DESCRIPTION AND APPLICATIONS

The water mist fire suppression systems, also known as sprinkler systems, are the most widespread in the world both in civil and industrial applications.
The sprinkler system is made of a pressurised pipe network with water (humid systems) or air (dry systems) connected with the sprinkler heads containing sensors reacting to heat.
The pipe network is linked to a water source designed to supply water to the sprinkler at a predetermined flow rate, pressure and duration. Its main characteristic is that it is completely automatic and needs therefore an automatic water supply system.

Its success is based in its simplicity, the sprinkler head in fact operates both as a detection device and as a starting signal of intervention.

The first examples of sprinklers (drop) go back to the beginning of XIX th Century. In 1878 Mr. Grinnell built the first modern sprinkler, then called with his name, shaping it in the way as we know it now.

Hand in hand with its application, technical regulations followed. The standard NFPA 13 dates back to 1896. European standards such as the English FOC, the German VdS and the Italian Concordato Italiano Incendi go back to the Fifties or Sixties years and all have an insurance origin.

The sprinkler systems are essentially based on the SPRINKLER HEAD.
There are two consolidated types:

Fuse head sprinkler.
Bulb head sprinkler ;

The first one is based on the fusion of a eutectic alloy duly calibrated at the activation temperature.
The second is based on the thermal expansion of a liquid caused by temperature until the quartz bulb breaks activating the water delivery.

The bulb system eventually has been recognised as the best and the more convenient.

All the sprinkler heads have been calibrated in the same way at normal activating temperatures (57 °C), intermediate and high (343 °C).Calibration is fundamental in establishing fire protection as it is related to environment, type of hazard, type of building, ceiling components etc.

Another important element is the deflector shape now consolidated into three shape heads, one upright, one pendent, the third conventional.

Apart these two elements, the sprinkler heads have been untouched for almost a Century until in 1980 a number of studies have modified them in the spray flow, size of drops and time of reaction.

At present sprinkler heads are as follows:

Sprinkler heads RTI, quick response
Sprinkler head Large Drop
Sprinkler head ESFR
Sprinkler head Residential
Sprinkler head Sidewall
Sprinkler head Receded
Sprinkle head Concealed.

The RTI head sprinkler (Response Time Index)

The large drop sprinkler heads have been designed for fire where a great amount of heat occurs with a rapid progression such as in stocked goods where upward diffusion of flame is favoured with subsequent increase of heat. These heads have a larger nozzle (5/8") and deliver a larger water flow with bigger drops capable to cope with rising smokes.

ESFR sprinkles heads (Early Suppression Fast Response) represent the best development of large drop heads. Nozzles vary from ¾" to 1". Their best use is in warehouses where goods are piled up above 10 meters without intermediate shelves.

Residential sprinkler heads are based on two principles: heads are quickly activated thanks to very sensible and small devices and deliver a water flow calibrated for residential buildings.

Sidewall sprinkler heads have a lateral flow, vertically installed and are horizontally fitted. Therefore they guarantee the same performance of the heads fitted in the centre of the room. They can be installed where it is not easy or forbidden to place heads in the centre of premises.

One of the specifications of sprinklers, which should be decided by the user, is its finishing. From an aesthetical point of view, there are several solutions; sprinkler heads can be painted or chromed, they can have a washer or be recessed or concealed.

Sprinkler systems are of three kinds: humid, dry and preaction.

Humid systems are the more common: pipes are constantly kept full of pressurised water and the opening of one or more heads causes the immediate exit of water.
At the same time a humid control valve provokes the acoustic alarm, through a hydraulic bell transmitting the warning signal to a remote manned place.

Dry systems are used in cold areas where freezing of water in pipes if feared. In this case pipes are in a constant pressure state thanks to a feeding device (compressor): the opening of one or more heads causes the immediate exit of air and subsequent reduction of pressure. A hydraulic control valve, kept closed by pressure under normal circumstances, allows water to flow in the pipes and reach the open heads. In these systems, however, there is always a delay in the exit of water, partly overcome by the installation of acceleration devices to be fitted near the control valve. The installation must be very accurate in order to avoid leakages and condensate that can block the hydraulic mechanism. In this case it is possible to install a drainage mechanism in all terminals of the system.

Preaction systems are particular because they require an additional and independent system for their activation, detecting the actual presence of fire before allowing the sprinkler system to operate.
It is therefore a more accurate system but it can also be less reliable because is depending on another system thus accruing the probability of faults (which becomes the sum of the two).

Another important factor is the water reserve of the sprinkler systems. In European standards the duration time of water supply is strictly and univocally fixed according to the reference classes; in the U.S. standards a bigger discretion is allowed.

Table A - Minimum duration time for water supplies