At our refinery we recently observed damage on the top of the radiant roof tubes of our VDU fired heater (2 parallel cabin radiant sections and 1 common convection section), clearly caused by long term operation at too high tube wall temperature. This is different from the issues we had in the past, where hotspots were mainly located on the bottom of the radiant tubes.
During the run we can't monitor this location with IR camera, thus the high temperatures could not be detected.
After restarting the unit the newly installed skin temperatures confirmed this area of the furnace is running at ca. 100 degC above the rest. Possible contributing factors are the presence of roof tiles just above the interested 8" and 10" tubes and the fact that the furnace was revamped to low-NOx burners 2 TARs ago.
Do you have any experience of high temperatures on the top side of the radiant tubes and possible mitigation measures / modifications to be carried out?May-2022
Jake Gotham, InSite Technical Services, firstname.lastname@example.org
If the overheating is on the top of the roof tubes, it sounds like you have stratified flow, hence less heat-sink at the top of the tube than the bottom. This would be the result of lower velocity or lower vaporisation. Are you operating at lower throughput than before? Or is there less light material in the feed (e.g. improvements in stripping at the atmospheric tower)? If your furnace has velocity steam connections, this could be used to increase the velocity and move back into a dispersed flow regime but obviously this would add to the load in the tower and ejectors. Some units also have the ability to send the overflash back to the furnace inlet – this might also help.
If the top of the tubes has been running hot for some time, you may have formed some coke in this area, and should plan to decoke the furnace at the next shutdown.
Nagarathinam S Murthy, McKinsey & Company, Chennai, email@example.com
Two independent radiation sections with common convection section is indicated as per design. This implies that there is likelihood of flow imbalance between the two. Also, please look into the COT of both the radiant sections... any significant variations will lead to such from overfiring in one.
Michael Olesky, Consultant, firstname.lastname@example.org
Are you running with very low excess air? Is it possible that you are experiencing afterburning at the very top of the radiant section due to air leaking through penetrations in the roof? The afterburning could certainly cause hot spots on the tubes. My thoughts on this matter ...