Out of roundness in skirt between new and old stacked vessels
Out of roundness in skirt between new and old stacked vessels
(OP)
Hopefully someone can point me in the right direction.
We have a set of two vertically stacked vessels and are replacing the top vessel only by cutting the skirt between the two. The problem we are facing is the out of roundness of the skirt, both old and new. The skirt is 5/8" thick and the OD of the skirt is 248 1/2". At the cut line, the new skirt is 1 7/8" out of round from a perfect circle. The difference between the new skirt and existing skirt at the worst place is 2 1/4".
Can anyone point me toward any information pertaining to this? How much out of round is too much? Any formulas for bending the skirt into place?
The party responsible for installation say they can dog it off and make it fit, they just need someone to give them the guidance on weld size, lug size, etc.
Thank you in advance.
We have a set of two vertically stacked vessels and are replacing the top vessel only by cutting the skirt between the two. The problem we are facing is the out of roundness of the skirt, both old and new. The skirt is 5/8" thick and the OD of the skirt is 248 1/2". At the cut line, the new skirt is 1 7/8" out of round from a perfect circle. The difference between the new skirt and existing skirt at the worst place is 2 1/4".
Can anyone point me toward any information pertaining to this? How much out of round is too much? Any formulas for bending the skirt into place?
The party responsible for installation say they can dog it off and make it fit, they just need someone to give them the guidance on weld size, lug size, etc.
Thank you in advance.





RE: Out of roundness in skirt between new and old stacked vessels
Regards
r6155
RE: Out of roundness in skirt between new and old stacked vessels
That kind of out-of-roundness wouldn’t usually be a problem under normal fabricating and erection circumstances, but I’m not sure what the various ASME and client/plant specs. might say about the problem or the new welded joint. Does this joint have to be a full pen. weld? Are both the new and the old/existing skirts .625" thick? Are they both 248.5" O.D., do they have the same outer circumference length? How far is it from the girth cut line to the attachment on the vessels, on each, the new and existing vessels? What are your wind and EQ loadings, any other loadings on the new girth joint, and how do they come into play in that joint design? A couple inches out of round in a 20.7' dia. cylinder which has been flame cut would not be too unusual and should certainly be able to be brought back into line to make the new weld. The biggest thing might be coming up with a scheme to temporarily reconnect the two skirts in a simple efficient manner so the top tank doesn’t have to hang from a hook for days. You might want to have access to the inside of the skirt during the fit-up and girth rewelding process, and you would want to prep the weld joints before you brought the two tanks together. Depending upon how you did the skirt cutting, you may have to true-up the old skirt cut edge so that it really ends up being in one horiz. plane, rather than all over the place, above and below that plane. You might add a few 20.7' minus stiffeners to the inside of the shells to bring them more closely into a round configuration. Then add some temporary vertical ears to one of the skirts at about 30 or 45̊ or closer intervals so that the top tank/skirt can, more or less, be dropped right down on the lower skirt and engage fairly accurately, with some min. shimming and adjustments. These vert. ears would be welded to one skirt and bent in 5 or 10̊ so that they slip inside the other skirt. Then you can make holding welds at these ears so that the upper tank isn’t swaying in the breeze. Then the final gagging and dogging will involve a min. effort, and the final welds can be made. Take some care to sequence the girth welding so that you do not pull the upper tank in one direction, out of plumb.
RE: Out of roundness in skirt between new and old stacked vessels
I'd replace one or the other conic plates completely.
Out-of-round by 2.25 inch in 245 inches diameter is less than 1% difference.
RE: Out of roundness in skirt between new and old stacked vessels
See UW-31
Regards
r6155
RE: Out of roundness in skirt between new and old stacked vessels
RE: Out of roundness in skirt between new and old stacked vessels