Vessel water mist fire extinguishing system explained…

On one of my previous posts, we have talked about CO2 fire fighting system which is present on all vessel types. As per rules and regulations, machinery spaces containing internal combustion machinery shall be provided with one of the fixed fire-extinguishing, but machinery spaces of above 500 m3 in volume shall, in addition to the fixed fire-extinguishing system required, be protected by an approved type of fixed water-based or equivalent local application fire-fighting system.

In the case of periodically unattended machinery spaces, the fire fighting system shall have both automatic and manual release capabilities. In the case of continuously manned machinery spaces, the fire-fighting system is only required to have a manual release capability.

Fixed local application fire-extinguishing systems are to protect areas such as the following without the necessity of engine shutdown, personnel evacuation, or sealing of the spaces:

  • the fire hazard portions of internal combustion machinery or, for ships constructed before 1 July 2014, the fire hazard portions of internal combustion machinery used for the ship’s main propulsion and power generation;
  • boiler fronts;
  • the fire hazard portions of incinerators;
  • purifiers for heated fuel oil.

Activation of any local application system shall give a visual and distinct audible alarm in the protected space and at continuously manned stations. The alarm shall indicate the specific system activated.

The water mist fire suppression system relies on the delivery of high-pressure water to mist nozzle heads installed at potential fire hazards sites. Through the atomizer heads (mist nozzles), water under high pressure breaks down into extremely small droplets. These droplets possess highly powerful fire-extinguishing characteristics, even against oil fires.

Water mist fire protection. Source and credit: MarioffHIFOG

The fine droplets remain suspended in the air and do not disturb the oil surface, they also do not settle to form a water layer upon which burning oil could float. Fine water droplets present a large surface area to the fire and so they exert a cooling effect as the droplets rapidly evaporate. The large droplet concentration and the evaporation have the effect of reducing the local oxygen
level. The cooling effect and the reduced local oxygen concentration result in the fire being extinguished.

As specified above, the protected areas consist of the separator room, diesel generator rooms, and areas above diesel engines and the boiler. A number of mist nozzle heads are installed in branch pipes that are connected to the main supply line to the compartment being protected. The primary supply line to the compartment has a section isolating valve that is operated by a solenoid. The water mist fire control system activates the solenoid valve when a fire is detected in the protected compartment; two fire detectors (one flame and one smoke) in a protected compartment must be engaged for water mist to be released into that compartment.

Example of a Water mist extinguishing system diagram

When the control system operates the solenoid valve it also starts the water mist supply pump, which pressurizes the water mist system. The water mist supply pump normally takes suction from the operating domestic fresh water tank, but in an emergency the pump may take suction from the sea suction crossover main. The pump suction valve from the fresh water system is normally locked open and the suction valve from the sea water crossover is normally locked in the closed position.

The suction from fresh water tank should be such that there is always a safe minimum quantity of water in the fresh water tank that will allow for 20 minutes operation of the water mist fire extinguishing system.

All the solenoid water mist release valves are located at a release block and the lines from the solenoid valves to the protected space nozzles are maintained in a dry condition in order to prevent corrosion.

Example of a valve block. Source and credit:

Each protected space has at least two heat, smoke or flame detectors. When the first detector is activated an alarm is initiated and when a second detector in a protected space is activated, the water mist pump is started and the protected space solenoid valve is opened.
For water mist to be released automatically into a protected space as described above, the system must be selected for automatic operation. The key switch at the control panel in the fire station must be in the AUTO position and the pump start switch at the local panel in the engine room must also be in the AUTO position.
Water mist may be released manually into a protected compartment by activation of the manual release switch located outside of the protected compartment. The water mist may also be released manually by the activation of the MANUAL RELEASE pushbutton, for a particular compartment, at the control panel in the fire station. For this manual operation of the system, the key switch and the pump starter switch must be set for AUTO operation as described above.
Outside of each protected compartment there is at least one pushbutton inside a glass fronted blue colored box. Breaking the glass allows the operator to press the pushbutton and manually release water mist into the compartment. Local manual release of water mist into a protected compartment is possible by turning the pump starter switch to the LOCAL position in order to start the pump; the solenoid valve for the protected compartment is then opened by turning the screw on the solenoid valve.
On some vessels, a backup pressurized water tank is fitted for emergency operation should the pump not function. The additional water tank is pressurized by nitrogen cylinders when emergency operation is required. The water tank must be maintained in the full condition and the nitrogen cylinders fully charged. The manual valves on the water tank and between the nitrogen cylinders and the water tank must be open at all times. Remotely operated valves from the water tank and between the nitrogen cylinders and the water tank are normally closed. The remotely operated valves are only opened when operation of the emergency system is activated. The water tank has capacity for 5 minutes water discharge without any power supply. The emergency system operates automatically should the pump fail to start.
The main control panel for the water mist fire extinguishing system is located in the fire station and the local water mist control panel, pump and solenoid release block are located in the engine room. The water mist fire extinguishing control system also shuts off the engine room ventilation fans when the system is activated.

The system should be tested on monthly interval for proper function. The testing procedure is, generally, as follow and must be coordinated with the officer of the watch on the bridge:

  • Close all manual outlet valves from the section solenoid valves and open the test valve at each solenoid valve. The blank at the end of the test valve drain pipe must be removed.
  • Activate one section solenoid valve manually by pressing the MANUAL RELEASE pushbutton at the control panel in the fire station.
  • Check that the pump starts and that the correct section solenoid valve opens. Water will be released at the drain valve and the control panel in the Fire Station should indicated that the section valve has opened.
  • At the fire station panel reset the alarm by turning the RESET RELEASE key to the RESET position and then back again or by pressing the RESET button.
  • Check that the section solenoid valve closes and that the pump stops.
  • Repeat the test for all of the other section valves.
  • Close all test valves and open all the manual outlet valves from the section solenoid valves.
  • Ensure that the operating domestic fresh water tank has sufficient water for operation.
  • Record the test in the log book.
Live test of water mist system by activating the fire sensors. Source and credit: ETO MELVIN CHRIS TV

If the vessel is equipped with an emergency water tank, after the individual section solenoid release units have been tested the water tank emergency system may be tested as follow:

  • All section line isolating valves must be closed and open the test drain valve on one of the section solenoid valves. The blank at the end of the test valve drain pipe must be removed.
  • Close the nitrogen cylinder valves and turn the pump switch at the control panel to the STOP position.
  • For the solenoid valve which has the drain test valve open, activate the release of the section solenoid valve manually by pressing the MANUAL RELEASE pushbutton at the control panel in the fire station and wait 18 seconds. Check that the nitrogen cylinder solenoid valve opens and that the solenoid valve from the water tank also opens. Water from the tank will flow through the open solenoid valve and the test valve to the drain line.
  • Reset the system at the remote panel by turning the RESET RELEASE key to the RESET position and back again or press RESET button. Close the test drain valve which has been opened and replace the drain line blank.
  • Check that all section solenoid valves are closed and that all section line valves are closed then open the nitrogen cylinder outlet valves.
  • Open all section outlet valves and the system is now in automatic operation.

If the water mist system has operated, the system must be returned to normal operation. After the fire has been extinguished, the section of line through which water mist has been released must be blown through with air for 2 – 3 minutes. The section solenoid valve which has operated should be manually opened, together with the drain valve and the line outlet valve. An air supply hose is connected and air is blown through the pipe system and nozzle heads in order to remove all water from the line section in order to prevent pipe corrosion. The air supply is then removed, the drain valve closed and the solenoid valve returned to normal operation.
If the water mist system has operated using sea water the entire system must be flushed through with fresh water and then blown through with air. It is essential that all traces of salt water be removed, as sea water can cause corrosion and salt deposits can cause blockage in parts of the system.

It is important that the nozzle heads and valves are checked at intervals in order to detect any leakage. The spray heads have plastic protective covers which should be kept in place to protect them from damage or being painted over, as the covers blow off when the system operates.

Also, it is important to note that the FW tank system is such that neither tank would ever be empty, as the tank not being used to supply water to the domestic system would be in the process of being filled by the FW generator. When changing over FW tanks for domestic purposes care must be taken to ensure that the water mist system is also supplied. There should be sufficient fresh water available to cover the largest section for a minimum of 20 minutes.

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