[Otto Nobedder]

Here it is, coming from the outside. First image is the outer structure I had to remove (pic is actually of an identical trailer... I forgot to snap one before I started.

GEDC0451.JPG (173.63 KiB) Viewed 1977 times

Next, the outer structure is (crudely) cut away, & I'll clean it at reassembly, with a 12X12" hole cut in the outer vessel. The insulation is removed to the edges of the hole, and fire blanket wrapping the edges of the insulation surrounding the opening. The red stuff is the penetrant dye.

GEDC0452.JPG (170.27 KiB) Viewed 1977 times

And the same view, with the crack stop-drilled (1/8") from the inside, and the penetrant cleaned up to see the extent of the external side of the crack. It travelled straight out, perpendicular to the vessel wall, so there is no "peel" involved, and the fix will be simple.

GEDC0461.JPG (201.64 KiB) Viewed 1977 times

The stuff after fixing this, however, will be time-comsuming.

Steve S.

[delraydella]

Do you know what caused it?

(I apologize if this has been discussed before, I'm too lazy to go back and look!)

[Otto Nobedder]

The most likely cause is a flaw in the original sheet.

The entire vessel underwent x-ray at some point, and there are a number of repairs apparent within the vessel. This particular flaw was directly under a piece of rolled angle used as a reinforcing ring inside the vessel. It was likely invisible due to the amount of metal in the way.

This was built in 1976, and the inner vessel spends most of it's time at -423*F. However, every five years it goes through a "retest" procedure defined by ASME and adopted by DOT. During the procedure (per customer requirement), the inner vessel is heated to 140-150*F to release moisture captured by dessicants within the annular space (space between the inner- and outer- vessels that is usually evacuated to high vacuum but is returned to atmospheric pressure for maintenance). The 500+ degree temerature change stresses the metal quite a bit, as does the pressure change from high-vacuum to atmospheric, which only happens during maintenance, meaning when the temperature is well above normal working temp.

It received it's last restest here, and didn't show a loss of vacuum until it was returned to service, i.e. returned to -423*F, so the assumtion is the temperature cycles finally opened the crack.

Steve S.

[delraydella]

What would be your theory as to why the sheet was flawed in that spot?

My only guestimation would be during the rolling process it may have gotten pinched a little in that spot and not been up to full thickness......?????

[Otto Nobedder]

My guess is an inclusion of some sort during the original melt or pour-- A grain of material from the refractory crucible, a grain of sand from an adjacent casting process, a shaving from a bearing at the foundry... Steel of any kind is difficult to maintain high purity outside of small laboratory batches.

That's why the entire vessel is x-rayed when manufactured, and why there are dozens of apparent small repairs that seem to be original to the vessel, which suggests it's fairly common.

I say "seem", because I'm not the first to enter this one; It's been opened before, for reasons unknown to me, but I wouldn't expect it to be for "dozens" of repairs. The main vapor piping appears to have been modified, so I expect that was what the last person was doing. BTW, THAT repair failed, as well. There is a long, visible to the naked eye crack in one of the welds on that piping to repair (actually, replace end-to-end), too. The crack originated at a bad tie-in between two halves of a cap pass over a less-than-impressive root. Because the vent piping within the vessel was not pressurized (sees vessel pressure inside and out at all times), it didn't fall under ASME codes and might not have even received a visual inspection. (That kills me... I ALWAYS ask a second set of eyes to look at my work on this stuff. I may be my own worst critic, but this work must be done right, and some else may (and have) see something I didn't.)

This is all speculation on my part, but it seems logical.

Steve S.

[Otto Nobedder]

Well, I finally closed the inner vessel today. Monday I went in with a longwave UV light for "oxygen clean" protocol, and by the end of the day had tacked the manway shut. Tuesday, I welded the root, pressurized the vessel to 5 psi, and "snoop" tested it for leaks, then vented down and put in my hot pass. Hot pass over root is shown here:

GEDC0482.JPG (216.31 KiB) Viewed 1919 times

That's not undercut in the root; It's a reflection of the root in the portion of the bevel that was buffed with a flap-disc. The root was actually "walked", while the hot pass was freehand, though it appears the opposite in the pictures.

The hot pass was "snooped" at 20 psi.

Today, I capped it out.

GEDC0483.JPG (174.32 KiB) Viewed 1919 times

It was then tested by dumping a bottle of helium in the inner vessel, and pressurizing to 65 psi (MAWP plus one atmosphere, as the vessel was not under vacuum), and using a "sniffer" on the mass spectrometer to test for helium passing through. The repair weld was tested at the same time. All passed. Once the outer vessel is sewn up, it will be tested at 63 psi (1.25 X MAWP) at full vacuum, for an effective pressure difference of 77.7 (approx) psi with the mass spec. on the vacuum pump.

The hard part is over!

Steve S

[delraydella]

Nice! I'll bet you'll be glad to see that trailer back out on the road.

[Otto Nobedder]

Boss always said, the two best days for any trailer are the day it arrives, and the day it leaves!

[weldin mike 27]

Hey,

Hey guys, has anyone seen my car keys? I last saw them in the.... oh damn.

Mick

[Otto Nobedder]

[delraydella]

Any updates on the total cost of the repair?

[Otto Nobedder]

Good question.

The boss is on vacation, but he'll be back before it's done.

I'll have to hit him up for that information!

[TamJeff]

Interesting job and nice work overall. I had no idea what those tanks were made up of.

[Otto Nobedder]

The boss is not forthcoming with the repair cost.

I think he's afraid I'll be offended at what a small percentage of the cost my wage is!

Steve S

[TamJeff]

You gotta make pretty good scratch working on stuff like that. My boss overestimates my abilities a lot. When I see that happening, I double my rate. He has taken some jobs I would have quit over. He forgets sometimes that it is just me there at the shop when "we" are to build or weld something. We build some BIG stuff. I'm not 40's anymore. lol

[Otto Nobedder]

I'm not uncomfortable at my wage.

I should be paid at least 25% more, though. Most people can be replaced, and I'm sure I can, as well, but it won't be easy.

The pool of people with the experience I've accumulated here is very small, and the instinct cannot be taught.

Steve S

[TamJeff]

Same here. I actually resigned in May of 2012 and out of some bent of loyalty, decided I would get him through the season because there is no other me, at least as a solo entity. I do the design, the layout, the fab and the welding. There is a stipulation though. That I get to use the shop for my side work and that's where I make it up. The economy, along with the Gulf oil spill nearly killed our industry and now many employers are riding on that "be lucky you have a job" mentality. I am planning a move to NJ, of all places, and hope to start my own small op up there, or at least find someone to work for who doesn't want just another welding robot.