Issues with equipment material selection
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For the gas liquid removal tank in the desulfurization unit, the operating pressure is 0.75 MPa, the operating temperature is 40 degrees Celsius. The gas mixture consists of 54% hydrogen, 0.4% hydrogen sulfide, and approximately 12% each of propane, ethane, and propane; there are also small amounts of isobutane and compounds with a molecular weight of C5 or higher. What material do you think would be suitable for use here?
Carbon steel + corrosion margin (appropriate) Carbon steel + anti-corrosion coating
Question: If carbon steel is coated with an internal anti-corrosion layer or combined with a duplex stainless steel composite plate, does the carbon steel still need to have resistance to hydrogen sulfide? Under wet hydrogen sulfide conditions, Q245R is recommended; Q345R should be used with caution. Low-alloy steels with higher strength than Q345R cannot be used. Since the walls of such high-pressure equipment are very thick, I would like to ask whether, in the case of using internal anti-corrosion coatings or composite steel plates, it is still necessary to take into account the effects of hydrogen sulfide on the base material
When selecting materials for carbon steel + corrosion resistance, wet hydrogen sulfide corrosion must be taken into account; this is not necessary for composite panels. Is the pressure high under your operating conditions? 0.75 – It’s too extravagant to choose a composite panel!
Thank you for your advice. I’m not the one who created the post. I was involved in the conceptual design for a project before; there was a high-pressure separator with a designed pressure of 22 MPa. Using Q245R material, the thickness required was over 200 mm, while with Q345R it was around 100 mm. It’s so thick – how is it possible to weld it? The inspector found it difficult to do the job and kept complaining about why he took on this task. We weren’t responsible for the detailed design that came later, so we don’t know how such a device was ultimately created. The working media for this project are wet natural gas and gas field water. In wet natural gas, the average molar content of CO2 is 0.017% to 2.07%, while the average molar content of H2S is 0.0369% to 0.0499% ； The pH of the gas field water is 5.18–5.22, the Cl- concentration is 9.17–9.20×104 mg/L, and the total mineralization is 1.68×105 mg/L; the water type is sodium bicarbonate type.
If the size is small, stainless steel can be considered directly. Or, for carbon steel, consider hydrogen sulfide stress corrosion (HIC).