Carbon steel process plant that operates with hydrogen at elevated pressure and temperature can be weakened by a phenomenon known as high temperature hydrogen attack (HTHA). The effect of hydrogen on carbon steel and its alloys has been known for some years, whereby at elevated temperatures and pressures, atomic hydrogen enters the microstructure of the steel, and reacts with the carbon present to form larger methane molecules. That process continues, with the hydrogen penetrating further through the steel structure, so that over time, tiny pockets of methane coalesce, leading to fissuring and ultimately crack development. If this phenomenon is taking place and continues undetected, it can potentially lead to failure of the process plant and a major accident.
At elevated temperatures and pressures experienced in refining operations, hydrogen at the surface of a metallic interface will be first adsorbed and then absorbed into steel. Diffusivity into steel is highly dependent upon a number of factors including: microstructure and alloying elements (material), and current temperature and partial pressure of hydrogen. Stress levels in piping and equipment impact susceptibility to HTHA. HTHA damage is irreversible and cumulative over the time.
The HTHA phenomenon differs fundamentally in the mechanism and conditions under which it occurs and should not be confused with hydrogen embrittlement.
High temperature hydrogen attack (HTHA) of steels
At relatively high temperature and low hydrogen partial pressures, the damage is manifested by surface decarburisation, whereas at relatively low temperature and higher hydrogen partial pressure, internal voids and microfissures are favoured. The more likely manifestation, dependent on conditions, is indicated on the Nelson Curves, which are published in API RP 641 “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants“. This document contains the basic guidelines for determining the risk of HTHA in installations operating at elevated temperatures. API RP 941 summarises the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low alloy steel in hydrogen service at elevated temperatures and pressures.
API Recommended Practice does not address the resistance of steels to hydrogen at lower temperatures (below about 400ºF = 204°C), where atomic hydrogen enters the steel as a result of corrosion or by electrochemical mechanisms.
What are Nelson Curves?
The Nelson curves have been developed for various carbon steel alloys and low alloy steels plotted against the axes of service temperature and hydrogen partial pressure, shown in a merged graph (the curves are based on the results of experimental investigations and actual data from operating plants). This was done with the aim of defining practical operating limits for carbon and low-alloy steels in hydrogen operation at elevated temperatures and partial hydrogen pressures.
The curves are not to be regarded as a ‘no attack’ line, below which the threat of HTHA is eliminated – operating close to but beneath the curves still comes at some risk.
If you plot the temperature and partial pressure of hydrogen for the process on this graph and the point plotted is below the specified metallurgy’s curve, HTHA will not occur. This graph also shows that HTHA doesn’t occur at hydrogen partial pressures below approximately 50 PSIabs, except at extremely elevated temperatures.
The time of exposure is an additional variable not currently taken in to account in the curves. It is anticipated that the time variable will be incorporated in to future editions of API RP 941.
Use of the Nelson Curves in practice
The curves attempt to describe limit conditions for different grades of carbon and alloy steel, where exposure to adverse conditions above the curve will lead to HTHA effects on the micro-structure.
The curves were drawn up with the aim of defining practical operating limits for carbon steels and low-alloy steels in hydrogen operation at elevated temperatures and partial pressures. The standard and curves should be applied with a degree of conservatism, especially where consequences of failure will lead to a major accident.
For new systems, the curves can be used to select materials and eliminate the risk of HTHA.
Existing systems at risk of HTHA should be appropriately installed and maintained to minimize exposure. Closer proximity to the applicable curve should involve higher levels of scrutiny, perhaps involving more frequent examination and testing.
Detection and quantifying of HTHA in steel
HTHA damage that has already occurred can be detected using suitable NDT procedures. The damage mechanism is very difficult to diagnose in the early stages, and unless newer, more advanced techniques are used at an appropriate time, may already threaten integrity by the time it is detected. NDT is continually developing, and currently requires complimentary, validated techniques to detect HTHA. Inspection methodology uses advanced techniques including Time of Flight Diffraction (TOFD), Phased Array (PAUT), Ultrasonic Backscatter Technique, Frequency analyses (FFT) and Advanced Velocity Ratio (AVR) measurements.
Partial pressure determination of hydrogen
For gases or mixed gases, the partial pressure of the hydrogen gas is the mole fraction of H2 multiplied by the absolute total pressure.
To determine the hydrogen partial pressure, multiply the mole fraction of hydrogen in the process stream by the absolute pressure of the stream.
If the stream contains liquid hydrocarbon and if hydrogen is dissolved in the liquid hydrocarbon, the hydrogen partial pressure is calculated differently. One method, cited in API RP 941, involves using the vapor pressure of the gas with which the liquid stream is in equilibrium. This is an important point since if there is a liquid stream, one might assume the amount of hydrogen present is very low, which could lead to underestimating the partial pressure of the gas.
Materials for which Nelson Curves are available
- Carbon steels (welded with no PWHT)
- Carbon steels (non-welded or welded with PWHT
- 1.25Cr-0.5Mo steel (Grade P/T11)
- 1.0Cr-0.5Mo steel (Grade P/T12)
- 2.25Cr-1.0Mo steel (Grade P/T22)
- 2.25Cr-1.0Mo-V steel
- 3.0Cr-1.0Mo steel (Grade P/T21)
- 6.0Cr-0.5Mo steel
Read More
EIGA Doc 243/22 Guideline on remedial action for HYCO Plant components subject to High Temperature Hydrogen Attack
New Part 15 of EN 13445 series for pressure vessels (“Specific requirements for hydrogen applications“
