Stress Intensification Factor for Socket Welds 4

[bulkhandling]

For the stress analysis of small piping, a 2.1 (some people like to use 2.0) SIF for socket weld is normally used. I know Code B31.1 or B31.3 allow us to use a smaller SIF=2.1*T/Cx (Where T is the pipe wall thickness and Cx is the heightof welds). So when the pipe stress is high at SIF=2.1, I tried to use the formula to calculate a smaller SIF (min. 1.3). When a SIF<2.0 used, I always require welder to control the weld thickness Cx to the height of the socket OD.
But a well-experienced stress analyst suggests me never use SIF<2.0. He said the site welder will often ignore the requirement for a bigger weld thickness (bigger than 1.09T as required by the CODE). He also believes the bigger weld height is often more expensive than using a thicker pipe wall or expansion loop for reducing the stress to within allowable.
Shall I follow his advice? What is your experience about it?

 

[StressGuy]

I would agree with the advice of your analyst friend. Stick with the 2.1 for socket welds. Since you're typically looking at this on small bore lines, it's a lot easier to get more flexibility and reduce your displacement stresses.

We must, as engineers, ultimately offer designs that are practical. Trying to specify a weld detail on small bore piping in the hopes that it will be followed by some guy in the field does not fall into the realm of practical, in my view.



Edward L. Klein
Pipe Stress Engineer
Houston, Texas

All opinions expressed here are my own and not my company's.

 

[bulkhandling]

Thank you, StressGuy,
But when I talked with another stress person, a specialist on thermo-piping, he just say &quot;Use it. Why not?&quot; Because the formula for the smaller SIF appeared only on the later versions of ASME codes (after 1990?), he believes that the stress engineers with longer experience would not be interested in it (overly conservative?). Code allows us. Why not?
Why?
Mike

 

[MJCronin]

bulk,

These are interesting questions and thoughts.....

My experience has been that using the various editions of the ASME code for stress analysis leads one to simply use the higher value and make final adjustments to the system design ( additional supports/piping loops)as necessary.

I suggest that you post this on

http://www.coade.com

and report back on your results

MJC

&quot;There comes a time in the affairs of man when he must take the bull by the tail and face the situation.&quot; W.C. Fields

 

[bulkhandling]

MJCronin,
I visited the COADE forum but think this website is more suitable for this topic.
But It seems there are not many stress engineers visiting this thread group...

For myself, I try my best to use SIF=2.1 for all the socket welds. In the case when no more room for extra loop or if a reasonable smaller SIF can prevent the requirement for higher end material (like jump from A106B to P22 or from P22 to P91), I do use a smaller SIF. As I mentioned, when a SIF<2.0 used, I always require a thicker welds.

I really hope to know how the other stress engineers do when deal with the issue.

 

[bvi]

If the fillet weld size is what, in my opinion is appropriate (e.g. something like what the fabrication rules of ASME B31.1 require), I believe, and there are some technical papers based on EPRI work that support it (by Ed Wais), a factor of 1.3 is most appropriate. However, B31.3 permits a very small fillet socket weld, based on the pressure design thickness of the pipe rather than the nominal pipe wall thickness. The requirement is rather problematic as it bases the rules for the fabricator for weld size on the pressure design thickness, which the fabricator does not necessarily know. This may change in the future, but as long as that requirement is in place, I would use the more conservative SIF value, or, preferably, in my opinion, specify a larger fillet weld size for the entire project as part of the piping specifications (e.g. based on B31.1 size) and use a lower SIF for the analysis of the entire project. The present B31.3 fabrication rules would let you use a 1/8 inch fillet even with XXS pipe, which I don't think is appropriate.

 

[bulkhandling]

cb4,
FIG. 127.4.4(C) in B31.1 gives the requirement for socket welding. It requires the minimum weld thickness Cx be the e 1.09 times of wall thickness or just pipe wall thickness, whichever is smaller. This requirement does not allow a SIF<2.0, according to the formula SIF=2.1*T/Cx.
I think B31.3 has a similar figure (FIG 328.5.2C--??)for socket welds but I do have the code beside.
Regards,

 

[bvi]

ASME B31.1 requires that the fillet weld be 1.09 times the NOMINAL PIPE THICKNESS. The SIF for the fillet weld provided in appendix D, for that weld, is 1.3 as long as the toe of the welds blends smoothy with no undercut in the pipe wall. So, per B31.1, a 1.3 SIF is appropriate for a weld of that size.

ASME B31.3 requires that the fillet weld be 1.25 times the PRESSURE DESIGN THICKNESS, which is typically on the order of hundredths of an inch for socket weld piping, or 1/8 inch, whichever is greater. The SIF provided in Appendix D of B31.3 is the equation you have cited. So, for B31.3, you could use a heavier weld and a lower SIF, or use the B31.1 SIF and weld size, suppported by the papers by Wais, and in consideration of para. 300c3 of the Code, and use 1.3.

 

[bvi]

A minor addition, you don't need to invoke para. 300c3. The appendix D SIF's in B31.3 are for use in the absence of more applicable data. You could go to the technical papers of Wais for &quot;more applicable data.&quot;

 

[bulkhandling]

Thank you CD4 for your helpful information. But do you mind sending me some of the technical papers of Wais?
minimoon06@yahoo.com
Regards,

 

[bvi]

Investigation of Stress Intensification Factors for Circumferential Fillet Welds; Wais, Jenkins and Rodabaugh, published in ASME PVP-Vol. 430, Pressure Vessel and Piping Design and Anaysis-2001.

 

[JohnBreen]

Hey Y'all,

One other thought about the fillet welds on the socket weld fittings: 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 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). Undercut is your enemy!

Regards, John

 

[bulkhandling]

Thank you John for your helpful information.

I received an email from Ed Wais, and paste it here for your reference if anyone is interested. And I'd thank Mr. Wais here.

*******************************************************
I happened to notice your questions on SIFs for socket welded fittings when I was looking at &quot;eng-tips&quot;. I am not a member so I could not respond directly. I noticed that someone had given you a reference on the subject. Actually a better reference is &quot;Evaluation of Stress Intensification Factors for Circumferential Fillet Welded or Socket Welded Joints&quot;, by Edward A. Wais, E.C. Rodabaugh and Robert Carter, PVP-Vol. 383, Pressure Vessel and Piping Codes and Standards, ASME 1999. This reference discusses the results of 183 tests.

The reference you were given focuses on undersized welds.

Ed Wais
*****************************************************

 

[llpcode]

Be careful, not all codes use 1.09 tn. ANSI B31.3 uses a higher figure 1-1/4 T for this dimension.