Stress Corrosion Cracking 1

[alex18]

Stress Corrosion Cracking (SCC)

Equipment: CS Off Gas KO Drum
Age: 34 years old

The equipment has suffered from SCC of weld heat affected zone.
About 20 cracks have appeared and many have gone through the wall causing small leaks.
I think there is no risk of vessel failure as the cracks are well inspected and are small through wall cracks - no longitudinal cracks appearing.

It is possible to weld repair these cracks with the vessel off line avoiding to replace the drum? Can anyone provide his feedback...I am not an expert in this area?

Regards,
Alex 18

 

[apiguy]

Alex,

As a short term fix you could repair the cracks and get another couple on months or hours. A vessel of that age has reached it's replacement point if you are experiencing a large degree of cracking. Check to see if the vessel was subject to PWHT when built. In a refinery situation, most off gas drums are subject to "Wet H2S Cracking". If you still have access to the inside of the vessel, I would recommend a UT examination looking for step-wise cracking. Scan the shell starting in the heat affected zone and progress farther away until a "clean" signal is found. Slower scan speeds than those used for welds should be used. The cracks are typically small (1/16"-3/32&quot. They normally appear like this ~ on the surface when viewed after a wet fluorescent magnetic particle examination.

If you search the web for wet H2s cracking you should find plenty of information.

hope this helps.... Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com

 

[rustbuster1]

The feasibility (and advisability) of repair depends on the loads on the welds that have SCC. If the loading is minimal, the SCC can be tolerated - as appears to be demonstrated daily on this vessel. Many components operate for years with SCC in welds.
Regardless of the SCC mechanism, you should be able to reinforce the vessel across the welds to provide sure safe operation. Repairs of this type can be made in accordance with prevailing code rules and with proper FFS assessment.
It is also advisable to prevent the SCC from continuing to form and grow by applying a suitable corrosion barrier inside - presuming this is accessible. There are many options, depending on service environment.