By dasimonds 
Date 06-06-2008 23:05
Edited 06-06-2008 23:14
As far as I know, there is no corrosion component with LNG(basically Methane) in either its liquid(-260C) or vapor phases(-190C), so in this particular situation, the arc strikes are of no concern. As you said, the fact the pipe has survived as long as it has is a testament to its toughness and ductility.
Thanks for confirming what I was thinking.
I can remember Prof. Crisi saying Aluminum stays ductile all the way to absolute zero(-273C,-459F).
Does 304L even have a ductile-brittle transition temp?
304 seems suitable for liquid hydrogen service,-252C,-423F.
I believe the extremely low or lack of a ductile-brittle transition temperature is the main reason that weld in the picture(there are more just like it in service on the same system) exhibits no problems, and probably never will.
Even considering that, there is no excuse for leaving work like this.
As a welder, I can't imagine leaving defects like these on a weld.
#1. Is there a time in a job that a guy wouldn't have time to remove these defects(start up)?
#2. Where was his foreman?
#3. Where was the QC?
The facility now does both PT and RT on all butt welds. I asked why and was told they had a weld that passed RT but leaked.
I come from a background, like many, that were taught that arc strikes were a bad thing, and you will probably get some, just make sure you remove them.
Thanks for the responses.
Dale Simonds
The problem with arc strike in general is there is no hard data on failures. Doesn't matter if it's riddled with arc strike, A failure usually is attributed to some other factor. Mechanically, I've re-created the experiment Mr wright performed on carbon. It did in fact have a notch effect. When I have time, I intend to get into the microscopic examination of it, but the lab I have access to is 100 miles away.
Service would play a large roll in it as well. My experience with LNG systems is that the players with the hard data on them, don't like releasing that data.
I've worked two LNG terminals/4 trains, finding data is like the proverbial needle in the haystack.
The one thing I've noted about cryogenics in general is, if it makes it past cool down, it's not very likely anything else is going to happen unless someone screwed up the installation of monitoring equipment. If one of the line I&C temp gauges is not working, it could be subject to in line localized uncontrolled thermal inversion as the operators would not have accurate temps from the top and bottom sides. Any notch in such a condition would be the likely fracture point if any. (if you've ever seen the aftermath of one of these, you would understand).
As for cracking, not all arc strikes create cracks, though they certainly have the potential for it. If it's been there 30 years, It will likely be there another 30 except for conditions I've already mentioned. For LNG, I would be more concerned with the notch effect. I seriously doubt the arc strikes depicted would be a problem as long as the instrumentation and plant operation are ok given the length of service already in.
A properly welded 304L weld / annealed base material will usually have very good toughness values at -196c, though the range servicable delta ferrite is not cut and dry with cryogenic temperatures. I've got hard data telling me 6 to 17 has acceptable values, 18 to 21 or so the toughness values are near to minimum. Then it goes back up after 21 before going back down shortly thereafter. That was for the particular source of filler/material and may not apply to all.
Clear as mud now.
Regards,
Gerald
