Page 270 - Introduction to Marine Engineering

Fire fighting and safety 245

Inert gas generator
The inert gas generator (Figure 13.13) burns fuel in designed quantities
to produce perfect combustion. This provides an exhaust gas which is
largely nitrogen and carbon dioxide with a very small oxygen content.
The exhaust gases pass to a cooling and washing chamber to remove
sulphur and excess carbon. The washed or scrubbed exhaust gas is now
inert and passes to a distribution system for fire extinguishing. The
complete unit is arranged to be independently operated in order to
supply inert gas for as long as the fuel supply lasts.

Funnel gas inerting
A system much used on tankers where boiler exhaust gases are cleaned
and inerted is shown in Figure 13.14. The exhaust gas is cleaned in a
scrubbing tower, dried and filtered before being passed to the deck
mains for distribution. The gas will contain less than 5% oxygen and is
therefore considered inert. It is distributed along the deck pipes by fans
and passes into the various cargo tanks. Seals in the system act as
non-return valves to prevent a reverse flow of gas.
The inert gas is used to blanket the oil cargo during discharging
operations. Empty tanks are filled with gas and the inert gas is blown out
when oil is loaded.
Inert gas-producing units have the advantage of being able to
continuously produce inert gas, A bottle storage system, such as carbon
dioxide flooding, is a 'one-shot' fire extinguisher which leaves a ship
unprotected until further gas supplies can be obtained.

Halon system
Halon 1301 (BTM) and Halon 1211 (BCF) are two halogenated
hydrocarbon gases with special fire extinguishing properties. Unlike
other extinguishing agents which cool the fire or displace oxygen the
Halon gases inhibit the actual flame reaction. As a result of its low
vapour pressure when liquefied Halon can be stored in low-pressure
containers. Alternatively if a standard carbon dioxide cylinder is used
then approximately three times as much gas can be stored. An additional
advantage is that the atmosphere in a Halon flooded space is not toxic,
although some highly irritant gases are produced in the extinguishing
process.
A Halon storage system would be very similar to one using carbon
dioxide except that fewer cylinders would be required. The liquefied
Halon is usually pressurised in the cylinders with nitrogen in order to
increase the speed of discharge. Bulk storage tanks of Halon gas are also
used with cylinders of carbon dioxide and compressed air being used to
operate the control system and expel the gas.