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Re: [PSUBS-MAILIST] O rings and pressure
Hey Guys,
I had to chime in on this one. If you use HP air to
blow ballast, and any portion of the pressure hull of
your boat is used as part of the ballast tank, it will
be subjected to that pressure as well and most likely
in a disproportionate way. This CAN'T end well. Just a
thought. I dive free flow hats all the time, same
concept. If the exhaust gets sticky, your hat blows
off of your melon, with the related pressure issues.
This really sucks. LP is probably safer inside the
sub, maybe with a backup means to open the HP to the
ballast tanks in an emergency.
Dewey
--- Ian Roxborough <irox@ix.netcom.com> wrote:
> On Thu, 20 Nov 2003 08:53:04 -0800
> "Brian Cox" <ojaibees@ojai.net> wrote:
>
> > Pressure:
> > Of course you will need air supply for
> life support so you
> > can't get around having those air lines to deal
> with inside the sub.
>
> But your life support air line doesn't have to be
> high pressure, you
> could use a regulator to reduce the O2 (assuming
> your life support
> system uses O2) pressure before the gas reaches the
> thru hull.
>
> > I had planned on not having any actual
> pressure lines (for
> > ballast) coming into the sub at all, that's what
> the thru hull shafts
> > are for. However I do need to look at a pressure
> gauge and unless that
> > is located outside and visible from my viewport
> then I would have to
> > bring a HP line into the sub to look at the gauge.
>
> Something that I'm considering is placing all the
> thru-hull connections
> inside a small pressure vessel mounted on the inside
> of the hull. So
> far I see this as a cylinder, about 8" diameter, 6"
> long, with one end
> welded to the inside of the pressure hull. All the
> thru-hulls come out
> inside the cylinder, which would have a pressure
> tight hatch on the
> other end (possible even a clear hatch to see
> inside). The valves
> would be operatored via electronic actuators, but
> should the electronics
> fail, the thru-hull safety cover could be removed
> and the valves
> operated by hand (with the use of special keys).
> With a see through
> cover, you could also mount the pressure gauges in
> there as well.
>
> > But for ballast air
> > it seems like it would be unnecessary to regulate
> that air down to a
> > lower pressure, the regulator would be just one
> more thing to go wrong.
> > However, If you are blowing your hard ballast with
> 2000+ air you better
> > make sure your water exit valve is open or you
> would rupture your
> > ballast tank. Also the pressure in that ballast
> tank is going to climb
> > very fast depending on the size of your water exit
> valve. so maybe lower
> > pressure would be better.
>
> If this is a hard tank, I don't see any reason it
> couldn't be designed
> to hold 2000psi of internal pressure.
>
> Ian.
>
>
> > ----- Original Message -----
> > From: Dan H.
> > To: personal_submersibles@psubs.org
> > Sent: Thursday, November 20, 2003 5:29 AM
> > Subject: Re: [PSUBS-MAILIST] O rings and
> pressure
> >
> >
> > Brian,
> >
> > I see you already have good recommendations for
> some O-ring
> > information sites. I would like to comment on
> some of the other
> > recommendations you were given though.
> >
> > True, many times an O-ring is installed in the
> shaft rather then the
> > housing. As was mentioned, It reduces the
> effective diameter of the
> > shaft though and caused critical stress points
> because of the nature of
> > the O-ring groove itself. Also, if you require a
> .500 inch diameter
> > shaft for the torque your applying and groove the
> shaft, you have to use
> > a shaft of maybe .625 or .750 inch to have .500
> where the groove is.
> > The larger shaft size is like a piston in your
> hull. With the water
> > pressure on the outside and ambient pressure in
> the inside, the larger
> > shaft diameter has greater longitudinal forces on
> it and will be harder
> > to rotate at depth.
> >
> > Machining internal O-ring grooves is more
> difficult then machining
> > grooves on a shaft but polishing a bored cylinder
> is more difficult then
> > simply buffing the shaft to a smooth finish.
> >
> > For most through hull applications put the
> grooves in the housing not
> > the shaft.
> >
> > I also think you should use a high pressure
> regulator on your air
> > supply. If find a time when you need 2000 pounds
> pressure to blow
> > ballast, your in deep ( I wasn't thinking water
> here). Regulate your
> > air pressure down so you can blow ballast at depth
> with some extra
> > pressure for a safety factor. The HP air won't
> freeze since the
> > majority of the moisture is removed if you have
> them filled at a dive
> > shop. But you just don't need to work with that
> high pressure in a
> > P-sub.
> >
> > Dan H.
> >
> > ----- Original Message -----
> > From: Brian Cox
> > To: Personal_Submersibles@psubs.org
> > Sent: Wednesday, November 19, 2003 10:07 PM
> > Subject: [PSUBS-MAILIST] O rings and pressure
> >
> >
> > Hi, I have a dumb question, I just machined
> my first sub part,
> > sort of a test part, a thru hull piece that will
> have a shaft going thru
> > it to turn a valve from the inside of the sub to
> the out side. I
> > machined an inside groove for an "o" ring, now
> are there recomended
> > specs for this "o" ring ? Like so many
> thousandths thickness etc..
> > and should the shaft be notched as well?
> >
> > Second dumb Question: Is all of your air
> pressure comming off the
> > high pressure cylinders (2000 psi) always dropped
> down to a lower
> > pressure (200 psi) with a regulator before it is
> used to blow out a
> > ballast tank? Is that high pressure too high to
> use for that (possiblly
> > freezing up lines?)
> >
> > Brian Cox
> >
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