Principles for pipeline material selection in hydrogenation units
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The corrosion environment in the high-pressure pipelines of hydrogenation units is quite complex. The main forms of corrosion include high-temperature hydrogen damage, high-temperature sulfur corrosion, hydrogen/hydrogen sulfide corrosion, wet hydrogen sulfide corrosion, NH4Cl/NH4HS corrosion, and polythionic acid stress corrosion cracking. Since one or more types of corrosion may occur in the same area, material selection should be based on a comprehensive analysis of the specific operating conditions, generally following the principles outlined below. (1) For pipelines carrying hydrogen-containing media at operating temperatures of 200 ℃ or higher, a suitable steel for use in hydrogen environments should be selected based on the highest operating temperature of the medium in the pipeline plus a margin of 20–40 ℃, as well as the corresponding hydrogen partial pressure, using Nelson’s curve from API RP941. (2) The operating temperature should be no lower than 240 °C. For pipelines containing sulfide-corrosive media, the corrosion rate and corrosion allowance for various steel grades should be estimated using the modified McConomy curve, with reference to the sulfur content in the medium and the operating temperature, so as to select an appropriate pipeline material. (3) For pipelines where the operating temperature is 250 ℃ or higher and the medium contains high-temperature hydrogen and hydrogen sulfide, the corrosion rate of appropriate materials in a high-temperature hydrogen/hydrogen sulfide environment should be calculated using the Couper curve, based on the operating temperature and the molar fraction of hydrogen sulfide in the medium, in order to select suitable materials. (4) The medium in the air-cooling system of the hydrogenation unit contains corrosive substances such as NH4Cl/NH4HS; appropriate materials should be selected based on factors such as pressure equilibrium constant, medium flow rate, and concentrations of NH4Cl and NH4HS. (5) When selecting materials for pipelines in a wet hydrogen sulfide environment, it is necessary to take into account both the uniform thinning of the pipeline wall thickness and wet hydrogen sulfide stress corrosion cracking. (6) For austenitic stainless steel pipes that may suffer from polysulfate stress corrosion cracking, stable stainless steel or ultra-low carbon stainless steel should be used.
The material selection for high-pressure pipelines in hydrogenation units should take the corrosive environment into comprehensive consideration and follow the following principles: 1. For pipelines operating in hydrogen-containing media at temperatures of ≥200°C, appropriate steel grades shall be selected based on the Nelson curve specified in API RP941. 2. For pipelines containing sulfides and operating at temperatures ≥240°C, select the material by referring to the modified McConomy curve, taking into account the sulfur content in the medium and the operating temperature. 3. For pipelines containing high-temperature hydrogen and hydrogen sulfide with an operating temperature of ≥250°C, the corrosion rate is calculated using the Couper curve to select the appropriate material. 4. For media containing NH4Cl/NH4HS in air-cooled systems, appropriate materials are selected based on factors such as pressure equilibrium constants and flow rates. 5. In a wet hydrogen sulfide environment, material selection must take into account uniform thinning and stress corrosion cracking caused by wet hydrogen sulfide. 6. If austenitic stainless steel pipes are at risk of polysulfate stress corrosion cracking, stabilized or ultra-low carbon stainless steel should be used. .