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High Temperature Hydrogen Attack (HTHA)

High Temperature Hydrogen Attack (HTHA)
Differing levels of HTHA damage in a specimen generated in TWI laboratory

Risks and Mitigation in light of the U.S. Chemical Safety Board (CSB) investigation into the Tesoro Refinery industrial accident


  • The Tesoro Refinery industrial accident in 2010 resulted from damage due to high temperature hydrogen attack (HTHA).
  • The risk of related failures was assessed using API RP 941 Nelson curves.
  • The U.S. 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 .
  • The CSB has identified at least eight incidents of HTHA occurrence below the carbon Nelson curve; i.e. which did not indicate high risk of HTHA. The 2016 edition of API RP 941 reports 13 new failures below  the carbon steel Nelson curve.

    CSB recommendation: For existing equipment operating at risk, all carbon steel equipment in hydrogen service should be identified. Those operating at elevated temperatures and greater than 50 psia should be replaced with inherently safer materials (see above).
  • The most effective safeguard is better design against HTHA; including the application of high chromium steels with greater resistance to HTHA.
  • An alternative approach to replacing all equipment with higher performance materials would be to improve detectability of defects at an early stage and focus replacement where the risk is greatest.
  • The effectiveness of Risk Based Inspection (RBI) is limited by current technology and the effectiveness of operator skill.
  • Advances in inspection system capability combined with effective training of operators coupled with effective RBI could mitigate the risk for existing carbon-steel plant.

Early Friday morning, April 2, 2010, an explosion and fire in the Naphtha Hydro-treater Unit at the Tesoro Refinery in Anacortes, led to the death of seven refinery workers in the immediate vicinity of the heat exchangers being returned to operation following maintenance. The subsequent investigation of the incident by the U.S. Chemical Safety Board found that the root technical cause  “was the result of the carbon steel heat exchanger being severely weakened by a damage mechanism known as HTHA” (Ref “CSB report 2010-08-I-WA”). High temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem which affects steels operating at elevated temperatures in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers (TWI FAQ).

Whilst the cause of the failure was linked to deficiencies in safeguards and hazard identification/control adopted by the operator, the CSB Report also called into question the efficacy of the use of Nelson curves for evaluation of design operating conditions. API RP 941 provides industry guidance to predict the occurrence of HTHA in various materials of construction by using the Nelson curves; which delineate the region of safe use for carbon steels, 1.25Cr-0.5Mo steels etc. within high temperature ranges. The report stated:

“The Nelson curves are predicated on past equipment failure incidents and are plotted based on self-reported process conditions that are ill-defined and lack consistency”

“The CSB has learned of at least eight recent refinery incidents where HTHA reportedly occurred below the carbon steel Nelson curve”.

“The CSB found that the carbon steel Nelson curve is inaccurate and cannot be relied on to prevent HTHA equipment failures or accurately predict HTHA equipment damage” (Ref: CSB report 2010-08-I-WA Section 1.2.3).

Corresponding recommendations by the CSB (Ref: CSB report 2010-08-I-WA Section 8.4) included the revision of API RP 941 to prohibit the use of carbon steel equipment in HTHA-susceptible service i.e. those operating above elevated temperatures and greater than 50 psia. API revised RP 941 and published the 8th edition of the RP in February 2016. The original recommendations together with API’s actions for each subpart are detailed in the “ CSB Status Change Summary 2010-8-I-WA-R10”, issued on 13 July 2016. The report noted that new API “design margins” are for the design of new equipment. Existing equipment that operates near the Nelson curves do not require an operating margin safety factor. This is a major ageing equipment issue, as older equipment is much more susceptible to catastrophic failure from HTHA than the new equipment to which this design margin would apply. All existing equipment is not required to incorporate a 50 °F safety margin. “CSB concluded that API took insufficient action on all parts of the recommendation”. In light of this, in August 2016 the CSB issued a “Safety Alert: Preventing High Temperature Hydrogen Attack (HTHA)”, which gives the following guidance to prevent HTHA Equipment Failure:

  1. Identify all carbon steel equipment in hydrogen service that has the potential to harm workers or communities due to catastrophic failure;
  2. Verify actual operating conditions (hydrogen partial pressure and temperature) for the identified carbon steel equipment;
  3. Replace carbon steel process equipment that operates at elevated temperatures and greater than 50 psia hydrogen partial pressure; and
  4. Use inherently safer materials, such as steels with higher chromium and molybdenum content

The CSB also pointed out that HTHA is a damage mechanism that takes years to form cracks that severely weaken vessels.

These recommendations to replace rather than monitor structures are driven by difficulties associated with inspection for microscopic cracks resulting from HTHA, also highlighted in the CSB report. This inspection is highly dependent on the specific techniques employed and the skill of the inspector.  TWI is looking to develop an alternative approach to reduce the risk associated with this failure mechanism by developing improved inspection techniques.

For more information, please contact us.

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