Selection of Boiler super heater tube temperature for creep. 4

[replica]

Hi all,

I am a little bit confused on the selection of super heater tube temperature to identify whether the tube is operating above or below the creep threshold temperature values. Should I use the steam temperature or flue gas temperature for the creep threshold value? If I take the replica at the outer diameter surface, should I use the flue gas temperature or inner steam temperature as the tube temperature? If we not have information on metal temperature of the tube because we did not have thermocouple attached to it which temperature can I used for the estimation of tube temperature?

Any comment is highly appreciated.

 

[metengr]

Based on my own experience and EPRI work, I would select the steam temperature and add 50 to 75 deg F on top of the steam temperature for the design metal temperature for gas touched tube surfaces.

 

[replica]

Thank you metengr, anyway is there any code or recommended practice (RP) some where that I can refer to?....

 

[Patafian]

Hi

You can refer to European Boiler standard EN 12952-3. You can find there so called temperature allowances. They set different temperature allowance for different boiler component and
heating mechanism. For example for tubes with superheated steam heated mainly by radiation temperature allowance is 50 deg C. This allowance shall be added to reference temperature which is basically steam temperature.

 

[davefitz]

Usually the boiler designer has a computer program that calculates the heat transfer along each section of each tube at steady max load . Once the heat transfer is determined, a margin is added to account for the "worst tube" vs average tube, and then the tube metal temperature is determined . Knowledge of the fluid temperature and internal heat transfer coefficient ,plus tube thickness and metal conductivity will allow determination of the "worst tube" metal temperature. One can then confirm if the selected tube material meets ASME allowable stress criteria or if a upgrade in material is required.

Transient events can cause a short term overheat much hotter than the full load case descried above. For example, a typical US coal fired boiler does not have a steam to reheater bypass system, so if the unit experiences a MFT at full load there is a 12 second period for which zero steam is flowing thru the reheater tubes yet they are exposed to full load fluegas temperatures. Overheat above the first critical temperature might occur, but severe ID oxidation does always occur with each MFT. Similar overheats occur to reheaters during "fast valving" of the IP intercept valves.

Additional overheat creep damage occurs if there is a significant accumulation of oxides or scale in the inside of the tubes. These accumulations insulate the tube metal from the cooling effect of the flowing steam, and the resulting tube metal temperature can be much higher than the original design's clean assumption.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[chicopee]

The National Board has a good article on creep and creep failure in their Bulletin archive at this site :

http://www.nationalboard.org