Skirt support, forging ring vs. weld build-up vs. fillet weld type

[jtseng123]

Dear all,

Many companies specify skirt support to be forged rig type or weld build-up for heavy wall (defined as thickness >1.5" or > 2" ) vertical vessels.
Questions:
1). What is the reason ?
2). What's wrong just using common support method, ie, fillet weld on the skirt outside only ?
3). Why does it need inside weld with round radius (for weld build-up type) at the skirt to head junction if not in cyclic service ?
4). Div. 2, Figure 4.2.4, sketch (f), is it weld build-up for skirt ? However, it says " weld overlay" with stripping on the vessel wall, I am confused where this sketch will apply. Normally for weld build-up type support, it is just great amount of regular weld, not weld overly nor stripping the vessel wall.
5) Can someone provide a weld build-up skecth from projects you have done ? Thanks.

 

[jte]

jtseng123

Interesting topic, you'll get various perspectives on this. My perspectives:

1) Reliability.
2) Thermal stress, particularly with transient loads. Cyclic service (e.g. coke drums) will be particularly unforgiving.
3) Transient conditions, even without service which qualifies as "cyclic". Also, many vessels which fit your criteria will also be of a material which requires PWHT. This type of design is cheap insurance against the "fun" one can have in repairing, and PWHT, in the field.
4) You may find, as I have, that there is considerable inconsistency in understanding the difference between "overlay" and "weld build". In my perspective, "overlay" implies a beneficial material, typically corrosion resistant, which is used to provide a relatively thin barrier over a base metal which is chosen for its strength. [I also worked on one design years ago in which we overlayed carbon steel onto stainless plate. It made sense in that circumstance.] On the other hand, "weld build" implies a weld rod of the same nominal chemistry as the base metal, and would be used to repair wasted areas or, in this case, to thicken a relatively small part of the plate in new construction. Note the (ab)use of "weld overlay" in PCC-2 article 2.2 through the 2013 Edition. See also the definitions in Article 2.11 section 1.5 which make sense to me. Note also that this is fixed for the 2014 edition. In the case of Fig. 4.2.4 sketch (f), I believe the word "overlay" should be changed to "buildup." But I'm not going to bother running to the committee to get it changed. I'll leave that to you! Seriously, it isn't that hard to send a note to the committee secretary, and you'd be doing the rest of us a service.
5) Not I. Unless you are working on a project for my company.

 

[GenB]

well said. In this case overlay and build up are the same. why bother.