William:

No disagreements here -- I dislike pipe thread fittings so much that when I
built gas mixing panels (that's what UAC did), I had custom-made manifold
blocks that were ported for #4AN (7/16-20 straight thread with an o-ring
boss). There is a standard for tightening pipe thread fittings -- I don't
know what it is for 1/2 NPT (like the tank valves), but for 1/4 NPT fittings
it is 2 to 2-1/2 turns past finger tight. This "standard" has a lot of
problems. Depending on the fit of the two pieces, which varies considerable
from manufacturer to manufacturer, it could be impossible to get 2 turns on
the fitting. Also, if you do a good job with the teflon tape and maybe put a
little O2 grease on the threads, the torque at 2-1/2 turns might seem so low
that you're tempted to run the fitting in 3 to 3-1/2 turns. That's when it's
easy to break the female fitting. Tapered pipe fittings are a pain!

Just for reference, I will probably use my four British 45s (the ones with
the 1/2 BSPT fittings) as crucibles in my aluminum and brass melting
furnace. Then I'll weld them back together and use them as scuba tanks again
(ha ha).

Sincerely,
C. Randy Bohrer
Underwater Applications Corp
voicemail/fax: (707) 222-7528
randy@mi*.co* <mailto:randy@mi*.co*>
www.mixedgas.com <http://www.mixedgas.com>
 


-----Original Message-----
From: William Gearman [mailto:gearman_1@ya*.co*]
Sent: Thursday, February 17, 2000 2:02 AM
To: Underwater Applications; Techdiver
Subject: Re: Rethreading steel 72s


Randy,

While I can relate to the intrinsic desire to
mechanically create things, being of like
inclination, this idea is still very flawed.

Precisely the "Tapered" threads are why most any
knowledgeable and reputable fill station operator
will refuse to service these type cylinders and
recommend their destruction.

The taper acts as a wedge applying "Lateral"
force across the crown opening from the outside
while gas pressure is doing the same thing
internally.  With standard type valves having
straight threads, the lateral force across the
crown is limited to internal and only "Axial"
against the valve itself.  This means the
internal gas pressure is pushing only against the
thread bearing surfaces and the underside of the
crown.

With 1/2 inch "Tapered" valves such ingenious
inventions as "Teflon Tape" add further to this
lateral force.  Add to this the fact I have never
seen a "Crescent" wrench with Torque increments
to facilitate uniform tightening, and the idea of
technicians with varying strength levels.  Even
then, the amount of torgue would change with each
subsequent installation because of the previous
wedging of the opening.  Thus, the opening is
constantly being opened wider and wider
increasing the future likelihood of a
catastrophic vessel failure.

One is still best advised to make them mail
boxes, bells, wind chimes, etc. as opposed to
using them for diving.  To really appreciate how
dangerous these things are one should see the
result of a cylinder explosion.

Sincerely

William



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