I feel like so many of you have become part of my life, and it has been a very difficult decision to move on from curves, but i have had a dream, and now that my kids are growing up its time to follow it. Additionally, the model accounts for other important factors such as. Api rp 941 steels for hydrogen service at elevated. In this case the htp is below the mhr, even though originally it would have been flagged as high risk using the standard nelson curve and operating conditions. For example, if the normal operating conditions are a temperature of 288 c 550 f and. New method for prioritizing equipment for inspection and. This rp is intended to help identify and manage equipment potentially subject to high temperature hydrogen attack htha by putting into place practical. Hightemperature hydrogen attack is also referred to as hot hydrogen attack or decarburization. Seminar presentation tesoro refinery explosion oil. Austenitic materials are satisfactory at all temperatures and pressures from hydrogen damage.
The api 941 nelson curves are an empirical formulation designed to identify safe operationl limits with respect to temperature and. The nelson curves are supposed to provide guidance as to the conditions that will cause no attack. In the 1990s, there was a growing number of cases of htha damage occurring in carbon. Appendix 8 case study assessing operating limits for c0. Htha damage compromised the impact properties drastically. Reducing the risk of high temperature hydrogen attack. Htha life assessment methodology an htha life assessment should provide guidance for classifying the likelihood of an htharelated failure per the owner or users process safety management psm risk matrix. This micrograph shows intergranular cracking caused by high temperature hydrogen attack in a c0.
Api rp 941 steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants, eighth edition, includes errata 1 2016 and errata 2 2018 standard by american petroleum institute, 02012016 amendments available. The nelson curve approach takes no account of the time in service. Variable operating conditions type and thickness of material base metal and cladding. Api carbon steel degradation in high temperature hydrogen. The driving force for void growth is the methane pressure, which is not assumed to be constant, arising from the interaction between carbon and hydrogen at void surfaces. Hydrogen service defined as a combination of h2 partial pressure and temp above the curve for cs per api 941.
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 microstructure. These nelson curves are detailed in a publication from the. All of the new reports of htha involve carbon steel equipment that was not postweld heat treated. Using api 941, if a piece of equipment or piping is operated above the api 941 nelson curve, then the material is not suitable for service under those conditions. Suggested revisions are invited and should be submitted to the standards department, api, 1220 l street, nw, washington, d. Prior to these recent reports, the only reported failures of carbon steel below the api rp 941, figure 1 curve were in cases of exceptionally high stress, as discussed in sections 5. This diagram has been updated a number of times by the american petroleum institute api and published in the api recommended practice 941. Ib160 alertpreventing high temperature hydrogen attack. A void growth model has been applied to hydrogen attack of plain carbon structural steels, and to calculation of the nelson curves. Hydrogen attack, detection, assessment and evaluation. For existing equipment operating at risk, all carbon steel equipment in hydrogen service should be identified.
Pdf api recommended practice 941 steels for hydrogen. For example, if the normal operating conditions are a temperature of 288. Hic, ssc, scc, hcl and htha corrosion engineering engtips. Api rp 941 provides nelson curves to predict the occurrence of htha in various materials of construction as a function of temperature and hydrogen partial pressure api rp 941 in 1970, the american petroleum institute api published recommended practice rp. Presently widely accepted specification against htha is api 941. High temperature hydrogen attack htha is an insidious condition that can occur in process equipment exposed to hydrogen at elevated temperatures at least 400f or 204c, under dry conditions, when hydrogen disassociates into nascent atomic hydrogen, which is then driven into the steel by the temperature and pressure of the environment. Although api states that there is a 50 f design margin for their nelson curves, no quantified.
Nelson curves found in api rp 941 are used to determine if a specific carbon steel alloy group is susceptible hydrogen attack, such as hydrogen embrittlement, at elevated temperature and pressure. Piping material and metallurgy basic guidelines for. Low alloy crmov steel plates for petrochemical reactors 22 aspects of interchangeability between 2. The first edition of the recommended practice was originally. Inline inspection for challenging pipelines fitnessfor.
It is the first change to the curves in the last 25 years. This set of curves represents the core information in the api standard. Status of the publication can be ascertained from the api standards department, telephone 202 6828000. These methods were developed using laboratory and failure data published in api 941 and have been validated against other published data. Api 941 steels for hydrogen service and elevated temperatures in addition, we provide thirdparty laboratory examination at our materials laboratory in san antonio, texas.
Steels for hydrogen service at elevated t emperatures and. Pdf components operating in hh temperature hydrogen service are susceptible to hydrogen attack. In the nelson diagram, boundaries are placed in a temperaturehydrogen partial pressure graph, which delineates the region of safe use for carbon steels, 1. How to use the nelson curve api 941 to confirm if the material shoosen is the right material. For many years, significant faith has been placed in the nelson curves 2 as described in api 941 to predict the likelihood of htha. Regarding subpart b of the recommendation, rp 941 did add a new carbon steel nelson curve for nonpostweldheattreated nonpwht carbon steel, rp 941 does not require the use of inherently safer materials. Carbon steel degradation in high temperature hydrogen. Using api rp 941 as a guide, if a piece of equipment or piping is operated above the api rp 941 nelson curve, then the material is not suitable for service under those conditions.
Api 941s nelson curves, based on industry experience provide guidance universally used for alloy selection. The csb has identified at least eight incidents of htha occurrence below the carbon nelson curve. Jun 04, 2017 many improvements have been made to the software, including the addition of the non post weld heat treat carbon steel nelson curve per the latest edition of api 9. Api rp 941, steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants, 7th edition, 2008, figure 1, shows the operating limits for steels in hydrogen service to avoid decarburization and fissuring from htha. Until recently, the use of api rp 941 nelson curves was thought to provide the margins and limits of process conditions relative to various alloy selections carbon steel and c0.
The hic resistance for crmo and carbon steel relates to pwht, sulfur content and inclusion shape. The most important aspect of the model is that it accounts for the timedependent nature of htha. Recent failures below the nelson curves and even below proposed revised curve offsets have raised questions as to the reliability of the nelson. The csb investigation of the tesoro accident identified that api rp 941 did not predict that htha could occur in the operating region of the vessel and the carbon steel nelson curve has repeatedly proven to be unreliable to predict htha. Steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants.
For example, the 2016 edition of api rp 941 reports new failures below the carbon steel nelson curve. The 2016 edition of api rp 941 reports new failures below the carbon steel nelson curve. Chemical safety board csb concluded that the carbon steel nelson curve methodology is inaccurate, cannot be depended on to prevent equipment failures due to htha, and cannot be reliably used to predict the occurrence of damage due to htha. In 2015, a significant change was made to the carbon steel curves in the 8 th edition of api rp 941. This recommended practice rp summarizes the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low alloy steels in hydrogen service at elevated temperatures and pressures. A catalog of api publications and materials is published. It occurs in carbon and lowalloyed steels exposed for an extended period to hydrogen under high pressure and at high temperature. At temperatures and hydrogen partial pressures below the curve, htha is not. Apr 05, 2018 one of the reason why hydrogeninduced cracking hic testing not required for austenitic stainless steel mentioned in nelson curve. Technical basis for api publication rp941 nelson curves. This rp is intended to help identify and manage equipment potentially subject to high temperature hydrogen. Ebsd showed that the lightetching phase adjacent to the fissures was cementite, as opposed to ferrite.
The focus of the paper is primarily on carbon steel and uns k12320 c0. Reducing the risk of high temperature hydrogen attack htha. In usa, api summarized these data as publication 941 steels for hydrogen service at elevated temperatures and. High temperature hydrogen attack htha degradation can build up to more extensive damage over time, risking costly shutdowns, slowing production, and creating a hazardous environment for plant personnel. Looking behind the curtain of api rp 941 high temperature. High temperature hydrogen attack htha cracking causes when steel is exposed to atomic hydrogen at high temperatures and pressures, hydrogen atoms may dissolve into methane, accumulating in bubbles that connect to create microfissures at steel grain boundaries. Included are comments highlighting some of the unknownmissing data incorporated into the nelson curves. A summary of inspection methods to evaluate equipment for the presence of htha is also included.
Life assessment methods for carbon steel and carbon. The api 9412016 nelson curve for carbon steel appears conservative, based on the results of this investigation. C0,5mo in high temperature h2 zapi 941 steels for hydrogen services at elevated temperatures and pressures in petroleum refineries and petrochemical plants 1st edition. The risk of related failures was assessed using api rp 941 nelson curves. A model calculation of the nelson curves for hydrogen attack. Api added a curve for nonpwht carbon steel, the 8th edition of api rp941. High temperature hydrogen attack htha at tuv rheinland. Api recommended practice 941 steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants. It was the first significant change to the curves in the last 25 years and is based primarily on damage of nonpost weld heat treated carbon steel. Free download api 941 latest edition programs viewbackup. Steels for hydrogen service at elevated temperatures and. Nelson curves was suggested in 1949 to define the operating limits for steels used in hydrogen service to avoid htha, a number of research and investigation activities on htha have been carried out around the world.
This is mainly determined by the temperature and partial hydrogen pressure, but other factors play a role as well, such as pwht. In 2015 a significant change was made to the carbon steel curves in the 8th edition of api 941. These nelson curves are detailed in a publication from the american petroleum institute api, recommended practice 941. Api rp 941 steels for hydrogen service at elevated temperatures.
Q, including code required thickness calculations, flange ratings, and direct access to the e 2. High temperature hydrogen attack htha programs mistras group. Nelson curve api 941 boiler and pressure vessel engineering. Many improvements have been made to the software, including the addition of the non post weld heat treat carbon steel nelson curve per the latest edition of api 9. High temperature hydrogen attack stress engineering.
June 1977 ylowered curve for c0,5mo failures in conditions around the curve 3rd edition. These curves are based on reported performance and have been subject to a number of significant changes as new experiences come to light. In addition, inspecting for htha is difficult because the. Api rp 941, steels for hydrogen service at elevated temperatures and. This curve was first published by api in 1970 and then underwent several changes after noticed cases of htha.
High temperature hydrogen attack, htha, nelson curve, carbon steel, c0. The nelson curves in api rp 941 do not account for this time dependence. Nelsons 3rd publication, thereby creating api rp 941 steels for. Pressure parts, fabricated from lowalloy steels, operating in hydrogen service at elevated temperatures the susceptibility of such components to hydrogen attack is currently determined by reference to the nelson curves api rp541 these materialspecific curves provide a go nogo criterion based on temperature and hydrogen partial pressure. High temperature hydrogen attack htha inspectioneering. A catalog of api publications and materials is published annually and updated quarterly by api, 1220 l street, n. Chemical safety board csb concluded that the carbon steel nelson curve. The appropriate alloy to select is shown as the curve immediately to the right or above the temperaturehydrogen partial pressure coordinates, which represent anticipated parameters of operation. The csb safety alert provides the following guidance to prevent htha equipment failure. Api rp 941, steels for hydrogen service at elevated temperatures and pressures in petroleum refineries and petrochemical plants, eighth edition, is a recommended practice developed and published by the american petroleum institute api. Bakerrisks examination room is suitable for housing interested parties of up to 20 individuals and their photographic equipment. What is high temperature hydrogen attack hthahot hydrogen. Mistras combines extensive engineering expertise with a wide variety of ndt to address high temperature hydrogen attack htha cracking in refining, manufacturing, and petrochemical plants. Failure of a hydrogenerator reactor inlet piping by high.
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