B31.1 SIF for butt weld =1.9?

[SverkerO]

Earlier I have mostly done ASME NC-3600 piping calculations and there the SIF for a butt weld is 1.

But now as I’m about to use B31.1 I noticed that the SIF for a butt weld could be as high as 1.9 depending on the mismatch. As I do not have any specification of the mismatch i feel forced to use 1.9 in every weld. This means that I would have to specify a SIF at each butt weld and that seems rather tedious.

Is specifying a SIF at every butt weld normal practice in B31.1 stress analysis or am I missing something here?

Best Regards
Sverker

 

[SverkerO]

I know found in B31.1 V127.3 (C) a criteria that max mismatch < 1/16”. That allows me to use SIF=1 for the butt welds with t>0.237 in.

But the questions remains for piping with t<0.237 in.

Best regards
Sverker

 

[StevenHPerry]

It has not been a normal practice in my experience when talking about the 99% of buttwelds that are same size and same schedule on both ends.

Here are some "paper fixes" that I think will mitigate your concerns:
1) B31.1 calculations are done on nominal dimensions.
2) The pipe material specifications (e.g. ASTM A106) probably do not allow dimensions to vary significantly enough to approach 1.9.
3) The welding procedure specifications have a limit on alignment as well.

The only time I've encountered trouble situations is on rolled fittings which were out of round at the end. In these circumstances, the SIF of the fitting was larger than the butt joint's SIF.

Rightly or wrongly, I would refer to this SIF equation only when something out of the ordinary piques my interest or if I'm trying to mix and match pipe schedules without a tapered transition.

I would love to know if I had been doing it wrong though.

- Steve Perry

http://www.linkedin.com/in/stevenhperry

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[SverkerO]

Thanks for the reply Steve,

Today I discussed this issue with a more experienced engineer at my office. He also had never specified a SIF for butt welds between same size piping, regardless of the thickness being less than 0.237 in.

It feels awkward to just discard the criteria just like that, I think I’ll look up some WPS for thin walled piping an check what mismatch that usually are allowed.

One more thing, In both B31.3 and ASME III NC the SIF=1. Does anyone know why this criteria has not found its way in to those codes?

Best regards
Sverker

 

[DSB123]

Sverker,
Have a look at the SIF's for bends and other fittings for the B31 Codes vs the ASME III Codes. The SIF for a butt weld is circa 2.0 but due to "normalisation" it is considered as being 1.0 otherwise you would need to know where every butt weld is in your piping system which is tedious unless it is Nuclear piping where you have to know the location as the SIF=2.0.