Cargo hold smoke detection system and its purpose

Smoke detection systems are typically installed for cargo holds and work in conjunction with CO2 fire extinguishing systems. Additionally, hold sampling pipes are utilized to deliver CO2 into the cargo holds as needed and they are connected to three-way valves which are used to effectuate the changeover. Smoke detection panels are positioned in the CO2 chamber, with an alarm repeater on the bridge and in the fire control station which is often located on the main deck level.

Three-way valves arrangement

The smoke detection panel is equipped with detection lines from the holds, each of which is connected to a separate detector box equipped with a visual detector on the inside and an airflow detector on top. The smoke detectors are self-contained in terms of internal dust residue, and their sensitivity is maintained constant up to a predetermined threshold of turbidity; when this level is exceeded, an error is presented on the LCD display. The smoke detection panel is equipped with connections for the main and emergency power supply, as well as for fault and common fire alarms. The alarm signal is communicated to the alarm and monitoring system.

Display panel with detection box unit

The CO2 room’s fan unit has two independent electrically driven fans for taking air samples from the holds. The exhaust sample air is routed to the atmosphere outside the compartment, which is typically aft on the main deck area. It is critical to know the exact location of this exhaust pipe because caution must be used in the immediate vicinity when poisonous vapours, in case of fire, are likely to be emitted by the fans.

Fan unit for smoke detection system

The three-way diverter valves used to link the smoke system to the CO2 system are typically positioned in the CO2 room, in the proximity of the sampling panel. A sampling isolation valve is located beneath each diverter valve. The primary sample/CO2 discharge pipes for the cargo holds goes through decks and any other compartments, depending on the vessel’s configuration, to the cargo holds. They are typically located on the first level of each hold and are painted red to make them easily identifiable.

The sampling/CO2 discharge pipes are routed through the hold compartment in an area that is unlikely to sustain damage during cargo operation. Automatic self-draining valves are installed in the lowest points of sampling pipes. The drain valves are normally open but automatically close when the line is pressurized, either during an emergency event involving CO2 flooding or during routine maintenance involving compressed air blowing through the lines. When the pressure is lifted, the drains will reopen.

The smoke detection system should not be used during cargo operations or painting in hold compartments, as the solvents emitted by the paint can contaminate the smoke detector. The procedure for operating the smoke detection system is quite simple and is generally common to all vessel, but it can vary due vessel layout or system maker specific instructions. So, in general, the procedure is as follow:

  • You must check and ensure pipe and hoses integrity, free movement of the diverter valves and they are set for air sampling.
  • Switch on the main and emergency power supplies to the detector panel.
  • Check and ensure that there are no faults present.
  • Switch on the air sampling fans and check if the fans are running.
  • After power switch on a “Normal operation” indication will appear on the panel display.
  • Perform a lamp and buzzer test to ensure their functionality.

During system normal operation, if smoke is detected in the air sample the detector will trigger an alarm and the display at the detector panel will indicate a fire alarm with the number of the cargo hold in which smoke has been detected. You will need to accept the alarm by pressing the dedicated button on the control panel and this will stop the buzzer and the indication red light will be steadily illuminated. If the fire alarm is not acknowledged within a certain period of time (2-3 minutes) by pressing the dedicated button on the duty alarm system the general emergency alarm will be activated.

The system is, generally reliable and doesn’t require extensive maintenance. The display should be checked on a daily basis to ensure that the unit is operational and there are no fault indications on the main panel or repeater panels. Similarly, the buzzer and control lamps should be checked for functioning on a daily basis. As per Class and manufacturer’s requirements, every three months the system should be checked as follow:

  • Check the air flow indicators by closing the isolating valves temporarily, one at a time.
  • Check the fan units and their drives
  • Blow through the sampling pipes with compressed air (close the valves to smoke detectors before this operation).
  • Check the smoke detector by opening one detection unit at a time unit and blow smoke into the outlet bore of one detection box and alarm should sound. Repeat the procedure for all detection boxes.

During normal operation dust can build-up on the smoke detectors which are self-controlled for sensitivity within a certain range. If deposits are excessive a fault is triggered on the panel display. The unit must be opened and smoke detectors cleaned.

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