Re: Ballast tanks

[Ray Keefer <Ray.Keefer@eng.Sun.COM>]

Hi All,

> A ballast tank open to the environment is possible, and easiest if the normal
> dive condition is a fully flooded tank.  In the absence of external controls
> for a tank open to ambient water pressure, any air inside the tank will
> compress with depth ( i.e. pressure), and so you lose bouyancy as you dive.
> Conversely, you gain bouyancy as you ascend if there is air in the tank. 
> This is not a good idea.
> 

There are three ways to use ballast tanks. 

The first way is what is used in a typical US sub and what Kittredge uses
in his MBT (Main Ballast Tanks). In this case, when the tank is full of air
it adds bouyancy to the submarine and makes it float. When the tanks are fully
flooded then the tanks do not contribute any bouyancy to the submersible and
the submersible is at nearly (+/- a few %) neutral bouyancy. These tanks have
flood holes in the bottom to let water in and out and blow valves in the
top of the tank to let air out and air lines in to let air into the ballast
tanks. 

In this use the tank is either fully flooded or fully blown. That way there
is no problems with bouyancy changing when an air bubble expands or compresses
with the changes of depth and water pressure.

One advantage to this approach is that the tank does not have to be 
constructed to withstand the massive pressure diffentials that the hull of a
1atm submersible has to withstand. It has to withstand the differential
pressure of the water pressue at the bottom of the tank (ambient pressure at
that depth) and be stiff enough to keep its shape when the submersible motors
through water, but it doesn't have to be made of thick hull steel. 

The second way to use a ballast tank is to keep a set water level in the
tank. Some wet subs, like the SportSubs, do this. The idea is when you go
deeper the ambient pressure of the surrounding water compresses the air in
the ballast tank. Some mechanism is used to automatically add more air to
compesate. As you surface the air expands and some must be release to keep the
expanding air bubble in check or you will get positively bouyant. 

This approach does work. The disadvantage is that if you change depth a lot
you consume your air faster then if you stay at a fairly constant depth.
Another problem is that if you are in the ambient ballast tank, like the main
cockpit of a SportSub, then you are limited to the physiological limitations
of the human body using compressed air at depth. Which is about 120-160 feet
depending on who you trust. Plus nitrogen abosrbsion limits you time or
you go into decompression diving limits. Fairure to be careful to limit time
and depth can lead to the bends.

At deep depths more air is used to fill a given volume so if you operate deep
you use your air faster.

The advantage is that the tank is at ambient pressure so it does not have
to be heavily constructed. Fiberglass can be used in the construction making
for a hull as stream lined as you want. 

The constant introduction of air make keeps the air breathable since the old
air is just bled away.

The actual condition for neutral bouyancy is adjusted by changing the level of
water in this tank.

The third approach to ballast tanks, often call trim tanks or VBT  (Variable
Balast Tank). These tanks are partially  filled with water or partially filled
with air. Depends on your point of view. The idea is to have enough water to
give you the weight or lack of bouyancy to give you neutral bouyancy. These
tanks take two forms. Hard and soft. 

If these tanks are outside the
submersible with the ambient  water pressure pressing on these tanks they need
to be made hard. That means they are as heavily constructed as the hull since
they need to withstand the same pressure differentials as the 1atm hull. The
reason is that the level of water in the VBT is determined at the surface then
the tanks flood and blow valves are closed. At this point the air in the tank
is as the local ambiient pressure that was near the surface. As the sub
submerges and pressure inside of the tank does not change, there is a pressure
differential with the outside water pressure. So the tank needs to be heavy.

As an alternative you can use a soft VBT. To be soft the VBT is internal to the
sub. You use the strenght of the hull and heavy duty piping and valves to the
tank. But the tank itself is not subjected to extreme pressure differntials.
That is as long as you don't need to be able to blow the tank at depths in
case of emergancies. In which case the tank will have to be heavy to handle
the air pressure to drive water out of the tank.

The advantage is the soft VBT is that it cuts down on external tanks causing
drag. You can also have a couple and pump water between them to adjust your
trim. Hench they are sometimes called trim tanks. When serveral tanks are
used one is made beefier then the others to water can be pumped to it to be
blown overboard with compressed air if needed, so only one tank out of several
needs to be heaviely constructed.

Multiple VBT allow for fine tuning of your trim. Though moving a weight can
so the same thing.

The disadvantage is that they take up space inside the submersible and add
more  plumbing.


> Military subs (at least WWII vintage diesel boats) have ballast tanks which
> are valved at top and bottom.  In dangerous situations, they will open the
> bottom vents while running on the surface to charge batteries.  This was
> called 'Riding the Vents" and allowed quick submergence because the tanks
> would flood as soon as the upper vents were opened.
> 

Ballast tanks of US subs were always open at the bottom. I know the US nuclear
submarines are and the WWII models were. Nuclear Soviet submarines use the
double valved ballast tanks. 

The only advantages of having a valve at the bottom is if the tank springs a
leek above the water line it won't flood while an open bottom tank will; and
the noise associated with the water flowing passed the flood hole of a fast
moving submersible is eliminated.

The disadvantages of a flood valve is that it restricts the flow of water into
the ballast and hince the speed of diving. For WWII subs this was important
since they had to get underwater in less then 30 seconds. So they had big
flood holes. Modern subs don't really worry about speed of submerging and
can take up to 10 minutes. 



> A ballast tank can be open to ambient if you control the water level in the
> tank, and simultaneously therefore, the air pressure in the tank.
> 

The SportSub does this in the main cabin.

> Is anyone in this group familiar with designs which actually do this?  Has
> anybody actually studied the ballast system on the Sport Sub, for example?
> 

I read the info packet. I have the video. Looks at the SportSub web site.
All put together with some logical deduction you can figure out a lot.

Look at some of the magazine articles in the Magazine Ariticle page. Some
of those articles state how to maintain a level of water. There is even an
ascii representation of one method.

> I have an idea that the water level and air pressure in an open ballast tank
> could be accomplished very easily with a scuba regulator, but the flow rate
> may be a limiting factor.

Air either needs to flow continuoisly or as needed. 

Regards,
Ray