I am having metal dusting issues in my 5 chrome crude vacuum furnace roof tubes and I was curious if any others have experienced this in their plants in crude service. From reading the MTI Awareness Bulletin 33 and several papers from RT Jones's work I have a good enough understanding of the mechanism and history in other services, but I have not seen much discussion about it happening in crude vacuum service.
We have verified the mechanism through a failure analysis and UT points showing short term corrosion rates up to 888mpy. The furnace is approximately 50 years old and has not shown any significant wall loss from carburization until 2018. No pitting found, more of a general wastage. The sulfur is currently low less at less than 50 ppm.
I appreciate any thoughts on trying to prevent the metal dusting. We replaced the tubes with 9 Chrome, but I was curious if anyone had any advice on preventing the 9 chrome tubes from metal dusting as well or if there are any other known cases in industry.
Was there a change in the sulfur content of the gas in 2018, or shortly before? Sulfur can protect against metal dusting at high enough levels, and maybe a change in the presence of sulfur contributed to your sudden experience of carburization.
Lead Materials Engineer
7201 Hamilton Blvd
Allentown, PA 18195
I am not aware of metal dusting case in crude and vacuum heaters even the same is not listed as damage mechanism in API 573 of CDU heaters. However, Carburization may occur in crude and vacuum heaters which is a known step that precede most of reported metal dusting failure cases.
In CCR units, DMDS are injected to minimize metal dusting since adsorption of sulfur, obtained from the decomposition of DMDS on metal, allow to minimize metal dusting (0.3 ppm found by experience necessary to prevent metal dusting in CCR). In contrast this option is not viable in many
processes due to the increased use of catalysts which are sensitive to sulfur content. (EX. Syngas Industry)
The same context of sulfur role, the influence of H2S on metal dusting of pure iron has been investigated at T 500 ̊C. The presence of sulfur retards the start of metal dusting. With increasing H2S content the onset of metal dusting can be retarded for longer times.
Hence, I believe that you need to review the change in process conditions and temperature and define a limit of sulfur content (i.e. IOW) necessary to protect your heater tube.
Hope this can help
Attached are some interesting papers for your kind info.
Senior Materials and Corrosion Engineer