how to fabricate the Skirt junction design details for Div 2 vessel in cyclic service? 1

[YuJie_PV]

Hi, all experts,
i am dealing with a similar vessels like that in this post:

https://www.eng-tips.com/viewthread.cfm?qid=461533

i dictate a simlilar design detail for the vessel under cyclic pressure on behalf of the owner.
and the manufacturer replied that it is unpractical for fabrication. the issues lie that, first, too much
buildup welding, second,the welding will definitely cause too much deformation of the bottom head.

if jtseng123 see the post, i'd like to know how did you accomplish your vessel finaly.
if anyone else who may experienced in the field, could you please shed some light on this issue?
Thanks so much.

 

[Geoff13]

That detail looks about what I'd expect for cyclic service. We've built similar for Coke Drums.
[ul]
[li]I'm not sure the outside face of the weld needs to go straight up. I would taper it in. The slope will depend on what your analysis of this joint allows. Depending on what it looks like you may want a gentle radius between the top of the weld and the outside of the head.[/li]
[li]The head looks much thicker than the skirt. I would think with proper weld control (smaller weld passes, lower heat input) distortion wouldn't be an issue. Of course the manufacturer will complain because they'd like to maximize pass size and heat input.[/li]
[li]I would add an arrow from the grinding note to the inside radius. If the manufacturer understood this has the same requirements (and it does) I would expect that to be their biggest complaint. Certainly that's what our field guys complained most about. We ended up adding a girth weld in the skirt about 1' to 2' down so they could weld the skirt to the head first, do all the inside grinding with small pencil grinders, and finally the skirt girth weld. The distance to the girth will depend on access to girth weld the skirt together (longer is easier) vs. access to the head weld for welding and grinding (shorter is easier).[/li]
[/ul]
This detail is not "impractical", but it's not as cheap as the manufacturer would like.

Good luck
Geoff

 

[DriveMeNuts]

How do you achieve welding the internal weld? That is a very tight angle. I have never seen a skirt of that design with an internal weld.
I have carried out the analysis for a similar skirt design, without the internal weld.
It achieves a 'reasonable' fatigue life.

 

[r6155]

1)This weld design is exaggerated. See PIP standard drawing VEFV1128 type B-2
2) Be care with the PWHT, not mentioned by (OP)
3) Nobody think in NDE?

Regards

 

[YuJie_PV]

thanks for all your reply.
@r6155, you mentioned VEFV1128, and i can't even find it when searching online. could you give me a link?

Thanks so much.

 

[r6155]

See

https://pip.org/practices/

Regards

 

[Trestala]

I've seen that done many times on cyclic service equipment on our projects.

Another option is to order a Forged Y-Ring of that shape, so your cylindrical shell, hemispherical head, and skirt are all butt-welded to it. If you do not have that much heavy thickness on the hemi-head, deformation would be an issue doing that weld build-up, so go with the forged Y-ring. Find a fabricator that have an actual experience of doing that configuration, or get your chosen fabricator to do a mock-up.

See Figure 4.2.4.(e)&(f) on ASME BPVC Sec. VIII, Div. 2.

 

[BJI]

I agree, this type of detail is quite common, often the junction is all weld build-up with the skirt weld below the inside radius. It is more typically used for operations with severe thermal cycles, so for cyclic pressure it may be overkill.

 

[YuJie_PV]

Thanks for your help, Trestala and BJI。
and @BJI, i just don't quite understand why the design detail is "overkill" for cyclic pressure rather than cyclic temperature. Could you please elaborate on this issue?

Thanks in advance.

 

[DriveMeNuts]

Weld build up is for high cycle frequency and high stress range fatigue.
A skirt with pressure cycling only, likely doesn't need weld build up, unless it has a high cycle frequency.

 

[BJI]

The internal radius is suited to reducing notch stresses for longitudinal bending, which is usually significant due to restrained radial thermal expansion at this junction. For pressure cycles, in the skirt at least, it is typically limited by restrained hoop expansion under pressure loading, which has a different loading direction and magnitude, so can be less sensitive to the notch effects at this location. This isn't to say that it won't be required, rather it should be driven by the design, not a standard detail.