FIRE DETECTORS
Engine room fires represent a hazard for crew members working onboard ships and heavy financial loss for the ship owners or operators. The investigation reveals that about 56% of all engine room fires were caused by the combination of oil leakage onto a hot surface. Removal of hot surfaces and correct installation of couplings and hoses in fuel oil system has reduced risk of fire significantly. In case of an engine room fire, a local water mist fire fighting or extinguishing system can be used to avoid the escalation of fire and keep the temperatures low.
System Components
Main Control Panel
Water Pump (For High Pressure Water)
Fire Detectors ( Detects Fire in Protected Area)
Solenoid Valve (One For Each Protected Area
Water Nozzles ( Produces Water Mist)
System Description
The water mist fire fighting system basically consists of fire detection part and fire fighting part. The fire detection part monitors and detects the fire condition of the protected area. If any fire is detected, it transits the information on fire location to the main control panel.
The fire fighting part activates the pump and solenoid valve(for the protected area) and discharges the water mist through the nozzles over the protected area. The water mist fire detection and fire fighting system can be operated automatically or manually.
Automatic Operation
The water mist fire fighting system consists of two detectors placed over the area to be protected (such as main engine, auxiliary engines, incinerator, auxiliary boiler, purifiers, etc.). Out of these two detectors, one is a smoke and other is a flame detector. The figure below shows smoke, flame detector along with the water mist nozzle fitted on top of the protected area.
If only one detector is activated in a protected area, it gives a fire alarm as a pre warning. In case both the detectors are activated, the information is transmitted to the main control panel of the water mist fire extinguishing system and it starts the water pump, and opens the solenoid valve to the particular protected area. Water at high pressure (around 10 bar at the nozzle) reaches water mist nozzles and forms fine mist which is sprayed over the area under fire. Water mist is discharged until stop. If the fire is not extinguished, the water mist can be discharged during 20 minutes according to international rule regulations.
The figure below shows solenoid valves which are electrically operated by the main control panel for directing high pressure water to the nozzles placed in protected area.
Manual Operation
In case water mist is to be manually sprayed over any protected area, the ‘START’ push button on the local or remote control panel for the particular area can be activated, which will start the pump and activate the solenoid valve for the respective area.
Requirement and Rules
According to SOLAS Chapter 2-2, Regulation 10.5.6 and 1.2.2.4, new vessels constructed on or after July 1, 2002, passenger ship of 500 gross tonnages, cargo ships of 2000 gross tonnages and some existing vessels should arrange this system.
Also in IMO MSC Circular 913and 668 Appendix A, it is mentioned this system should provide localized fire suppression in are, without the necessity of engine shut down, personnel evacuation, shut down of forced ventilation fans or the sealing of the space and component manufacturing standards of equivalent water based fire extinguishing systems.
Bimetal strips form the basis of thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature. When temperature increases, the bimetal curves as the metal with higher coefficient of expansion lying on the outer side of an arc undergoes a greater increase in length. With one end fixed, the movement of the free end of the strip can be arranged to close an electric circuit that operates an alarm. This principle is utilized in thermal fire detectors designed to operate at a fixed temperature or a ‘rate of rise’ in temperature.
RATE OF RISE THERMAL FIRE DETECTOR
Figure above shows how a ‘rate of rise’ thermal fire detector operates using two bimetal strips. One bimetal strip has a higher thermal inertia either because it is lagged, as shown in the figure, or because it is thermally shielded from the space being protected. On a appreciable rate of rise in temperature, the contact ‘B’ on the faster response bimetal strip closes on contact ‘C’ of the slow acting bimetal strip. This causes an alarm signal to be produced by an alarm circuit connected between points ‘A’ and ‘D’. In the case of a very slow rate of rise in temperature, the difference in movements between contacts ‘C’ and ‘B’ will be such that a high temperature will be reached before alarm sounds. To ensure that the alarm signal is initiated before a temperature of 78 degree Celsius is reached, a second contact ‘F’ is provided on the slow acting bimetal strip. At the required space temperature, contact ‘E’ closes to contact ‘F’ and n alarm signal is initiated. At low rates of temperature rise, (less than 1 degree Celsius / minute) the alarm should not operate until the temperature exceeds 54 degree Celsius. Without the insulation, the upper bimetal strip would act as a basic fixed temperature thermal detector.
Thermal fire detectors are least sensitive type of detector. They have a high thermal inertia and the fire has to produce large amount of heat before the temperature at the detector is sufficient to cause it to operate. Consequently, they are normally used in spaces such as laundries, drying rooms, galleys, and pantries, where other detectors are susceptible to false alarm from water vapour or smoke.
In the most recent detector designs the bimetal strips have been replaced by thermistors (solid state devices of, for example, nickel, manganese and cobalt, whose electrical resistance changes significantly with temperature ). The principle of operation is, however, no different as one thermistor is exposed to the air and one is shielded.
Smoke Detectors
The working principle of light scattering smoke detectors is as follows. When the sizes of small particles are greater than the wave length of the incident radiation, the light is scattered in different directions. Known as the ‘Tyndall Effect’, named after its discoverer, this is used for smoke in several different arrangements, however, all depend on light being scattered by smoke onto a photoelectric cell that is obscured from the light beam in normal circumstances.
In the arrangement shown above, the smoke detectors are placed in line with the pulsed infrared light source. In the absence of smoke, the masking disk casts a shadow over the circular region of diameter ‘AB’ and prevents light from emitting diode from falling onto the detecting photocell. In the presence of smoke, light is scattered and some of it falls onto the photocell. The evaluation circuit is arranged to trigger an alarm if the threshold value is exceeded for a predetermined number of consecutive pulses.
A diagrammatic arrangement of a light obscuration smoke detector is shown in figure above. An infrared light beam, at an operating frequency of 1000 pulses per second, is received by a photoelectric cell and analysed. In a fire, smoke rises and spreads below ceiling level and the intensity of the light falling on the receiver is reduced due to light scattering or absorption. The fire alarm sounds when the signal strength is reduced to between 40 – 90 % for a period of about 5 seconds. Using a pulsed light source saves power and allows the use of a receiver turned to the pulse frequency to reduce false alarm caused by sunlight or illumination. Light obscuration smoke detector is suitable for covering large areas with flat ceilings. They are not suitable for outside use.
Ionization Smoke Detector – Fire Detectors
An ionisation using a single radioactive source is shown diagrammatically in figure above. The ionization smoke detector detector has a chamber that is open to the air and is divided into two regions by a perforated electrode known as the collector. A small radioactive source ionises the air in these two regions and the electrical potential between them is balanced on the collector. When smoke enters the chamber, the balance is disturbed by an amount dependent on the smoke density. This change forms he basis of the analogue output to the trigger circuit.
Sensitivity and False Alarms
Ionization smoke detector responds best to invisible (below 5 micro meter diameter) particles. The sensitivity can be varied by adjusting the threshold value or varying the configuration of the closed chamber characteristics. Their sensitivity is very high for particles of one micrometer and below but falls off with increase in particle size.
Susceptibility to false alarms is decreased by the use of pulse type detectors in which the voltage is applied in pulses and the alarm trigger is operated only after the threshold potential has been maintained during a specified percentage of pulses.