One of Inspectioneering's editorial themes this month is High Temperature Hydrogen Attack (HTHA). It is a highly contentious issue that the industry is still struggling to fully understand. If you have any questions or experiences with HTHA that you would like to share with the Inspectioneering community, please feel free to post a comment below. Here are three tips for addressing HTHA at your facility.
1. Know what you are dealing with.
High Temperature Hydrogen Attack (HTHA) is an insidious condition that can occur in process equipment that is 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. That atomic hydrogen then reacts with unstable carbides in steel to form methane gas, which then causes hydrogen gas pockets to form that leads to material degradation, cracking, and failure.
2. Do not underestimate its potential severity.
HTHA is a time-temperature-pressure function, which basically means the longer that a piece of equipment is exposed to temperatures above its resistance limit in a certain hydro-process environment, the more damage to the steel will accumulate; and the higher the temperature rises above the limit of the steel, the more rapidly the damage will occur. The damage that results from HTHA ranges all the way from superficial surface decarburization of the ID surface to severe material embrittlement, loss of mechanical properties, and cracking. This degradation has led to catastrophic equipment failures, some involving multiple fatalities, such as the tragic event in a Pacific Northwest refinery back in 2010, when a heat exchanger shell burst open during operation.
3. Know where to look for it.
HTHA affects carbon and low alloy steels, but is most commonly found in carbon steel and carbon-1/2 Mo steel that is operating above its corresponding Nelson Curve limits. However, there is now a building body of evidence that carbon steel, like carbon ½ Mo before it, may suffer HTHA below its current Nelson curve. Areas that are hotter, often near the outlet nozzle of catalytic equipment or the inlet nozzle of an exchanger that is cooling the process, are two specific areas of concern for HTHA. In addition, welds often suffer from HTHA degradation preferentially in the HAZ, especially if they are not postweld heat treated.
The information provided above has been derived from several past articles in Inspectioneering Journal, including those listed below. They make it clear that the potential for HTHA should be taken very seriously. We recommend reading additional material on the subject, as we have only begun to scratch the surface in this Blog. Here are links to a few articles published in Inspectioneering Journal that cover HTHA:
- Avoiding HTHA Failures: An Owner-User Perspective of the HTHA Risk Mitigation Process - Part 1 by Mike Urzendowski, Valero Energy
- Avoiding HTHA Failures in Existing Equipment by John Reynolds, Intertek
- 99 Diseases of Pressure Equipment: High Temperature Hydrogen Attack (HTHA) by John Reynolds, Intertek
Interested in reading more about high temperature hydrogen attack, corrosion and materials, and many other asset integrity management topics? Click the button below to download a free copy of John Reynold's updated and revised 101 Essential Elements in a Pressure Equipment Integrity Management Program guide.