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Re: [PSUBS-MAILIST] ASME PVHO



In a message dated 11/3/2010 2:47:39 P.M. Eastern Daylight Time, JimToddPsub@aol.com writes:
  I'd love to see posts about what others have encountered during hull fabrication
With Git Kraken I ordered rolled parts ( two 84 inch dia. heads ) from a supplier and to get exact measurements I used the "Egyptian Method" of assuring roundness.
I first measured the circumference of both pieces with a flexible steel tape measure held flat against the edges with small magnets placed at intervals. Luckily the two heads were within 5/32 of an inch which is good considering the circumfrence is 22 feet 5/8 inches. I marked off increments ( match marks ) and important center and compass points around both perimeters for future use as " match lines ".
Then to check roundness I cut an "arc section" out of 24 gage ( thin ) sheet metal that was about 50 or 60 degrees. Using this by hand I slid it around the inside of the tubs ( tank heads ) and used a soap stone to mark any areas that appeared to be out-of-round, judging by the presence of a gap or high spot that showed as the arc section template was dragged around the "lip" of the tub.
I was again pleased to find that the two heads were nice and round with only a few areas that showed a tiny gap or flat spot. Never more than an 1/8th of an inch.
After making sure the lower tub was perfectly level, the next step was to lift the other tub up and set it on the lower tub so the two "raw" ends were lined up. I used wooden blocks between the two tubs to maintain a space of 6-1/8 inches to allow room for the short 6 inch straight section that would be installed.
Once the upper tub was positioned EXACTLY centered over the lower tub, I welded 1/4 inch by 2 inch flat bar
spars to the inside of the two heads ( using a small mirror to see the arc ) spanning the 6-1/8 inch gap. Once the two tubs were welded to the spars, the wooden blocks were removed and the three rolled sections of 6 inch hull plate were "laid" into the gap and tacked in place. This formed the 6 inch straight section between the two tank heads.
I wrapped the 6 inch wide "band" with a chain and used a come-a-long to squeeze it down so it fit flush with the upper and lower sections of the hull ( tubs ) and by using a small peanut grinder with cutter blade I adjusted the length of the band to fit both tank heads. A straight edge spanning the two edges was used to ensure a flush surface and small weld tacks were placed all around the perimeter. The first series of tacks were on 12 inch centers, both upper and lower seams. I used the soap stone "match marks" to keep everything right where it needed to be as I worked my way back and forth adjusting, hammering, prying, and coaxing the two seams into near perfect alignment before tacking in place, making sure all the "match marks " lined up.
Eventually I ended up with weld tacks at 2 inch spacing all around the two perimeter seams.
At this point I had the basic hull sitting on the floor all tacked together.
I next laid out the hole for the hatch and cut out the 24 inch hole in the top tub.
I crawled inside and removed the weld spars that spanned across the 6 inch middle "band" section, ground off any weld lumps or spatter, and checked the inside surface to ensure it was as " flush" as the outside.
I took the sheet metal arc section and slid it around inside the hull again on the two tank head edges and the newly installed center band. There were a couple spots that had flattened just a bit on the perimeter so I used lengths of wood 4X4, and wedge blocks knocked into place on the inside to "stretch" the hull sections round again. This sounds rough but the damn thing was almost perfect anyway so it didn't take much to push it where I wanted. I never had to push more than an 1/8th of an inch out.
I cut a length of 2X2 wood to just under the inside diameter and slid it around the inside, hammering the wedges in and out until the 2X2 just touched all the way around.
At that point the hull was less than 1/8 inch out of round with an 84 inch diameter.
I climbed out and tacked the two seams at 1 inch intervals, climbed back in and checked the roundness again. It hadn't moved.
I went outside and tacked the two seams at 1/2 inch intervals, and checked the roundness again. It was GOOD.
Now remember these two seams are 22 feet long each so although it sounds simple in the telling, it took a long time. I used a TIG torch to set all the tacks, and all the edges of the hull parts were pre-beveled to the correct angles, and there was a uniform 1/16 inch air gap between the two tank head edges and the center band. I used small pieces of  1/16 inch dia. weld rod to maintain the gap.
I welded the root passes with TIG using an alternating pattern, starting with 4 compass points, moving to 8, then 16, then 32, etc. until both seams were completely welded closed.
Many times during this stage I was checking the inside for roundness. As I added more points to the root passes, the diameter would shrink slightly but flex back most of the deviation as subsequent root welds were added around the perimeter.
After it was all welded I found that the inside diameter had shrunk by almost 3/32 inch. I sanded the end of the 2X2 and checked it again. It was still very close to round so I knocked out the wedges and blocks, checked it again and found it was still within 1/8 inch across the 83-1/8 inside diameter.
Using the sheet metal arc section on the smooth inside, it didn't even show as much as a credit card thickness of out-of-round  gap and the 2X2 rod "just touches" all the way round.
I used the same method of pattern welding as more weld beads were laid down over the root welds.
I am confident that the out-of-round on my hull is well within the required tolerances. 
As more things are welded to the hull, if I expect any stress to be applied from a particular weldment, I install blocks and wedges to hold the hull dimensions true and tack everything into place and then remove the blocks to see what's happened.
I had to re-stretch some areas of the tubs when I cut and welded the big window in the front, but it wasn't too hard to knock the wedges in until the hull was right back where it started once everything was tacked in place, ready for final welding. 
Once the big window assembly was welded in, I removed the blocks and it was still within the 1/8th inch tolerance across the 84 inches.
You need to get a feel for the elasticity inherent in the steel and use it to your advantage. By using the heat of the welds to pull the hull into the shape you want, and hard blocks to prevent movement where you don't want it, a very round vessel can be made.
My treatise on " The Egyptian Method "
Frank D.