Swaged Tubes in Boilers
Swaged Tubes in Boilers
(OP)
We have an installation that has swaged tubes installed in the lower furnace. Recently we have experienced some failures of these tubes. The boiler rep is pointing towards possible water treatment issues, however, we have had no failures away from the swaged tubes. The tubes were not stress releaved after manufacturing. Any one have any input?
thread794-163695: Swaged Tubes in Boiler
thread794-163695: Swaged Tubes in Boiler





RE: Swaged Tubes in Boilers
Just to clarify, are you referring to actual swaged tubes where the OD changes in the tube circuit, and not rolled tubes in a lower drum? Also, what is the tube material?
RE: Swaged Tubes in Boilers
RE: Swaged Tubes in Boilers
I would strongly recommend you have a metallurgical failure analysis performed on one of the swaged tubes. There could be several reasons for the tube failure, so without a detailed failure analysis I could only guess at what the problem is.
RE: Swaged Tubes in Boilers
Does the boiler rep have anything to support his claim of boiler water chemistry?
RE: Swaged Tubes in Boilers
We are in the process of doing several metallurgical analysis both internally and a third party. I was hoping to find others that have experienced similar failures in case the metallurgical analysis proves inconclusive. It appears that high residual stresses play a key role. Initally we thought that the mechanism was SCC with the high residual stresses and free caustic building up from DNB in pitting on the hot side of the tube. Unfortunately we have now found cracking on the cold side where you would not expect to see caustic concentrating to levels where you would see SCC. Other areas of the boiler are well passivated and the water chemistry appeared to be well controlled.
RE: Swaged Tubes in Boilers
If so, in my opinion, swaged zone connections are failing due to differential thermal coephicient between carbon steel and stainless cladding and are also stressed by carbon steel membranes. How does look like your failures? Cracking on swage membranes?
Let us know of your findings.
Good luck
luismarques
RE: Swaged Tubes in Boilers
The boiler tubes should be cladded on the OD not the ID surface.
RE: Swaged Tubes in Boilers
RE: Swaged Tubes in Boilers
We're about to embark on cutting out a tube so we can apply metallurgical analysis. Cutting out a tube is a big deal because the package boiler is a positive pressure boiler where the wall tubes are membrane welded together to create a gas side pressure envelope.
The idea of DSAE and/or SCC of some kind has crossed our minds. We're also concerned that some combustion product is accreting on the mud drum surface and becoming acidic when offline and absorbing moisture... the pH of the combustion product we've scraped off is as low as 3. (The steam boiler only ever goes off line for inspections or ancillary equipment problems)
The other concern we have is that we have no idea how many swage/bends may be impacted... short of plugging a whole bunch of tubes to bring back reliability in the short term - some kind of major overhaul is looking likely.
Sooo, I'll be watching this thread closely and looking with interest at any shared experiences. I'll be sure to come back and share ours.
Thanks.
Rob