back strips for pressure vessel

Date 08-23-2004 12:20

For a proper answer to this, you need to provide much more information. In some applications the answer is yes, in many the answer is no.

Charles

By jiweiming (**)

Date 08-23-2004 14:01

hi,charles:
would u tell me about it in detail? in which case can it be used?
any answer from u will be appreciated.
regards
jimmy

By chall

Date 08-23-2004 14:13

Referring to ASME Section VIII, Division 1, Table UW-12 (Maximum Allowable Joint Efficiencies):

Type 2 Joints: Single welded butt joint with backing strip (other than those included in Type 1. This authorizes the use of backing strips (and we have used this before for seam welds in vessle shells). However, there is a limitation applicable to offset plates. Furthermore, joint efficiency must be considered. If your application requires 1.00 joint efficiency, this isn't allowed (the max efficiency is 0.9 in this application, when the joint is subjected to 100% RT).

I'm not comfortable going into many details, but jacketed vessels, addressed in Appendix 9 have some allowances as well.

What exactly are you working on, and do you have the current accepted edition of ASME Sec. VIII, Div 1?

Charles

By TimGary (****)

Date 08-24-2004 12:11

About 4 years ago I was involved in a project like this. We were fabricating 60" dia carbon steel pressure vessels for water treatment. If I remember right, the heads were around 5/8" thick while the shells were 1/2" thick, grade 70 carbon steel. We had been edge prepping for a double bevel, flux core welding the inside bevel by hand, back gouging the outside and finishing with SAW. Very labor intensive, but effective.
Then we got to thinking about cutting costs and decided to take the shortcut of using ceramic tape on the inside and doing the complete weld from the outside with single pass SAW. We made a mock up vessel to try it out and qualify the procedure. After wasting 2/3 of the available weld bevel with defective welds, we finally got all of the settings just right and finished up with a beautiful single pass, no defects, x-ray clean weld that also passed the bend and tensile tests.
This success made the Production Manager very happy as we had figured out a way to dramatically cut labor and material costs.
Then came the first job...
We welded three vessels with the backing tape; at the settings we had perfected earlier and produced two and a half dismal failures. The first weld was fine, but on the rest, the tape either pushed away from the root resulting in excessive weld penetration, or the root weld metal didn't evenly penetrate all the way to the ceramic tape resulting in incomplete penetration. Now we had to go on the inside and back gouge in order to make repairs (the welders really loved that!). After a solid week of two shifts making repairs and about 50 failed x-rays, we finally cleaned out the welds, but in overall appearance, the new vessels now looked like garbage.
The same Production Manager who was very happy with this new technique at first completely lost his cool and swore to fire the next man he saw with a piece of backing tape in his hand.
What went wrong? I wish I knew. We weren't able to continue with the process in order to work the bugs out. That final test weld we made sure was pretty though.
So anyway, I'm sure that it is possible to make these welds, but it's important to do thorough testing and qualification and to have a backup plan for defect resolution.