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 be considered.

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 the tank.
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.