High temperature hydrogen attack (HTHA) is a problem which concerns steels operating at elevated temperatures in hydrogen environments. It is not to be confused with hydrogen embrittlement or other forms of low temperature hydrogen damage.
HTHA is a time-temperature-pressure function. These, basically means the longer that a piece of equipment is exposed to temperatures above its resistance limit in certain hydro-process environment (see API941-NELSON Curve), 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.
Monitoring the integrity of equipment in HTHA-Service is necessary to avoid accidents, especially if one of the following reasons are true:
- HTHA in many old plants -is true for many ammonia plants- can occur especially after a long onstream-time, if the incubation time ended.
- However, there have been several cases where HTHA was found, even though operating conditions were below the Nelson Curve.
- There are new findings since 2016: After explosion in the TESORO refinery with 7 dead, it must be assumed that HTHA in C steel can already be expected at > 204 °C and a hydrogen partial pressure of pH2 > 3.45 bar.
It is strongly recommended to monitor the integrity of HTHA vulnerable equipment!
Due to the age of many ammonia plants, a lot of equipment may need to be replaced.
For avoiding HTHA of new equipment, use the recommendations for fabrication of new equipment in the hydrogen service according API 941 and API 934A!