PWHT of HRSG tubes in place (Section I)

[boofi]

One of our bundles in our HRSG has had a couple of failures on the top return bends. These are 2 1/4 Chrome tubes, .105 wall, with a design temperature and pressure of 700ºF at 790 psig. As it turns out, these tube to return bend welds were not PWHT during fabrication. The bundle is about 2 years old. The code does not require PWHT as far as I can tell, but after some analysis on the failed weld, the hardness came back twice as high as our standards allow. So now I would like to pursue doing a PWHT in place. The contractor came up there with me to look. Doing the circumferential band around each weld (what the code recommends if doing a PWHT in sections) on each return band would be a logistical nightmare. Access is also extremely limited to the welds themselves due to tube supports. I estimate about 900 welds that we'd have to stress relieve. The contractor suggested stuffing insulation down below the welds and laying the beads directly on top of the return bends and heating down towards the welds. The code doesn't explicitly forbid this but I'm kind of skeptical about how effective his idea is. However, I can't see any other way to do it. The other problem I have is if we're heating to 1200+ degrees, how would that affect the hanger supports? These are carbon steel and they butt up right near the return bends. We probably wouldn't do all the tubes at once; we'd mostly likely do them in 6 or 7 sections, depending on what the contractor can manage. Anybody ever do anything like this before, or have any insight?

 

[metengr]

What does the tube failure analysis show regarding the previous failures? I would suspect mechanical/thermal fatigue as the problem given this is an HRSG. There is no reason nor requirement to perform PWHT on tube to tube butt welds either on straight or on hair pin bends. If you required PWHT during fabrication, this is a contractual matter and should have been verified during construction.

I would not waste time, money and risk of collateral damage to perform PWHT on tube welds on bends or other in-situ. It is unnecessary with P-5A material as-welded, especially with a 0.105" tube wall thickness. Leave it alone.

 

[boofi]

There were a couple things that our lab discovered:

1. The weld failed due to environmental cracking
2. The failed weld had extremely high hardness in the weld and HAZ, which contributed to the environmental cracking.
3. The most likely environmental cracking agent high pH in the boiler feed water. These pH issues have since been mitigated.

I'm somewhat confident in our most recent repair. We did not PWHT but we did specify a 300ºF preheat and a 500ºF interpass temperature. The x-ray showed no defects. We did not obtain hardness readings after repair which in hindsight we definitely should have done. I'm worried about the optics; we did not PWHT the return bends on fabrication (even though we technically didn't have to. This bundle was fabricated before I moved to this position), which may have led to high hardness, which due to a high pH, may have cracked the weld. Then on our repair, we still didn't PWHT even though we were technically correct in specifying a preheat and interpass temperature.

 

[metengr]

This sounds like corrosion fatigue cracking from the tube to tube weld ID surface? Are you sure the cracking did not initiate and propagate from the tube OD surface to the ID surface, a likely spot given it was circumferential and the local hardness mismatch? The local high hardness was probably the result of lack of preheat from welding.

So, you have eliminated the contaminant associated with EAC or CF, there should be no issues because you only need to remove the contaminant and the mechanism terminates. Unless it was OD cracking from thermal/mechanical stresses.

 

[boofi]

The lab suspects that the cracking initiated from the weld ID (with pictures showing initiation points). If I had to guess, the cracks were possibly initiated at stress concentration points caused by suck back during fabrication; the lab report showed an x-ray where there was an incomplete weld on the ID. However, the lab report only mentioned the lack of penetration on the weld; nothing about stress risers. When we were having a bundle for another one of our HRSGs fabricated, the fabricator emailed me saying they had to redo a bunch of welds due to a suckback issue.

So I guess we currently have a weld that was not PWHT (did have an appropriate amount of preheat), but our x-ray shows that the weld is complete without defects. The previous failure had a defect initially, that was exacerbated by high pH water.

Going back to the original question; I was talking to a couple of more experienced mechanical integrity engineers at our site. One of them agrees with you-- stress relieving all those return bends might cause more problems. Especially if we go back and x-ray even 5% of the welds only to discover flawed welds. Then we'd have to repair them and we'd be stuck in this terrible feedback loop lol. The other one thinks that it is worth doing since the whole reason we PWHT is to reduce hardness. But again, our standards would require us to x-ray these welds after stress relieving and we'd be in the hole again.

I've reached out to a contractor so I suppose we'll see what they come up with. I'll talk it over with our senior engineers and we'll just go from there. It seems likely that we will not pursue PWHT in place. Anyway, I'm just rambling at this point.

 

[metengr]

Thanks for the follow-up. I think your decision is wise to not attempt in-situ PWHT and focus instead on proper repairs and ensure weld quality. You may have several more of these one-off failures from workmanship but the good news the failures will significantly diminish over time.

 

[weldstan]

You never did state the hardness and pH levels. You should be able to achieve as welded hardness values less than 280 BHN (closer to 250 BHN) with proper preheat and interpass temperature controls.

 

[boofi]

The highest hardness the lab saw was 380 brinell in the weld. Just for kicks, the lab heat treated the return bend they had and the brinell hardness went down to 220 in the same location. I do not know exactly how high the BFW pH was, just that it was beyond our typical operating range for an extended amount of time.

 

[metengr]

Not surprising and related to lack of preheat. But even at this hardness value you should still be at low risk of cracking given proper FW treatment and no significant weld defects.