Chapter-II: Fire Suppression Methodology
A basic necessity of Fire suppression system is the detection of fire conditions as early as possible, so as to provide enough time for Automated Systems/ Fire personnel for effective counter actions.
It is extremely essential that addressable microprocessors based devices be used, so that the locations of fire may be easily located. These devices (or, Detectors) are provided all over the power plant for detection of fire as early as possible.
Fire detectors are designed to detect one or more characteristics of fire, i.e. smoke, heat or flame. Any single type of detector is not suitable for all applications and the final choice will depend on the individual circumstances. Usually combined use of different types of detectors is made to achieve appropriate standard of protection. The detectors working on different principles of operation will respond differently to a given fire situation. Also, a particular type of detector cannot detect a fire equally efficiently in all situations. Detectors are classified into the following categories:-
- Smoke Detectors
- Heat Detectors
- Flame Detectors
- Linear Heat Sensing Cables
These devices are controlled by Fire Alarm Panels, which also monitors all local control panels, all fire pump control annunciation panel and all addressable initiating devices such as isolator modules & manual call points.
The Fire Alarm Panel consists of a Monitoring station, a Master CPU, Loop Cards & Supervisory Control Modules. The Fire Alarm panel should be able to:-
- Detect fire quickly enough to fulfil its intended functions.
- Transmit the detection signal.
- Translate the detection signal into a clear alarm indication that will attract the attention of the user in an immediate and unmistakable way and indicate the location of fire and initiate operation of ancillary service, such as fire extinguishing system, etc.
- Remain insensitive to phenomena other than those which its function is to detect.
- Signal immediately and clearly and supervised fault that might jeopardise the correct performance of the system.
A number of external “Hooters” or “Visual Emergency Indicators” are generally required for protective premises. The number and distribution of these alarms in the premises should be such that the sound level/audibility requirements are met with a view to alert the occupants and initiate fire fighting actions with the least delay.
The grouping of external fire alarm sounders can be done in either of the following way:-
- Sounding/ Glowing of alarms can be so arranged that any alarm operates all the sounders throughout the premises. This grouping is particularly suitable for smaller premises.
- Sounding/ Glowing of alarms can be so arranged that the alarms operates initially in the sector of fire origin or in this zone and on its adjoining areas, or in this zone and in specially selected areas of high flammability.
General fire alarm system through external sounders can be provided in either of the following ways:
- Single Stage Continuous Alarm- In this scheme, a continuous alarm (evacuation alarm or signal) sounds in the sector/zone of fire. This is normally done, particularly in smaller premises, sparely populated premises or storage premises.
- Two-Stage General Alarms- In this scheme, two-stage alarm is provided in which a continuous evacuation alarm is immediately given in sector/zone of fire or in a restricted area, together with intermittent ‘alert’ signal in other parts.
Protection techniques for various systems are briefly discussed below:
- Hydrant system-
Protection in a Hydrant system is achieved by operating of Hydrant Valves, Landing Valves and Water Monitors by Fire & Safety personnel. This protection system is entirely manual.
- High Velocity Water Spray System.
HVW Spray System is used mainly for the protection of Transformers, Lube oil System of Boilers, Boiler Burner Fronts, etc.
As it is hazardous for fire fighting personnel to go near such areas, an automated system is provided with Deluge Valves by spraying water through the spray nozzles which are attached to the spray rings. The Spray rings are erected around the protected instrument along with the Detection system, which uses Quartzite Type Bulb detectors for detection system. The bulbs burst up in case of fire conditions, thus actuating the Deluge valves.
- Medium Velocity Water Spray System.
MVW Spray System is used mainly for the protection of Cable Galleries, Conveyors, Fuel Oil Tanks, etc.
As it is hazardous for fire fighting personnel to go near such areas, an automated system is provided with Deluge Valves by spraying water through the spray nozzles which are attached to the spray rings.
This system employs the use of addressable detectors, Linear Heat Sensing cables, etc for actuating the Deluge valves.
- Sprinkler System.
In a Sprinkler System, the area to be protected is pressurised with the alarm valve secured in the open position. When a fire sprinkler is exposed for a sufficient time to a defined temperature rating of the heat sensitive element (glass bulb or fusible link) it bursts out, allowing water to flow from only the affected sprinkler.
When this occurs water from the water supply will pass through the alarm valve to the affected fire sprinkler and also past the alarm bell. The resultant pressure drop will also activate the alarm pressure switch, which in turn will activate an alarm thus gathering attention for the area.
- Fixed Foam System.
The foam system consists of foam bladder tank, foam proportioner, foam discharge outlets, deluge valves, isolation valves, interconnection piping and instrumentation. The operation of fixed foam system is made automatic with the aid of probe type heat detectors provided in the HFO, LDO and HFO Day Oil tanks.
In the event of fire in any HFO/LDO tank, the detectors in the tank will detect the fire and will actuate the Deluge valve of respective tank through the solenoid valve provided in the respective DV trims. At the same time water from the deluge valve downstream flow into the bladder tank and the water enters the area between vessel wall & bladder, applying pressure to the bladder .The foam concentrate is forced out of the bladder through the foam outlet pipe & flow into the inductor through hydraulically operated ball valve.
Water from hydrant header starts flowing to the proportioner and foam concentrate from foam bladder reaches the inlet of hydraulically operated ball valve. As soon as pressure develops at inlet of proportioner, the ball valve opens automatically thereby injecting foam concentrate into water flowing through the proportioner. The rate of injection of foam concentrate is controlled by the Pressure Operated valve (Balancing valve), ensuring mixing of foam concentrate with water in the right proportion.
The foam water mixture flows to the foam makers provided in the affected tank, where it is converted into foam and discharged in to the tank. Once the foam is discharged in to the tank, it spreads on the oil surface forming a blanket and extinguishes the fire.
- Inert Gas System
The operation of Inert Gas System is made automatic with the aid of various detectors, Manual Call Points employed in various zones of Fire Alarm Systems. The Inert gas (NN100) fire extinguishing system consists of the following:-
• An automatic fire detection system
• A control panel to release the system
• Cylinder units (Storage cylinders, Pilot cylinders, Manifold pipe, Rack and others)
• Actuating cylinder cabinets
• Selector valve unit
• Manual actuation devices/ abort switches
• Audio visual safety warning devices
• Instrumentation associated with the control systems
• Discharge nozzle with orifice
• Pressure Relief Vent as per requirement for various areas.
When fire is detected, a detector sends a signal to the control panel which registers there is fire. When the signal from the first detector is received, an alarm sounds in the control panel to alert the people. When a second verified signal is received from the second detector in the same area, the extinguishing sequence begins, an alarm sounds in the area to warn to evacuate the people. Output signals will be initiated to shut down the Air Conditioning system of the fire affected area. Simultaneously Fire Dampers will also be shut down to prevent any air ingress/avoid any gas leakage from the protected areas.
In the next chapter, there will be a brief description of Fire Alarm System.
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