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The stripping arrangements for
specialist vessels such as oil tankers, chemical tankers, gas carriers must
comply with the requirements of Marpol Annex II, reg 5.
All ships must not retain residue in
excess of
·
0.1m3 for category B
·
0.3m3 for category C
in the associated piping and immediate vicinity of the pump suction.
The stripping system is required to
ensure that there is not an excessive cargo quantity remaining before tank
cleaning or ballasting commences.
If the vessel does not use cargo tanks
for ballasting and washing is only carried out prior to repair or dry-docking,
a stripping system is not required as long as the effluent is discharged
ashore.
A stripping system also ensures that
the maximum cargo quantity is delivered.
As the majority of pumps are of the
centrifugal type a stripping system is required. There are three systems
available;
a)
Pressurising the tank and stripping by means of a small diameter pipe led
from the bottom of the tank well. A slow process that requires large quantities
of inert gas.
b)
Using a stripping box in the tank. The box is alternately subject to vacuum
and pressure. The flow being controlled by non return valves.
A deepwell pump running against a closed
discharge valve. The upper part of the discharge column is pressurised with
inert gas, the stripping line leads from the bottom of the discharge column to
a point after the discharge valve
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