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THE MH SYSTEMS' BALLAST WATER TREATMENT SYSTEM DESCRIPTION
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(Note:
The Authors are cognizant that a large tanker of the size
as 300,000 DWT may not be an ideal candidate for ballast
water treatment features. However, this hypothetical design
study can be easily modified for smaller tankers.)
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Figure
2.
[click image to enlarge]
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The MH Systems Ballast Water Treatment
System is a combination of two other effective treatment
systems, i.e. deoxygenation and carbonation. It also is
an extension of the MH Systems American Underpressure
System - AUPS (Husain et al. 2001). The inert gas, supplied
by the standard marine gas generator, is 84% nitrogen,
12-14% carbon dioxide and about 2% oxygen. This inert
gas has all the ingredients necessary to combine the two
very effective treatments of hypoxia and carbonation at
a very reasonable cost. The laboratory tests at Scripps,
described previously, show that this gas needs very little
contact time to be effective. The analyses described earlier
established the flow rates and control time for hypoxia
carbonated conditions.
Each ballast tank has rows of pipe
at the tank floor with downward pointing nozzles. The
pressurized inert gas is jetted downward out of the piping.
The jets stir up the sediment for contact with the inert
gas bubbles. The bubbles then rise through the ballast
water to the space above the water surface, which
has previously been underpressurized to -2 psi. For
the purposes of this paper, a 300,000
DWT single hull tanker was
used for design studies of this system to test
practicality and
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Figure
3.
[click image to enlarge]
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affordability. Applicability to a 300,000
DWT double hull tanker was also examined. Figure 2 shows
inboard profile, deck plan view, piping layout, nozzle
detail and section through
ballast tank. Figure 3 shows schematic of the system and
Figure 4 shows isometric of one tank.
A 300,000 DW T double hull tanker
has somewhat less installation costs since the tank bottom
is smooth as shown in Figure 4.
For the 300,000 DWT tanker, there are
8 ballast tanks as follows in Table
2. From analyses and experience (Tamburri et al. 2002),
it is estimated the hypoxia and pH conditions can be set
in at least 8 hours, even in the largest tanks, B3 Port
and Starboard. The flow rate is 1350 cfm for each of these
tanks. With one 1500 cfm marine gas generator, and treating
each tank sequentially, it is stimated that all 8 tanks
can be in a hypoxia, low-pH (5.5 - 6) condition in less
than 48 hours. Contact time for essentially total lethality
may not require more than another 24 hours although the
remainder of the 2 to 3 week voyage is available.
The space above the liquid in each
tank is underpressurized to about -2 psi and maintained
throughout the voyage. As the gas bubbles rise up to the
surface, they are evacuated by a blower to maintain the
underpressure of the inert gas blanket at the surface.
The underpressure further facilitates the solubility of
the oxygen (see analysis) and tends to compensate for
the oxygen captured in the bubbles as they rise.
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Figure
4.
In a double hull vessel
the piping
system is simplified by installing the nozzle
grid on the tank bottom
without any structural interference.
[click image to enlarge]
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Since the ballast tanks are treated
sequentially, only two 700 cfm compressors are required
to compress the gas. The gas
is compressed enough to offset
the hydrostatic head plus an additional 25% psi to
provide a jet force for stirring the
sediment. Two compressors are
provided for redundancy. If there are some concerns with
the dumping of hypoxia and
carbonated treated water, it is easily
countered with the system discussed in this paper. The
compressors will shift over
from the gas generator to atmospheric and the ballast
water will be oxygenated within just a few hours. In this
same period of time the CO2 is readily washed out since
the air contains no CO2 component.
Sensors are needed to monitor the pH to ensure that it
never goes below about 5.5. Sensors will measure dissolved
oxygen content to ensure an adequate deoxygenation is
established. Sensors will also
monitor the underpressure. The
control system will remotely start and stop the gas generator,
the compressor and the blower.
The control system also remotely controls the valves off
of the inert gas manifold to
each ballast tank and the valving
for the underpressure manifold.
It is expected that system will be
controlled by a suitably designed
arrangement of programmable logic controllers (PLCs).
These devices are commercially available. They are also
easy to program and maintain.
A control console with displays will
integrate the functions of the inert gas generator and
the AUPS ballast water treatment system as well as provide
for monitoring, status displays and manual override, if
required.
Tests were conducted with the AUPS
System installed on naval reserve fleet tanker. They verified
the structural capability of tanks to withstand the pressure
of -3 psi and the controls needed to maintain the required
underpressure. These findings are applicable to the equipment
and controls that will be used for the ballast water treatment
system. |
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