Pipe to Flange Design Question

[doezer]

Hi all.

i have very limited experiecne in weld design, and have been assigned a task to look at the design of a flange/pipe welded connection.
im reading all over that a full penetration butt weld is the best.
But why is that?

When I look at a socket weld, the weld seems to be so much more supported, structurally I mean.
You have essentially an inner sleeve located within an outer sleeve. So any structural loads occuring (bending moments etc) are not being taken directly by the wleded joint.
Whereas with a butt weld, its fully 'bendable' at the joint.

I understand you are creating a crack, but if you can verify that the crack is ok for the duty cycle involved and the fluids and pressure involved,
then surely you have a stronger joint structurally??

by the way, this pipe/flange joint will see very high operational loads, not just pressure load, also structural loads due to operation.
the pipe will vibrate and apply bending whilst the flange is secured, so the weld needs to withstand it etc.

hope this makes sense? as I say am new to welded joint design

thanks
D

 

[LittleInch]

Ok,

1) "Flange with pipe stub" doesn't exist. It's either a flange or its a bit of pipe which is then the same as any other bit of pipe
2) Your calculation for end force only on the pipe thickness only works if the pipe was an interference fit inside the flange - Not possible. For slip on flanges and socket welds, there is a gap between the OD of the pipe and the ID of the flange / socket weld. Therefore the weld sees hoop stress.
3) As we've all hammered on about already, slip on / socket weld fillet type welds create significant stress concentrations at the weld root when the connecting pipe is subject to any sort of vibration and bending moment. Many company specifications won't allow such connections for high stress / vibrating service.
4) Unless you can get the fit between the pipe and slip on / socket weld flange to be a near interference fit, the strongest weld between pipe and flange is a butt weld.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

I don't like the excessive change in wall thickness of the stub where it connects to the flange either.

 

[doezer]

ok thanks.

the excessive change in thickness wont be there.. it will neck down, sorry I should have drawn it that way.
the pipe will be ~zero clearance. we think we have a way
its not cheap and a bit experimental, but we think can work.

yet another possibility is hydroform the pipe into a groove on the ID of the flange.
so then the weld at the bottom becomes just a sealing weld?
all this beacause we cant butt weld
once again - thanks for this feedback and i cant get over what a resource this forum is

 

[doezer]

youve written 'not possible' re pipe interference fit into a flange
we do have some samples made up like that already, is it generally something considered not possible??

 

[LittleInch]

I meant not normally possible.

Clearly for a thin tube the amount of interference fit is limited otherwise it crumples, plus this make it a special. I was working on the basis of a standard off the shelf components.

You will need to get specially made fittings drilled to exact limits on tolerances of both the fitting and the OD of the tube / pipe.

Maybe that's worth it for your item - I don't know.

You will need to look closely at the tapering of the flange to meet the pipe to reduce stress concentration and I suspect some sort of FEA analysis to prove it can stand the forces and vibrations.

I would just go find a material I could weld without issues, but I must say 3mm seems a very low limit for a material like aluminium.

One issue always brought up is how you NDT the weld. Normally very easy to x ray a but weld and / or UT it, but a fillet weld is much harder to get high levels of quality assurance.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[doezer]

ok thanks
for sure, increasing the 3mm constraint and staying with a butt weld is the easiest way to go.
we need to push back on that a bit before embarking on this whole other road.

D