Luis Thank you for giving information on boiler tube burst . It looks like a peculiar problem in a boiler tube in radiant horizontal straight leg portions. Your analyses and observations from micro-tests , RT confirming possibility of turbulent flow of steam water droplets eroding a portion of the straight leg at a particular position in the 0 degree position is an interesting information. Forum partcipating members - i am sure - will appreciate your sharing of this interesting failure and your analysis on possible failure mode and likely extension to other tubes in your boiler. Also a good info for other users to look from a different angle of boiler tube failures. You would appreciate that in a discussion forum, however knowledgeable and experience persons may have, you may not get the near or exact correlation or typical sharing of problem solving for a similar failure situation in some other parts of the world. It is quite possible that turbulent flow of steam droplets can cause inpingement and vortex effect at localized areas and tube erosion at a particular position (due to vortex effect) in the straight leg of radiant tube. Impingement erosion-corrosion may not be restricted to only a narrow band of 0 degree (12' O clock position). It can happen usually not at specific 0 degree position (12'O clock) but usually will be spread around 330-30 degree (11' O Clock to 1' O clock positions). For specific effect of only 12' O clock position to get affected in your tubes - i guess - there could be possible areas of transition bend angle is too either too sharp or the angle of positioning the tube straightness has changed due to tube sag or tube holding cleat welds had given way etc. This is my only my guess which could be totaly wrong. Other possibility is stoppage of boiler and re-start during recent years would have cumulatively affected vortex effect on particular tubes where tube bends and orientation effect had changed over years. "Fish mouth" rupture could be the result of sudden failure at the maximum thinned portion. This portion might have given way under sudden start up stress and impingement conditions (due to impingement effect at localized areas and mechanical hoop stress on the tubes). Still there could be some effect of plastic deformation on the tube. This could be posssibly quantified or detected if you can do further analysis by X-ray stress analysis or atomic absorption spectra analysis at failred area of the tube samples. Finally, thank you for sharing to the forum your insight into the boiler tube failure. This - i believe - is a good way of sharing information on failures to avoid similar failures in some other plants of a possible different mode of impingement erosion-corrosion. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India July 27, 2006 E-mail: nishi@vsnl.com > > >Some days ago we have had a water boiler tube burst. Burst >location was in a radiant rear water tube at the top >convection zone on an almost horizontal part of the tube, >about 1,5 m from the top water tubes header. First visual >inspection of damaged tube was a typical failure of a >"thin-lipped" burst of the boiler tube. This type of failure >results in a ductile rupture of the tube metal and is >normally characterized by the classic %E2%80%9Cfish mouth%E2%80%9D >opening in the tube where the fracture surface is a thin >edge. > After a closer microscopic microanalysis of the thin edge >and good tube zones we didn%E2%80™t found any differences in the >microstructures between thin edge and adjacent good tube >zones. No evident distortion of ferrite and perlite was >seen. > For that reason we think the failure has occurred well >below the lower critical transformation temperature with no >evident distortion of ferrite and perlite. > >Adjacent tubes don%E2%80™t reveal any plastic distortion or >diameter tube growth. After digital Rx examinations we found >that adjacent tubes of boiler tube burst showed great >thinning on top of part of the tubes at 12 o%E2%80™clock >location, the thinning was about 250mm long. Nominal tubes >thickness is 5.6mm and thinner point in 250mm length was >1.5mm.Thickness of bottom part of the tubes were ok. > >Due to the facts my conclusion is: tube failure cannot be >explained by a short term overheating. > >My question is can we explain the thinning on top of the >tubes by cyclic turbulent erosion of wet drops of steam? > >Tube goes 15m in a vertical run and then it turns almost >horizontal within 2m after connecting with the upper water >boiler header. On the vertical part of the tubes there is a >equilibrium between water and diffused steam bubbles, but on >the horizontal part of the tubes about 1m of the turn bend, >by differences of densities the water runs on the lower part >of the pipe and the drops of wet steam run turbulently above >the water inducing high erosion on that 12 o%E2%80™clock >location of the tubes. > >My intention is to warning other operators for this kind of >failures and to have comments from the forum on my >conclusions. > >Tanks > >Luis Marques