Waste Heat Boiler - tube failures 3

[KDiz]

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!

 

[MisterDonut]

KEnglish,

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.

 

[chicopee]

Elucidate me on what you mean by "thick-lipped stress rupture". Must be some coinage not use in the USA.

 

[KDiz]

It's a typical failure - looks a bit like fish lips and is also called a fish-notch rupture.
In our case, creep was the main mechanism leading to failure.

 

[MJCronin]

KEnglish...

I believe that the more commonly used term is "fish mouth" rupture.

See Figure #5

http://www.swcc.gov.sa/files\assets...E FAILURE OF BOILER No 1 ECONOMIZER TUBES.pdf

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

 

[metengr]

The cause of SA 178 carbon steel tube failures can be from several reasons; poor circulation in the boiler circuit, partial flow obstruction from debris in the tube circuit or upstream tube leaks that can rob water/steam flow in the same tube circuit downstream of the leak.

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.

 

[chicopee]

by the way what was the state of the medium flowing in the failed tubes, liquid, saturated or superheated vapor?

 

[rmw]

If it is in a roof panel, could there be circulation issues in that zone?

rmw

 

[ashaari]

KEnglish,

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.

 

[chicopee]

Interesting report from MJ Cronin. Economizer is notorious with this type of boiler design. I have seen a similar failure in MA back in the '80's because of the sulfur attack in an area of the boiler that is relatively cold.

 

[chief]

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