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tbedford (Mechanical) (OP)
2 May 07 19:58
Hello everyone,

There is a significant difference between the two welds regarding cost and inspection. Does anyone know where I can quickly source requirements for capping a 4 inch steam line at 600 psi? Am I required to use one weld in favour of the other and what is maximum pipe size requirement?

Thanks,Tom
metengr (Materials)
2 May 07 21:56

Quote:

There is a significant difference between the two welds regarding cost and inspection

Yes. Butt welds require more effort and welder skill to complete. Socket welds by design are simply fillet welds.

Regarding inspection - butt welds in most service conditions are inspected by surface and volumetric methods, again more cost.

Regarding design use, butt welds are treated differently in comparison to socket welds for specific service applications.

Quote:

Does anyone know where I can quickly source requirements for capping a 4 inch steam line at 600 psi? Am I required to use one weld in favour of the other and what is maximum pipe size requirement?

Review ASME B31.1 Power Piping Code.
imctmga (Petroleum)
3 May 07 3:54
Hi metengr,

Yes, there are some difference between butt and socket.

There more cost in butt weld than in socket, usually the welder is more expensive for butt weld and the inspection too.

The END are more demanding for butt than socket, for example you may need to do a 10% of RX in buttweld and a 2% in socketweld. It depend of specific service, the product and the material of the pipe, in some cases you must check all the welds, but in this case is cheaper socketwelds.

I don´t be sure, but usually in refineries you can use socket welds for pipes since 3 inch, for diameters aboves you need butt welds. Of course it depend the specific service.


This is only my opinion.
Helpful Member!  Montemayor (Chemical)
3 May 07 7:35

I would never employ a socket weld on a 600 psig steam service weld cap - or any other type of fitting.  This is just a fillet weld - something a high school welding school can easily handle.  I consider this type of weld merely one of aesthetics and never meant to lend mechanical strength to a joint.

For strength and integrity you have little choice but to opt for a 100% penetration, butt weld.  For this type of service (where the end cap may be subjected to steam or condensate hammer) I seriously recommend a butt weld.

And I haven't even gotten to the part where you suffer crevice corrosion within the socket joint - especially under certain steam services.
JohnBreen (Mechanical)
3 May 07 7:45
Hi Tom,

It is still true - "you get what you pay for".  The (expen$ive) well executed girth butt weld is stronger than a fillet weld.  For 600 psig steam I would always use a girth butt weld.  Generally, this would especially be true where there is any bending moment as the girth butt weld will give you better fatigue life.

ANSI B16.11 Socket Wekding Fittings are made up to NPS 4 size so you are looking at the largest of these.  If you use socket welding fittings make sure that the design/analysis includes the larger Stress Intensification Factor that the Code recommends for the fillet welds where the pipe meets the B16.11 components (reference B31.1, Table D-1, note 11). Remember, by far the most common mode of failure that we see in the field with socket weld fittings is fatigue - crack propagation that begins with a crack at the toe of the fillet welds (usually at a fillet weld undercut stress riser). If this is a system that would be expected to see vibration or if it is B31.3 “cyclic service” (also see B31.3 severe cyclic conditions), don’t do it. Specify a weld profile 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).

Ed Wais developed a report for Epri that you might find interesting - go here and download:

http://www.epriweb.com/public/000000000001006867.pdf
(Stress Indices for Circumferential Fillet Welded and Socket Welded Joints)

Regards, John.
davefitz (Mechanical)
3 May 07 8:45
I agree with most of the above reservations on socket welds, if there was being attached a piping segment. In this specific instance, if only a cap is added, I do not think there is going to be significant bending moments applied to the fillet weld , so the opening of the pre-existing crack might not be a significant issue. Thermal stresses can cause internal bending moments, so if it is a heavy schedule pipe, then a full pen weld / butt weld would be preferred.

For smaller dia pipes, it is difficult to align the pipes  for a butt weld, so for smaller pipes, the socket weld is usually preferred.
JLSeagull (Electrical)
4 May 07 7:50
Tom gets a star for the question.  The responses suggest that I too could stand to learn more about the mechanical integrity aspects associated between the welded fittings.
BronYrAur (Mechanical)
4 May 07 12:39
I have seen socket welds on hydraulic lines with 6,000# fittings.  Pressure was probably 3,000 psi and piping was 1-1/2" or smaller.  Would butt welds have been betetr for this system?   
Casimo5 (Chemical)
4 May 07 14:01
In my plant, we don't use socket weld fittings for anything above 1-1/2".  The welders like it because it is faster to put together, but as stated above, butt welds are better.
metengr (Materials)
4 May 07 14:03
BronYrAur;
The design concept of a butt weld is that you want to achieve 100% fusion in the weld joint to assure no reduction or penalty in load carrying capacity in service. Fillet welds by design are subjected to significant reduction factors for shear and tension.

Yes, you can account for socket welds in piping design. However, my experience has been that if you have unaccounted for thermal/mechanical bending stresses, the socket welds may be the first to develop problems in cycling related service. So, for components subjected to cyclic service stresses the fillet weld just doesn't exhibit the fatigue strength endurance as a full penetration butt weld. Whereas, under static load conditions or few cycles, socket welds may be appropriate.

It really comes down to design philosophy and past experience.
hydrae (Mechanical)
5 May 07 12:46
One more caution on the socket weld, ensure the pipe is not homed into the fitting, even minor thermal expansion can cause the pipe to grow faster than the fitting, and if the pipe is bottom out it will put an unexpected stress on the weld causing failure.
Hydrae
ccfowler (Mechanical)
5 May 07 23:11
Montemayor has stated the most significant issues.  Crevice corrosion is a major problem in steam service, and the results can be most unpleasant.
Helpful Member!  tbedford (Mechanical) (OP)
6 May 07 9:25

Thanks for all the great info and your time.
Tom

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