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Re: rusty tanks (was: Trojan)
Gary,
I agree about the inevitability of physics. I make my living with
physics and reckon we ignore it at our peril. Sorry. Didn't know you were
doing absolutes.
Of course, you COULD blow something up. 80 cubic feet of gas
suddenly appearing at high pressure out of nowhere against your basic
incompressable material will exert lots of force and ultimately transmit it
SOMEWHERE. But, in practice, it takes a while to blow 80 cubic feet of air
out of any tank as the throats are small and the tubing imposes big
restrictions. So, while the blow is gathering steam, the water is
transmitting pressure to the shell and to the vent aperture and at the same
time, that water is starting to move south.
I have given the tanks on both types of sub very hard shots indeed
without problem, but the technique is to give small shots, increasing in
force as you get things moving, and then not overdoing it because you will be
rising pretty quickly and might want to slow down by some technique other
than punching through the bottom of your neighbor's cabin cruiser.
Ascent rates are usually held at 60 feet per minute or thereabouts
(easy to do as bubbles handily rise at about that rate and you just follow
them up) unless you are coming up from pretty deep in which case nearly twice
that is possible. Regardless, you will slow down in shallower water because
things can get out of hand very quickly indeed if you do not. The volume of
air in your MBTs will double every once in a while enroute. This gets to be
a very important thing to remember as you near the surface. Volume doubles
at 60 from 120--again at 30 from 60--again at 15 from .... well, you'll be
skyrocketing and out of control by then and Katie-bar-the-door because you're
coming through!!!!! The clever psubber will of course remember that the
shallower he gets (during ascent) the bigger his MBT bubbles get and the
faster he goes the faster the bubbles get bigger and the faster the bubbles
get bigger the faster the sub goes and pretty soon he can't dump enough
through the vents to slow down and there's nothing left but that cabin
cruiser between him and a space shuttle rescue as he's likely to be in low
orbit before the thing slows down.
In point of fact, you will be pretty steadily venting squirts of air
from the tanks as you rise to maintain that 60 feet per and watching upward
pretty carefully for that cabin cruiser. Then you can stop at 50 feet or so
and let the bubbles go past and make a nice surface marker so your support
crew can see you and make sure the area is clear. Much better than
contracting NASA for a rescue.
I HAVE blasted hard on the bottom a time or two while attempting to
bust out of a tangle of 1/4" polypro, using the upward jerk and the main
motor to give the well know mighty heave (It worked, by the way). The point
being that you have to build a staunch tank, whatever the material, so the
pilot can blow his brains out without worrying whether or not the seams will
hold or not--while being aware that they just might. Even on the surface the
pressure differential between tank top and vent ap is probably a couple of
pounds and over the inner surface of the tanks that probably works out to
upwards of 5 tons of push (not to be confused with lift). Hooray for
distributive laws! Something else that takes some figuring is wave slap
and big dumb feet lumbering around on the tank tops--these babies need
STRENGTH for some of that abuse and never mind their primary function.
Vance
In a message dated 5/6/99 11:39:09 PM, protek@shreve.net writes:
<<Vance,
I'm not saying that it will rupture every tank. But I know that in
theory if you try to accellerate water or any other massive fluid suddenly
where the pressure is applied as an ideal step function you will have a
spike. The lines you were using with the 3000 PSI tanks were surely
restricted at least. If you have an unlimited flow ability at 3000 PSI and
a full BT you better have a 3000+ pound test BT. The reason that the
inside pressure might not be 3000 PSI at any time is the fact that the
volume of the high pressure source is controlled.
Just because neither of us knows of a problem caused by this, and I
don't, doesn't mean that the physics can be discounted here. Water is not
compressable for practical purposes, but it sure has mass.
Gary Boucher
At 04:35 PM 5/6/99 EDT, you wrote:
>Gary,
> Hyco & Perry used 3000 psi systems for hp air storage and we blew
>MBTs with shots of air straight off the manifold, using 1/4" ball valves.
>Hyco MBTs were fiberglas and Perry's were 10 gauge 316L. Never had any
>problem. We'd give it a squirt of air to get the sub moving, then a bit
more
>until we had everything underway, and then blow like hell at the surface.
>Makes gorgeous bubble patterns in clear water.
>Vance
>In a message dated 5/6/99 2:01:37 PM, protek@shreve.net writes:
>
><<David,
>
> No, soft tanks refer to tanks that are usually open to the outside water
>at the bottom. To blow these tanks requires pressure just somewhat higher
>than the outside pressure at that depth. Normally, they are 100% full of
>water. As Vance said they can be of light weight material due to the small
>differential pressures needed. If you put this tank inside the sub in one
>atm of pressure, the tank would have to withstand a pressure larger than
>the outside pressure to blow the tank. Here is something to consider for
>designers I bet a lot of people have never thought about: Imagine you have
>a "soft" tank outside, 100% full of water, with a 4 inch hole in the bottom
>to allow water in and out. You are submerged. You open a valve to your
>air supply and to allow air pressure inside the tank to force the water
>out. The air flows in at a certain constant rate pushing the water out.
>At the instant that the air is applied, when the tank if totally full, you
>will have a differential pressure on that tank equal to the air line
>pressure minus the outside water pressure for that depth. This is because
>it takes a short period of time for the water to begin to flow out of the
>tank due to the inertia of the water itself. Even if the total air flow
>under pressure is slow enough overall to promote a reasonable evacuation of
>the tank, there will, or can be, a pressure spike in the tank. If this
>pressure spike ruptures the tank at or near the top it will allow the air
>to flow out and destroy the use of that tank. Just something to keep in
>mind. This effect can be reduced by slowly allowing air to flow into the
>tank at first followed by a fixed volume of flow. It would not hurt to put
>a return pressure gauge on the tank to observe the tank and make sure that
>during blowing it does not exceed the design limits. This can rip a tank
>apart even if it is very open at the bottom. If so, the air can be
>restructed or regulated. If anyone is contemplating the use of scuba tank
>air, unregulated, for such a tank, I would not.
>
>Gary Boucher
>
>
>At 01:19 AM 5/6/99 -0500, you wrote:
>>When you say "soft" tanks, it gets me thinking of balloons or innertubes or
>>something, which makes me think of those bladders inside household
>>captive-air pressure tanks -- those households which have their own wells,
>>that is. Anyway, it's a steel tank with a heavy rubbery bag inside. I
>>haven't thought it out very far, so I don't know just how... but I'm
>>wondering if there's some way such a replaceable liner could be used to
>>deal with the rustiness in the dark insides of ballast tanks. By unscrewing
>>a small port at one end it was attached to, one could inspect or swap
>>liners as often as one liked. The tank itself could be pretty crude, but
>>the inside of the liner would be fresh and clean.
>>
>>Gary said this:
>>> There are two things however that I would have done differently with
>>>Harold's. First the windows. Second, his saddle bag ballast tanks were
>>>sheet steel formed and welded in place onto the 1/4 inch hull itself.
>>>There is no rust protection inside his tanks and no real way to install it.
>>> I tried to talk him into an access door into these "soft" tanks for
>>>inspection and coating but he has not done this. He did tell me that rust
>>>is ejected out of the tanks on each blow. I have access doors on my soft
>>>tanks. It is a genuine pain to get to them but they are there.
>>
>>
>>---------
>>David
>>buchner@wcta.net
>>http://customer.wcta.net/buchner
>>Osage MN USA
>>
>>
>>
>
>
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Date: Thu, 06 May 1999 17:38:48 -0500
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Subject: Re: rusty tanks (was: Trojan)
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