Re: rusty tanks (was: Trojan)

[protek@shreve.net]

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
>
>
>