Nelson curves and HTHA damage

High Temperature Hydrogen Attack (HTHA) is one of the Damage Mechanisms that affects Hydrogen service Steel equipment which exists mainly at Refineries, Methanol and Ammonia plants . HTHA occurs at elevated temperatures with Hydrogen service. HTHA vulnerability for equipment depends on three major factors, namely, Metallurgy type (composition), operating temperature and Hydrogen partial pressure. HTHA is mostly found in equipment made from Carbon Steel as well as Carbon ½ Mo.
Damage mechanism
· HTHA can occur in hydrogen atmosphere at elevated temperatures (at least 400 °F or 204 °C) and a hydrogen partial pressure of pH2 > 3.45 bar.
· Thermal dissociation of Hydrogen.
· The atomic hydrogen (i.e. H+) diffuses through steel.
· This atomic hydrogen then reacts with unstable carbides in steel and forms methane (CH4) gas pockets within the metal structure. Since Methane gas can’t diffuse through the steel, it creates stresses that result in cracking and failure of the vessel eventually.
8H⁺+ Fe₃C + C→ 2CH₄ +3Fe

How can HTHA be prevented?
Typically, HTHA can be avoided by choosing the proper alloy steel or stainless steel cladding to resist the combination of hydrogen partial pressure and temperature, or by adjusting the operating conditions to stay below the Nelson Curve limit for the existing materials of construction.

What is Nelson Curve?
The standard API RP 941 uses “Nelson Curves” to predict the operating conditions where HTHA can occur in different types of steels. The Curve was revised in 2016 after Tesoro incident to consider Two carbon steel Nelson curves, distinguished by whether the equipment has been post weld heat treated (PWHT) or not.

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