Halon system for Ship fire protection

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Halon system for Ship fire protection

Halon systems

Halons are included in the Montreal protocol as gases with ozone depletion potential (ODP) and must not be specified for use in fixed fire fighting installations. Compared with R12 which has been assigned an ODP of I, Halon 1211 has been given an ODP of 3 and Halon 1301 a figure of 10. Existing fixed halon fire protection systems will need a replacement fire fighting medium for which adaptations will be required.

Halon systems with Halon 1301 have been fitted to a large number of ships. Halon is the name for halogenated compounds made by the replacement ot hydrogen in methane or ethane with one of the halogens. Fluorine, chlorine and bromine are halogens.

Halon 1301 has the chemical formula CF3 Br being known as bromo-trifluoromethane. It is a colourless, odourless gas with a density five times that of air and extinguishes fire by breaking the combustion chain reaction.

Other halogenated hydrocarbons such as methylbromide and carbon tetrachioride have been used in the past as fire extinguishing agents but have been banned by various authorities because of their extremely toxic nature. Halon 1301 however, is classed by Underwriters Laboratories as least toxic' (Group 6) and properly applied discharges of the gas allow people to see and breathe permitting them to leave the fire area with some safety.

It must be pointed out however, that when Halon 1301 is exposed to flame or hot surfaces above 480°C halogen acids and free halogens having a higher level of toxicity are produced. A self- contained breathing apparatus or a fresh air mask is therefore essential equipment when entering a space which has been flooded with Halon 1301.

In some small machinery spaces Halon 1301 systems may be found in which the Halon is stored in a sphere within the machinery space. In larger installations the storage battery is similar to that used in CO2 systems. The gas may be stored in 67 litre cylinders at a pressure of 40 bars. This equates to 75 kg of Halon 1301.

Figure : Halon 1301 cylinder valve with electric actuator (Fire Fighting Enterprises (UK) Ltd)

Figure 14.18 shows the release gear used by Fire Fighting Enterprises (UK) Ltd. In this system a cam on the end of the horizontal pin pushes the vertical valve rod downwards, when the pin is actuated. The pin may be moved by a pull wire or by one of a range of electrically or pneumatically operated actuators.

As soon as the valve rod opens the cylinder valve, gas from the cylinder is admitted to the top side of the piston and holds the valve open. When the gas pressure falls away a spring below the piston re-seats the valve. Only one cylinder needs to be fitted with the horizontal pin, shown in the illustration. The valves on the other cylinder can be linked to it with small bore copper pipe so that gas from the master cylinder opens all of the other valves simultaneously. Most halon cylinders are fitted with pressure gauges so that leakage can be more readily detected. The cylinders are also fitted with bursting discs

Halon system operation

The halon release arrangement shown in Figure 14.19 consists of a storage tank, two sets of CO2 operating cylinders and a manual release cabinet. The halon is stored at a pressure of 14 bar in the holding tank, which has a pressure relief, filling valve and level-indicator.

Release procedure is much the same as for COr When the cabinet is opened, the alarm operates, fans stop and dampers or vent flaps will close. With all entrances closed, the handles (1) and (2) are operated in succession. Handle (2) can only be moved when released by the blocking mechanism.

The contents of the CO2 bottles opened by handle (1) pressurize the pipeline between the halon tank and the master valve causing the bursting disc to rupture and allowing the halon to flow as far as the master valve. The pressure build up in this line acts on the blocking device to permit operation of handle (2). The latter opens the master valve to the engine room distribution pipe and also opens the CO2 bottles (2). CO2 from these ruptures the bursting disc at the top of the storage tank and then assists in expelling the halon.

The discharge must be completed in 40 seconds but the alarm sounds as the cabinet door is opened. Personnel must evacuate the space when warned. The 5% concentration gives a risk because the gas is toxic and must be treated with the caution as CO2.

Figure : Halon system release

Portable halon extinguisher

The use of halon in portable extinguishers should be discontinued when suitable alternatives are available because of the high ozone depletion potential.

Summarized below general cargo ship fire protection equipments & guideline:

Fire main system & related mechanism

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CO2 fire extinguishing installation

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Fire fighting equipments

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Details of fire detectors

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Machinery space fire & use of Walter Kidde CO2 system

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Fire protection system for cargo holds

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Low pressure CO2 storage

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Halon system

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Multi-spray system for the machinery spaces

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Automatic sprinkler system

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Details various portable extinguishers & how to use ?

.....more

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