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Brent,
The problem is not the porosity of the weld
directly.....it is the effect the porosity creates. The weld leaking is the
minor part.....the structural integrity is the problem. A weld with voids and
flaws is not as strong as one without. The ideal pressure vessel would be made
seamless.....but in the real world.....that is unlikely.
A weak weld can (notice ....I said "can".....not
"will") cause catastrophic failure which would, in most cases, kill you
instantly. This is not saying that a catastrophic failure is probable. The
problem is the slight chance of it happening. The failure could happen without
any forewarning.
James Long
Owner/Designer
Lil Brother LLC
(Instrument Division)
----- Original Message -----
Sent: Monday, August 06, 2007 2:52
PM
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.
Regards,
Brent
Hartwig
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