WRC-107 Pad Plates 2

[Wideflange81]

I am designing a lifting trunnion with a pad plate in accordance with WRC-107. My question is with respect to the attachment detail of the pad plate. The vessel is 54" in diameter and I'll be using a 1/2" x 16" pad plate with an 8" diameter x-strong pipe.

Can I consider the pad plate as continuous and weld the pipe to the plate and then the plate to the outer shell or do I have to interrupt the pad plate and weld the pipe directly to the vessel wall and then the pad plate.

From past experience I remember some rule of thumb that the pad plate is considered as continuous and that you do not need to cut a hole for the trunnion to slide thru.

Thanks

 

[MJCronin]

Wide,

There is no general "rule of thumb" about this matter that will address all geometries and all loading cases,.

Tony Paulin has written a superb (and reasonably priced) FEA tool to handle just this type of vessel attachment..

http://www.paulin.com/prg/paper/NP06-2004.pdf

I do not know of any similar engineering software

I suggest that you contact the people at this firm and ask questions.

Please come back with your final decision....

regards


MJC

 

[TGS4]

Be very very very careful about applying an FEA-based solution to this problem. The pad-on-vessel problem is an extremely complex one that involves non-linear behavior such as contact as well as indeterminant geometries such as fillet welds.

I have tried to evaluate a similar situation using FEA (real FEA programs such as ANSYS, NASTRAN, COSMOS not some bodged together wannabe) and found an "easy" solution to be extremely difficult to find. There is very little in the engineering/scientific literature about how to do this problem using FEA.

My two cents for a rule-of-thumb (one cent for each rule):
- determine the stresses at the edge of the attachment on the pad ALONE (pad thk = WRC 107 vessel thk). If these are OK, then check an attachment with the same dimensions as your pad with the actual vessel thk. If all OK then go ahead an attach the pad then the trunnion.
- If the stresses at the edge of the attachment for the pad alone are excessive, then you must make the pad more integral with the vessel. That means cutting the pad around the trunnion (which is welded directly to the vessel wall) and doing the appropriate welding on the inside and outside of the pad.

It MAY be inappropriate to assume that the pad is integral with the vessel wall. I know - it's done all the time, btu that doesn't make it right.

 

[JStephen]

Read the fine print in WRC 107 and WRC 297, first.

If I remember correctly, one of the load cases considered in WRC 297 is a "rigid" plug (IE, infinitely stiff penetration). If using that case, you can design the vessel using that assumption, and then design the pad to carry the load from the point of application to the perimeter of the pad. You wouldn't need to weld the trunnion itself to the shell unless the pad design gave an unreasonable pad thickness.