metal degradation in components of a refinery

This event regards the degradation of steels used in the heat-exchangers of a reformer of a refinery.
An inspection by non-destructive ultrasonic techniques revealed material degradation of the steel components. The opening of the equipment confirmed the analysis: blisters were visible on metal sheets and cracks were present on welded parts. The material was an 0.5% Molybdenum steel.

The INITIATING CAUSE was degradation of gas pipes and other components due to high-temperature hydrogen attack

This is a near miss. If the degradation of the steel components would have not been detected on time, the hydrogen attack would have proceeded until a loss of confinement, with release of hydrogen-reach process gas I the heat exchangers. Similar incidents have brought to extensive fire and damage to plants.

The ROOT CAUSE would be an inadequate design, tens of years before, due to lack of scientific knowledge on the sensitivity of the specific steel used to hydrogen attack.

metal sheet, steel components.

The non-destructive controls performed were:
1) AUBT =Advance Ultrasonic Backscattering Technique for metal sheet,
2) TOFD =Time Of Flight Diffraction for welded parts
3) magnetoscopy for steel parts containing 0,5% Mo, to detect high temperature hydrogen embrittlement.

This is a near miss

An emergency shutdown was executed and relevant equipment were purged with nitrogen.

This is a near miss: if not timely detected, the metal degradation would have brought to a loss of confinement with hydrogen (and possibly other process gases).
At the time when the plant was built, back in the 1970, there was less technical knowledge than today on the sensibility of specific steels to hydrogen attach and how to choose the optimal steel composition. The design of steel components was based on the ‘Nelson curve’ method, a guide helping to determine the suitability of steels for high-temperature hydrogen service. In 2005, when this event occurred, there was a more advanced knowledge on the effect of individual stell additive to their hydrogen resistance.

Specifically on Mo in steel, used in this event, ARIA wrote a lesson learnt related to the need to carefully monitor old components using materials operating at high temperature and in presence of hydrogen.
"The resistance of 0.5% Mo steels to hot hydrogen attack has been gradually called into question since the 1970s. It is currently considered comparable to that of unalloyed carbon steels. On older units, these materials were chosen in accordance with the Nelson curves in use at the time of construction, but they are potentially sensitive to hot hydrogen attack. 0.5% Mo equipment working under hydrogen must therefore be identified and classified according to its criticality, inspected by AUBT (Advanced Ultrasonic Backscatter Technique) and weld control (TOFD, Magnetic Particle Testing) to look for hydrogen degradation, and systematically replaced for the most critical ones if degradation appears."

The operator replaced urgently the corroded equipment with 1.25 Cr 4 steel shells.