Distance of Piping weld to weld as per ASME B 31.3

Date 11-26-2007 05:27

Minimum distance between girth welds is usually covered in the fabrication or installation specifications and is not a code requirement. If you want to add a weld not shown on the isometric drawing, you usually have to get the approval of the piping designer since it may affect the piping stress analysis.

By G.S.Crisi (****)

Date 11-26-2007 20:41

Marty is right. There's no Code requirement for the minimum distance between welds.
A figure "pulled off the air" that is very used in prefabricating shops and jobsites is that the minimum distance should be three times the pipe diameter. Now, as I said, this figure has been "pulled off the air", i.e., there's no sound technical reason to support it.
As Marty correctly says, the piping specifications issued by engineering firms and highly reputable industrial companies (Exxon, Dupont, Dow Chemical etc. etc.) use to state what the minimum distance should be, BUT with a warning: if the distance can not be maintained, then it can be shortened with the client's approval. And the client almost always approve, for there's no other chance to do the job.
Giovanni S. Crisi
Sao Paulo - Brazil

By 803056

Date 11-26-2007 20:48

Nicely stated.

Al

By Flash Date 11-30-2007 11:08

Interesting
Australian Standard 4041 Pressure pipe uses 4 x pipe thickness or 40mm whatever is greater betweeen the toes of the welds
this is to ensure you do not get an overlap of HAZ, and/or create a stress raiser
R
Flash
www.technoweld.com.au

By G.S.Crisi (****)

Date 11-30-2007 13:22

Fine! This is the first time I see the minimum distance between pipe welds written in a Standard.
Aussies should spray their standard all over the world (articles in technical magazines, papers in technical conferences etc.) to put an end to endless (and sometimes angry) arguments at prefabrication shops and jobsites.
Giovanni S. Crisi

By js55

Date 11-30-2007 15:07

Giovanni,
If I understand the last two posts correctly (and I'll combine my critique between yours and Flash) I would have to disagree.
Its one thing to have some metallurgical logic as to why something is done, like overlapping HAZ's, stress risers etc, but unless there is some hard data, and the chance of failures, there is no point in imposing code limitations (and I'm not saying there isn't hard data, necessarily, but it seems ASME hasn't seen it).
You have overlapping HAZ's in multipass welds(in fact, overlapping high temp HAZ's-not the overlapping low temp type that is most likely to occur from near proximity welds). I fail to see how overlapping HAZ's from near proximity welds is any different. Logically if you're limiting due to overlapping HAZ's then you would have to limit multipass welds as well. In fact, its even worse, since base metals most often can accomodate HAZ variances better that the overlapping HAZ's in weld metals. As for stress risers the very existence of a weld is a stress riser.
Its not a quesiton of whether or not there is stress risers. There's always stress risers. Slag is a stress riser and its allowed to some extent. So is porosity. So is undercut. Non metallic inclusions, etc. Its a question of, is there verfiable or research justifiable concern for failure based upon that stress riser. Such as undercut for cyclic regimes, or extreme thickness transitions in block forged fittings. These you will find in ASME.
Also, what I've found in my experience is that it is Engineering firms that impose the arbitrary, and I emphasize arbitrary, 3X/4X stuff, and it is the very same engineering firms that miss the fact that they've designed a piping system that won't allow the fitting take offs and 3X/4X dimensions at the same time.
There are a lot of welds being made close to each other out there because of this take off situation (and cost quite frankly for extremely expensive alloys) and are there any failures due to it?
I think this is the reason why this is still a controversy. There are a lot of people that say don't do this and impose restrictions (mostly because they just don't like it) and yet nobody can definitively prove why not? So it keeps being allowed.
Bottom line for me, though logic is an invaluable tool, especially with the vast experience of committee members, we should be hesitant to impose code restrictions based upon it alone.

By G.S.Crisi (****)

Date 11-30-2007 18:01 Edited 11-30-2007 18:38

js55,
English being not my mother language, sometimes I'm not able to express in writing or speaking what I'm really thinking.

Let's see.
You say that the 3X/4X figure is arbitrary, and you repeat "arbitrary". In my first posting, I said that it's usual in jobsites and prefabricating shops to use a figure pulled off the air consisting in 3 diameters, and I kept on saying that "pulled off the air" means that there's no sound technical reason to support it. So, we're saying the same thing.
You also say that the arbitrary figure is imposed by engineering firms, forgetting that in their isometrics there's no way of following what they themselves have imposed. So, when this situation shows up, they (the engineering firms) have nothing to do but approve the change. In my first posting, I said that firms with a high reputation like Exxon, Dupont etc. have issued piping specifications where they state distances between welds, BUT allowing changes with the previous client's approval. I kept on saying that, as usually there's no other way to do the job, the approval is always granted. So, we're saying the same thing.

Back in my days of erector engineer, I took part, as I said in my second posting, in endless and sometimes angry arguments between contractor and client's inspector about the minimum distance between welds. Those arguments happen because, to their knowledge (contractor and client), there's no widely accepted technical standard which states that. We must agree (and excuse me, Flash), that Australian standards are not as widely known as American or German ones.
So, I felt happy at knowing that Aussies have issued such a standard. It means that the best piping and welding engineers in Australia put their heads together and have decided what Flash has reported in his posting. And if it is good enough for the best Australian engineers, it is also good for me.
Maybe the Committee that writes the ASME Code for Pressure Piping can study the Australian standard and see the conveninence of incorporating it in ASME B.31. There will be no language barrier in this case.

Giovanni S. Crisi

By js55

Date 11-30-2007 21:21

Giovanni,
Perhaps we did have misunderstanding. The main crux of my post was my thinking that you were advocating a code imposed standard for weld proximity.
And while I have been involved in a few of those disputes myself (no fun at all-well, unless you win), I wouldn't wish resolution of disputes to be criteria for code inclusion. If thats what you meant.
If not, I apologize.
If Engineering firms wish to impose something, thats a different issue. But its also possible that continuing issuance of something that seems to have no technical basis may be the very cause of the disputes in the first place.
The result is, we are left with very energetic arguments and nobody really knows why.

By G.S.Crisi (****)

Date 11-30-2007 22:08

js55,
I've understood your point.
Best wishes
Giovanni

By js55

Date 11-30-2007 23:09

Giovanni,
No problem.
thanks

By CWI555

Date 11-30-2007 18:42

The question was related to "that if we had weld pipe to fitting and short piece was the dimension as shown at ISO drawing"
It's a pipe to fitting

"319.3.6 Flexibility and Stress Intensification Factors.
In the absence of more directly applicable data, the flexibility
factor k and stress intensification factor i shown
in Appendix D shall be used in flexibility calculations
in para. 319.4. For piping components or attachments
(such as valves, strainers, anchor rings, or bands) not
covered in the Table, suitable stress intensification factors
may be assumed by comparison of their significant
geometry with that of the components shown."

Therefore, This particular case may be related to piping flexibility calcs performed by the engineer of the referenced system.
I agree that general rule of thumbs 3x 4x etc have no basis typically, but in this for instance, It would be a mistake to tell
this gentlemen it's something pulled out of the air in my opinion.

Suggest the person runs this by engineering before making any rash statements.

Regards,
Gerald

By gpsvn Date 01-04-2008 19:36

I see many times the distance between welds are specified in project specifications but did not know this is not a code requirement. Thank you all for this.

My question is if the codes ASME B31.3, B31.4 and B31.4 allow welds to be overlapping.

By js55

Date 01-04-2008 20:04

Geralds caution is well taken. It may in any given instance be fine or NOT be a good idea.
Run it up the engineering flagpole first. Fact is, if you gotta weld that close someone dimensioned something wrong and it needs to be considered anyway.
Overlapping welds are common with circumferentials on seamed pipe, jacketed pipe and ID/OD pipe overlays (think about spiraled circumferential ID overlays), but these things are engineered and intended.