Welding Tips and Tricks

While I'm waiting on materials for my other repair, I began the leak investigation on another trailer.

Through tenting the entire trailer in a HUGE tarp, I proved the outer vessel was leaking, and narrowed it down to here: or more specifically, here: This is not the end of the search, however. This is just where helium can get through the MIG welds that attach this ring. The leak in the outer vessel may be several inches from this point (though not much farther, because the leak detector responds almost immediately).

Sometimes, I love my job.

Steve S.

This one arrived with a suspected INNER vessel leak, and the evidence and history suggest this is true. However, before beginning to search for inner vessel problems, I must first investigate the outer vessel and prove it to be leak-free, or find and repair any leak, before beginning to investigate the inner. (It prevents chasing ghosts... Once helium is introduced to the inner, it takes a great deal of nitrogen to purge it sufficiently to be sure a reading I'm getting is NOT from the inner vessel, whereas on the outer vessel, the atmosphere takes care of "purging" overnight.

Steve,

I certainly appreciate you sharing the interesting jobs you get to do. Keep us posted !



Shawn

Do you have down time between these leak jobs, or is this something that happens a lot so you end up doing this every day? If the former, what do you do at other times?

First,

Thanks, Shawn! I enjoy this kind of work a great deal, and enjoy sharing it. If I were the type to go to the bar after work (Well, I have been that type, when working on the road, but not so much now that I'm home and can have a beer with a home-cooked meal.) I'd probably drive people nuts with it. Unlike here, where everyone welds.

Jeff, I'm not sure yet how common these leaks are. Only since I returned to this job (I quit and went on the road for a year.) has the client begun sending us almost all, if not ALL, their vacuum work. I take that as an "attaboy", as I've always had a talent (instinct might be a better word) for it, and their chief engineer is impressed with me.

When I'm not doing leak-chasing, I'm usually doing ASME/DOT retest on the vessels, sometimes major modifications to the external piping and valving, and, less often, any other job required. I have begun to suspect I'm the only one in the shop qualified for every last item we do. I've also become the shop's ad-hoc engineer. As an example, Meritor redesigned their air-ride suspension, and all our drawings for the air-ride upgrade were suddenly obsolete. The trailer manufacturer is completely uncooperative, protecting their own business. It fell on me to redesign the subframe structure (based on Meritor's design and the previous configuration) to meet the engineering requirements of the suspension, and the client's requirements for equipment including an antilock braking system. About the only time I find my job dull is when I'm doing "production" work, like building new cabinet doors, or rebuilding axles.

I sometimes do simple fabrication. Sometimes instrument fitting. Sometimes pipe (sometimes vacuum-jacketed). Valves. AOVs (air-operated valves). Pneumatic plumbing. Sometimes basic knuckle-busting mechanics. Sometimes training. Sometimes supervising.

Damn. I need a raise!

Steve S

Could you actually (if applicable) get yourself certified in this particular specialty (these gas trailers and all of the ancillary aspects) and become a consultant of sorts ? There can't be too many folks that do this highly specialized work or am I wrong ?

You're right, Shawn, in that very few do this.

There is no specific certification available, though I am ASME certified to perform repairs on the inner (pressure) vessel.

Helium leak detection is not that uncommon, either. RedIron881, a member here, is familiar with it, as he welds vacuum vessels for the semiconductor industry.

My particular application is a bit unusual, though. "This thing leaks... Find it", and I have several thousand square feet to search.

Steve S

Otto Nobedder wrote:My particular application is a bit unusual, though. "This thing leaks... Find it", and I have several thousand square feet to search.

Steve S

That was my initial interest in your posts here. That's pretty amazing, at least to another welder. Personally, I would almost have to be in the right mood for such a task, without having an attack of some perhaps, new found ADD skew my process.

Impatience is my biggest weakness.

While performing a test, I'm just standing there, drinking too much coffee, smoking too many cigarettes, and watching a bar-graph on a machine. My mind is already on where I'll test next, and second-guessing the wisdom of my last test.

I do keep a very thorough log of everything I do and all machine readings. Often while testing, the "background level" will drift up. Each time I complete a test, the area where I've confined helium ("bagged" is the term we use, and for some items, it's an actual bag. Usually plastic sheeting taped over a suspect area) is opened up, so the helium is released.

When the background level changes, I'll look at the log to see what area I was testing in the previous hour or two, and this helps narrow it down to a "region" as I look at air movement in the shop (here, the cigarettes actually come in handy, as I can go to a previous test point and see which way the smoke drifts, as helium rises,diffuses, and carries on the breeze much like smoke once it's above the convection zone above the cigarette). and follow the path.

Steve S.

BTW, it's actually illegal to smoke in the workplace here, with the exception of some bars.

Nobody in the company, from the president on down, including my non-smoker co-workers, cares, though. All a bunch of conservatives, who believe the Gov't overstepped it's bounds telling us how to operate.

Okay, I put my finger on the leak. I sectioned out 4" of the ring, cleaned it up, and hit it with dye-penetrant. I found this crack, with spider-webbing at the ends, an inch and a half from my starting point. Like hitting the lottery, on a Monday morning! After grinding edge-on with a 1/4" wheel 'til about 2/3 through the material, I hit it with the dye again, and found this: The space between the (now) two cracks is about 1/4". As I finally ground through, I shot it again (I'll look at the result in the morning). If I've ground out the entire cracked area, I'll TIG it up open-root. If I have to grind more out, I'll open it up enough to get a small backing-bar in it.

Isolating this leak and retesting the rest of the area showed this to be the only leak, so it was a great Monday!

Steve S

Do you get time parameters for the repairs or is it "it's done when it's done" ?

This kind of work is all "time and material". No pressure.

I like it like that!

Steve,

When you have time could you give your opinion as to what particular flaws were in the original work, noting what prevented this nuisance leak from becoming a catastrophic failure (say a burst which would require an emergency response as a tanker driver or yard manager) since you are obviously up close and personal with the "patient" !


Thanks,



Shawn

Shawn,

This is, in part, speculative.

I don't believe there were flaws in the workmanship. The origin may have been as simple as a tiny inclusion in the plate this vessel head was made from. I saw a YouTube video of the manufacturing process. These heads are cold-formed, and in the video I saw, done in a huge CNC hammer mill, though the uniformity I see suggests something more like a huge English Wheel, or perhaps a drop-forging process. Regardless, the new stresses around an existing (if microsopic) flaw provide the starting point.

The MIG (or possibly FCAW; both were used at different times) welds of the "wagon wheel" structure added another distortion component.

The final straw is the purpose of the wagon wheel. The center (hub?) is connected to the inner vessel, and is the only thing that absorbs front to back movement of the inner vessel and it's contents. The cargo at full load weighs about 7400#, and I'd estimate the vessel at 10000#, so the wagon wheel is absorbing the inertia of almost nine tons when the truck accelerates or brakes.

The main reason this doesn't lead to catastophic failure is the extreme over-engineering required to permit rocket fuel on the highway. The load is spread over such an area that it's like trying to break a paper clip by bending it a degree or two, slowly, for thirty years. Even the paint helped protect it. The vacuum loss increased 10-fold after I wire-wheeled the paint off the leak area. Also consider that the space between the inner and outer vessel is under vacuum, so the air we breath is pressing in on this rounded surface at approx. 14.7 PSI. (For a 10" X 10" area, that's amost 3/4 ton mashing the crack shut!)

Steve S

That's wicked. That small amount of metal is probably more expensive than it's weight in gold by now. What would be the full benefit of a backer in such a situation in the event you had to use one? Admittedly, only time I ever used them was for certain open root joints in school but then we washed the backers off with the torch afterwards, leaving the root.

I did end up using a backer, and in this application, it's permanent. By the time I'd removed all signs of cracking, I had an open root slightly more than 3/8" at it's widest. Certainly doable, but at that size, it's big enough to get a thin (14 ga.) backing strip slightly larger than the root in there.

Often, backing bars are required because the insulation (200 layers of mylar/tissue wrap) is so close to the weld zone, it needs protection from direct welding heat. This was not the case on this rear head; The space for piping and the longitudinal support structure means nothing was within the heat zone.

I welded half a SS lock-washer to a 1/2 X 2" strip of 14 ga, tied it with safety wire, and fished it in the hole. Pulled it tight, levered the wire over some 3" angle, and weighted it with a 3" 150# blind. Centered it with needle-nose pliers, tacked it in, then broke off the washer, and ready to weld.

Seen here, if Jody has the pic thingie fixed: Steve S

That's a neat trick. I can see why you enjoy your work.

Creative thinking required.

That's exactly why I enjoy it.

Very cool Steve,

Thanks for the follow ip....it's very interesting as always....and exposes me to thinking creatively to solve problems !


Shawn

One other benefit to the backing bar...

Nitrogen is more than adequate for the back-purge. The flow of metal is no longer influenced by the back-purge gas, and all I need it for is to prevent sugaring.

I'm getting too used to this, or I'd have mentioned it in the first place. It'd have taken a bunch of argon to purge that volume.

Steve S

Hey that really cool Steve I have never heard of using nitrogen as a back up purge. Does it leave a silver backside or just prevent sugaring?

I only use nitrogen for backpurge when I'm either using a backing bar, or welding vent piping. In the vent piping, the back of the root is not quite as shiny as with argon, and the puddle doesn't seem to flow as nicely (less consistent roots).

With a backing bar, I'm just avoiding sugar, which is kind-of like porosity on the back side.

Stainless steel (At least 300 series, not sure about others) can suffer "nitrogen embrittlement", which is a potential problem for very low temperature processes like the inner vessel and piping, but it's non-critical on vent-side piping.

Steve S

Would nitrogen work a exhaust system for purging, I normally use argon. Is nitrogen cheaper?

Yes to both questions.

I don't know what they'll charge for a bottle of compressed nitrogen (we have a 4000 gallon liquid tank at work), but it is far cheaper than argon, and would serve the purpose just fine. If you have a high-pressure bottle of any kind (CO2, 75/25, etc) you're not using, you can dump it and have it filled with N2. They might not even charge much to purge it to N2, though I doubt that's needed for exhaust work. Many folk don't even purge exhaust tube, though I would.

While I'd have to study to see if 404 is suseptible to nitrogen embrittlement (400- series being more common for exhaust tube), it shouldn't be an issue at all for a high-temperature application. A nitrogen pack-purged weld tends to become more brittle when cold, and more closely match the parent properties when warm.

Steve S