Active Fire Safety Systems

Active systems are both automatically and manually operated fire control systems and these can include sprinklers, risers and hydrants, hose reels, Heat detectors, Smoke detectors and portable fire extinguishers.  Information and  management systems are also included and this could be demonstrated with visual, audible and informative emergency control systems, fire detection alarms and communication with the fire service.  Control interfaces provide air handling, door releases, smoke control and emergency power supplies. 

Smoke Detectors.  

If smoke detectors are required to activate pressurization systems in fire isolated exits or smoke control systems these detectors should be positioned adjacent to each bank of lifts and set back from the door openings by not more than 3 m.

(Ref BCA E2.2a AS/NZS 1668, AS 1851, AS 1670, MS Schedule 3.7).

 

Photo Electric  smoke detectors consist of a light source and a photo cell.  These detectors are based upon light scattering and are designed so that light from the source cannot be seen by the photo cell except when scattered by smoke.  As the smoke increases the amount of light scattered by the smoke onto the photo cell increases, resulting in an increased signal from the photo cell.  If the electrical signal exceeds the preset threshold an alarm is activated.  These smoke detectors are more sensitive to visible larger smoke particles of 1.0 micron in diameter; these smoke paticles are typical of a smouldering fire.

Ionization  type smoke detectors consist of 2 chambers in series with each other.  One chamber is open to allow smoke to enter whilst the other is semi sealed to prevent smoke from entering.  Within these chambers a pair of electrodes exist and an electrical voltage is maintained between the two.  The ions are generated by a radio active source, ‘Americium 24’, and they move across the chamber resulting in a small ionization current.  If smoke enters the open chamber the current is reduced so inducing an electrical imbalance between the two chambers.  When a change in the preset threshold, corresponding to specific smoke concentration, is exceeded it generates an alarm.  These detectors are sensitive to small invisible smoke particles 0.1 micron in diameter typical of a fast burning fire. 

 

Beam type smoke detectors comprise of a transmitter and a receiver units and are separated by some distance.  A transmitted light beam is pulsed to the receiver and processed to convert it to an electrical signal.  If smoke enters the beam the light is reduced by scattering and absorption resulting in the reduction of the processed electrical signal being received.  When the signal drops below a defined threshold an alarm is generated.  These detectors are especially suited to large volume buildings such as warehouses.

 

Aspirating type smoke detectors work by sampling air from the protected area by a system of sampling pipes.  The smoke sensing component of these systems are based on  the light scattering principle, similar to photo electric smoke detectors, except that they use an intense light source and a highly sensitive photo cell.  Early ‘VESDA’ systems worked on a principle of three different alarm systems dependant on smoke concentration.  These type of detectors are particularly suited to computer and clean rooms because of the high rate of movement of air.

 

(A bank of VESDAs).

 

 

 A Vesda sampling pipe and fire wire on the ground floor protecting an atruim of 13 floors.

 

 

A VESDA going into alarm stage 1 and 2.

 

 

As such smoke particle size affects the performance of different smoke detection systems.

 

 

(Photo Optical Smoke Detectors)

Heat Detectors.  (BCA E, AS1603, AS1851 & AS 1670)

 Heat detectors can be classified as ‘rate of rise’ and ‘fixed temperature’ because of their mode of operation and sensitivity.  Rate of rise detectors are designed to operate when they detect a rapid rate of rise of > 10 Deg C a min.  Fixted temperature generate an alarm when the temperature reaches a pre set threshold.

Electro Pneumatic heat detectors work on a principle of expansion of a flexible diaphragm when subject to a rapid rise in temperature of more than 10ºc /min.  When this occurs it causes the flexible diaphragm to bend forward so pushing electrical contacts closed which activate the alarm.

Electronic type  heat detectors consist of electrical components.  When these components are subject to a temperature change the electronics in these detectors monitor the rate of change and when this exceeds a preset threshold an alarm will sound.  The majority of these detectors manufactured today are thermistors. 

Bi metallic type heat detectors use bi metallic strips or disc that change their shape when temperatures change, so closing a pair of electrical contacts to generate an alarm.

 

Pair wire heat detectors are as the name suggests two sprung wires that wrap around each other but are separated by insulation.  If the wires are subjected to high temperatures the insulation melts, the wires come together so producing an electrical short circuit to generate an alarm.

Sprinklers.  MS SA 76 Schedule 3.5 (g).  (Ref BCA E1.5, AS 1851, AS 2118 & AS 2218).In 1806  John Carey from England made the first automatic sprinkler.   Since then the basic sprinkler design has not changed a great deal.   Today there are many different types of systems and heads giving varing responses that are designed for differing types of buildings.   Sprinklers have a success rate of more than 97% worldwide.  No multiple deaths have been recorded in a sprinkler protected building (1 – 3 persons).  Upon activation the sprinkler initiates suppression reducing the Heat Release Rate (HRR) thus reducing the amount of smoke produced. They can also be connected to the fire indicator panel that calls the fire Service automatically, alerts the occupants in the building and starts the smoke management systems.  A standard sprinkler can deliver 20 - 40 Gallons of water a minute to the fire affected area.  A quick response sprinkler has a 3 – 5 mm frangible glass tube which in laboratory tests have been proved to respond 5 - 10 times faster than a standard sprinkler, these are not to be confused with the Early suspression Fast Response sprinklers (ESFR).  ESFR sprinkleres are rated by the K factor (this is a constant that describes the discharge orifice).  They discharge large droplets of water that delivers 100 Gals of water a min they can also be positioned higher in a building.  The quick response and ESFR sprinklers allow a greater Available Safe Egress Time (ASET) for building occupants.

 

A Fire Sprinkler requires a clear space of not less than 500mm below the level of the sprinkler deflector throughout the area.  For high piled combustible stock a clearance of not less than 1 metre.  (REf AS 2118).

 How They Work.

The frangible glass bulb contains a fluid and when the heat from the fire reacts with the sprinkler the fluid inside the bulb expands this consequently causes the glass bulb to shatter.  This action releases the plug allowing the water to be discharged over the deflector.  The water discharging over the deflector causes an umbrella effect. 

 

 

 

Some types of Sprinkler Systems.

Dry Pipe Systems - The system is charged with air or nitrogen under pressure holding the water back via a valve.  Once a head is broken the air/nitrogen rushes out followed by the water.  This type of system would be suited in areas that would be likely to freeze, for example freezer compartments etc.

Wet pipe- These systems contain water under pressure up to the sprinkler head, once the bulb shatters the water is delivered immediately to the affected area.

 

Pre action - This type of system contains air which is not necessarily under any pressure. Once a detector detects smoke or heat it will open a valve allowing the water to flow into the system up to the sprinkler heads.  When the heat of the fire is hot enough the bulb will shatter and operate the same as the dry pipe system.  Theses systems are used in areas that are susceptible to either accidental or malicious damage, so not to cause any serious damage by water.

Deluge - The sprinkler heads are open at all times (no bulb fitted).  However the water is held back by a closed valve.  As with the pre action system a detector detects smoke or heat that will automatically open a valve that will then allow water to be discharged by all the heads.  This type of system would normally be used in areas with high hazards i.e. LPG stores, Aircraft hangars etc.

 

Some different types of sprinkler heads.

 

Concealed Pendant.

Upright.

Early Suspression Fast Response (ESFR).

Recessed Pendant.

Horizontal Sidewall.

Window.

 

 

 

 

 

 ESFR Sprinkler Head.

 

 

       

        Wall wetting sprinkler head

 

 

 

 Recessed pendant standard response sprinkler head.