2.11 STRIPPING TANK WASHINGS
Effective cleaning of cargo tank bottoms requires that they be continuously
drained while washing. The ship's stripping pumps or eductors are used
to remove tank washing water throughout the tank washing process. For
the pumps or eductors to do the job properly, a number of factors must
2.11.1 Washing trim
The ship must be correctly trimmed and listed. The best tank washing
trim is a compromise among ship speed, hull bending and shear forces
and effective tank draining. The ballast arrangement must not exceed
maximum acceptable seagoing stress numerals. Excessive trim will reduce
speed more than necessary. The trim must be enough so that the wash
water flows with enough vigour to carry sediment to the stripping suction
inlet. 4 meters of trim may be adequate for a 40,000 tonne ship, while
8 meters may be necessary for a 200,000 tonne tanker.
The ship should be listed so that the tank will drain toward the stripping
suction. If stripping suctions are located in the port corners of the
tank, then a port list of three or four degrees is needed.
Poor draining will result if the ship is rolling. Additional list may
be required so that the tank bottom is always inclined toward the suction
inlet while rolling.
2.11.2 Stripping pump operation
Check the setting of the stripping system valves carefully before tank
washing begins. All suction valves on the stripping line should be verified
closed and the appropriate discharge valves open to transfer washings
to the selected slop tank. Verify that the overboard discharge and overboard
block valve (if fitted), are closed and lashed. Finally open the suction
valve to the tank(s) being washed.
The stripping pump should be started before the tank washing machines
are turned on to remove any residual oil from the tank before washing
begins. It should be operated at moderate speed and frequently checked
to see that it is not vapour bound, but is transferring the tank washings
effectively to the slop tank.
While hot-water washing, the water from the stripping pump discharge
should be warm or hot. If it is cold, the stripping pump is not keeping
up with the washing operation. To prime the stripping pump, close the
suction from the washing tank momentarily and open the stripping suction
from an appropriate ballast tank. Then close the ballast tank suction
and resume stripping the washing tank.
Float-type ullage gauges should be retracted to the stowed position
when tank washing, to prevent damage from the washing jets. If these
gauges need to be used to check for wash water accumulation, then the
tank washing machines must be shut off before lowering the float.
If an eductor is used for stripping, good suction will be available
unless the mechanism becomes clogged with scale or other material from
Continue to run the stripping pump on a washed tank after the tank washing
machines have been stopped. Watch the suction and discharge gauges of
the stripping pump to detect when it has removed all free liquid from
the tank and lost suction. If using a reciprocating pump with a stroke
counter, the stroke rate will increase when the tank dries.
Do not sound the tank with portable ullage equipment to see if it
is dry! Sounding tanks immediately after washing could produce
an electrostatic discharge.
Do not sound tanks until they have been ventilated for one hour, or
five hours have passed without ventilation.
2.11.3 Stripping pump capacity
The capacity of the stripping pump is not as great when tank washing
as when stripping cargo. It cannot be operated at high speed while tank
washing and part of the time there will be no liquid at the suction
for it to pump. To ensure that the pump will keep up with the tank washing
machines, it should not be expected to pump more that one half of its
rated capacity while tank washing.
To determine the number of machines a single stripping pump is capable
of serving, the chief officer must refer to the tank washing machine
pressure-to-volume curve. This curve indicates the volume of water each
machine will spray into the tank as a function of the tank washing pressure.
A standard portable tank washing machine will use approximately 30 tonnes
of water per hour at 12 kg/cm sq. washing pressure. If four such machines
are used in a single tank, the stripping pump must be capable of pumping
240 tonnes per hour, rated capacity, in order to keep the tank properly
drained while washing.
The best stripping pump will not provide effective tank draining unless
the stripping lines are in good condition (free of holes) and the tank
bottom framing limber holes are clear. If limber holes are clogged with
sediment or rust, the tank must be washed, ventilated and de-scaling
first, then the tank bottom must be re-washed after sediment/scale have
been removed. All tanks should be periodically de-scaled to ensure free
drainage of cargo and tank washings.