POWER April 2011 - 61

FIRE PROTECTION
a discharge, the accepted practice is to dry
the generator by running it while it is electrically
isolated for at least 24 hours before
re-energizing it.
Water Mist Smothers Fires
Water mist is a relatively new player in the industrial
fire protection market. This technology
discharges about one-tenth or less of the
quantity of water of a water spray system and
is thus a good choice when ample fire protection
water is not available or when larger
quantities of water discharged are undesired.
Water mist offers two distinct advantages:
a plentiful agent supply, and the enclosure
" tightness " is not critical.
The operating concept is that a small
quantity of water discharged at high pressure
through very small orifices produces a fine
mist, which is more efficient in fire suppression
than larger water drops. The primary
extinguishing mechanism consists of heat
extraction and displacement of air by the water
vapor. The system design and installation
are governed by NFPA 750, Water Mist Fire
Protection Systems.
Water mist systems are among the most
expensive fire protection options because of
proprietary parts, their extensive use of stainless
steel materials, and the requirement of
a full-scale fire test for each manufacturer's
system. Unlike the other systems, which are
engineered on a case-by-case basis, each manufacturer's
water mist system is evaluated and
tested to prove that it will be able to extinguish
fire within the expected parameters. Therefore,
the system listing/approval stipulates limits as
to the type of hazards and enclosure size that a
particular system can protect.
There are currently few manufacturers
whose systems hold listings/approvals from
two agencies (Underwriters Laboratories and
FM Global) for the protection of machinery
enclosures. These include Marioff, Securiplex,
and Tyco Fire & Building Products.
Particular attention should be given to the
listing/approval limitations of any water mist
system to ensure that it is appropriate for the
protected hazard and that it is also acceptable
to the stakeholders.
There have been numerous water mist installations
on air-cooled hydroelectric turbines,
both in the U.S. and Europe, in recent years.
Clean Agent Gases Are
Now Available
There are a variety of halon-alternative clean
agent gaseous systems, including halocarbons
such as DuPont's FM-200 and inert gas agents
such as Ansul's Inergen. The design and installation
of these systems is governed by NFPA
2001, Clean Agent Fire Extinguishing Systems.
Clean agents generally have the advantages
of being nontoxic at their design concentrations
and leaving no residue. Clean agents
are not specifically listed for generator protection,
and testing of halocarbon agents
on Class C (electrical) fires has shown that
5. A hybrid system. This spray mist system uses high-pressure water and special spray nozzles
to produce a water mist that can blanket and smoother a fire. On the left is the stainless steel piping
system that forms the mist. On the right are nitrogen-filled cylinders that provide the pressure to
atomize the water droplets. Although it is an effective fire suppressant system, the hybrid microfog
design is the most expensive of the fire protection system options. Source: Dominique Dieken
flame can rekindle if the equipment remains
energized. Therefore, it is imperative to deenergize
the generator prior to actuating the
fire suppression system. If this cannot be
done, then the concentration of fire suppressing
agent should be increased accordingly.
The cost of clean agent systems is generally
higher than that of water spray or CO2
systems. In addition, as with CO2
, the integrity
of the generator enclosure is critical, as the
agent concentration must build to prescribed
levels to extinguish flames.
Due to the prohibitive cost of discharging
the agent for the purpose of measuring
its concentration during an acceptance test,
NFPA 2001 allows an enclosure integrity test,
also known as a door fan test. By measuring
various pressure differentials while pressurizing
and depressurizing the room using a
calibrated fan, a computer program predicts
the retention time of the minimum required
concentration. Although NFPA 2001 specifies
the minimum protection duration as 10
minutes, it is prudent to increase this to 20
minutes, following the methodology for CO2
systems. Because it is highly unlikely that a
single-shot system will maintain the concentration
for that length of time, an extended
discharge time should also be considered.
As with CO2
, special care must be exercised
when evaluating the generator cooling air arrangement.
Because of the many drawbacks,
there are few, if any, clean agent systems installed
on hydroelectric generators today.
Consider a Hybrid
Suppression System
Hybrid systems use a dual fluid, water, and
inert gas (typically nitrogen) to generate a
micro fog. They share many properties of
water mist and clean agent gas systems, but
they operate at lower pressures than either
system and produce even smaller water droplets
than water mist systems (Figure 5). The
extinguishment mechanism is through flame
cooling and reduced oxygen concentration
by displacement of air from water vapor
and inert gas. The design and installation of
these systems is governed by NFPA 750, Water
Mist Fire Protection Systems and NFPA
2001, Clean Agent Fire Extinguishing Systems,
with certain exceptions.
Similar to water mist, listed/approved systems
are currently only available from two
manufacturers: Ansul (AquaSonic) and Victaulic
(Vortex 1000). As with earlier systems,
a hybrid system must be designed, installed,
and tested in accordance with the manufacturer's
instructions and its listing/approval.
Detect, Then Release
The objective of the fire detection system is
twofold: to notify plant operators of a fire and
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