Waste Heat Boiler - tube failures
Waste Heat Boiler - tube failures
(OP)
Currently have SA-178 Gr. A tubes installed in a waste heat boiler quench zone roof panel.
We've had 2 thick-lipped stress ruptures in the last few months and the panel is being replaced this spring. The question is: do we stay with SA-178 (a common boiler tube material) or go with SA-213 T11?
The gas can reach a temperature up to 1800F.
Other suggestions on how to prevent tube failure are also appreciated!
We've had 2 thick-lipped stress ruptures in the last few months and the panel is being replaced this spring. The question is: do we stay with SA-178 (a common boiler tube material) or go with SA-213 T11?
The gas can reach a temperature up to 1800F.
Other suggestions on how to prevent tube failure are also appreciated!





RE: Waste Heat Boiler - tube failures
You could have a circulation problem. 1800F is not unusually hot for the gas side of SA-178 tubes. If there is no water on the indside, however, the tube metal temperature will be much higher than the designer intended.
T11 tubes might withstand this condition, or they might not.
Also consider stresses induced by differential expansion, particularly if the unit cycles frequently.
RE: Waste Heat Boiler - tube failures
RE: Waste Heat Boiler - tube failures
In our case, creep was the main mechanism leading to failure.
RE: Waste Heat Boiler - tube failures
I believe that the more commonly used term is "fish mouth" rupture.
See Figure #5
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The only "thick lipped" rupture that I am aware of occured on Mick Jagger (Rolling Stones) while hitting a high note during a rock concert... : )
-MJC
RE: Waste Heat Boiler - tube failures
Thick-lipped tube failures can be caused by long term creep rupture or from graphitization in carbon steel boiler tubes. Both of these damage mechanisms are caused by exposure of the boiler tube to metal temperatures in excess of 800 deg F.
You really need to conduct a proper failure analysis of the tubes to evaluate the general metallurgical condition in addition to determining the cause of the thick-lipped tube rupture. Once you have this information, you may find that the cause of long term overheating could be corrected without upgrading tube material. If the tubing is exposed to long term overheat conditions because of poor selection by the original designer, upgrading is possible and should be optimized based on an approximation of the mean wall tube metal temperature. Grades T11 or T22 are possible.
RE: Waste Heat Boiler - tube failures
RE: Waste Heat Boiler - tube failures
rmw
RE: Waste Heat Boiler - tube failures
I agree with several posts above that thick-lipped fish mouth failure is normally due to long-term overheating.
The normal cause of the problem is due to circulation, as also mentioned above. The fluid inside the tube is actually the cooling agent. If circulation is lower than the design range, then the tube won't have sufficient cooling thus overheating occurs. If i'm not mistaken this phenomena called departure from nucleate boiling (DNB) or critical heat flux (CHF).
I'd like to suggest that apart from checking the cooling of the tubes, i.e the fluid circulation, please also check abnormalities from your heat source.
If your waste heat boiler has a supplementary burner firing system, you can check the complete firing system. I've come across a tube overheating failure where the main reason was due to off-spec fuel gas composition, which subsequently made the flame length longer than its design and caused overheating to the tubes.
Also you can check the temperature and pressure readings of your exhaust gas for abnormalities. If you can check the exhaust gas flow rate even better, so that you can compare with the flow rate design.
RE: Waste Heat Boiler - tube failures
RE: Waste Heat Boiler - tube failures
I presume your boiler water treatment and testing programme is up to modern standards, and there is not a build up of scale on the inside of your tubes.
The other cause of a boiler tube failure is of course poor circulation. Check your circulation pump pressure and flow rate.
Flame impingement is another possibility, if your burners are not directed properly.
Of course you are sootblowing regularly and preventing build up of soot on tubes which can cause localised overheating.
Offshore Engineering&Design