Butt Welded or Socket Welded? 2

[StoneCold]

What applications would you use socket welded fittings on 2" pipe or less, instead of butt welded fittings? I resently finished a small project using 1.5" butt weld fittings and it took a significant number of welding hours. TIG was the welding process used to avoid slag inside the pipe. I probably could have went with stick welded socket fittings since the pipe just carries heat transfer oil. I have been reluctant to use this method due to corrosion in the socket. What makes you choose one or the other?

Regards
StoneCold

 

[unclesyd]

We have a very large thermal oil heating system where all piping under 4" is socket weld fittings. 4" and above is butt welded. Cost is and was the primary reason for this application. The RTJ system is a better sealing and fire resistant system.
We had a verifiable savings of 45% over a full butt welded system measured on two new units.

A little information on the system.
We have both vaporized fluid and condensate. 300?C @ 70 psig and FV. 90% of the heating is with vapor and 10% with liquid. The vapor systems are pressure controlled with the condensate at the boiling point. Essentially we only use latent heat.

In about 1/2 the system all pipe 1" and under has table bends.
On the latest systems (4) we started with 100% radiography and backed down as we weeded out welders that couldn't make the prescribed socket or butt welds.
The system is all RTJ and is tested with vacuum and He prior to commissioning and after every turnaround. I have run pressure decay with He and N with good results.
We require a discernable gap in all socket weld fittings and a 1/2 pipe dia. engagement, not absolute.
There only been minor leaks on some of the older systems due to slag when there was only visual inspection.

 

[JStephen]

A lot of times, you'll use little fittings, 2" and smaller, for fairly low pressures, maybe 10-50 PSI. For small fittings at pressures like that, the required thickness of the pipe (or attachment weld) is just next to nothing. Since you don't gain anything structurally by using a butt weld, you use whichever is quickest, and thus the socket weld.

 

[JohnBreen]

Hello StoneCold,

Something to think about:

"Remember, by far the most common mode of failure that we see in the field with socket welds is fatigue at the toe of the fillet welds. If this is a system that would be expected to see vibration or if it is B31.3 “cyclic service” (also see B31.3 comments on severe cyclic conditions), don’t do it. Specify a weld like the illustration in B31.1 Figure 127.4.4(A)(d). Specify the examination of all finished fillet welds with repair for any undercut. Make sure that the welder provides the 1/16-inch gap shown in B31.1 Figure 127.4.4(B)(c)."

The B31 Piping Codes prescribe a Stress Intensification factor of 1.3 for a fillet weld (at a socket weld fitting)with a smooth transition of the weld leg parallel with the pipe axis (achieved by grinding and finishing the weld). In other words, a very gradual concave weld surface should be achieved to minimize the stress intensification at the socket connection fillet weld.

The prescribed SIF for an unfinished fillet weld at a socket weld fitting is 2.1. That is a big difference.

This difference is because the unfinished fillet weld is prone to undercut and because the fillet weld geometry is prone to fatigue failure. If you are going to use socket weld piping fabrication you must include an NDE plant that will find any undercut and have it repaired (ground-out and "blended") Otherwise be prepared for cracking at the joints.

Regards, John.

 

[StoneCold]

Unclesyd, JStephen
Thanks for your response!

What does RTJ stand for?

Thanks

StoneCold

 

[unclesyd]

RTJ Ring Joint Flanges
All flanges in Heating Fluid service are Ring Joint.

 

[StoneCold]

JohnBreen
Thanks for the reply.
What does SIF stand for?

Thanks

StoneCold

 

[moltenmetal]

When do you NOT want to use socket-welded fittings?

- in severely vibrational services as others have said

- in severely corrosive or high purity services, because all socket-welded fittings contain a deep crevice ideal for corrosion

- for high toxicity services where a radiographic inspection of each weld's integrity is required

- where the flexibility analysis tells you that they're no good because of the stress intensification issues indicated above (a rarety if the piping is otherwise designed and supported correctly)

- in services with expensive materials of construction (i.e. titanium etc.), because the extra material required in the SW versus BW fittings will often eat any savings in welding cost (combined with the corrosion issue above!)

- any time your welders are not familliar enough with them to realize that they must be fit with a shrinkage gap- it's the fittings which have been bottomed in the socket before welding that are most prone to failure

- beyond about 3" pipesize, regardless

When do you WANT to use socket-welded fittings? Virtually in all other circumstances where threaded connections have been ruled out. Socket-welded piping is quicker and easier to both fit AND weld and hence is far less expensive than the butt-welded alternative. Savings of 50%+ in installed cost can be expected in practice relative to butt welding.

Clearly your results may vary, but that's my take on the issue. Be careful about axiomatic statements, whether from me or others, that you might hear on this site! Case in point: unclesyd's "RTJ flanges are used on all heating fluid services". Although I bow at unclesyd's feet as he is an extremely knowledgeable engineer and I listen carefully to what he says, there are plenty of places where ordinary raised-face gasketed ANSI flanges are perfectly fine for use with heating fluids. Even 150# flanges, though I know there are people and companies which just say no to them for hot oil...

 

[unclesyd]

moltenmetal[/],
I should have stated our company’s piping standards call for RTJ's for all heating oil piping either liquid or vapor. We never used Class 150 flanges in oil or steam. On other systems, CS and SS, we very careful with Class 150 especially the 3" and 8" sizes.

All thermal heating fluid manufacturers will tell you that there is no problem using raised faced flanges in there fluid service. The only problem is that experienced users can argue the point. Our system is one of the larger vaporizing heating old systems and was inherited from a legacy system and is now has a 50 year service record with no major flange problems. We have had many an internal arguments to point of pencil breaking and hat throwing, concerning the merits and cost of each type piping configuration. It got to the point that central management was going to force us to accept what our division called substandard system, mainly RF flanges. We were saved by a phone call I received from a colleague at Solar One in California concerning all the problems they were having with leaks and fires on their thermal heating fluid circulating system with RF Flanges. Saved the day for our site.
We have found that RTJ’s under our service conditions are the safest, easiest to install, most economical to use.

Solar Ones’s problem with their RF Flange system was savaged with the following.

http://www.selcoseal.com/index.htm

Even though it has been stated many times the design and implementation of a bolted connection is an extremely important function of the engineer. It is a lot more than merely the type of flange and gasket. I’ve always taken the approach that a flange or gasket failure is unacceptable under any circumstances.

You can rest assured that your design will get tested and it want be the hydro tester that puts it to the max.

 

[mike372]

I am working on an experimental project for a university that requires a weld test procedure for .093 wall in 3/4 ss pipe-butt weld. what gap/bevel angle/tig rod dia. should be used ??

 

[ahmad7114]

StoneCold,
SIF stands for stress intesification factor jost like factor of safety, it is always used when branch connection is involved, this takes into consideration compansation for the areas of stress concentration.

 

[KernOily]

A lot of times it's a constructiblity issue and an issue with the QA of your contractor and construction process. As moltenmetal alluded to, the fillet weld of a SW fitting can't be radiographed so you are counting on one of the other methods for your NDT, e.g. mag particle or dye-pen, to name two.

Do you not have a standardized pipe specification/line class specification for this particular fluid service? If you call up your local friendly engineering contractor, he can probably provide you a typical, tried-and-true, user-friendly line class spec for heat transfer oil.

mike372 - thread hijacking is not allowed. You will now receive 40 lashes with a Lincoln SA-200 ground lead. Suggest you go back and post your question as a new question in the forum.

Thanks!
Pete