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Inert gas system for machinery installations onboard

Inert gases are those which do not support combustion and are largely nitrogen and carbon dioxide. Large quantities suitable for fire extinguishing can be obtained by burning fuel in carefully measured amounts or by cleaning the exhaust gases from a boiler.


Inert gas generator

The inert gas generator (Figure below) 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.

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The complete unit is arranged to be independently operated in order to supply inert gas for as long as the fuel supply lasts.

Inert gas generator
Fig 1: Inert gas generator


Boiler exhaust gas inerting system

A system much used on tankers where boiler exhaust gases are cleaned and inerted is shown in Figure 2 below. 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.

Funnel gas inerting system
Fig 2: Funnel gas inerting system

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.

Funnel inert gas system
Fig 3: Funnel inert gas system

The Fig 3 shows an inert gas system from a main boiler. Gas from the boiler uptakes is taken via pneumatically operated high temperature valves, where it then passes through a scrubber tower.
In the scrubber tower, seawater is sprayed for cooling the gas to around 3 or 4° C above the seawater temperature, the seawater is also used to remove most of the soot and sulphur from the gas. The gas then passes through a demister, which can be cleaned by means of back flushing. After the scrubber tower, the gas analysis is as follows: Two centrifugal fans then supply the dry, clean inert gas at 1.2 ­ 6 bar via a deck seal, non-return valve and pressure vacuum valve to the cargo tanks for inerting. Safety features include :
Fixed installations

A variety of different fixed fire fighting installations exist, some of which are specifically designed for certain types of ship. A selection of the more general installations will now be outlined.

Types of portable fire extinguisher

There are four principal types of portable extinguisher usually found on board ship. These are the soda-acid, foam, dry powder and carbon dioxide extinguishers. Details as below :
  1. Soda acid portable fire extinguisher
    2. The container of this extinguisher holds a sodium bicarbonate solution. The screw-on cap contains a plunger mechanism covered by a safety guard.

  2. Foam type portable fire extinguisher
    3. The main container is filled with sodium bicarbonate solution and a long inner polythene container is filled with aluminium sulphate

  3. Dry powder fire extinguishers
    4. The outer container contains sodium bicarbonate powder. A capsule of carbon dioxide gas is located beneath a plunger mechanism in the central cap

  4. CO2 portable fire extinguisher
    5. A very strong container is used to store liquid carbon dioxide under pressure




Fixed fire extinguishing installations

A variety of different fixed fire fighting installations exist, some of which are specifically designed for certain types of ship. A selection of the more general installations will now be outlined.
  1. Fire main system for cargo ships

    2. An outbreak of fire requires a source of ignition, the presence of combustible material and ample oxygen. Of the three factors, oxygen is provided in large quantities in machinery spaces, accommodation, dry cargo holds and tanker pumprooms by ventilation fans. Air supply trunkings are not only a source for a supply of oxygen to feed the fire but also have potential for carrying smoke from one area to another....

  2. Automatic water spray & water mist system for machinery protected area

    3. The automatic spray or sprinker system provides a network of sprinkler heads throughout the protected spaces. This system may be used in accommodation areas, and in machinery spaces with certain variations in the equipment used and the method of operation. ....

  3. Automatic foam induction system for machinery space fire

    4. Foam spreading systems are designed to suit the particular ship's requirements with regard to quantity of foam, areas to be protected, etc. Mechanical foam is the usual substance used, being produced by mixing foam making liquid with large quantities of water. Violent agitation of the mixture in air creates air bubbles in the foam. ...

  4. CO2 fire extinguishing installations for machinery spaces

    5. Fire extinguishing installations employing CO 2 stored under pressure at ambient temperature are extensively used to protect ships' cargo compartments, boiler rooms and machinery spaces. When released the CO 2 is distributed throughout the compartment, so diminishing the relative oxygen content and rendering the atmosphere inert....

  5. Inert gas systems, inert gas generator

    6. Inert gases are those which do not support combustion and are largely nitrogen and carbon dioxide. Large quantities suitable for fire extinguishing can be obtained by burning fuel in carefully measured amounts or by cleaning the exhaust gases from a boiler. ....

  6. Fire fighting Halon system

    7. 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. ....



Other important fire & safety equipments



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