Aircraft Freon Discharge Cartridges

Freon Discharge Cartridges

The service life of fire extinguisher discharge cartridges is calculated from the manufacturer's date stamp, which is usually placed on the face of the cartridge. The manufacturer's service life is usually recommended in terms of hours below a predetermined temperature limit. Many cartridges are available with a service life of approximately 5,000 hours. To determine the unexpired service life of a discharge cartridge, it is necessary to remove the electrical leads and discharge hose from the plug body, which can then be removed from the extinguisher container.

Care must be taken in the replacement of cart­ridge and discharge valves. Most new extinguisher containers are supplied with their cartridge and discharge valve disassembled. Before installation on the aircraft, the cartridge must be properly assembled into the discharge valve and the valve connected to the container, usually by means of a swivel nut that tightens against a packing ring gasket.

If a cartridge is removed from a discharge valve for any reason, it should not be used in another discharge valve assembly, since the distance the contact point protrudes may vary with each unit. Thus, continuity might not exist if a used plug which had been indented with a long contact point were installed in a discharge valve with a shorter contact point.

When actually performing maintenance, always refer to the applicable maintenance manuals and other related publications pertaining to a particular aircraft.

Freon Containers

Bromochloromethane and freon extinguishing agents are stored in steel spherical containers. There are four sizes in common use today ranging from 224 cu. in. (small) to 945 cu. in. (large). The large containers weigh about 33 tbs. The small spheres have two openings, one for the bonnet assembly (sometimes called an operating head), and the other for the fusible safety plug (figure 5.6). The larger containers are usually equipped with two firing bonnets and a two-way check valve.

The containers are charged with dry nitrogen in addition to a specified weight of the extinguish­ing agent. The nitrogen charge provides sufficient pressure for complete discharge of the agent. The bonnet assembly contains an electrically ignited power cartridge which breaks the disk, allowing the extinguishing agent to be forced out of the sphere by the nitrogen charge.

A single bonnet sphere assembly is illustrated

. The function of the parts shown, other than those described in the preceding para­graph, are as follows:

(l)         The strainer prevents pieces of the broken disk from entering the system,

(2)       The fusible safety plug melts and releases the liquid when the temperature is between 208° and 220° F., and

(3)       The gage shows the pressure in the container. In this type of design, there is no need for siphon tubes.

In some installations the safety plug is connected to a discharge indicator mounted in the fuselage skin, while others simply discharge the fluid into the fire extinguisher container storage compart­ment.

The gage on the container should be checked for an indication of the specified pressure as given in the applicable aircraft maintenance manual. In addition make certain that the indicator glass is unbroken and that the bottle is securely mounted.

Some types of extinguishing agents rapidly cor­rode aluminium alloy and other metals, especially under humid conditions. When a system that uses a corrosive agent has been discharged, the system must be purged thoroughly with clean, dry, com­pressed air as soon as possible.

Almost all types of fire extinguisher containers require re-weighing at frequent intervals to deter­mine the state of charge. In addition to the weight check, the containers must be hydrostatically tested, usually at 5-year intervals.

The circuit wiring of all electrically discharged containers should be inspected visually for condi­tion. The continuity of the entire circuit should be checked following the procedures in the ap­plicable maintenance manual. In general this con­sists of checking the wiring and the cartridge, by usⁱng a resistor in the test circuit that limits the circuit current to less than 35 mill amperes to pre­vent detonating the cartridge.