This document describes an Automatic Unloading System (AUS) that improves the efficiency of stripping work by automating most of the stripping process using a cargo oil pump. The AUS uses a separator to separate gas from pumped cargo oil and vapor, and uses a vacuum pump to extract the separated gas. It also uses a discharge control valve connected to a level transmitter to monitor the liquid level in the separator and adjust the discharge valve opening to prevent gas from being sucked into the pump as the liquid level drops, which would stop pumping. This allows stripping to be completed using just the large capacity cargo pump instead of smaller reciprocating strip pumps.

1. AUTOMATIC UNLOADING SYSTEM (AUS)
1. General
This system is provided to improve the efficiency of stripping work. It can perform
complete stripping work by the cargo oil pump (hereinafter called "pump'') alone.
The features of this system are as follows:
(1) Since most of stripping work is automated, pump operation is very easy.
(2) Since stripping work can be done by large capacity pumps alone instead of
conventional small capacity reciprocating strip pumps, unloading time can be
reduced.
2. Basic principle and actions
The basic principle of this system is to automatically prevent the suction of gas into
the pump. By doing so, the completion of stripping by the pump alone becomes
possible.
(1) Removal of gas from pump suction pipe
The gas sucked in from the suction bell mouth and the cargo oil vapour produced
in the suction pipe are separated from the cargo oil in the separator and gathers
at the upper part. These gases are extracted by the vacuum pump out of the top
of the separator.
Discharge control valve (hereinafter called 'discharge valve'‘) for preventing pump
from sucking in gas.
When a large volume of gas comes into the separator in the stripping stage,
its liquid level becomes very low. When a large volume of gas is sucked into
the pump, pumping becomes impossible.
In order to prevent this condition, the discharge valve is controlled to the degree
corresponding to the liquid level in the separator (''hereinafter called separator
level") and the pump discharge flow is adjusted.
When the separator level falls excessively, the discharge valve is throttled to
decrease the sucking amount of gas from the bell mouth and it prevents the
suction of gas into the pump.
When the separator level falls below the minimum set value, the discharge
valve is fully closed.
3. Function of components
(1) Separator -
This is a tank to separate the gas sucked in through the bell mouth and the
cargo oil vapour produced in the suction line from cargo oil. The separated
gas is extracted by the vacuum pump through the gas extraction pipe
connected to the top of the separator.
(2) Level transmitter
This consists of a main body (differential pressure measuring portion and
pneumatic signal converting portion) and low/high pressure detectors
(diaphragm seal type). Pressure detector is connected with main body via
pipe (filled with sealed liquid) and low/high pressure detectors are fitted to
upper and lower part of separator. Static pressure (basic pressure) sealed

2. liquid of pipe and static pressure of separator liquid level which is transmitted
·from high pressure side diaphragm are acting to the main body. The gas
pressure in the separator acts evenly on both sides of the low/high pressure
detectors and thus its effect is cancelled, so that the main body only
measures the differential pressure of both static pressures. This measured
differential pressure is converted into pneumatic signal as a separator level
and transmitted to cargo control console.
(3) Discharge valve
This is of butterfly type valve driven by pneumatic cylinder and controls the
pump discharge capacity. This valve is remote-controlled by the valve
opening signal (automatic control or manual control signal) from AIM
selector.
(4) Valve position transmitter
This transmitter, which is installed on the discharge valve, converts the
opening of discharge valve into pneumatic signal and transmits it to the valve
position gauge on the cargo control console.
(5) Vacuum pump
This is of horizontal water ring type. This is driven by the motor through the
intermediate shaft which passes through the bulkhead, and extracts the gas
separated in the separator and discharges it to the slop tank. This pump is
automatically started and stopped by the pressure switches which are
operated by the pneumatic signal from the level transmitter. By means of the
control switch, manual start and stop can be also made.
(6) Sealing water tank
This is a tank to separate the gas from sealing water which is discharged
from vacuum pump and to hold the sealing water necessary for the vacuum
pump function.
(7) Suction valve
This is of screw-down check angle valve fixed on the suction flange of
vacuum pump When the vacuum pump stops, this valve closes to prevent
sealing water and gas from flowing back to the gas extraction line.
(8) Gas extraction valve
This is of pneumatically operated piston type installed on the gas extraction
line and is opened and closed by the solenoid valve (D18) controlled through
the pressure switch (PRS1). This valve is in the same system as the automatic
start and stop circuit of vacuum pump and opens when separator level is less
than 50% and closes when it returns to 10% or more.
(9) A/M selector
This is used to change over the discharge valve to automatic or manual
control. By setting the A/M selector at "MANUAL'', it can be remotely
operated bind loader to get any opening.

3. (10) Solenoid valve
The valve is controlled by the pressure switches (PRS1) operated through
the pneumatic signal from the level transmitter, and opens and it closes the
gas extraction valve by changing over the gas extraction valve operating
pneumatic line
(11) Positioner
This is fitted on the discharge valve and it controls the discharge valve
opening to the opening signal from the A/M selector.
(12) Booster relay
The booster relays, which are provided on the open side and shut side of
pneumatic cylinder respectively, change over the loading pneumatic line of
pneumatic cylinder through the pilot signal transmitted from the positioner
to open or close the discharge valve.
(13) Lock valve
This is fitted on the discharge valve and it locks the discharge valve opening in
case of the driving air pressure drops abnormally low.
(14) Speed controller
The speed controllers, which are provided on the open side and shut side of
pneumatic cylinder respectively, controls the opening/closing speed of the
discharge valve.
4. Operating procedure of system (AUS)
(1) While the liquid level in the cargo oil tank (hereinafter called tank level) is high,
this system does not work. The cargo pump is operated at 700/o or above of separator
level.
(2) As the tank level falls, the suction pressure also falls and approaches the vapour
pressure of the pumping liquid. In this case part of the pumping liquid will tum to vapour
and accumulates in the top of the separator and as a result the separator level begins
to fall.
(3) When the separator level falls down to below 50% the pressure switch
(PRS1) is actuated by a pneumatic signal from the level transmitter and the vacuum
pump starts. At the same time, the gas extraction valve opens and the discharge
valve is throttled
(4) When the separator level recovers above 70%, the gas extraction valve closes
and 10 seconds later, the vacuum pump stops. The discharge valve opens.
(5) Every time the vapour of pumping liquid accumulates in the separator top, the
above mentioned conditions (3) and (4) are repeated and. thus the unloading work is
advanced while the suction of gases by the pump is being prevented
(6) When the tank level further falls (e.g. when the bottom longitudinals of the hull
appear above the oil surface), eddy begins to occur around the bell mouth. Soon, when
the bottom of the depression of this eddy (gas column) reaches the bottom face of the
bell mouth, gas begins to be sucked into the bell mouth.
(7) The gas sucked in is separated from the pumping liquid in the separator and
accumulates in its top lowering the separator level. Hereupon the action described in
(3) and (4) above is repeated.

4. (8) When the tank level further falls, the liquid surface around the bell-mouth comes
to wave violently and a large volume of gas comes to be sucked in directly from the
bottom face of the bell mouth. Under such condition, even though the vacuum pumps
are operated, the separator level falls to a great extent.
(9) As a result, the signal air pressure of the level transmitter falls excessively, greatly
reducing the opening degree of the discharge valve, namely the pump discharge flow
decreases considerably. In consequence, the speed of the pumping liquid flowing into
the bell mouth decreases and the wave motion of the liquid surface around it becomes
small. Thus the volume of the gas sucked into the bell mouth decreases.
(10) When the volume of the gas sucked in becomes less than the, extraction capacity
of the vacuum pump, the separator level begins to rise. At the same time the discharge
valve opens gradually and the pump discharge flow begins to increase.
(11) As a result, the volume of the gas sucked in begins to increase again, the
separator level falls and the discharge valve is throttled.
(12) In this way the conditions described in (10) and (11) are repeated and the tank
level further falls. During this time the vacuum pump is continuously operated. When the
tank level falls excessively, the separator level falls excessively due to excessive volume
of the gas sucked in and the discharge valve comes to be completely closed. Hereupon
the separator level rises a little and as the discharge valve opens a little the volume of
the gas sucked in becomes excessive again and the discharge valve is completely
closed again. When the separator level becomes below 5%, the red lamp (LOW
SEPARATOR LEVEL) lights, showing that the unloading work has entered the stripping
stage. Thus as the stripping of cargo tank advances even though the vacuum pump is
continuously operated the separator level does' not rise, as the result it so happens that
the discharge valve (81) remains completely closed from time to time. The continuation
of this condition for a long time means that there is no residual liquid to be unloaded.
When the red tamp (LOW SEPARATOR LEVEL) is ON for a certain time (3 min.) the
red lamp (COMPLETION OF STRIPPING) flickers and the buzzer sounds by each
action of the pressure switch (PRS2) and time- lag relay, thus showing the completion
of unloading work