DIL and O2 center section connections (Swagelok VCO)

hoppyinca

okay, here is the pic...

you can barely see the weld, it was actaully welded on the face, then re-faced (to ensure a good seal), then pressure tested albeit only to 150psi.

tigWeldVCO.JPG

hoppyinca

Hootis

Dave,

Just checked again to be sure...many (maybe all) of the fitting on all 4 of my MK15's are TIG welded.

I checked on Swagelocks site, and in the application guide. They talk quite a bit about welding, but don't mention silver soldering.

Not interested in splitting hairs, but I was going to rebuild some of the tubes I have apart for cleaning right now, and planned on TIG welding the fittings to the tubes. Why did you recommend against welding? (data?)

There is a lot of information about welding Swagelok fittings, but not much (really anything credible) on silver soldering. Can you provide a link or something?

Respectfully,
Ryan

Dave Sutton

Regarding Swagelok: They do not "gladly" support non compression fittings as they have all of their investment in the compression tube connection industry. Gladly for us, Parker has a line of products that are interchangable with the VCO fittings and you can find 'em here:

http://www.parker.com/tfd/cat/pdffiles/seallok.pdf

Note on the assembly area that they discuss both compression fittings and brazing, but not a word about welding....



To finish with Swagelok: I don't think that Swagelok distinguishes between the various thermal methods in their catalog (welding v/s brazing), and most of the methods used in industrial production is selected by the pipefitter anyway, based on experience and local engineering requirements and not "what's in the catalog". I'm not sure that the catalog is very complete in this area, and it's not a technical manual in any case. Swagelok will provide engineering services if required and leaves a lot unsaid in the catalog, and in any event what they sell now as face-seal components tends to be a LOT larger in physical size than the small diameter tubing of a Mark-15. I'm on the boat right now and don't have a catalog handy, but I'll be back next week and will look and see what I can show you there.


In respect to the Mark-15 in particular, I cannot ever remember seeing any TIG welded gas line ends, and I'd be *really* interested in a detailed photo of any VCO fitting that you suspect is welded. I could be way off base here, but... heck, let's all learn together. My bet is that they are all brazed. I am certain that I've never seen one welded. And, not particular to the Mark-15, but I personally silver-brazed 100's (probably 1000's) of stainless fittings of all sizes to stainless steel tubing from 1/8 to 1 1/2 inches in diameter when we built the Viking Piper saturation system, and we would have tossed anyone who suggested welding into the looney bin.... it's just not a practical or desirable method.


Grabbed this off of the net:

"Silver brazing requires high heat of around 1350° F and is very strong, It is fairly corrosion resistant, but will tarnish with age.
Silver brazing is sometimes called silver “soldering”, but that is bit of a misnomer, because soldering is a low temperature process. It is even more confusing, because another type of silver soldering involves the use of low-melting point Silver/Tin alloy. So let’s call silver brazing by its more descriptive name. Silver brazing is a high temperature process and somewhat time-consuming because of the time required to heat up a part. The joint area is well-fluxed with a borax paste that is able to clean oxides off the metals when it is molten while resisting boiling off in the flame for a short while. When the materials are up to almost dull-red, silver alloy wire is fed into the joint area. Over-heating can cause oxidation, which will keep the metals from being wetted by the solder and flux.

Small parts are easier to braze than large ones because of the heating time or total heat needed. A good deal of cleanup after brazing can be needed, including acid treatment of oxides and/or buffing of the assembly. Parts which are in a hard condition will be annealed by the process. Silver brazed brass parts can become fully annealed or distorted by the process. A very controlled joint can be the result of silver brazing, and its strength is certainly a factor in its favor".

"It is not advisable to use 303 stainless or hardenable alloys when TIG welding. The very characteristic of 303 stainless which makes the chip break up while machining, makes it crack when it is welded. Silver brazing can be a better way to join 303 stainless parts."


Silver soldering (properly "Silver Brazing") is a very well known method of connecting stainless steel tubing to end components, and it is considered to be superior for many applications as the heat applies is insufficient to melt the actual *components* to be connected. Bottom line: You cannot weld tubing without the potential of destroying it. There is a VERY high possibility of blowing thru the tube, and every piece needs to be inspected by X-Ray or other NDT in order to determine if it's a good part. The technique that was used above, of face-welding and then refinishing the face is far too time consuming for production quantities of anything. Silver soldering avoids all of these issues, as the heat used is high enough to melt the solder, but is not high enough to do any damage to either the tube diameter or the end fitting.

The desired goal of the job is to produce a gas-tight seal that is strong enough to hold the tension applied when the fitting is pressurized. Both of these are easy to do with silver solder. Note that the solder type is critical: Soft lead-based solders will NOT work and silver solder is REALLY expensive to buy. It takes a high heat to melt (requires oxy-gas, not gas alone) and is very strong mechanically.

Method is easy, most fittings designed for soldering have a step inside the bore that is designed to accept the tubing. Scrub it out with a small brush, flux it well with a flux designed for silver soldering, and clamp it in a vise. Emery paper the end of the tubing and flux it as well. I use powder flux, meaning that I heat the end of the tubing with a torch and then dip the end of the tube into the powder, which melts and then re-cools, leaving a crust of flux. Paste flux works just fine too. Insert the tube into the fitting and make sure it bottoms out. Sometimes you may need to gently heat the assembly to melt the flux so the tube can bottom out completely in the fitting. Heat with a torch, and you can be very aggressive with the heat. It needs to be HOT!. When it's heated fully, apply the end of the solder rod to the joint, and as the solder melts, it will be pulled into the joint by capillary action and will form a perfect radius of extra solder at the joint where the tube enters the fitting. This is NOT done while the torch is being applied: You are either adding HEAT or adding SOLDER but not both, as you do not want the velocity of the gas from the torch to blow away the solder. You can "maintain heat" by applying the torch to a section of tube remote to the actual brazing, but if it's hot enough to begin with, it's probably not needed on a small tube.

Common errors: Not fluxing correctly, and not getting it hot enough. If you get it right, which is easy to do, it'll be stronger that the tubing itself.

This can be learned in about 10 minutes, and if I showed you how, you could do it the first time. Welding it? Good luck... even if you are an expert welder.


I'll drag out a set of Mark-15 pipes when I get back and will have a detailed look again... but dollars to donuts the center section face seal components are silver soldered to the tubes. There *is* TIG welding on the 50cc accumulator.... different application.




Dave