20" Dia CS elbow with multiple cracks on the extrados (Rich Benfield Solution)

[Alpha Charlie]

We have a CS 20"-12.7mm THk elbow installed on rich benfield solution. Operating Temp: 123.5 C while pressure: 29.5 Kg/cm2. Line is stress relieved as per process design conditions. Benfield CS lines are monitored against thickness loss. Particular elbow was found with multiple surface cracks on the extrados and few cracks on the intrados. All cracks are in the direction of flow and longest is 4" long but estimated approx. depth is not more than 2-3mm based on hot penetrant test bleed out. CS elbow was insulated with mineral wool.

Any idea what might be the reason of these surface cracks since we never experienced such an anomaly before. Elbow material was confirmed to be CS by PMI of removed shavings.

 

[r6155]

Can you send us photos?

Regards

 

[Alpha Charlie]

r6155, Pictures have been uploaded. Pls confirm

 

[Alpha Charlie]

 

[r6155]

Good photos
Investigate with the following.
1)Search the manufacturer test report.
2)Thickness near cracks.
3)Actual Hardness.
4)Metallography.
5)Stress relieving report.

Contact a welding engineer for a possible repairs

Regards

 

[GD2]

Alpha Charlie,
Is this an elbow or a bend? Usually, bends are manufactured by induction process and are better controlled than elbows in terms of material toughness.
I am not sure if you will get a lot of quality reports for an elbow.
One thing I can draw is that the surface cracks are a result of stress (often due to strain hardening) and material toughness.
Cold forming is usually the cheapest way to produce these kind of fittings but always a concern with the acceptability of the metallurgy of the finished product.
I would guess that the elbow is a ASTM A234 or equivalent and long radius one.

Recommendation: Change the elbow with an induction bend (ASME B16.49 or equivalent) if practicable or conduct period FFS.

GDD
Canada

 

[Alpha Charlie]

r6155 and GDD thanks for your comments.

GDD you are right about elbow manufacturing. Its a long radius ASTM A234 WPB elbow. Though there are 7 other elbows of same size/material on the same circuit and all are perfectly ok in VI. Unfortunately quality reports are not available as this fitting was installed back in 1979. I think elbow failure is mainly attributed to strain ageing which is more pronounced on cold worked fittings.
Regards

 

[weldstan]

Was this made from welded pipe or seamless?

 

[Alpha Charlie]

It is an old elbow (1960), manufacturing data is not available. Elbow itself is seamless.

 

Smells very much like a manufacturing defect.
Given the vintage I would recommend replacement rather than attempt weld repairs, which looks like it could become a bottomless pit.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

Lots of inclusions those days. Could be squeezed to the surface during forming.

 

weldstan,
Inclusions will indeed limit formability, but that's not what they do. I'm guessing there are reams of sulphide stringers in this steel.

Recommend doing:
1) Chemical analysis on those shavings.
2) In situ metallography on the extrados surface containing the cracks (but avoid the big, open ones - too messy).

Still, I advise replacement. 61 years is way beyond end of life.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[weldstan]

ironic,
That's what I meant. Loads of sulfide stringers, some coming to the surface. And we don't know the forming process of the original pipe nor for the bend. Concur with your recomendations.

 

[Alpha Charlie]

PAUT was performed to check external cracks depth and presence of cracks on ID. No crack was detected on ID surface while cracks on external surface are too tight. Calibration of the PAUT tool was done and checked on hot metal surface with cracks on it.

Since Benfield service requires heat treatment so weld repair would not be beneficial. Is it possible to monitor these cracks for 2 years as we have planned shut down after 2 years and then we can replace it.

Ironic Mettalurgist:
We are interested to remove shavings from the crack area for chemical analysis. How much %age sulphur content are we looking for and what is this phenomenon?

 

Can't give you a hard number but I can offer these generalizations:

1) ASTM composition limits reflect the manufacturing technology of the day. Metallurgists and welding engineers are well aware that the upper limits for impurities (specifically P and S) are higher than is desirable, so they frequently restrict them to lower than ASTM limits in technical specifications. Steelmaking technology has improved significantly since 1960, meaning steels have become cleaner. For constructional steels I would define 'low' S as < 0.010%, 'medium' as 0.010 to 0.020%, and high as > 0.020%. But everything depends on the application of the steel and how you want to process it, and for most non-critical applications, high S can be totally benign. High S will adversely affect formability (your elbow, which I would define as a critical component), and it has a dramatic adverse effect on impact toughness.

2) The quantity and size of MnS stringers (inclusions) is roughly proportional to the S content (although there are other types of nonmetallic inclusions).


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[r6155]

1) This elbow is a waste and needs to be replaced now.
2) The crack is the most dangerous defect.
3) Two years for a replacement? !!!!!, meanwhile contact a lawyer.
4) Try the Acoustic Emission Test before replacing this elbow, just to get your doubts out.

Regards

 

[weldstan]

If this ell has been in service for nearly 60 years, don't you think it's time for replacement?

 

It's unanimous: scrap & replace.

Possibly the entire section.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Christine74]

I agree that the elbow should just be replaced but I'm not sure I agree that since equipment that's 60 years old is overdue for replacement.

In the US, the newest major oil refinery was built in 1977 and many are decades older than that. I would guess that the average refinery in the US probably has hundreds of tons of pressure equipment that is over 50 years old. If no defects or corrosion problems has been identified, no one seems to be concerned about the physical age of the equipment. One might even argue that if something hasn't failed in 60 years it probably is going to provided the operating service hasn't changed.

It's also very unusual that cracking appears on the exterior of the elbow. Does anyone have any thoughts as to how that might have happened? It doesn't appear to be a Corrosion Under Insulation problem.


-Christine

 

[weldstan]

While refinerys may be old, that does not mean most of the piping is as old. Having worked for a construction company doing refinery maintenance, we replaced lots of piping over the years in all plants serviced. And I've worked on turnarounds where much of the piping was replaced.