Date Announced: Tue, 06 June 2006
Date Completed: Thu, 18 June 2009
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 above 400 °F 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