Laminations found in a Pressure Vessel in Wet H2S service 2

[cespi82]

Good morning Gentlemen,

The situation is as follow:

- Glycol Absorber Column in a Gas Dehydration Unit operating at 1100 psi and 40 Celsius degrees
- Service: Wet Gas with around 400 ppm H2S
- Commissioned in 1979, over 35 years in continue operation
- Material of Construction: SA-516-70
- No History of failures or corrosion issues
- Internal visual inspection revealed no presence of bulging / blisters

During the current Turnaround, a UT scanning was conducted for the 1st time at the “Chimney” Tray level. This Chimney Tray is basically where liquids (Glycol + Water) accumulates. So I presume if there is a potential of Wet H2S damage to occur, it is going to be here.

As a result of the UT Scanning several indications, lamination/inclusion like, were found in this area. Phased Array UT scanning was carried out finding a great extent of indications that were classified by the NDT technicians as laminations and clustered inclusions. After finding this, inspection was extended to other areas of the column (away from the chimney tray) and laminations were also found. Presumably we are talking here about dirty steel with poor control back in 1979.

They reported a total of 7 considerably big laminations. The report included the lamination sizes (they grouped nearby laminations since some of them were very close to each other and classified them as single ones), and the locations and estimated area of the clusters of inclusion.

With the outcomes of the report I ran FFS as per API-579 Part 13 Level 1 and everything is satisfied but the last condition which refer to a maximum lamination size if the service is Hydrogen Charging. I continued further to Level 2 and did not pass either.

If we take this literally, we have to repair the absorber column and eliminate these indications, which obviously implies a huge and long job with a lot of things to consider.

My question is: if this column have been in this service for such a long period of time (35 years) and these laminations have been there since then and there’s no presence of bulging/blisters, why should we consider repair it? Only because we ran FFS and did not pass?

All parameters for susceptibility of hydrogen charging are met (I think), but there hasn’t been any evidence of this to be happening.

Your comments are highly appreciated!!

 

[dhengr]

R6155:
Try this on for size, I’m really saying the same thing as TGS4, but in slightly different words. And again, you guys know your ASME codes better than I do, I don’t have them in front of me or work with them everyday. Your AE testing is a great ‘gross’ inspection method, it shows general anomalies and general shapes and locations. It’s fairly quick and can cover the entire vessel if you wish. But, then you have to use a finer inspection/testing method like UT or RT to start to hone in on finer size/mapping, location and depth of what you saw with your AE. And, RT doesn’t do a particularly good job of depth determination, but it’ll nail a small bubble in the plan/surface view. Neither of these finer methods will always tell you what the defect actually is, i.e. lamination or slag inclusion, etc., although some techs get pretty good at interpreting what they think they are seeing. Finally, you have to take a cut-out sample to a met. lab, for a final determination of what you really have. This progression is particularly true if you think the defect might be detrimental to the vessel. The OP’er. has already gone to my level 2, so AE won’t show more detail, although it might cover a greater area on the vessel.

 

[EdStainless]

All that any NDT system does is provide indications that there is a difference or discontinuity in material.
Knowing the size, morphology, cause, and more importantly if the indication is a defect is all beyond the scope of any NDT system. That is what engineers do.

If this were my vessel I would run, and in 1 year repeat the test.
Make sure that everyone knows that if there is growth of these that dooms the vessel.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

Indeed Ed, but this is what it appears r6155 is driving at with AE in a roundabout way. It may be possible to monitor the defects with AE to ascertain whether they are actually growing, and under what conditions they grow. They are restricted to a manageable zone within the complete vessel, but it could be a tough call

http://info.inspectioneering.com/blog/acoustic-emission-monitoring-for-hic-affected-vessels

Perhaps, a more reliable operational picture could be obtained if AE were combined with hydrogen permeation measurements, see for example, NACE Corrosion 1998, Paper 98395

I didn't notice any internal inspection for surface breaking flaws which would help to ascertain whether a continuous leak path would develop should the identified planar defects start to link up and further growth occurs towards the outer surface.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

I have been hunting for an old paper and I can't find it.
In the paper the plant removed plugs of material from a vessel. Their diameter was less than the wall thickness.
They did hydrogen analysis and metallographic work on the plugs.
And since the holes were small the weld repairs were fairly simple.
Doing full (for areas of concern) UT and/or AE, along with a few samples, lets you put the unit back into service and build a plan for future inspection and testing that is based in facts.

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P.E. Metallurgy, Plymouth Tube