Re: [PSUBS-MAILIST] warning for smyth

["Adam Lawrence" <adteleka@in-tch.com>]

Hi all,



This is a very good topic to discuss, personally speaking, the fabrication
of the pressure hull is the most critical aspect to be researched by the
builder. If you are not qualified to weld on a pressure vessel, find someone
who is. This means that the "welder" will have certifications to perform
your welding procedure. I used to get all excited when two pieces of metal
stuck together; I was a welder, well I thought I was a welder until I went
to a high power welding school and see what it was really about.





Since none of us have fully researched the metallurgy, weld design, or
derived all equations necessary for pressure vessel design, we take ASME's
word for it that they have done this work and we use their codes as
recommended practices.

Grinding

Weld reinforcement in a fillet weld orientation is different to that of a
butt weld like those found when joining a tank head and a cylinder. In the
butt weld design, the area of weld that extends above the vessels surface
offers no additional strength to the vessel. I worked as a welder for
several years and by default you assume that the more you put on, the
stronger the joint will be. Well it might be true if you've filled the joint
with bolts, as filler, and are trying to cover them over. But for pressure
vessels, our goal is 100% weld soundness. Grinding is usually only done
before welding is begun to help with fit up and so that the joint (bevel
and/or groove only) is all shiny metal that has just newly been introduced
to the atmosphere. ASME and such codes specify weld reinforcement to be no
less then flush and no more then 1/16", depending on base metal thickness.
The reason for no less then flush is obvious. The excess weld limitation is
based on several factors. If you are a good welder you will easily be able
to make (manual or semiautomatic) continuous welds that show no indication
of where starts and stops occur, the edges of the weld will blend into the
base metal with a nice uniform bead contour and you can satisfy the
respected code. If any grinding is required the welders qualifications
should be questioned. The weld might look good after grinding but incomplete
fusion usually accompanies poor bead contour.

Grinding is permissible and usually required for a head and cylinder butt
joint in a P.V., where the root is accessible. This is because welding is
usually done on both the inside and the outside of the joint in order to
achieve full penetration. Back gouging with a grinder or air carbon arc is
necessary to remove the backside of the root of the initial bead since it
has been exposed to an unshielded environment and shiny base metal is
required for verifying full penetration (A discussion on other ways around
this would take a while). Grinding a weld flush is acceptable but not
recommended, it is sometimes done for NDE, but this introduces either stress
risers from grinding marks or weld edges and you usually end up grinding
slightly below flush. A weld machined flush is very common but who can
operate a grinder like that?  The bead contour and amount of reinforcement
should be the same on both sides of the joint. This is to minimize the
stress in the base metal caused by the weld pulling more on one side then
the other.



A more important issue is arc strikes. If an arc has been struck anywhere
outside of groove, the integrity of the vessel has been permanently affected
(a discussion on failure mechanisms would take awhile too). There are
procedures for pinpointing and repairing weld defects within the weld, but
when you take a welding cert test one notices that if a arc is struck
anywhere outside the groove the coupon is rejected. If an arc is struck
anywhere outside the groove when your welding up your SUB should you junk
your investment and start over? Well I guess that's a question to be asked
between the owner and the welder before the welder is hired. PQR's allow
grinding because the process, material, and filler are being tested. Welder
qualifications are testing the welder and no grinding is allowed.



ASME and other standards are just the minimum requirements for safe
operation. So one way to look at welding is if you can't meet the code
yourself, improve your skills or find someone who is qualified.



Regards,

Adam