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Re: [PSUBS-MAILIST] over pressure hard tank
Just to expand on the point about salinity changes - if you are
transiting from salt water (~1025 kg/m^3) through a region of fresh
water (~1000 kg/m^3), your boat is going to lose about 2.5% of its
buoyancy. Depending on your overall displacement, that could correspond
to a significant volume of ballast water, but for any sub operating in
salt water I would consider this to be an absolute minimum VBT volume.
Other factors to consider include payload (passenger and cargo, fuel,
water, etc) compensation - where you do not wish to spend time adding or
subtracting hard ballast (lead) as your manifest changes, and hull
volume changes - regardless of your hull design, pressure at depth will
cause small scale deflections which will change its shape and
consequently reduce its buoyancy by a small amount. Consider too that a
sub is not an entirely closed system, and that the loss of gases (from
HP gas flasks, MBTs, etc.) to the sea is weight that needs to be
accounted for. For example, a standard 80 cu. ft. aluminium SCUBA tank
loses about six pounds full to empty.
With respect to blowing MBTs at depth, hopefully a scenario in which
this action is necessary will never arise, but in consideration of the
possibility, a few things to think about:
1) That the release of high pressure air into the MBTs will not create
an overpressure in excess of their design strength in consideration of
the maximum rate that water can be blown out, and the presence /
effectiveness of any incorporated pressure relief mechanisms in MBTs
that are not normally open to sea,
2) That in the event of an emergency MBT blow at depth, the sub is
designed to maintain both adequate stability and attitude control
throughout the resultant ascent and when breaking the surface, and
3) Under circumstances where a MBT blow at depth is desireable, but
there is not an immediate threat to life support or pressure hull
integrity which demands surfacing at the maximum possible ascent rate,
and that a greater volume of air is not required to initiate ascent,
that a means of controlling the volume of HP air admitted to the MBTs
exist such that the volume of gas admitted to the MBT at depth is not
greater than the MBT capacity when expanded at the surfaced pressure (so
ballast air is not wasted).
-Sean
ShellyDalg@aol.com wrote:
Hi Brian. That's how it COULD work. I don't think that's how best to
use it though.
As I understand the variable ballast tank ( VBT ) it's used as a fine
adjustment for buoyancy. Changes in buoyancy occur on several
occasions. The salinity of the water, like if you go from a fresh
water harbor to salt water, or even pass through a salty layer in the
ocean, or a fresh water current in the ocean near a river mouth. These
things will have an effect on how buoyant a sub is.
Water temperature can make a difference too. Although not much, it
gets a little more dense when the temp goes down.
I think the most common way to handle the VBT is to adjust the volume
of water in the tank while submerged at a shallow depth, to achieve
neutral in the water column. Once the sub is neutral, the valves are
closed and you're good to go. Now, a "hard tank" is the same, but is
stronger. Let's say you are adjusted to neutral, and driving along
submerged at 300 feet. There's not much pressure in the tank, and it's
half full, with the valves closed.
You come to an area where the buoyancy changes for what ever reason,
and the sub starts to float up. You want to add more water to the
tank, so you crack the water-in valve and add a little. The pressure
of the surrounding water compresses the bubble in the tank, and the
sub gains a little weight. Again you are buoyant neutral. You close
the tank valves again to maintain that level of buoyancy.
You're moving along and the sub starts to sink, so you want less water
in the tank. Read the surrounding depth/pressure on the depth gage,
add some high pressure air to the tank to just overcome the ambient
pressure, then dump a little water. Again, you keep the valves closed
when not actively adjusting the tank volume.
Now you've got some serious pressure in the tank, but it's
water volume is constant. OK, now it's time to end the dive and
surface. Use the motors to go up, not the tanks. Once you get near the
surface, dump the water from the VBT as it's already full of high
pressure, and the water will blow right out. now you're buoyant
positive and the sub should float at the surface. At this point, you
blow the main ballast which raises the tower/hatch as far out of the
water as possible. If you just want to look around and go back down,
you wouldn't blow the MBT's. Save that air until you're ready to open
the hatch.
You've gone down and up, and still have most of your HP air in the
scuba tanks.
Now, if a situation develops where you want to go up really fast, then
blow the VBT at depth and still use the motors. Maybe give the MBT's a
little squirt of air to help, but be careful. A fast ascent can be
very dangerous. If it's not an emergency, use the motors to go up and
down. Save the air as much as possible. Blowing either tanks at depth
and rising in the water column, the bubble expands unless it's valved
off.
This can lead to unsafe control issues. Frank D.
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