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measure. Behavioral studies have shown that most Today, more than 100 tunnels worldwide are equipped with
people do not panic in a fire, even when they are an active fire protection system. Although a few automatic
unable to see. sprinkler systems have been installed in tunnels, most systems
6. The use of water-based fixed firefighting systems could are deluge systems. Water mist systems may require less water
cause the delamination of the smoke layer and induce per zone. Storage tanks, pumps, and pipes can be smaller, sav-
turbulence and mixing of the air and smoke, thus fur- ing on costs. However, to protect the small nozzle orifices
ther threatening the safety of persons in the tunnel. higher-quality materials, such as stainless steel, are required,
This has been shown not to be a valid concern. Fire which add to the costs.
tests have demonstrated that smoke does not usually
form a layer at the top of the tunnel but quickly fills the The type of ventilation system influences the type of sprin-
cross section. Normal air movement in the tunnel accel- kler system and the sprinkler system design impacts the venti-
erates this process. A water-based fixed firefighting sys- lation system performance. Some of the challenges faced with
tem reduces temperatures and the risk of fire spread to considering ventilation and fixed fire suppression systems in
other vehicles. the tunnel are:
7. Testing of a water-based fixed firefighting system on a
periodic basis to determine its state of readiness is · Selection of the type of fixed fire suppression system
impractical and costly. depends on the type of tunnel ventilation system.
A full discharge test is normally performed only · Wet fixed fire suppression systems can be activated
at system commissioning. During routine testing, the before ventilation and can control fire growth rate, fire
system can be configured to discharge flow to the drain- size, and the overall smoke production rate at an early
age system. stage of fire development.
· Activation time of a fixed fire suppression system may
differ depending on the type of ventilation.
SUMMARY
· For longitudinal ventilation, the sprinkler zones may need
PIARC, NFPA, and several European countries are rethink- to be switched or additional zones may be required once
ing their position on fixed fire suppression system application ventilation mode is in full speed.
for their tunnels. It is recognized that sprinklers are able to · With transverse ventilation, a short system activation
reduce fire size and temperature and prevent fire from spread- delay may need to be considered.
ing. In addition, it is recognized that timely activation of · Delay with the fixed fire suppression system activation
active fire protection systems will limit damage to the tunnel will require additional water supply because of the larger
in the event of a fire. However, there were also some earlier fire size at the time of activation.
lessons, which are to be reviewed when making a decision: · Extended delay with a fixed fire suppression system may
result in its inability to control fire, in structural damages,
· Once the sprinkler system is activated, it shall not be and a possible explosion. A reliable automatic fire-
turned off until a fire source is completely extinguished detection system is essential.
and the tunnel determined to be safe.
· With timely activation of a suppression system, the The undesirable consequences of fixed fire suppression
HRR is reduced. With delayed activation fire over- system activation, such as smoke destratification, increased
whelms and the suppression system may not be effec- humidity, and decreased visibility, are hopefully outweighed
tive. Extended delay with a fixed fire suppression system by its other positive outcomes of fire growth rate control,
may result in its inability to control fire, in structural containment of fire spread, and reduced temperatures.
damages, and possible explosions. A reliable automatic
fire-detection system is essential. The questions that need additional investigation are whether
· It is essential that the detection system is capable of the fixed fire suppression system can replace other tunnel fire
detecting a small fire (in the order of 15 MW) life safety systems, such as ventilation and passive protection
· Accidental activation of the sprinkler system is un- systems, or whether the size and requirements for such systems
acceptable. can be reduced.
