equivalent is less, and the single austenite structure is also more stable. Therefore, the corrosion resistance of the material is better. When austenitic stainless steel stays at temperatures between 450 to 850°C, carbon in the material will combine with chromium to form chromium carbide, which will make the intergranular lack of chromium and cause intergranular corrosion. This is the sensitization of austenitic stainless steel. Austenitic stainless steel is also more sensitive to chloride ions, and its resistance to chloride ion corrosion is poor.
