Re: [PSUBS-MAILIST] O rings and pressure

[Ian Roxborough <irox@ix.netcom.com>]

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
>