Boiler Tube repairs

Date 10-06-2007 15:50

Repairs to boilers must be done according to the standards specified in state and local laws. In the U.S. and Canada, this would be the ASME Boiler & Pressure Vessel Code and the National Board Inspection Code. It would be advisable to check with the Chief Boiler Safety Engineer for the state or local government where the repair is to be performed before attempting this.

Having said that, if the leak looks like a "pinhole" on the outside, the inside may have a larger area of wall thinning that will cause the tube to blow through when you weld on it. The E6010/E7010 electrodes have deep penetration and fast freezing characteristics that will cause it to burn through easily and leave porosity in the weld. Also, if "re-weld on weld-seam to overcome lekage" means can you weld it under pressure to seal the leak, the answer is no because the tube will rupture and probably burn the welder severely. The recommended repair would be to cut the tube and install a new section of tube.

By mhanif Date 10-10-2007 06:23

Dear Sims,

Thanks for reply. Boiler is in shut down condition and pinhole so small that water comes from pinhole only on pressure i.e. above 5 bar. Any suggestion/recommendation other than replacement of tube.

Thanks

By js55

Date 10-10-2007 17:09

Isn't UT a common practice used for tube wallthickness assessment?
The data then being compared to thermal cycling history and an estimate of life being made?
To just weld a pinhole in a boiler seems pretty 'seat of the pants' for something as sophisticated as a boiler.

By G.S.Crisi (****)

Date 10-10-2007 21:55

If I were you I'd do this:
take two flat pieces of steel of the same (or at least similar) composition and tensile strength than those of the boiler tube. The length could be, for example, 4 inches or 100 mm. The thickness should be the same than that of the tubes.
The width will be calculated as follows: [one half the outside radius of the tube x 3,14] minus 3 mm or 1/8 inches.
Now have a blacksmith (do blacksmiths still exist?) round up the pieces to make half a cylinder out of each one. The two half cylinders should fit more or less exactly on the outside surface of the tube. Make the necessary adjustments if they don't.
Now install the two halves one in front of each other, so they will embrass (or wrap) the leaking part of the tube. Weld the two half cylinders to the tube and one to each other.
Select the electrode, or MIG wire, according to the opinions of the other gentlemen who answered your question.
Before putting the boiler in operation again, don't forget to make a hydrostatic test.
Now that you have plugged the leak, you must correct the problem that caused the tube corrosion, and the problem is poor water quality. So, talk to a competent chemist, i.e., one that knows well how to treat boiler feed water and what precautions must be taken when the water is already into the boiler, and ask for his recommendations.
Giovanni S. Crisi
Sao Paulo - Brazil

By dasimonds (**)

Date 10-11-2007 02:36 Edited 10-11-2007 03:07

I don't believe your approach would "fly" in my part of the world(New England, USA). Perhaps a "pad" weld, but certainly not the type of repair your suggesting. Keeping in mind the type of boiler involved.
Never happen in a Black Licquor Recovery Boiler. Even a trash burner would do a dutchman in order to stay on line till the next scheduled shutdown.
A "pad" weld means adding weld metal of the same omposition as the base metal over the leaking area as to increase the tubes thickness and to stop the leak until the time comes for an appropriate repair to be done(Dutchman).
Dale Simonds

By G.S.Crisi (****)

Date 10-11-2007 22:40

I didn't know mhanif was referring to a black liquor recovery boiler. From the way he wrote his question (10 bar boiler), I supposed he was talking about a boiler producing saturated steam at 10 bar (roughly 150 psig) for heating purposes (heat exchangers in a chemical plant, for example).
If the boiler is a black liquor recovery one, then the problem is more serious. In fact, they burn black liquor, a byproduct of pulp and paper mills, whose flame and combustion gases are corrosive. In this case, the corrosion that made the pinhole isn't due to poor water treatment, i.e., acting from the tube inside, but to the corrosiveness of the black liquor combustion gases, i.e., acting from the tube outside.
Giovanni S. Crisi