Waste Heat Boiler Tube Leakage

[sahsanb]

We recently had to shutdown our plant due to tube leakage in our Waste Heat Boiler(WHB). Upon vessel opening we observed visible leakage from at least 01 tube as the refractory and heat shield adjacent to it was damaged. Evidence suggested that upon leakage BFW water flashed and damaged the refractory on shell and tube sheet. Thermal shock caused by the cooling effect of flashing developed cracks on adjacent tubes, ferrules and tube sheet.

Boroscopy was carried out of the leaking tube and it was observed that the source of leakage was a small hole of 1.73 mm diameter at 3’o clock position in immediate downstream (0.9” away) of the installed ferrule at hot end. Location of the leaking tubes is showing in attached tube sheet map from hot end. It is important to mention here that we did not observed any signs of thinning on either side of the hole. No damage was detected on the leaking tube on Cold End tube sheet. Equipment has been in service for nearly 6 years and no concerning observation was made during previous inspections.

In consideration of the aforementioned inspection findings, satisfactory inspection history, tube defect location and the leaking tube request your expert opinion on the possible damage mechanism responsible for this leakage.

Attached are pictures taken during our inspection as well as brief equipment introduction

Regards,

 

[metengr]

The tube may have contained a part through defect from original manufacture - I have seen this before on new tubes supplied with a boiler. Second, this could have been a damaged tube from original installation. I encountered this once where a plug welded tube leaked. The plug weld was identified by destructive examination.

To determine the root cause, this tube should be removed for metallurgical analysis. Otherwise, you can speculate and it means nothing.

 

[sahsanb]

metengr,

Thank you for the response. Unfortunately due to time constraints we could not remove the tube itself, however based on your experience, please share besides the obvious suspicion of a manufacturing defect what possible damage mechanism could have been responsible for this defect. Does the location of defect in immediate downstream of ferrules on hot end gives us any clues to the problem?

Regards,

 

[davefitz]

If the ferrule is upstream of the defect , then the origin of the thinning at 0.9" may be due to the recirculating eddies and max fluid turbulence caused by the "vena contracta " effect. Same damage occurs at tube inlet end of ACC air cooled condensers and also in copper alloy surface condensers.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[metengr]

sahsanb
I could not open your attachment. Does the tube ID surface contain deposits or other pits? The only other possibility is under deposit corrosion caused by feedwater contaminants that plate out on the hot end of tubes.

 

[sahsanb]

For an unknown reason I am unable to attach the file in a way it can be viewed by you. However I have uploaded it on google drive and hopefully you will be able to see the pictures through it.

https://drive.google.com/file/d/0B_RKjhcx7xaDNWhCUVBKR0VsN2c/view?usp=sharing

Looking forward to your expert feedback.

Regards,

 

[metengr]

After review of your attachment, it looks to me the hole shown along the side of the tube wall appears to have been a defect that opened over time. Since this is a firetube boiler (feedwater on shell side) and process gas on the tube ID, it does not look like a corrosion pit from the OD to the ID.

Have you performed Eddy current testing to evaluate the condition of the surrounding tubes in this section of the WB?

 

[sahsanb]

We performed RFT of the adjacent tubes and could not find any detectable defect. We are troubled by the fact that besides the leakage defect there is no evidence that points to the cause of the leakage.

 

[EdStainless]

There are a lot of possible things that could have been in the original tube that lead to this.
And they were probably well below the level for rejection when the new tubes were tested.
The one other possibility is that during fabrication someone hit the tube with a welding rod. the arc strike would change microstructure and lead to localized attack.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[metengr]

As I repeated above:

To determine the root cause, this tube should be removed for metallurgical analysis. Otherwise, you can speculate and it means nothing.