Aerosol fixed fire extinguishing systems involve the release of an agent to extinguish a fire by interruption of the chemical process of fire. There are two methods considered for applying the aerosol agent to the protected space as defined by the IMO (see MSC Circular 1270) as:
Condensed aerosols – created in pyrotechnic generators through the combustion of the agent charge.
Dispersed aerosols – these are not pyrotechnically generated but are stored in compressed gas containers with carrier agents (such as inert or approved halocarbon gases) with the aerosol released into and distributed throughout the space through valves, pipes & nozzles.
Systems will be installed on the vessel based on the requirements in this MGN and the equipment approval certificate and accepted by the attending surveyor. Systems are to be installed in accordance with the requirements of each manufacturer’s design, operation, and maintenance manuals. Installation details including position of generators relative to entrances, exits and walkways and the concentration design calculation should be supplied along with a declaration from the competent installer that it complies with the MCA requirements and accepted by the vessel’s Certifying Authority before the system is installed.
The installation plan including the aerosol generator type, size and position along with the calculations for the protected space volume and concentration should be provided to the vessel as well the approved body approval certificate. The installer will provide a declaration that the system is suitable for the protected space and has been installed in accordance with MCA requirements.
Installation is only permitted in machinery spaces that are not normally occupied. Such spaces are protected spaces that are occupied for limited periods only, in particular for monitoring, or undertaking maintenance, and where the continual presence of persons is not necessary for the effective functioning of the equipment.
The machinery space housing the Fixed Aerosol Fire Extinguishing System is referred to in this document as the protected enclosure or protected space. The protected enclosure volume should be calculated by the system supplier or installer and agreed by the vessel owner, equipment supplier and supplied to the surveyor for acceptance (as noted in section 5.2). The required concentration of the aerosol varies between manufacturer / system types, and agreement of the design concentration required for the space should be sought from the manufacturer or their agent and be based on fire extinguishing tests as defined in this document. The methods used for calculating the design concentration shall be included in the vessel’s application for certification. The quantity of extinguishing agent for the protected space should be calculated at the minimum expected ambient temperature using the design density based on the net volume of the protected space, including the casing. The net volume of a protected space is that part of the gross volume of the space, which is accessible to the fire-extinguishing agent.
Where the protected enclosure contains multiple spaces where mixing of the aerosol and air may be inhibited, such as a partially enclosed room, the partially enclosed space should be treated as a separate space for design concentration calculation purposes. For example, using an aerosol with a design concentration of 100 g/m3, a 30 m3 enclosure could be protected by ten 300 g generators but if there is a partially enclosed space of 5 m3 within, where mixing is significantly inhibited, 25 m3 of the space could be protected by eight 300 g generators plus one 100 g generator for example, and the remaining 5 m3 space protected by one 300 g generator plus one 200 g generator for example. These would be controlled from the same control unit; the space is only treated as separate for design concentration calculations.
Where the space being protected has the possibility of being entered, an isolation switch will be provided that should be used to deactivate the system whenever personnel enter the space. The system isolation switch shall be situated outside the protected space and close to the system or control panel or adjacent to the main entrance to the protected space. The isolation switch shall only inhibit the discharge of the system, fire detection and alarm systems shall not be impacted and will continue to work normally during isolation. The isolation switch shall electrically isolate and earth each conductor of the wiring to the generator units / discharge device. When the isolation switch is active a visual indication will be provided at the system control panel.
Only control panels approved by the manufacturer and included in the system’s approval certificate shall be used. Control panels approvals are not in the scope of the approval tests in this MGN, however control panels for use with each system must be approved by the manufacturer.
Control wiring for the system should be shielded or run in separate conduits from power cables or other alternating current (AC) cables.
Clear instructions for installation, maintenance, testing, and operation applicable to the specific system installed on the vessel should be provided on board.
Clear safety notices should be placed at the entrance to the protected space, inside the protected space, at the control panel and at the system isolation switch. These will include notices to the nature of the hazards, including hot aerosol release and potential for carbon monoxide to be present in quantities which may present a danger to life.
Generator units should be positioned and orientated such that the aerosol discharge stream shall not be across any entrance or exits to the space or over any walkways through the protected space unless set a safe distance from them. The manufacturer shall supply safe distances for positioning of generator units in their manual. The aerosol discharge stream temperature shall be provided in the manual for each generator and temperatures must not exceed 75 °C in the stream when it crosses walkways, entrances/exits to the space or anywhere people may be exposed to the stream.
The system may be activated by an automatic system (where approved by the manufacturer). The automatic discharge must be automatically disabled when the isolation switch is engaged. Where an automatic release system is installed a pre-discharge alarm must sound for a minimum of 20 seconds before release. This is to allow any personnel present evacuation time, and time to close openings prior to activation. Where there is an automatic activation system in place the system shall not activate when isolated. Following an attempted automatic activation impeded by the isolation switch being active, the system shall discharge immediately when the isolation is removed.
The space to be protected should be provided with means to close all openings which may admit air to the protected space. All openings should be closed before the system is discharged.
Class A fire materials such as wood, paper, plastics, rubber, or textiles are to be stored outside the protected space. Regular inspection and control of Class A materials present shall be undertaken in the protected space.
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