The automatic water sprinkler system is an automatic fire detecting, alarm and extinguishing system which can deal quickly and effectively with any outbreak of fire that may occur in accommodation or other spaces on ships.
The system consists of several sections comprising a number of sprinkler heads mounted on the pipes, each section being connected through a section control valve to a sprinkler main which in turn is connected to a pressure tank and a pump. Number of sprinkler heads per section to be not more than 200. Each section has an alarm system.
The entire system is initially charged with fresh water at a pressure of about 8 bar by a tank (tank is half filled with fresh water), maintained under pressure by compressed air. This should be such that the pressure at the highest sprinkler head in the system is not less than 4.8 bar. It is also connected to an independent sea water pump with its own sea water suction. Each sprinkler head has a quartzoid bulb which retains a diaphragm seal in the outlet of the water pipe. This bulb is partially filled with a special fluid so arranged that a rise in temperature in the compartment concerned will cause the liquid to expand and entirely filled the space, the bulb burst, the water pressure forces the diaphragm out and water flow out from the sprinkler. Under the specific pressure maintained in the tank by air pressure, the water from the sprinkler is deflected outwards and broken into a fine spray by the serrated edge of the sprinkler base and will adequately cover a floor area of about 16 square meters at a rate of 5 L / m2 / minute.
When the sprinkler head comes into operation, the non return alarm valve for the section opens and water flows to the sprinkler head. This non return valve also uncovers the small bore alarm pipe lead and water passes through this small bore alarm pipe to a rubber diaphragm. The water pressure acts on this diaphragm and this operates a switch which causes a break in the continuously live circuit. Alarms both visible and audible, fitted in engine room, bridge and crew space are then automatically operated.
Thus the pressure drop at the release of the water causes an alarm to sound on the bridge and indicates on the board, the zone in which the ruptured sprinkler heads are situated. When the system pressure dropped to 5 bar, the sea water pump will start automatically and continue supply with sea water. A hose connection is also provided so that water can be supplied to the system from shore when ship is in dry dock. The sprinkler heads are spaced not more than 4 meters apart and 2 meter from any bulkhead or part of the ship’s side which forms a boundary of the protected space.
Stop valves ‘A’ and ‘B’ are locked open and if either of these valves are inadvertently closed, a switch will be operated that brings the alarms into operation. The alarm system can be tried by opening valve ‘C’ which follows delivery of water similar to that of one sprinkler head to flow to drain. At least two sources of power must be provided to operate the sea water pump and automatic alarms. The component should be tested weekly.
What are the requirements of automatic water sprinkler system on ships ?
- Each sprinkler head must be capable of delivering water at a rate of 5 L / m2 / minute.
- Each sprinkler head is sufficient to cover the area of about 16 square meters.
- One section is permitted to contain maximum 200 heads.
- The pressure at the highest sprinkler head in the system is not less than 4.8 bar.
- Sprinkler heads are spaced not more than 4 meter apart and 2 meter from the vertical wall.
- There must be at least two powers source for the sea water pump, alarm system and FW pump.
- This system consists of independent sea water pump with its own sea water suction.
- Section alarm can be tested individually by test cock on each section.
What is the principle of shattering quartzoid bulb in sprinkler head ?
Principle is ‘Boiling Liquid Expanding Vapor Explosion’ (BLEVE). It is an explosion caused by the rupture of a vessel containing a pressurized liquid above its boiling point.
How the sprinkler head operates in automatic water sprinkler system on ships ?
Each sprinkler head is made up of steel cage fitted with a water deflector. A quartzoid bulb, which contains a highly expansible liquid, is retained by the cage. The upper end of the bulb presses against a valve assembly which incorporates a soft metal seal. Refer to the diagram below.
When quartzoid bulbs are manufactured, a small gas space is left inside the bulb so that, if the bulb is subjected to heat, the liquid expands and the gas space diminishes. This will generate pressure inside the bulb and the bulb will shatter once a predetermined temperature is reached.
Once the bulb shatters the valve assembly falls permitting water to be discharged from the head which strikes the deflector plate and sprays over a considerable area.
Generally the operating temperature range of quartzoid bulbs is 68 °C to 93 °C but the upper limit of temperature can be increased. Quartzoid bulbs are manufactured in different colours which indicate the temperature rating of the bulb.
‘SHIP SAFETY AND ENVIRONMENTAL PROTECTION’ by GAUTAM SEN
Oral Guide by – MIN ZAR TAR