Max. pressures for socket welds. 1

[Carlloss]

Hi,

I have read previous posts about socket weld vs. butt weld and I am still in situation for which I am looking to gain extra information.

I have to design/select weld connection for pressure regulator body (body + pipe section + flange) on which I will have to use 2 welds on each side.

I would describe my experience with weld connections as a basic, more theoretical knowledge that practise

What I am interesting to know is the max. pressure limit for which socket weld could be used?
Any restrictions?

Plus if anyone have some suggestions based on past work experience with this type of welds?

Product specification:
Body: SS316
Pipe section: SS316, SCH XXS, 2"
Flange: SS316, CL 2500, SCH XXS, 2"
Working conditions: Pressure: 350bar, Medium: Nitrogen air, Temp. not more that 50 deg. C

At this moment I do not have any other information e.g. how the unit will be mounted, etc.

I have attached rough hand sketches to illustrate the concept of this design. Personally concept 1 will be more beneficial from preparation point of view but in my opinion we should stick with concept 2 which give us better strength values.

Thank you.



Karol

 

[XL83NL]

There may be a lot of recommendations for max pressure socket welds on this board, may be as much as there are members.
This will be based on practical aspects and user experience. Furthermore socket welds have the potential for crevice corrosion, which also plays an important role for serviceability.

In general, the standard to which the socket weld complies specifies the equivalent wall thickness of the socket weld fitting.
That equivalent thickness can be used to determine the MAWP of the fitting. This is outlined in the standard, e.g. B16.11 para 2.1.1.

As an example for a 1" Cl. 3000 B16.11 socket weld fitting, e.g. half coupling, the standard defines in table 7 the correlation between the class designation and wall designation used for rating basis.
This would be Sch. 80/Sch. XS.

 

[moltenmetal]

At 2500# class, you can get by with 6000# SW fittings. But even I would butt-weld at 2500# class- the stress intensification factors are just too high in SW.

 

[racookpe1978]

I think you are (mentally) combining the pipe internal pressure capacity - a function of the pipe wall outside diameter, pipe wall thickness, pipe wall strength - with the weld capacity and strength.

For small bore pipe (1/2 to 3 inch for example), you can find combinations of pipe material and pipe wall thicknesses able to handle almost any internal pressure. And, at those diameters, socket welds can also hold almost any pressures with low cost but have the problems noted above with the crevice inside. (But, threaded pipe ALSO has crevice corrosion and vibration issues and stress risers!)

At a larger diameters and at larger pressures, you will find no socket welds that will be suitable. I am now working with 10 inch diameter pipe of 2 inch wall thickness, 6 inch diameter of 1-1/5 wall thickness: Could those be sized for a socket weld? Of course not!

 

[Carlloss]

Thank you for your all replies.

Racookpe1978

My main consideration is weld itself, connection that will be created between pipe and body. Normally with butt weld we will get full material penetration. Weld than can be X ray to check it. I am unsure how to check SW and how ‘big’ the weld should be to provide ‘decent’ connection?

I know that pipe can hold pressure, same is with flange.


Karol

 

[XL83NL]

how ‘big’ the weld should be to provide ‘decent’ connection?

There are codified rules for the minimum weld leg size. See e.g. B31.3:2012 328.5.2C.

I am unsure how to check SW

Well, you usually dont, simply because its impractical. You can RT an SW weld, but the required exposure time is soo much, it's not practical, and most probably requires special techniques.
Furthermore, it'll only tell you if the gap the welder left before making the fillet, is sufficient.
That an important aspect to check though, that gap, otherwise it'll be a matter of time before all your welds break.

 

[Duwe6]

"I am unsure how to check SW "

In-process weld inspection. You need to have a roving inspector(s) checking cleanliness of the weld-prep areas, pull-back of the pipe into the fitting, proper filler metal being used, proper preheat and interpass temperatures.

Quality cannot be "inspected in" after all the work is done. Making a weld is a step-by-step process. Somebody [cognizant & qualified] needs to be there while these steps are being performed to verify that they were properly performed. This is something you get 'for free' when your work is performed in a shop that holds an ASME code-stamp. They have already proven that their welders know the proper steps, that there are an adequate number of QC inspectors, that QC knows what the steps should be [and NOT be], etc. That is why hiring 'just another welder' is so much cheaper than having the work performed by a Code shop. There is no infrastructure with 'just another welder' to pay for. And, sorry, but most engineers are not sufficiently cognizant of the detailed requirements to perform as a QC department, even if they had time to spend watching the fitters and welders.

 

[XL83NL]

My second quote is wrong, it should have been the part "Weld ten ..... check it".

Anyway, I had a good laugh when duwe said that quality comes for free when you have a stamp. Sure, a stamp is some sort of assurance a shop has a quality system that meets up to ASME standards. But really? Gimme a break.
I would not be amazed if the average west-european non-code shop will at least perform equally to that of an american code shop.

 

[gtaw]

The applicable construction code, i.e., B31.1, B31.3, etc, may have limitations on when integrally reinforced fittings are permitted. If I remember correctly, there are restrictions on the diameter of the branch versus the diameters of the run pipe and there are other restrictions depending on the fluid service category when working to B31.3 requirements.

Best regards - Al

 

[moltenmetal]

XL83NL: there's no requirement in B31.3 to check socket welds for residual gap- gap before welding, not after welding, is the specified variable. RT on socket welds is useless and a pure waste of money. And the failure of welds made on bottomed-out socket welds is not a certainty, i.e. "just a matter of time".

Again, at 2500# class with XXS pipe being required for pressure, SW is probably a poor choice.

 

[XL83NL]

hi molten, my reference to RT of socket welds was merely meant to say it is possible. I absolutely agree it's a waste of time and money to do so however.

At these pressures, I believe sockets might not be the way to go (no experience for me at this special case, so Ill take your word).
For low(er) pressures and moderate temperatures, combined with some temperature cycling, having no gap at all will put unnecessary excessive stresses at the fillet weld, which is undesirable.

 

[Carlloss]

Thank you all for your comments and suggestions. I will make a weld between pipe section and body as a 1/2 V butt weld instead of socket.

Karol