Head to shell weld 1

[mariolucas75]

Dear All,

Your opinion needed:
This is an elliptical horizontal vessel head-to-shell weld which developed a corrosion ~ m long.
Vessel thickness 15mm, corrosion allowance 3mm depth on marked location on photo is 5mm, rest of the pits are shallow.

What would be your opinion?

As per API:
"7.4.3 Evaluation of Pitting
During the current inspection, widely scattered pits may be ignored as long as all of the following are true:
a) the remaining thickness below the pit is greater than one-half the required thickness (½ t
required), and greater than 1.6 mm (0.062 in.);
b) the total area of the pitting deeper than the corrosion allowance does not exceed 45 cm2 (7 in.2) within any
20 cm (8 in.) diameter circle;
c) the sum of the pit diameters whose depths exceed the corrosion allowance along any straight 20 cm (8 in.)
line does not exceed 5 cm (2 in.)"

So would not it need an intervention / repair ?

But on the other hand - it follows the weld and looks as if some sort of "undercut" type corrosion ?
What would you advise ?

 

[weldstan]

Because you have not provided info as to material,length of service, service environment, I would advise making the repair. It is assumed that the corrosion is at the bottom of the vessel.

 

[r6155]

Any radiography?

Regards

 

[mariolucas75]

This is a crude oil... Material SA516 Carbon steel...it is a bottom - 6 o' clock location ....
Will non-weld repairs - so called cold bond composites do or weld repair would be preferred ? and Why ?

 

[jt1234]

I have gone through this on many new vessels due to extreme poor quality from shop. We did repair pitting and porosity. Attached is our action on each problem on several vessels. You shall be able to find one that fits your case. If pitting on plate, grind smooth the pitting and MT or PT the surface. Since the thickness reduced, there is a criteria somewhere talking about how much thinout % vs required thickness, area and locations are permitted without weld buildup. It will take time for me to dig out as I did perform that years ago. Someone may know right away.
If you leave pitting alone because of the API you mention, the issue is how do you know how deep it is without grinding off ? It can have minor crack or propagation beyond the pin hole. We don't treat it lightly. We don't run the risk. We grind it to reveal what's hidden under the pin hole. Vendor won't dare to ask why.

 

[mariolucas75]

jt1234

Million thanks your file will help me much .....

 

[mariolucas75]

Dear jt1234,

Again thank you a lot for your assistance.....and your file that is very helpful...

Another question - if I have pits "here and there" wide spread - still the best practice would be to excavate them (blend-grind) and NDT them ? Even if they do not violate the API pitting assessment criteria ?
What then API 510 wording meaning would be "widely scattered pits may-be ignored" and especially word "ignored"? If still it is better to excavate them for NDT ....

 

[jt1234]

Its wording "may be" is the key. That means, you decide and take the consequence. We ask vendor to grind/polish to reveal any defects beyond it, and do surface MT/PT. So it is your call.

 

[r6155]

Ultrasonic scanning or radiographic profile techniques are preferred where corrosion is localized or the remaining thickness is approaching the required thickness.

Regards