POWER April 2011 - 60

FIRE PROTECTION
Evacuate Affected Spaces. Given that
the quantity of CO2
discharged is usually for
a relatively small enclosure, the likelihood of
CO2
gas migrating to lower levels at harmful
concentrations in the large volume of a
powerhouse is remote. Nevertheless, in the
event of a CO2
system discharge, it is prudent
to evacuate levels in the powerhouse beneath
the generator. Personnel entering these lower
levels after the discharge should be provided
with oxygen monitors and, preferably, selfcontained
breathing apparatus. The powerhouse
should be thoroughly ventilated and
atmospheric checks made before other personnel
are allowed to reenter.
Use an Odorized CO2. Although not required
by NFPA 12, CO2
gas with an added
wintergreen odorizer can enhance personnel
safety. Much as the natural gas industry adds
mercaptan to odorize natural gas to produce
that rotten eggs smell as a safety measure,
the smell of wintergreen quickly warns staff
that carbon dioxide gas is present. Odorized
CO2
can be difficult to obtain from local suppliers,
as it is not a common industrial gas.
A better alternative may be to install a commercially
available odorizer assembly, which
injects a scent into the CO2
discharge.
stream during a
3. Deluge system. A stainless steel deluge
water spray system with spray nozzles
protects an air-cooled generator. Courtesy:
Public Utility District No. 1 of Chelan County
Perform Baseline Tests. A discharge/
concentration test is an ideal time to also
measure oxygen levels throughout lower areas
of a powerhouse to provide a baseline for
which areas of the powerhouse could become
dangerous in the event of a CO2
system discharge.
From
a design perspective, the initial concentration
of CO2
when released must reach
30% after 2 minutes and 50% after 7 minutes
to effectively extinguish an open flame,
in accordance with both NFPA 12 and FM
Global Data Sheet 5-3/13-2. Next, the concentration
must remain at a minimum of 30%
for 20 minutes or the wind-down time of the
generator, whichever is greater, to allow for
cooling and to prevent re-ignition. These
specifications could pose special problems
inside a generator enclosure if generator
cooling air is drawn through the enclosure
using fans. If this is the case, the design must
consider additional CO2
quantities to make
up for losses caused by cooling air until the
generator is brought to rest.
New CO2
systems require a full discharge
test as part of the acceptance testing
requirements. This test measures the
concentration within the protected space
with an analyzer to ensure that the design
concentrations of CO2
Water Spray Douses Fires
Water spray systems are the second-mostcommon
fire protection system, assuming
that an adequate water supply is available,
Because, like CO2
systems, water spray systems
are engineered using common components
and water is abundant at hydroelectric
plants, spray systems are also the least expensive
for air-cooled generators.
Water is a highly effective fire extinguishing
agent, but the generator should be de-energized
prior to the application of water-a
disadvantage compared with CO2
systems.
Water spray systems extinguish fire by direct
cooling, by placing calibrated quantities of
water directly on the fire. In most cases, water
spray is applied through directional spray
nozzles located on a ring just above the stator.
The two water spray options have different
system designs:
are achieved. If
this test was not initially performed, or if
the generator was recently modernized, an
ideal time to perform such a test on highpressure
systems (the gas cylinders) is after
12 years from the last hydrostatic cylinder
test date, the date when the cylinders must
be emptied and tested anyway. In the event
of a discharge, the doors to the generator
enclosure should remain tightly closed for
at least 20 minutes after the discharge. Only
then should be generator enclosure be carefully
inspected to ensure that the required
residual concentration of CO2
Many older CO2
is present.
systems use components,
4. Preaction system. This is a close-up
of a water spray line and a spray nozzle inside
an air-cooled generator. Courtesy: Public Utility
District No. 1 of Chelan County
such as squibs on the discharge heads, that
may no longer be commercially available.
Squibs can be replaced with modern electromechanical
actuators. In addition, these
older systems typically lack the personnel
safeguards required by NFPA 12. As long
as they are properly retrofitted and/or maintained,
older CO2
systems can provide a
level of safety and reliability equivalent to
modern systems.
CO2
continues to be an appropriate fire
extinguishing agent for generators, and with
the proper controls and safeguards, its hazards
can be minimized. A safety poster seen
during a recent plant visit encapsulates the
purpose of these safety systems: We do not
work in a dangerous environment; we work
in an environment where hazards are recognized
and controlled to safe levels.
60
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■ Deluge systems. A flooding device called
a deluge valve uses an external fire detection
signal to charge the piping with water.
A deluge system uses open nozzles; once
the deluge valve is actuated, all nozzles
spray water (Figure 3). The deluge system
gives a better level of protection because
the entire stator is sprayed in the event
of fire. The disadvantage is an increased
chance of inadvertent discharge. For unoccupied
plants, cycling deluge valves are
available, which automatically shut off
the flow of water when the fire is extinguished,
thus minimizing water damage.
The system design and installation are
governed by NFPA 15, Water Spray Fixed
Systems for Fire Protection.
■ Preaction systems. A preaction system
uses closed nozzles. The piping integrity
is supervised using air pressure, resulting
in a supervisory alarm signal if the piping
system is breeched by leak or failure. Preaction
systems require two separate events
to cause a discharge and are thus considered
to give the highest protection against
inadvertent discharge. The preaction valve
admits water to the piping system, but
there is no water discharge until the fire
has generated sufficient heat to actuate
the thermal element in one or more spray
nozzles. The disadvantages are the inherent
delay caused by the two separate sensors
and the likely lack of uniform water
distribution onto the stator, which could
result in increased fire damage (Figure 4).
Although there are no known cases where
water spray from a fire protection system
has caused significant damage to a generator,
that doesn't mean damage isn't possible.
Therefore, water discharge in the absence
of a fire should be avoided. In the event of
POWER | April 2011
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POWER April 2011

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