Vessel Repair and Failure Analysis
Vessel Repair and Failure Analysis
(OP)
Hi All,
My company has a Section VIII Div 1 vessel that has been in steam service for 2 years and has developed a crack in the jacket. I attached a picture of the defect for reference. The vessel is a flat wall cross-section with a stainless steel inner wall, 516-70 jacket (1/4" thick) and A36 support bars between the vessel walls. I've attached an illustration of the cross-section of the construction. The picture from the field shows that the defect is about 1" below the seam weld. The seam weld is backed by an A36 bar that is 1.5" wide. The defect runs parallel to the seam weld/backing bar and is about 3/4" below the edge of the backing bar. I'm trying to figure out what happened here. We have thousands of vessels of this design that have been in the field for 40+ years and we have never seen this type of failure before. In terms of operating conditions, the jacket sees saturated steam service at 20-40 psig and has a proof test pressure over 1000 psig.
My intuition says that this crack is likely a material defect - either an impurity or void that was captured in the rolling process. The material receiving inspection reports don't note anything peculiar during the visual inspection about this plate. I was hoping that if something was rolled into the plate that there may have been a tell tale sign on the surface of the material but this doesn't appear to be the case. Has anyone else seen anything like this happen? What could be the cause?
In terms of repair, we were going to grind out the length of the crack plus about 2" on either end. Then we were going to drill the ends of the ground area to blunt the crack in case there's a hairline fracture that we can't see. After cleaning and other weld prep, we were going to perform a weld repair. Are there any suggestions of other items to examine or procedures to perform prior to repair?
I appreciate the assistance!
Best,
Steris
My company has a Section VIII Div 1 vessel that has been in steam service for 2 years and has developed a crack in the jacket. I attached a picture of the defect for reference. The vessel is a flat wall cross-section with a stainless steel inner wall, 516-70 jacket (1/4" thick) and A36 support bars between the vessel walls. I've attached an illustration of the cross-section of the construction. The picture from the field shows that the defect is about 1" below the seam weld. The seam weld is backed by an A36 bar that is 1.5" wide. The defect runs parallel to the seam weld/backing bar and is about 3/4" below the edge of the backing bar. I'm trying to figure out what happened here. We have thousands of vessels of this design that have been in the field for 40+ years and we have never seen this type of failure before. In terms of operating conditions, the jacket sees saturated steam service at 20-40 psig and has a proof test pressure over 1000 psig.
My intuition says that this crack is likely a material defect - either an impurity or void that was captured in the rolling process. The material receiving inspection reports don't note anything peculiar during the visual inspection about this plate. I was hoping that if something was rolled into the plate that there may have been a tell tale sign on the surface of the material but this doesn't appear to be the case. Has anyone else seen anything like this happen? What could be the cause?
In terms of repair, we were going to grind out the length of the crack plus about 2" on either end. Then we were going to drill the ends of the ground area to blunt the crack in case there's a hairline fracture that we can't see. After cleaning and other weld prep, we were going to perform a weld repair. Are there any suggestions of other items to examine or procedures to perform prior to repair?
I appreciate the assistance!
Best,
Steris





RE: Vessel Repair and Failure Analysis
This is why you MT or PT the crack. I strongly recommend that you perform this to ensure that you have removed all of the cracked material prior to weld repair.
RE: Vessel Repair and Failure Analysis
The best way to excavate cracks is with arc-gouging - CAC, 'air-arcing'. The crack opens up due to the heat stress and the welder can see the crack fairly well, and can see when the crack has been eliminated. Additionally, the arc-gouge leaves rounded ends, unlike the sharper excavations you get with a grinder or a burr-bit. And if there are internal material flaws, like slag-lines and delaminations, the arc-gouge will find it.
RE: Vessel Repair and Failure Analysis
RE: Vessel Repair and Failure Analysis
Thanks for the tips! PT is a great idea and you are absolutely right that it should be performed.
In terms of the potential causes, I would be surprised if this had experienced fatigue failure. We are running at temperatures of 60F-280F and all temperature changes are very gradual - there is no thermal shock on this vessel. Additionally, prior to failure, this vessel could not have seen more than 2,000 pressure/temperature cycles. Given that the stresses are low and the cycle count is low, I don't know what the mechanism would be to create fatigue failure. Is there a reliable way to determine if thermal fatigue was the root cause? Are there any other explanations that could describe why this failure occurred?
RE: Vessel Repair and Failure Analysis
Residual stress from forming or handling coupled with the temperature variation in service (despite gradual), can result in this type of cracking. You would need to extract a small boat sample capturing the crack and have a proper lab analysis performed to characterize the cracking mechanism - straight, branching, transgranular, intergranular, etc.
RE: Vessel Repair and Failure Analysis
"Whom the gods would destroy, they first make mad "
RE: Vessel Repair and Failure Analysis
RE: Vessel Repair and Failure Analysis
Thanks for all of the feedback! I really appreciate all of the information.
I am reluctant to believe that this crack is a design flaw mainly because our company has been using this exact vessel design for 40+ years and has 1000+ vessels of this exact design in service without ever seeing this happen before. If this was a design issue, I would expect that we would have seen this failure mode occur many times before. I don't see how I can make a case for a design flaw when there is such a long and successful service history of the design. As such, I'm forced to believe that something unique happened on this vessel whether with the materials, manufacturing or use. Could this have been caused by an excessive HAZ either from plate cutting or welding? I'm wondering if a higher welding heat was used, could there be embrittlement of the base metal which led to premature thermal fatugue? Is it possible for this to be a defect in the base metal from the rolling process? Are there other explanations for this other than design flaw?
Thanks again!
RE: Vessel Repair and Failure Analysis
Unless you remove a material sample capturing a portion of the crack, whihc you can weld repair the slightly larger excavation, you cannot determine root cause, period. Why beat your head against the wall looking for an answer????
RE: Vessel Repair and Failure Analysis
Thanks for the advise. I apologize for repeatedly asking the same question. We will have a sample analyzed however in the meantime management and the customer are pushing hard for a possible explanation. I was just trying to give them a few possibilities to tide them over while the the analysis is performed.
RE: Vessel Repair and Failure Analysis