Page 318 - Ship Construction.DJ Eyres 6Ed

Ch26-H8070.fm Page 307 Wednesday, October 18, 2006 7:01 AM
Pumping and Piping Arrangements 307
of delivering foam to the entire cargo tanks deck area and into any tank the
deck of which has been ruptured.
Oil tankers of 20 000 tonnes deadweight or more are to have a fixed deck
foam system and a fixed inert gas system.

INERT GAS SYSTEM The inert gas system is to be so designed and
operated as to render and maintain the atmosphere of the cargo tanks
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non-flammable, other than when the tanks are gas free. Hydrocarbon gas
normally encountered in oil tanks cannot burn in an atmosphere containing
less than 11 per cent of oxygen by volume, thus if the oxygen content in a
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
cargo tank is kept below, say, 8 per cent by volume fire or explosion in the
vapour space should not occur. Inert gas introduced into the tank will
reduce the air (oxygen) content.
On an oil tanker, inert gas may be produced by one of two processes:

(1) Ships with main or auxiliary boilers normally use the flue gas which
contains typically only 2 to 4 per cent by volume of oxygen. This is
scrubbed with sea water to cool it and to remove sulphur dioxide and
particulates, and it is then blown into the tanks through a fixed pipe
distribution system.
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(2) On diesel engined ships the engine exhaust gas will contain too high
an oxygen level for use as an inert gas. An inert gas generating plant
may then be used to produce gas by burning diesel or light fuel oil. The
gas is scrubbed and used in the same way as boiler flue gas.

Non-return barriers in the form of a deck water seal, and non-return
valve are maintained between the machinery space and deck distribution
system to ensure no petroleum gas or liquid petroleum passes back through
the system to the machinery space.
The double hull and double bottom spaces of tankers required to have an
inert gas system are to have connections for the supply of inert gas.

CARGO TANK PURGING AND GAS FREEING When the tanker is fitted
with an inert gas system the discharged cargo tanks are purged until the hydro-
carbon vapours in the cargo tanks have been reduced to less than 2 per cent
by volume. Gas-freeing may then take place at the cargo tank deck level.
For tankers not provided with an inert gas system the flammable vapour
is discharged initially through the tank venting outlets specified by MAR-
POL. When the flammable vapour concentration at the outlet has been
reduced to 30 per cent of the lower flammable limit, gas-freeing can be con-
tinued at cargo tank deck level.
A range of portable gas freeing fans with associated equipment are
readily available with typical air movement ranging from 8 000 to 15 000
cubic metres per hour.