Nelson curves

Nelson Curves" have been published in API Publication 941, First Edition, July 1970. 

To avoid decarburization and Assuring of the carbon and low-alloy steels, which is cumulative with time and, for all practical purposes irreversible, the limitations of the Nelson Curves should be followed religiously, as a minimum. Suitable low-alloy plate materials include ASTM-A204-A, B, and C and A387-A, B, C, D, and E, and similarly alloyed materials for pipe, tubes, and castings, depending upon stream temperatures and hydrogen partial pressures, as indicated by the Nelson Curves. 

The Nelson curves in API 941 (published by the American Petroleum Institute) list the operating limits that should be followed to avoid de-... 

The Nelson curves, utilizing the maximnm operating temperature plus 25°F (14°C), should be used. 

If hydrogen is involved in the process, the material selected for resistance to sulfur must also meet the minimum requirements, based on the Nelson curves. In many cases, combined hydrogen-sulfur or hydrogen-hydrogen sulfide service require cladding or overlays, to provide resistance to the effects of high-temperature sulfidic corrosion and hydrogen attack, at an affordable cost. 

Item 3 discusses a 1970 American Petroleum Institute recommended practice (API 941), which predicts HTHA in various material of constmction using Nelson Curves. The document was determined to be inadequate for this situation. 

Damage mechanism hazard reviews called corrosion reviews, were periodically performed by Tesoro and the prior owner/operator Shell Oil. But, these corrosion reviews did not identify HTHA as a probable failure mechanism for exchangers B and E. The Nelson Curves" which are relied upon by maiy metallurgists were inaccurate for this situation. 

Brear, J.M. and Church, J.M., Technical basis for API Publication RP941 (Nelson Curves) , Third Int Conf Engineering Structural Integrity Assessment, Cambridge, September 1996... 

Hydrogen diffuses rapidly through porous materials and through some metals at red heat. It may leak out of a system that is gas tight for air or common gases at equivalent pressures. It can diffuse into carbon steel and combine with carbon to form methane, which then causes lamination and loss of strength in the steel. A Nelson curve indicates the proper alloy to use depending on the temperature and pressure. 

Hydrogen gas is noncorrosive and may be contained at ambient temperatures by most common metals used in installations designed to have sufficient strength for the working pressures involved. Equipment and piping built to use hydrogen should be selected with consideration of the possibility of embrittlement, particularly at elevated pressures and temperatures above 450°F (232°C). A Nelson curve should be consulted to select the proper alloys. 

It follows that the position of the experimental points, relative to the Martinelli-Nelson curve, cannot be due to errors in the calculated values of x resulting from the assumption of stable equilibrium,... 

From these discussions we conclude that the variance between the authors results and the Martinelli and Nelson curve is due to shortcomings in the latter. However, our results indicate that the use of the Martinelli and Nelson correlation can give results with 10 to 30 per cent accuracy when obtaining a value of from a given value of range of... 

The conditions under which different steels can be used in high-temperature hydrogen service are listed in API 941. The principal data are presented in the form of Nelson curves, as shown in Figure 2.11. The curves are based on long-term refinery experience, rather than on laboratory studies and are periodically revised by the API Subcommittee on Materials Engineering and Inspection. The latest edition of API 941 should be consulted to ensure that the proper steel is selected for the operating conditions encountered. 

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