Basis for selecting piping materials in the low-temperature methanol washing process
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How should pipeline materials be selected for piping in the low-temperature methanol washing unit? Are there any resources that can be used as reference? The materials commonly used for low-pressure pipelines are 304, 16Mn, and 20#. Generally, 304 is used at low temperatures, while 16Mn and 20# are more suitable for high temperatures. So how should one choose between 16Mn and 20#, and why?
What I have encountered are 20304L and A333 Gr.6. 304L was chosen due to its corrosion resistance and the requirement to operate at low temperatures of -70°C (I have also seen 304 being used; this may be due to budget constraints, but in certain pipelines containing H2S, the probability of H2S-induced stress corrosion is much higher for 304 compared to 304L). A333 Gr.6 is used instead of 16Mn because the lower temperature limit for 16Mn is -40°C, whereas the operating temperature of many pipelines in low-temperature methanol washing processes is -40°C, with a design temperature of at least -45°C. 20 is used for medium to high temperatures, media with moderate corrosivity, and utility process media. Low-temperature methanol washing does not involve high temperatures; 16Mn is primarily used in low-temperature operating conditions here.
What is the difference between 16MN and 20#? Is 16MN more corrosion-resistant than 20#? The upper temperature limits seem to be similar. The lower limit depends on the fact that 16MN can tolerate lower temperatures to some extent, right?