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 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 is consisted with 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 lead(filled with sealed liquid) and low/high pressure detectors
are fitted to upper and lower part of separator. Static pressure (basic
pressure) of sealed liquid of lead 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 A/M
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 discharge 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 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 control
led through the pressure switch.
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 100% 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 by manual loader to get any opening.
(10)Solenoid valve
The valve is controlled by the pressure switches operated
through the pneumatic signal from the level transmitter, and opens and 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 corresponds the
discharge valve opening to the opening signal from the A/M selector.
(12)Booster relay
The booster relay, 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.
(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 turn 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 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 ctops.
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.
(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 the 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.
(13)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.
(14)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 remains completely
closed from time to time.
(15)The continuation of this condition for a long time means
that there is no residual liquid to be unloaded. When the red lamp (LOW SEPARATOR LEVEL)
is ON for a certain (3mm.) the red lamp (COMPLETION OF STRIPPING) flickers and
the buzzer sounds by each action of the pressure switch and time-lag relay,
thus showing the completion of unloading work.
