Lifting Lug Safety Factor

[chuckoehler]

When designing a pressure vessel with 4 lifting lugs, what is the safety factor for each lug. I have read that it should be 5 to 1 of the empty weight, meaning that each lug should be able to withstand the entire weight of the vessel during lifting. Is this overkill? This tank is not an ASME vessel.

Additionally, we are using 4 lifting lugs shaped like tombstones welded to the shell of the tank by the head. The tank will be lifted from a horizontal position to a vertical position during fabrication. Are these the better lugs to use due to the axial force that will be applied during lifting, or should we try a trunnion and tailing lug design? What are the benefits and drawbacks?

Thanks

 

[jte]

chuckoehler-

Take a look at thread794-103507 for code "guidance." This should cover your first question.

I'm not sure where your four lugs would go. I'm used to seeing two lugs or trunnions and a tailing lug (one point pick) or beam (two point pick) or frame (4 point pick). The choice of a one, two, or four (ok, I've also seen 3 point pick designs) point picks for the tailing end is a function of the vessel weight, diameter, and base ring design.

Given a choice, I'd go with two lifting lugs before resorting to trunnions. The lugs will transfer the load into the vessel primarily through shear while the trunnions will have moments to be concerned with. OTOH, I recently designed a dead vessel pick (the vessel was not repairable due to metallurgical issues) with holes cut in the vessel and a 8" pipe placed through the shell but not welded. We couldn't make a weld to the shell which wouldn't crack within 48 hours. Added some collars to each side of the pipe to keep the rigging from slipping off. Worked fine. I'd go to a trunnion approach only if you have a relatively flimsy skirt or baseplate and you need to reduce the load at the bottom.

jt

 

[codeeng]

The lifting factor of safety comes from you local Safety Act not the Code. This can vary between about 5 and 8 depending on your location so you should check this first. If you are lifting from the horizontal then 2 lugs and a tailing lug would be better. Check the lifting/tailing lug load every 10 degrees of rotation. I've seen 4 lugs on large tanks which are lifted vertical only.