High Temperature Hydrogen Attack (HTHA) - CSB Recommendations

In 2010, the Tesoro refinery in Anacortes, Washington experienced a catastrophic rupture of a heat exchanger in the Catalytic reformer/Naphtha treater unit releasing highly flammable hydrogen and naphtha at more than 500 °F which ignited and caused an explosion that lasted for more than three hours and fatally injured seven employees working in the vicinity.

The US Chemical Safety and Hazard Investigation Board CSB has identified the causes of the accidents related to integrity programs, operating procedures, monitoring parameters, safety protocols, inspection and regulatory deficiencies, but what concerns us here today is their technical report about material selection and damage mechanism involved in this disaster.

For more information about the accident, refer to the full report on CSB website in the references below.

High Temperature Hydrogen Attack is a damage mechanism that doesn't result from corrosion reactions, however it takes place in temperature above 260 °C (500 °F) and with hydrogen partial pressure above 689 KPa (100 psia), and it's especially a concern in hydrotreating and hydrocracking units.

Hydrogen attack generally starts by depleting carbon in pearlite leading to decarburization, which if extensive will reduce material strength.

Under these conditions, hydrogen diffuses into steel and reacts with carbides to form methane with large molecule size that prevents diffusion to the surface then causing blisters, fissures and cracking.

Susceptible materials in increasing resistance order (with Cr-Mo content) are: non-PWHT carbon steel, non-welded and PWHT carbon steel, C-0.5Mo, Mn-0.5Mo, 1Cr-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo, 2.25Cr-1Mo-V, 3Cr-1Mo, 5Cr-0.5Mo. 300 SS, 5Cr, 9Cr, and 12Cr are not susceptible.

Nelson Curves from API RP 941- are a set of curves showing temperature and partial hydrogen pressure safe operating conditions for carbon steel and low-alloy steels, where the conditions under the curve are considered safe.

CSB recommendations after the accident found that the heat exchanger B and E were constructed using carbon steel with the hottest areas SS cladded. According to API RP 941, carbon steel with in the safe operating conditions under the Nelson curve. CSB suggested that these curves were "plotted based on self-reported operating conditions that are ill-defined and lack consistency". It states that 2016 edition of API RP 941 reported 13 new failures below the carbon steel Nelson curve.

In its report, CSB conclusion was that Nelson curves for non-PWHT carbon steel in the 2016 API RP 941 - in spite of the curve improvements - was not reliable as it didn't take into consideration the operating conditions that contributed to this catastrophe. Furthermore it recommends prohibition of carbon steel use above 400 °F and 50 psia hydrogen partial pressure, and revising API RP 941 to require the use of inherently safer materials.

Inspection of affected equipment is not effective in preventing these failures due to its intricate nature, and it requires special expertise to detect it. The most effective method of preventing this damage is material selection and monitoring and control of operating conditions under the safe zone.

References

ANSI/API RP 571 - Third Ed. 2020, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry

ASM International, 1992, ASM Handbook - Volume 13 - Corrosion , 07-Corrosion in Petroleum Refining and Petrochemical Operation

https://www.csb.gov/tesoro-refinery-fatal-explosion-and-fire/