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Shown above is the CONVAC control box and the bypass valve assembly. The control box has displays
for Vacuum, Velocity and Bypass Position and becomes the primary operating instrument. The bypass valve is a linear
slide with a hydraulic actuator to precisely meter the bypass water. Other CONVAC system components include a dedicated
hydraulic system and a pump inlet sensor.
Dredge operation without CONVAC
Dredge pump inlet pressure (vacuum) is the operator's main source of information as to when and what should
be done to maintain dredge production. Inlet pressure is an indicator of the density of the slurry that is flowing
up the suction pipe to the pump inlet. With a constant pump speed the inlet pressure can be varied to maintain a constant
velocity. When this occurs, the slurry density remains nearly constant in the suction pipe and solids production
will be nearly constant. The operator's task is to maneuver the suction inlet so that the inlet pressure is
maintained as required to maintain production at the desired rate.
Slurry density is determined by the ratio of solids to water in the mixture flowing in the suction pipe. Moving the
suction inlet changes the ratio by increasing or decreasing the amount of solids taken into the suction inlet. The
desired production rate and velocity determines the correct amount of solids. The suction inlet is moved into the
material bank to increase the amount of solids and moved away from the material bank to decrease the amount of solids.
The operator must constantly monitor and adjust the inlet pressure to maintain the desired rate of production.
If the operator does not make the correct adjustment the system will either overload and plug or production will
be less than the desired rate.
If excess pumpable solids are available at the suction inlet the CONVAC does the major part of the operator's task
by automatically adjusting the amount of water entering the suction pipe.
Dredge operation with CONVAC
With CONVAC the operating goal remains the same - keeping the proper amount of solids coming into
the suction. The difference is that the CONVAC will automatically regulate the solids to water ratio in the suction.
The operator inputs the maximum vacuum setpoint on the control box touchpad. The suction is maneuvered to create
excess pumpable solids at the suction inlet and the vacuum reading increases to the setpoint. At this time the
bypass valve opens only as much as required to prevent the vacuum reading from rising above the setpoint. The
position of the bypass valve is indicated on the bypass valve position display.
The position display becomes the primary display to indicate when the excess solids are available at suction inlet.
Ideally, excess solids will be available and the bypass valve will be "modulating" to control the vacuum. When this occurs there will be continuous production at the desired rate.
If a cave-in covers the suction inlet the bypass valve will open further to assure that adequate water is available
to maintain system flow. If the cave-in does not stall the cutter the operator can leave the suction in the
material and let the CONVAC adjust the solids to water ratio and maintain uninterrupted production. If the cutter
is stalled the ladder need only be raised as required to unstall the cutter.
In addition to the vacuum setpoint, the CONVAC control box also has a setpoint for low system velocity. If the
system velocity drops below the setpoint the bypass valve will open until the system velocity rises. The low system
velocity setpoint is a fail-safe feature to prevent pipeline plugging if the system becomes overloaded.
The CONVAC system takes the drudgery and stress out of dredge operation by continuously monitoring and maintaining
the desired vacuum. Even the most attentive and caring operator cannot duplicate the relentless control of the
CONVAC. The CONVAC system will pay for itself by increasing production and preventing pipeline plugging. Profits
will rise when CONVAC controls the rate of solids intake.
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