Machinist

[Lawrence London <lfljvenaura@gmail.com>]

http://www.practicalmachinist.com/vb/general/ok-weld-black-iron-pipe-air-lines-259242/

here's a few comments from the original thread, "
Thread: OK to weld on black iron pipe air lines?":
[some great information here that you might be able to use]

Q:
"OK to weld on black iron pipe air lines?

Shop air lines, under 200 psi. I'm working on a new install and rather than use clumsy space consuming plumbers' bracketry to support valves and etc, I'd like to attach fabricated steel tabs and brackets to fittings to mount them. The easiest and cleanest way is to just TIG weld them on to either the pipe or the fittings. The material welds nicely. Is this safe considering the material and the pressure involved? It's been done for years, suddenly I'm wondering about it."

"As said you can weld on the pipe however... Don't do 1/4 inch tacks. The material like any steel can form hard and brittle martensite at the spot where the tack chilled down. Do at least a one inch long bead. Size of fillet is not important but length ensures that the area cools relatively slowly. TIG is ideal since it is slow and chances of martensite formation are nil. "

"Welding on the pipe itself isn't a problem. The only difference between plain end pipe for welding and threaded pipe is the thread on the ends. Same material otherwise.
Assuming you're using screwed fittings, I'd avoid welding on any common ells, tees, or crosses. They'll normally be malleable iron rather than steel. Welding on couplings should be okay since the majority of them are now steel rather than malleable iron. The MI ones are easy to identify by a raised band around each end whereas a steel coupling just looks like a piece of pipe with an internal thread. You could pick up a few forged tees and/or ells for use in places where you want to weld on these fittings. They're substantially more expensive as they're forged steel and rated for use at 2000 or 3000 psi, so you probably wouldnt want to spend the money to use them everywhere, but its okay to mix them in with malleable fittings as necessary. I'd also recommend sticking with domestic pipe from a known mill like Wheatland Tube, Sharon Tube, or similar known brands. The foreign stuff will weld just fine, but a lot of it won't thread worth a damn. Generally very little difference in the selling price of import vs domestic, but its easy to sink a bunch of extra labor into chasing leaks due to poor threads on the import material. Also, if you buy your pipe from a steel supplier rather than a pipe, valve, and fitting supply house, make sure they have A53 pipe. A lot of them will stock A500, which is structural pipe. Structural pipe is stronger and has a nicer finish on its OD (looks a lot like DOM), but it doesn't thread as well as A53 when using normal pipe threading tools."

"Since this is a "Pressure Retaining Vessel", you should follow Section IX of the ASME Boiler & Pressure Vessel Code. You should go out and hire a welding engineer to qualify the procedure, the welder and issue a stamp. Then hire a quality engineer to check the incoming material, the welding documentation, welder prolongation, weld filler material, and the X-rays. You should also arrange for an independent 3rd party inspector to review you process. We use Hartford Steam Boiler. You should also get a PMI gun to verify base and filler material. I would get a Niton for about $20K
You should probably look at TWI software for all of this. This is great stuff and will help track all of you WPSs, PQRs, WPQs, and NDE reports.
I can't think of anything else at the moment. You might want to hire and executive assistant to help you will all of the above."

"Don't forget to re-hydro the pipe since you've welded on it! 150% of operating pressure for 8 hrs. "

"To go along with JR's tongue-in-cheek post, it is worth noting that ALL the important lines in a refinery are welded (as opposed to threaded), but then they know exactly what they are doing - like JR's post illustrates. (be sure you get your hot work permit first )"

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"I am an old time mechanical engtineer, in practice over 40 years. I've designed a lot of piping in and around powerplants and other jobs. I had my best teachers in the form of some fine pipefitters, and learned from seeing other people's good designs as well as their mistakes. I've run plenty of piping on my own as an erecting engineer overseas, and on other small jobs. Here is some quick insight:

"Black Pipe" is known as "carbon steel pipe". Welding of carbon steel piping is more common than not in powerplant work. As a point of fact, in powerplant work, and extrapolation from the "Power Piping Code" , typically, any carbon steel piping 2" and under in a powerplant is run using schedule 80 (extra strong) pipe, and any piping over 2" can be schedule 40 (standard weight).

The reason for going with schedule 80 on the 2"-and-under piping is multiple:
-the wall thickness of 2" and under pipe is not all that thick to begin with. If this pipe were threaded, the roots of the threads reduce the
wall thickness considerably.
-pipe threads are a sharp vee form, so are inherently stress risers, or potential failure points
-vibration and thermal stress are actually more likely to produce a failure in screwed standard wall "small" piping than the actual internal
pressure.
-the thicker wall allows for wastage due to corrosion over time, something condensate in air or steam lines will produce.

Welded carbon steel piping is run using two principal types of fittings:
-forged steel, socket welded
-butt weld fittings using an open root weld or "chill ring"

Socket welded fittings require the pipe be seated in the counterbore of the fitting and pulled back 1/16"-1/8" before welding. This allows for thermal expansion of the pipe during welding and prevents cracking at the weld. Socket welded fittings use drilled passages, and as a result, produce turbulence and friction losses. In addition, they tend to "pocket" or "harbor" dirt, tramp oil or condensate in the "pullback" space between the pipe end and the shoulder of the counterbore in the fitting.

Socket welded fittings are forged steel, normally rated for 3000 psig fluids and gases. Socket welded fittings get a fillet weld. Mostly, this is run using TIG, but it is also acceptable to use SMAW (stick) with E 7018 or E 6010.

Butt welded fittings for small bore pipe make a nicer job on lower pressure, which is what shop compressed air would be. For best results and a clean bore, butt welded fittings for small bore pipe (2" and under) should be welded using TIG (GTAW) with an open root and a back purge. Butt welded fittings should also be less costly than socket welded fittings. The difference is in the weld and fitup.

Bending of piping is done quite often on power and process piping. As long as the pipe is not flattened or kinked in the bends, and as long as no cracking occurs, cold bending is acceptable.

If you want to attach pipe hangers or supports by welding tabs or lugs to the pipe, consider the following:
-the welded attachment point produces high localized stresses in the pipe wall, particularly on a compressed air system where there may be
vibration and thermal stresses.
-standard weight pipe has a relatively thin wall to start with. Any undercut in the weld and you have created not only a thinner wall, but a
bad stress riser in an area where there is a high stress concentration.
-Welding a lug or tab to the piping to support it creates what is known as a "restraint" or anchor. This may or may not work in your favor,
depending on location on the piping and length of pipe between such lugs or tabs. The pipe needs to be able able to move lengthwise for
expansion. Too rigid restraints in close proximity to cast (or molded plastic) air line parts (lubricators, regulators, filter/separators, etc) can
throw a bad strain into those components and result in cracking them.

If you do go with welding support lugs or tabs to the piping, be careful to avoid putting a strain into any system components. My own recommendation on welding the lugs or tabs to any piping 2" and under is to use Schedule 80 pipe. In that way, you have the wall thickness
working in your favor and you greatly reduce the stress concentration and chance of in-service cracking over time.

Compressed air lines will sometimes pick up and transmit vibration or pulsation. A good way to support compressed air lines and allow for expansion as well as to dampen any pulsation or vibration, and reduce the chances of throwing a strain into air system components is to use a cushioned U bolt or a split block type of pipe mounting. The split blocks are molded resin, and make a neat job. they are made by Stauff, and I've designed them into many small bore pipe and tubing jobs. The cushioned U bolts are made by a variety of firms, and "Cush Clamp" is one of them. If you fabricate neat pipe support brackets using angle and flat bar to anchor to the walls, radius'ing the exposed corners and using either the split clamp blocks or cushioned U bolts, you will have a good job that is less prone to pipe vibration and strain.

If you do go with welded pipe joints, remember to put in unions to allow the components to be maintained/replaced. I generally spec 300 lb screwed black malleable iron fittings for black carbon steel pipe. Do NOT use the imported (Chinese, Indonesia) black iron fittings. They are unpredictable in quality and are often s--t. Bad threads. Go the extra bucks and get Ward, US made black malleable iron screwed fittings if you need screwed fittings.
Beware of any old line US valve or fitting manufacturer (such as "Anvil") who uses the word "International" in any item they list. I got stuck on one job with "Anvil International" screwed black malleable fittings. I thought I was getting Anvil, US made. I got Anvil Internation and the fittings came from Indonesia and China and were S--T. We'd get joints made up, properly cut threads on the piping, use a good pipe dope, and despite our best efforts, including "taking up" on the joints with plenty of beef on the wrenches and "remaking the joints" with fresh and different pipe dope, we'd get leaks. Compressed air will find leaks that water would not pass thru.

300 lb black malleable iron fittings are rated for 300 psig saturated steam, so are good for a lot higher cold fluid or compressed air pressure. They will take thermal stress, vibration, and the occasional person who has to climb on the piping.

We use a "Hossfeld" bender at the powerplant for bending small bore piping with the right dies. It works quite well. I'd shy away from using a rigid conduit bender as I think there may be some slight flattening of the pipe in the "throat" of the bends.

Here's a few rules for running compressed air piping, which a lot of people who should know better tend to overlook:
-always make the runs as straight as possible.
-pitch horizontal runs about 1/16" -1/8" to the foot downhill in the direction of flow for condensate drainage
-provide "drip legs" at the downhill end of horizontal runs, with a blowdown valve. An automatic condensate drainer can be added
(timer + solenoid valve, or some variety of float trap)
-make connections into and off of horizontal runs or air piping off the top of the line (12:00) or off the side (3:00 or 9:00) to avoid condensate collecting in the branches.
-make sure to ream and chamfer the ends of any pipe used for screwed or socket welded joints
-avoid screwed close nipples wherever possible. Schedule 40 screwed close nipples are an in-service failure waiting to happen. There is little or no
pipe left that is its original diameter/wall thickness. It is one continuous stress riser. If you must use a screwed close nipple, use a schedule 80
nipple.
-for first connections off air receivers, particularly to condensate drains, I use schedule 80 nipples. Less chance of a vibration failure, less chance
of condensate rusting through the nipple, more "meat" to put a wrench on to remove the nipple for maintenance or replacement after condensate
has laid it it for years of service.
-If you use ball valves, try to get what is known as a "full port" ball valve. Many ball valves have a reduced bore thru the ball. On compressed air in particular, this can create turbulence and friction losses, reducing pressure and flow downstream of the valve.
-If you need to throttle, or control flow of air, use a globe valve with a "regrinding seat" or a needle valve. Ball valves, while handy and common, are not meant for controlling flow or pressure.

It is Ok to "mix n' match" pipe joining and types of fittings ( i.e.- socket or butt welded and screwed). Do not play "mix n match" with pipe materials. IMO, stick with black carbon steel piping and black MI or forged fittings. Bronze or brass valves are OK as well. Do not throw in screwed galvanized fittings or galvanized nipples and pipe. I call this "Hardware Store" pipe fitting.

I prefer to use a "flex" line between the compressor and downstream piping on larger compressors with remote receiver tanks. I also use a flex line between the receiver tank and rigid system piping. The flex line is made of corrugated bronze or stainless steel, sheathed with a braided wire jacketing. When installing a flex line, care must be taken to get the flex line installed so it is straight and in its neutral (unstressed/free) length.
The flex line allows for thermal expansion and isolate vibration from being transmitted thru the piping. A flex line is NOT a cure for mis-aligned or badly
run pipe.'

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x2 on not welding hangers and brackets

long way to go for little return

minnies and beam clamps (you won't even need the later) support millions and millions of feet of pipe in north America, done right its looks good

and works