Welding Tips and Tricks

The first image is of the valves I have to jamb into the second image. I'm changing out a number of valves notorious for losing parts into the process stream, but I don't have the luxury of spooling all this pipe on the bench as the original builders did.

Sorry about the quality of the second image. I took two shots, and thought one might be good.

Sooo... restricted welds with a "capital 'R'."

It actually makes for a pretty pleasant week, 'cause nobody bothers me (afraid I'll ask them to help) and I get to test all my skills, including close-quarters mirror-welding.

I'll try to get a better shot of the gallery the valves are going in.

Steve

Here's the body of the first 2" valve, in it's final position.

Steve

What is it?

Liquid Hydrogen tanker-trailer.

Hey Steve,

What kind of NDT is required on a job like this? Also how do you cut out the old valves?

Mick

Hi, Mick,

Strangely, all work done downstream from the safety interlocks, called "fire control valves", is only inspected visually and with a bubble leak test, and I do both myself. I was prequalified for the welds, but it's generally up to me to do it right and prove it. The only part of the trailer that falls under a qualified welding code is the inner vessel, covered under ASME section IX. I also perform the five-year retest under that section, with an exemption. The normal procedure requires test at 1.5X max working pressure. Under the exemtion, I test to 1.25X MAWP.

For the 1.5 and 2" valves, I simply hack out the old valves with a sawzall to eliminate the weight, then trim pipe ends to length with a cut-off wheel (The new valves' bodies are a few inches longer than the originals, requiring major refit). The small-bore valves are much closer in size to the ones I'm replacing, but are socket-weld, so when I have decent access I cut the welds to remove the valves. When I don't, I'll section out the pipe to the points easiest to weld in that line.

In either case, I vacuum out the debris, and at the end of the project pressure the trailer with nitrogen and go through a procedure to blow out each affected line.

Steve

Very cool looking project. Looks like a lot of fun.

Thanks Steve,

I was expecting at least a dye pen. Have fun.

Mick.

Got the second 2" valve in and welded out today, and the first 1/2" valve fitted up and in for a test-fit before welding it out and installing it permanently. The "angle" between the two valves is an illusion of perspective. Both valves are tilted toward the camera. The one on the left DOES lean to the left to provide the spec'd 2" minimum clearance between handles (or between a handle and any other obstruction), but only about two degrees. It is also tilted toward the camera about 5 degrees more than the one on the right, because the one on the right had to be moved out 1 1/4 inches to clear an obstuction.
Steve

Today's progress:

The 1/2" valve was welded out, and the last 2" valve was fitted, and tacked off in the photo. I had the root passes in the field welds before the day was done. Tomorrow, after I cap those welds, I'll fit and weld a 1 1/2" Wye valve in the left of the image (you can see the hacked-off pipe stub), and fit one more 1/2" valve where the green tape is, then its on to the other half of the gallery. Here's a better pic of the other half of the gallery, where all but one of the remaining small valves go. It's been two years since I've done this changeout, and I'm really enjoying it, so far.

Steve

How much room do you have to work in? It looks pretty tight.

Steve, my second pic today should give you an idea.

First, here's the last two valves in this section. The 1/2" is welded out. The 1 1/2 body is welded out behind the valve; The fittings in front are only tacked. The stright pipe at the bottom is a dummy, representing where the pipe leads to the pressure-building coil that will sit beneath the floor pan. I'll pull that section out and weld the three 90's on the bench. Here is a pic of the weld for the inlet side of the last valve. (Dead-center in the pic.) The quality of the pic is due to the fact that I cannot get my camera and my head into the space at the same time. I actually had to tape a #10 welding lens (4X5) to the pipe & angle in the foreground and stuff my head in there to weld it. The bottom side wasn't so bad, I did it from a creeper. My arms were extended around all the other crap in the pictures, so, no, it's not the picture. That weld looks crappy.

After posting, I'm struck by how the portion (12-3 o'clock) I did right-handed reflects so much brighter than the (9-12 o'clock) part I did left-handed. BTW, thats a camera flash effect. It doesn't look like that under normal light.

Steve

Here's that half of the gallery, all fitted out (minus one small pipe support I had fabbed but not installed before the day ended. The only valve in this picture that will be disassembled again is V-3, on the far right. It comes out and gets blinded off for the heating procedure, which releases any moisture trapped between the inner and outer vessels. This trailer is like a HUGE Thermos bottle. Between then inner and outer vessels, there is a wrap of at least 50 layers of alternating Mylar and tissue paper, and the space is evacuated to high vacuum (less than 20 microns).

Steve

Steve,

Fascinating thread.

Can you elaborate on the reason for having to replace all the valves? Do they become embrittled or otherwise deteriorate because of the hydrogen?

mArc wrote:Steve,

Fascinating thread.

Can you elaborate on the reason for having to replace all the valves? Do they become embrittled or otherwise deteriorate because of the hydrogen?

Good question. I hadn't thought to mention that.

The Powell valves I'm replacing have a tendency to lose parts into the process stream. The way the poppet seals are retained is apparently succeptable to loosening from extreme thermal cycles (ambient to -423F almost instantly) and the vibrations any part on a moving vehicle suffers. On three of the 1/2" valve in this trailer, the poppet seal, seal retainer, and retainer nut were gone. I did find one retianer in the liquid sample line, because the connection port was too small for it to escape.

The new valves are designed specifically for cryogenic temerature cycles, down to liquid He (-458F). They've been commonly used on another manufacturer's version of the same trailer for years, and in the rebuilds I've done I've never seen a "loss of parts" failure.

Steve

wowww....very good job. must be tigged I supposed???

Thanks! It's a fun project, and I have a few more like it coming up.

Yes, it's all sch. 10 stainless, TIG all the way.

For me, as a beginning welder, that looks like a night mare! First, I don't know how to tig. Pipe welding is even harder. I was just proud to be able to fix the leaks in my compressor. I had to remove the motor mount off the top and was pleased that the welds held nicely. It was real difficult to get them off. This week I'll be in Vegas, but, when I get back, It's "Trailer Time". I got some great info for some 3/16" x 4" chanel. When I start, I'm gonna have alot of questions.

Fire when ready!

I know more about trailers than I'd like to admit. On my current job I had to design a sub-frame for an air-ride suspension on a 47,500# (empty) trailer. I did a lot of basic utility trailers on my way to that point, though, and many others here have experience on trailers of all sizes.

You may get more advice than you want.

Steve

Wow. . .holy manifold batman. I would love the symmetry of such setup. I would be one of the one's who noticed. It's like a curse with having an eye for such things. Top notch stuff right there.

Strangely, no two of these are ever identical, despite being assembled in the same place, by the same people, from the same prints. Each job always presents unique challenges.

Keeps it interesting, at least...