Brent,
Head over to the American Bureau of Shipping web site and
download their rules for submersibles and pressure vessels (they are FREE!).
They give some numbers in there for what is allowable for welds but fall back
on the ASTM PVHO document (which I haven’t had the opportunity to
purchase as of yet). The point with porosity is not so much weepage
through the seam but it is a focal point for stress and may be the origin for a
propagating crack.
While we may not be certifying our hulls to the ABS Rules, they
provide great guidance based upon the collective knowledge of a large number of
authorities and experience gained over time. I would recommend anyone
getting started in PSUBs to read these and follow these Rules. It would
save many of us from having to respond to the same issues repeatedly that are
brought to the list by new members just starting to contemplate PSUB design.
Be careful with vacuum testing as different stresses are placed
on the viewports than when performing a hydrostatic test.
R/Jay
Respectfully,
Jay K. Jeffries
Andros Is., Bahamas
A skimmer afloat is but a submarine, so poorly built it will not
plunge.
From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Brent
Hartwig
Sent: Monday, August 06, 2007 3:53 PM
To: PSUBSorg
Subject: [PSUBS-MAILIST] 100% MIG Welding PV Seams
From what
I've gathered so far about welding pressure vessel seams, if looks like the
main focus is on non porosity, penetration, in some cases annealing, shielding
from oxidation, and material choices.
In
regards to porosity, I was working on a large building complex for a company
that seals very large CA (Controlled Atmosphere) cold storage buildings, made
from stand up concrete slabs. The cold storage rooms are for storing mostly
apples and such, and when they are full, then all the air is removed and
replaced with CO2. We usually would insulate and seal the concrete with a
industrial spray on urethane foam, and then cover that with a fireproofing
material and protective coating for the foam. But on this one monster project
we sealed the seams with a large rubber type band with a sealing adhesive. For
the walls we sprayed on a thick rubber like paint and back rolled it
on the large wall surfaces. Even with all that, the rooms leaked, when a vacuum
test was performed. I was instructed to fill all the millions and millions and
millions of tiny holes in the painted concrete slab surface, from open bubble
voids in the concrete surface.
So I used
a 6 inch putty knife and a caulking gun and went to work. It didn't take very
long before I realized I was wasting my time. The concrete was like a closed cell
foam. Just because some cells were open on the surface, doesn't mean they go to
the other side of the wall. Long story short after a week of complaining that I
was wasting my time, the rooms were retested and a powder duster test was done
to the seams and they found out that they guy doing the seams had not used
enough sealing glue.
So with
regards to voids in a weld bead, if they are very small, and the weld has great
penetration, wouldn't that still make a good PV weld bead? A tiny bubble
on the surface and/or inside the bead, I wouldn't think would usually produce a
pin hole leak. Once you have finished all your welding you can install the view
ports and perform a vacuum test for leaks.