Boiler Tube / Tube Plate Weld Cracks 2

[smegfab32]

I have radial cracks that presumably are thermally induced in my aged boiler tube to tube plate welds. On examination of the bolier tube plate tube ends there was white deposits on 12 tubes end welds. Suspecting problems MPI inspection has been carried out the to find the minor cracks in 25 tube to tube plate welds in total. I would be interested on the best practise from now. Is it best to grind out the welds. MPI to check the cracks have gone and then reweld followed by further MPI on the finished weld? That is the way I expect to proceed. And does anyone have any thoughts as to the need to hydraulic test the boiler shell side as a final inspection to check we have caught every defect and got the welding right? The high operating temperature/pressure is a problem though.

 

[unclesyd]

I strongly recommend that you get a boiler inspection company involved as the presence of the "white deposits" could mean major trouble in you boiler.
Not wanting to sound too pessimistic, white deposits in an area of cracking in a boiler or steam system usually indicate the presence of caustic embrittlement. If you have such a case it will take some expertise to determine even if repair is possible.

Can you comeback with your boiler operating conditions and water treatment?

 

[metengr]

Based on the information in your post, your repair approach sounds correct.

One question, are the boiler tubes rolled and seal welded or are the boiler tube welds considered strength welds (partial penetration) in the tubesheet? If the tubes are rolled and seal welded it is best to remove the entire defective seal weld and re-weld, do not perform local grinding and leave the remaining seal weld intact.

You may need to actually apply a preheat to the tubesheet to drive out residual moisture in the tube to tubesheet joint. This is not going to be an easy weld to make because the tube to tubesheet joint will have crud that could interfere with welding. You will need to work around this problem.

After all of the defective tube welds are repaired, I would only MPI. There is no need to perform a hydrostatic test. Check to make sure that the repairs are performed by an organization that has a valid Certificate of Authorization to Repair Boilers/Pressure vessels issued by the National Board of Boiler and Pressure Vessel Inspectors.

 

[deanc]

This sounds like a firetube boiler. If so you probably have a build up on the waterside that allowed the tubesheet/plate to be become overheated and crack. We used to call them "firechecks",tough to weld repair as it is full of residue and the carbon may be cooked out.

Suggest you tear all the tubes and check the sheets after a good cleaning. May be time to replace,esp. the rear one. You can try to weld repair and/or patch but just does not work that well. If you got to have it back on line fast,remove the leaky tubes,weld repair,use copper ferrules in the tubeholes,and retube.

Check your blowdown procedure and the location of the blowdown fitting. Your water chemistry may need a look as well.

A good boiler repair firm can handle this rather common repair.

 

[deanc]

One more thought---You may be able to reroll the tubes and get them to seal. It will be short lived and depends how bad you need this boiler.

I would do a hydro to working pressure when I had it torn down. You may find weeps or other problems.

If you plan weld repairs without removing the tubes---you are likey wasting your time.

 

[smegfab32]

Unclesyd : I need to check some info and come back to you, thanks.

Matengr & deanc : valuable comments, again let me come back to you

Thanks All

 

[smegfab32]

Unclesyd: the operating pressure is 250lbs. Water treatment can be a problem but I don't think we have a high pH issue.

Metengr: majority of the tubes are expanded and seal welded with the exception of the the stay tubes which have strength welds. I have defects in both types. Why don't you see a benefit of a hydro test?

Futher info : the front tubesheet has expanded tubes only.

Deanc: no obvious build up or carbon. What do I need to look for and what would cause the build up?

Thanks

 

[deanc]

The build-up would be hard or soft scale on the waterside of the tubesheet. This is not uncommon on the back sheet. Scale of even .10" can cause enough overheating of the base metal around the tubes to decarbonize. If you have spider cracks from the tubeholes(LP or MT)that is likely the cause. They will crack through the seal welds.

Try a "reroll" before you go further. Make sure you have someone whos knows what they are doing. Hydro at working pressure and if it leaks---you need to pull some tubes to really check this. Also check the seating area of the roll joint. If this is the case--weld repairs are seldom long lasting. Best to replace the sheet.

Water treatment/enough bottom and surface blows will help. If you have poor water or lots of make-up,plus run your boiler hard you must have good water chemistry.

Rear tube sheet beading/seal welds make the pull out value higher and keeps the tube ends from burning off. In a old boiler a "seal weld" seldom seals. If your hole is bad use a copper ferrule,a good (maybe old) boiler guy will know about this.

Call a "GOOD" boiler repair firm--you will be glad you did and cheaper in the long run.

Sorry you asked???? Have fun and let us know what you find.

 

[smegfab32]

Thanks deanc