External attachment welding for high temperature or cyclic service 1

[jtseng123]

Dear all,

I have a project with the following requirements:

Clips or brackets for platforms, ladders, davits and pipe supports shall not be welded to the vessel shell or heads if any of the following conditions are presented:
a) Vessel design temperature of 1000°F (537 °C) or more
b) Vessel is in cyclic service

Based on your EPC experience, do you agree with a) and/or b) ?

 

[TGS4]

Yes to both.

 

[loft]

so how should the clips be attached to vessel in these conditions ?

 

[TGS4]

They should be avoided. Period.

 

[jtseng123]

TGS4, shall internal attachments also be avoided, such as support ring, support clips and beam seats ?

 

[TGS4]

Those are generally necessary and unavoidable. However, they should be designed to minimize the potential thermal stresses. I would lean towards continuous beam rings as opposed to individual beam chairs. Both creep design and fatigue design require specialized skills. Get an expert involved early, or the owner will have to get that expert involved when things go wrong in service.

 

[DekDee]

Please advise if I have misinterpreted this but my take is all clips, brackets etc must be welded onto a pad - not directly onto the shell or head.

 

[TGS4]

DekDee - generally, yes. But I would recommend to not use fillet-welded pads for temperatures in excess of 650F. Definitely would be forbidden if in creep service, and forbidden if in cyclic/fatigue service.

 

[jtseng123]

Thanks for the opinion. but:
We normally do full penetration weld and grind fillet weld to a smooth concave curvature for internal and external attachment for cyclic service. As for high temperature, we may allow to weld external pipe support such that piping from the vessel will grow at the same rate as the vessel.

Internal attachments can have all kinds of shape based on process and mechanical purpose, are unavoidable, and can be a large quantity, such in reactor. By that I see no reason why external attachment is prohibited. The above a) and b) requirements are from our client and I am seeking if anyone does have that experience applying both criteria, or they are overfilled.

The cost to comply with these requirements is huge, since we have to build a structure around each vessel to serve as platforms as well as pipe support.

 

[TGS4]

In high temperatures, external appurtenances will act as cooling fins. I can guarantee that they will crack. And the allowable stresses in creep are much lower, so the additional reinforcement is much larger. Yes it is expensive, but the owner will be much better off with a separate structure.