Mechanical Properties of Heat Exchanger Tubes

The heat exchanger tube of the high-pressure heater bears high pressure of the water supply. Therefore, when materials are selected, the mechanical properties of the heat exchanger tube are one of the important indicators. At a specific temperature, the allowable stress of heat exchanger tubes of various material grades is shown in Table 4
 
Table 4 Allowable stress of each grade of heat exchanger tubes at a specific temperature/MPa

 
It can be seen from the data in Table 4 that when the temperature is below 375°C, the allowable stress of SA-556 Gr.C2 is the highest, followed by 16Mo3, while the allowable stress of stainless steel is relatively low. It should be noted that 16Mo3 is the selected material in EN10216-2 standard. In the EN 10216-2 standard, the yield strength of the 16Mo3 is given, as shown in Table 5.
     
Table 5 Yield strength of 16Mo3 at different temperatures

 
In the ASME standard, the yield strength is used as the allowable stress for selection, and the safety factor is taken as 1.6. Therefore, the allowable stress of 16Mo3 is also calculated according to the ratio of yield strength and safety factor, as shown in Table 6.
 
Table 6 Allowable stress of 16Mo3 at various temperatures

 
It can be seen from Table 6 that when the temperature is higher than 300°C, the allowable stress value of 16Mo3 is lower than the value given in ASME II Volume D. Therefore when 16Mo3 is used, if the allowable stress is selected according to the ASME standard, the minimum value of the yield strength of the material must be artificially increased before purchasing, which cannot meet the requirements of the EN10216-2 standard.
 
Thermal power generation units are often divided into subcritical units, supercritical units and ultra-supercritical units. The design pressure and design temperature of the tube side of the heat exchanger are selected according to the highest value given by the design institute. The design pressure and temperature of the tube side are shown in Table 7. Since the design pressure of the double reheat unit reaches 45 to 50MPa, it is a special working condition.
 
Table 7 Design pressure and temperature on the pipe side of various types of units

 
Table 8 Calculating the wall thickness of heat exchanger tubes in various materials (The outer diameter is 15.88.) Unit: mm

 
According to the design pressure and design temperature in Table 7, calculate and select the wall thickness of heat exchanger tubes in various materials, as shown in Table 8
 
From the calculation data in Table 8, in the subcritical unit, the difference in the thickness of the heat exchanger tubes in various materials is not big due to low pressure, which is 2 to 3mm. If it is used in an ultra-supercritical unit, the thickness of the stainless steel tube and low-alloy steel pipe has reached more than 3.0mm, except for 16Mo3.