Process gases leak in a reformer tube

This event affected a reformer furnace part of the hydrogen production unit of a refinery.
During steady state operation, a leak of process gas occurred near the outlet of one reaction tube. After inspection, three additional reaction tubes showed damage, although they had not ruptured.

Previous inspections of the reaction tube had not detected any increasing degradation or damage. A post-incident microstructural investigation revealed penetrating cracks and bulging observed on the furnace floor portion of the tube (40–220 mm from the bottom).
The wall thickness had not thinned in the area around the leak area or on other tubes, but there were multiple line-type defects and cracks (not oxidised) on the inner surface near the leak area.
All through-cracks, non-through-cracks, and the internal cracks were intergranular.
The microstructure corresponded to an aged steel material with second phase carbides precipitated.

The INITIATING CAUSE was a release of process gases from a longitudinal crack developed on a reformer tube.

Since no corrosion or wear was observed, the damage was not due to corrosion induced thinning, wear, or material embrittlement. The combination of plastic bulging, cracks originating from the inner surface, and intergranular cracking indicated high temperature material creep as failure mode.
The potential degradation by creep was known and had been partially considered by design and by regular inspection. However, the design was not based on msot advanced methods and the inspection tools (visual, dimensional, wall thickness, destructive tests) used to assess the life-time of the components could not detect the type of degradation produced by creep.

The unit affected was the onsite hydrogen production unit of a refinery.
Operating pressure: 2.31 MPa
Operating temperature: 904 °C
Gas composition: hydrogen 45 %, methane 3 %, CO 10 %: CO₂ 7 %
High pressure gas production capacity of the unit: 3,249,200 m³/day

The reformer tubes were 13 m long and made of KHR35CT steel: a micro-alloyed, centrifugally cast, heat-resistant cast austenitic stainless steel of approximated composition: 24–27% Cr, 33–37% Ni, Nb + Ti.
It is used for high-strength, long-term service up to almost 1100 C. Thanks to its high creep-rupture strength, it is often used in ammonia, methanol, and hydrogen reformer furnace tubes.
[https://dl.asminternational.org/alloy-digest/article-abstract/48/10/SS-766/6257/KUBOTA-ALLOY-KHR35CTHeat-Resistant-Alloy?redirectedFrom=PDF https://dl.asminternational.org/alloy-digest/article-abstract/48/10/SS-766/6257/KUBOTA-ALLOY-KHR35CTHeat-Resistant-Alloy?redirectedFrom=PDF]

The damage to 4 tubes amounted to approximately ¥9.3 million (the costs of the replacements of all tubes is not provided)

Cronology of the event:
01:40 In the instrument room, a faulty temperature indication on the furnace’s temperature gauge was discovered. An employee inspected the site and visually confirmed gas leakage from near the outlet of a reaction tube (located on the 2nd floor furnace, near the 1st floor upper section).
01:50 The on duty staff was notified via internal telephone.
01:54 Public emergency number called.
01:55 The hydrogen production unit shut down.
03:45 Public safety number (Coast Guard Headquarters) was called.
8 to 20 August: cooling work, removal of pipe insulation, and leak testing were performed.
21 August: a vertical crack of about 20 cm observed at the lower part of the reaction tube.

The KHK report lists the following learnt lessons:
(1) Thermal stress driven creep was the dominant failure mode for these reformer furnace reaction tubes. The tubes were exposed to a temperature gradient between the inner and outer surfaces, which generated large thermal stresses. This is the reason why the tubes failed by thermal stress induced creep rather than by internal pressure creep.
(2) Consequently, design cannot rely on simple analytical formulas; finite element type analyses are required. Repeated heating cooling cycles (start up/shut down) cause cumulative creep damage. A reliable method for predicting thermal stress creep life is required.
(3) The creep data of the steel used in the tubes was owned by the manufacturer and were not publicly disclosed. When performing creep analyses, users must ensure they have up to date, trustworthy creep data rather than relying solely on data supplied at the time of manufacture.
(4) Surface conditions on the inner side of the tube (e.g., coke deposition, oxidation, carburization) can affect both thermal stress and creep damage. These surface phenomena should be incorporated into remaining life predictions.
(5) In addition to conventional sampling inspections, full length, full coverage non destructive examinations can significantly improve the accuracy of remaining life assessments and are recommended for critical components.

An advanced inspection technique was applied to four tubes found affected by creep damage. This technique is based on full component electromagnetic eddy current measurements combined with deformation measurements. LEO SCAN (LASER EDDY CURRENT OUTSIDE SCAN). This non-destructive technique proved able to provide creep life assessments equivalent to those obtained by destructive examinations.

Therefore, the company adopted the full length creep life evaluation for inspections aiming at assess the remaining service life of the reformer tunes.
The plant replaced all reaction tubes one year later and scheduled the first post replacement inspection in 5 years.
Other similar plants planned to inspect their tubes during the next scheduled overhaul.