Inspectioneering Journal

Avoiding HTHA Failures: An Owner-User Perspective of the HTHA Risk Mitigation Process

Part 2

By Mike Urzendowski, Technical Advisor at Valero Energy. This article appears in the January/February 2014 issue of Inspectioneering Journal.
This article is part 2 of a 2-part series.
Part 1Part 2


The views expressed in this article are those of the author and do not necessarily reflect the views of Valero Energy Corporation.


In my article published in the last issue of Inspectioneering Journal (Nov/Dec 2013) I began the discussion on what it takes to establish an effective High Temperature Hydrogen Attack (HTHA) risk mitigation program. I not only provided a brief introduction and historical summary of HTHA, but I also discussed how to properly define the scope of work, collect critical data, determine hydrogen partial pressure and process temperatures, and use this data to compliment your site’s Positive Material Identification (PMI) program. In Part II, I will be highlighting the importance of several other recommended aspects of an effective HTHA risk mitigation strategy. The intent of these two articles is to share lessons learned from recent experiences setting up a comprehensive HTHA review process across multiple refineries, and to help other operators define and mitigate the HTHA risk to an acceptable level.

Figure 1. Failed NHT feed / effluent exchanger shell from the Tesoro event. Note the wide-open rupture at the right end of the shell.
Figure 1. Failed NHT feed / effluent exchanger shell from the Tesoro event. Note the wide-open rupture at the right end of the shell.

Quantifying Risk

Now that scope is defined and data is collected and analyzed, what next? Not all the items in the original scope may be susceptible to HTHA and for those that are, there are varying degrees of HTHA likelihood, depending on how severe the service. API has developed methodologies for determining likelihood and priority using many of the items listed in the data section of this article. These methods are highlighted in both API RP 941 and API Technical Report 941. Perhaps the simplest, and one that is used by many owner-users, is to determine HTHA likelihood according to how close the operating conditions are to the Nelson Curve for a given material. Although the Nelson Curves have been developed based on real data accumulated over time and the confirmed presence of HTHA, a few failures have been documented for materials below their respective curves. Some of this may be due to incorrect assumptions of operational data and some may be due to the curve itself. Because of this, operating carbon steel just below the carbon steel Nelson Curve is not a guarantee that HTHA will not occur. One needs to assign some “cushion” below the curve for temperature, and to the left of the curve for hydrogen partial pressure to be not just conservative, but also realistic. Some owner-users have used a starting point of 25°F below and 25 psia to the left as the first likelihood hurdle, while others have used 50°F and 50 psia. Some apply the former to heat treated carbon steels and the latter to non-heat treated carbon steels. Each company developing their program will have to decide what works best for them, depending on the certainty of data and their risk tolerance.

Carbon - 1⁄2 Mo steels and Mn- 1⁄2 Mo steels are normally treated a little differently. Since 1970, several (27) accounts of HTHA have been reported on C - 1⁄2 Mo steels occurring below the 1977 published 1⁄2 Mo curve, which already had been lowered from the 1970 version. This resulted in the removal of the 1⁄2 Mo Nelson Curve from the other curves in 1990. It can still be located in Annex A of API RP 941, but the uncertainty of the HTHA resistance of C - 1⁄2 Mo steels makes determining the likelihood less clear. Many owner-users have referenced the position of the C – 1⁄2 Mo operating parameters above the carbon steel curve. The likelihood of HTHA increases as conditions plot further above and to the right of the curve. As with carbon steel, where the risk lines are drawn depend on the specific company’s protocol and risk tolerance. The most conservative approach is to identify any C - 1⁄2 Mo component operating above the carbon steel curve as having HTHA likelihood, with that likelihood increasing as you move up and right on the curve.

Note that per the latest edition of API RP 941, no reports of Mn – 1⁄2 Mo steel experiencing HTHA below the 1⁄2 Mo Nelson Curve have occurred, therefore determining the likelihood of HTHA for this material should take that fact into account.

This content is available to registered users and subscribers

Register today to unlock this article for free.

Create your free account and get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.

Comments and Discussion

There are no comments yet.

Add a Comment

Please log in or register to participate in comments and discussions.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Training Solutions

Improve your skills in key mechanical integrity subjects.

Case Studies

Learn from the experience of others in the industry.


Inspectioneering's index of mechanical integrity topics – built by you.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.


Read short articles and insights authored by industry experts.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.


Downloadable eBooks, Asset Intelligence Reports, checklists, white papers, and more.

Videos & Webinars

Watch educational and informative videos directly related to your profession.


Commonly used asset integrity management and inspection acronyms.