[b]ASME Section I, NDE & Qualification for Strenght T/TS Welds[/b]

[Lmarcano]

Hi, friends. We experienced recently a recurrent leak problem with tube to tubesheet welding joints in a sulfur recovery unit waste heat boiler (firetube, Rentech). After 4 years in every shutdown (cooling) welds began to leak, we made several repairs before take a decision to retube.Tube attachment is like Fig PFT-12.1 (g)in code. During repairs work we found a lot of joints with lack of penetration, a metalurgical evaluation taking boat samples before extract old tubes show cracks beganing due that (stress raiser) and no fusion in bevel was another indication of welds problems.
Right know I´m working on a root cause anlysis and found no specs for NDE in this type of welds or Test for Welders Performance, some manufacturer documents talk about mock up test in order to relief requirements of ASME VIII div.2 (art F3) but nothing more.(I don´t have this article)
Do you know how to guarantee a full penetration/fusion on this joints, a NDE type or code examination?
What is the best way to ensure welders performance in this type of joint?
Do you know about similar problems?
I´m really worry about it, the same manufacturer it´s working on 2 new boilers for revamping our Sulfur recovery Units. I want to reinforce our quality control specs in order to avoid this problem again in operation.

 

[metengr]

I reviewed ASME, Section I, PFT-12 for tubes. According to your post, you have welded tubes into the tube sheet using a partial penetration weld (illustration g in Figure PFT-12.1.

The method of attachment is a partial penetration weld where the tubesheet is beveled to accommodate the weld. The depth of the weld into the tubesheet is the tube wall thickness or 1/8" minimum (whichever is greater) and the cap is the thickness of the tube wall.

With that being said, you have something to compare with in evaluating the failed welds. Lack of penetration would be a weld profile that is less than the depth of the weld into the tubesheet as required by Figure PFT-12.1 g. The lack of fusion is a weld defect.

Regarding NDT, you really do not have any method that would assure weld depth profile and detection of lack of fusion type weld defects other than hold points during various aspects of the job.

A hydrostatic test could locate defective welds but not weld preps that were inadequately prepared in accordance with Figure PFT-12.1 g. The only other NDT option I would recommend is to have a wet fluorescent MT of the tube to tubesheet welds when the re-tube is completed.

I would request a mock-up as part of a weld qualification program. Frankly, this type of weld joint should be relatively easy to make with minimal problems. The only problem that I could see during fabrication was that the contractor did not take the time to clean the tube to tubesheet joints prior to welding. Lubricants used for machining the weld joint bevel or used for tube insertion could easily result in porosity or lack of fusion. If this is the case, a hold point for cleaning is in order and possible the use of preheat to bake out the joints prior to welding.

 

[deanc]

Hmmm! Sulfur? does not sound good for repairs. Is your tubesheet cooked out? Fire cracks/checks? May have a carbon problem from overheating.

How about a roller expansion joint,seal weld,and reroll. Maybe use copper ferrules in the retube. Weld metal? Have had good luck with 6010 P5/IP.

Contact a good boiler repair firm,they can help you.

 

[GenB]

1-preparation: includes grinding tubesheet before tube insertion, clean tube ends before insertion, sandblasting may be required.
2-roll tubes before welding: Code required.
3-weld per Code (good experienced welder needed)try the refinery welders for their miracles.
4- reroll after welding.
The mfr is responsible for weld quality so you should audit their procedure.
may be the tube thickness is not sufficient for the job.
ER