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Lifting Lug with Shell Corrosion (Codeware COMPRESS)

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ColinPearson

ColinPearson

Petroleum
May 1, 2011
142
Hey Y'all! Looking for a little guidance on designing lifting lugs here. I do this routinely and the lug calcs (by hand) are no problem for me. That said, I'm not a vessel guy, so the local stress on vessels is a bit outside of my wheelhouse. In this case, the client has indicated they have significant, known corrosion issues in the existing vessel.

What I'd like to do is run lug calcs in COMPRESS as if the corrosion had already reduced the vessel wall thickness. However, I want the full weight of the vessel to be used in the calcs to check everything b/c obviously I can't know the true extent of corrosion.

My thought was to "fake" it by using a wall thickness equal to the original minus the corrosion that the client has told us to expect. Then I would take the difference in weight between the original and corroded conditions and apply that as a liner with the density manipulated to make up that difference. If I'm thinking right, that would run the local stress calcs on the thin metal but the lugs would be loaded with the original/uncorroded weight. However, COMPRESS appears to force me to use a min thickness that is greater than what the approx corroded thickness is.

What do y'all think about that, sound reasonable?

Thanks for your help, have a great day and stay safe!
 
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If I recall correctly, COMPRESS performs calculations for the support lugs (and local stress in the shell) for a number of different conditions, including new versus corroded, empty versus operating, with wind or seismic, etc.

As far as COMPRESS forcing a (nominal) thickness greater than your input, it sounds like you are using "design" mode and the input corrosion combined with the 'tr' found by ASME Code rules is greater than what you want to use. You can switch to "rating" mode or take a closer view of the report for the support lug to see if your actual condition is investigated properly without artificially ginning up the corrosion.

You could also try contacting the people at Codeware for advice on this: support@codeware.com or call them.
 
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See UG-54(a). Remember the full weight of the vessel at hydrostatic test.
 
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@TomBarsh , thank you, I will look into switching modes as you suggest.
@oknow, I'm not sure I follow why the weight during hydro would be important?
 
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ColinPearson- ASME Section VII Div.1 UG--54(a) states the "maximum imposed loadings" must be considered when designing supports (UG-22). This is a weight issue and the addition of the water weight at hydrostatic test needs to be included in the calculation. Depending on the size of the vessel this could be a substantial amount of weight. I have seen these lugs fail both in the shop and field because for a various number of reasons the vessels were lifted when full. ASME is a Safety Code.
 
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With all respect, if the shop lifts a vessel after hdyro without draining and they tear up the vessel, then they deserve to eat the cost. This vessel will only have a portion replaced and after R-2 Alteration, we hope to be allowed NDE in lieu of hydro. In any event though, we will remove the lugs once the new section is replaced and no attempt to lift the entire vessel (full or empty) will be made.
 
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Well if they hurt or kill some one in the shop they may be eating much more than the cost of the Vessel. There are legal folks who specialize in Pressure Equipment and have experts on retainers. A few questions about the design of the lugs
will close the case quickly. Negligent with the design IMO.
 
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You asked a question and I answered. It is about compliance. You choose not to accept it that is your choice. Save your insults for FB, tells me all I need to know.
 
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Read again, "oknow, I'm not sure I follow why the weight during hydro would be important?" Clearly I have waisted my time as your replies paint a picture.
 
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@oknow read again VIII about designing supports for the maximum imposed load, and perhaps you’ll realize lifting lugs are not supports. You might also read again my response including the words “with all respect” before you came back saying I’m being negligent in my design. At that point it was clear that you were the one “waisting” MY time. You’re just wrong and that’s okay. Now if you’ll excuse me, I have to go call the fabricator and inform them that they need to remove the walls from their shop and plan the hydro for bad weather so we can make sure we test the wind resistance of the lugs while we’re lifting the vessel during hydro. Pretty sure it says to do that in VIII too, right?
 
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Gents, I am reading the comments under this post...
FYA, Lifting lugs are normally designed for empty lift with applying certain load factors only as necessary. Designing the lifting lugs for hydro static (water filled) case would cause considerable increase in the cost, thus, equipment would be commercially inefficient.

Best Regards,
Ramil Abbaszade CEng MIMechE, MSc
Pressure Vessel, Pipe Stress and Mechanical-Piping Design Engineer
 
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Rami Abbaszade I agree with you on what normally done for the design. Based on the reasons I have stated above I believe it is an oversight in ASME Section VIII Div.1. IMO the lifting lug is a support it is just that that the forces and moments are in a different direction but the Vessel is supported by the lugs when lifted. I hope we can agree to disagree without any rants.
 
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Lifting lugs are not supports.

The role of an engineer is to use proper judgement in gathering inputs to generate safe and economical designs.

At the outset you must determine what loads are reasonable to occur simultaneously. For example, equipment is not normally designed for 1/50 year wind and 1/50 year earthquake events occurring simultaneously. ASME VIII-2 takes this into account when defining load cases in Tables 5.3, 5.4, and 5.5 for example.

Normal practice in the pressure equipment industry is to not lift a vessel when it is full of water. That is the baseline assumption that 99% of our industry will make.

That's not to say you never need to consider this case, and that comes down to using proper engineering judgement. If for example you are designing water tanks that are to be filled, transported, and unloaded while flooded, then it would make sense to design the lifting lugs for a flooded condition. This is an unusual design case however, and would need to be specified by the customer or end-user.

Cheers,
Marty
 
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