We recently repaired a waterwall tube on a superheater boiler. The repair was approx. 45' from bottom of slag
tank, just above the slope in a transition corner ( the corner is not a 90 degree, it is about 45 degree for manway access. starting vertically up the wall. The rupture and the location is the problem.....the rupture was approx. 6 1/2" high, with 3/8" rip going vertically up the tube. BUT, the whole face of the tube, membrane to membrane, was gone. SA-210 grade steel. No preexisting padweld was seen on the tube, but with 6 1/2" of tube dissapeared, we cannot be sure..The wall of the dutchman we cut out to repair, was still the wall thickness the prints showed. I suspect hydrogen embrittlement in the ID of the tube from lack of purging before and after boiler shutdown.
I could use other opinions on this matter because I'm sure it won't be the last time I'll see this.
Regards
By -
Date 11-28-2001 21:08
I think the real answer to this question is that a metallurgist should perform a failure investigation to come to an informed conclusion. There are so many issues relating to boiler tubes, that it is not reasonable to give a conclusive answer to your question.
Having said this, we can obviously speculate, as long as you understand that it is speculative.
The fact that the wall of the tube was still the same thickness in the area, would suggest that there was not a corrosion mechanism operating.
The fact that it did not fail with a "fish-mouth" fracture with a lot od wall thinning would suggest that it was not a short term localized overheating of the tube.
The other major failure mechanism that thus remains is creep. As creep generally occurres over relatively wide areas and generally does not show up as a great wall thinning, it is possible for a "window" to shoot out.
My best guess would thus be that it was a creep failure. If this is so, then you are still sitting with a problem with the remaining tube and probably other tubes seeing the same service. I would suggest that a metallurgist examines the tubes to establish if they are suffering from creep.
Let us know if you find out anything more.
Regards
Niekie Jooste
Boilermaker, have seen similar failures in one of our units. From your description I think you have a corrosion fatigue failure caused by breakdown of magnetite layer on tube ID. Failures occur in water touched tubes, are longitudinal, and ID initiated. Were you able to recover the missing piece of tube or was it still attached like a "hinge" to one side of the tube? Did the failed piece have longitudinal cracks on the ID of the tube? Best to take Neike's excellent advise and send failure to metallugist for analysis.
