When attaching hemi heads which are thinner than the shell plate, some fabricators cut back the hemi head and taper the shell plate to the head thickness ( 3: 1), while others put the hemi head, without cutting it back, on the shell plate, and use weld metal to provide the required taper. Is there any advantages to one over the other? How is it possible to reduce the shell thickness of the shell without compromising it's integrity?
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UW13.1 2
- Thread starter arcer
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The transition in thickness must take place in the head region and not extend past the tanline. The head and shell plate centerlines must also be within Code tolerance. I usually start off by laying out the shell and head plate centerlines. Where the ID of each plate coincides is where the circ seam should be. The transition is then taken off the shell on the outside only making sure that this does not extend past the tanline. You can juggle around with the centerline tolerance to optimize this attachment.
Another star to codeeng for highlighting an often overlooked Code requirement about tapers. As noted by codeeng for hemi heads: "The transition in thickness must take place in the head region and not extend past the tanline."
Just another bit of Code trivia... the minimum thickness of the skirt of a formed head is determined as that required for a seamless cylindrical section. A Code book is not in-hand, but that's in UG-32 somewhere. UG-32(O), maybe?
Steve Braune
Tank Industry Consultants
Just another bit of Code trivia... the minimum thickness of the skirt of a formed head is determined as that required for a seamless cylindrical section. A Code book is not in-hand, but that's in UG-32 somewhere. UG-32(O), maybe?
Steve Braune
Tank Industry Consultants
I agree that this detail is one which I see done wrong occasionally. As codeeng pointed out, the transition (for hemi, 2:1 SE, F&D etc) must be tapered on the side of the component which is thinner. See the last page of Fig UW-13.1.
As Steve Braune pointed out, the sraight flange of a head is basically considered to be a shell course by the code. As far as the code is concerned, the head ends at the tangent line. UG-32(l): "... When a skirt [straight flange] is provided, its thickness shall be at least that required for a seamless shell of the same inside diameter." [But what do you do with say a two piece 2:1 SE head which has circ seams on the straight flange?]
But to answer arcer's question: In theory, both approaches produce the same result. I've seen tapers provided both ways. My preference, however, is to have the taper machined from a thick plate instead of by weld buildup. Simply because I believe there is less likelyhood of quality issues and potential stress issues which could come up with welding. If its a relatively small change in thickness as you might see with a 2:1 SE to a shell, that's one thing. For a hemi head, I presume you have relatively high pressure, and the change in thickness between the shell and head is more substantial.
jt
As Steve Braune pointed out, the sraight flange of a head is basically considered to be a shell course by the code. As far as the code is concerned, the head ends at the tangent line. UG-32(l): "... When a skirt [straight flange] is provided, its thickness shall be at least that required for a seamless shell of the same inside diameter." [But what do you do with say a two piece 2:1 SE head which has circ seams on the straight flange?]
But to answer arcer's question: In theory, both approaches produce the same result. I've seen tapers provided both ways. My preference, however, is to have the taper machined from a thick plate instead of by weld buildup. Simply because I believe there is less likelyhood of quality issues and potential stress issues which could come up with welding. If its a relatively small change in thickness as you might see with a 2:1 SE to a shell, that's one thing. For a hemi head, I presume you have relatively high pressure, and the change in thickness between the shell and head is more substantial.
jt
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