POWER April 2011 - 59

FIRE PROTECTION
when de-energized. " That means, in addition
to the winding insulation, other
combustibles within the generator-such
as various fiberglass parts, lubricating oil,
and dust-must be carefully considered in
any fire protection design.
Unfortunately, NFPA 851 is open to interpretation,
and to my knowledge there are
no representative tests that would quantify a
specific threshold in the full geometry of an
installed stator. For example, the 2005 edition
of NFPA 851 stated that " [t]hermoset
[insulation] does not require fire suppression
systems. " However the technical committee
that reviewed the standard reconsidered this
position after receiving reports of several
self-sustaining fires involving thermoset insulation
and has deleted that sentence from
the 2010 edition.
One major property insurer, FM Global
(formerly Factory Mutual), takes a somewhat
different approach to evaluating potential fire
losses. The current FM Global standard, Data
Sheet 5-3/13-2, Hydroelectric Power Plants,
requires fire protection for generators rated
50 MVA and over, regardless of winding
composition.
Good design practice is to determine if
the risk of an extended outage for repairing
or replacing a generator is unacceptable. If
so, protection should also be provided for
smaller machines. Accordingly, the U.S. Bureau
of Reclamation guidelines state that fire
protection should be provided for units over
10 MW.
In the end, a determination that fire protection
is needed will probably be driven more
by the potential for huge monetary losses
during a lengthy forced outage than by physical
damage to the generator alone.
The rest of this article looks at the types
of fire suppression systems available for aircooled
hydroelectric generators. The table
summarizes the advantages and disadvantages
of each option discussed.
CO2 Suffocates Fires
Although carbon dioxide (CO2
) is a very effective
agent, the fire extinguishing success
of CO2
systems requires enclosure integrity
and the proper application rate of the gas.
CO2
systems extinguish fires by starving
them of oxygen, so a release of the gas into
an enclosed space poses a risk to humans.
Hydro air-cooled generator fire protection system options. Source:
Dominique Dieken
System type
Carbon
dioxide
Low agent cost
Electrically nonconductive
No residue
Effective whether or not generator is
deenergized
Water spray Low installation cost
Low maintenance cost
Unlimited agent availability
Enclosure integrity not a factor
Safe for occupied spaces
Water mist No agent cost
Enclosure integrity not critical
Water damage unlikely
Safe for occupied spaces
Clean agent Electrically nonconductive
No residue
Safe for occupied spaces
Hybrid
systems
Enclosure integrity not critical
Electrically nonconductive
No residue
Low agent cost
Safe for occupied spaces
April 2011 | POWER
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59
High installation cost
Limited choice of system manufacturers/
installers
High maintenance cost
High agent cost
Requires " tight " enclosure
Need to increase concentration if generator
cannot be deenergized immediately
Same as for water mist, plus requires inert gas
cylinders
Requires inert gas cylinders
2. Orderly CO2 storage. A well-arranged
fire suppression system manifold for a generator
enclosure, including high-pressure CO2
cylinders designed to provide an initial and extended
discharge. Source: Dominique Dieken
Electrically conductive
Potential negative long-term effects following
water discharge
Requires " drying out " of generator
Advantages
Disadvantages
Lethal at fire suppression concentrations
Requires " tight " enclosure
Requires numerous personnel safety provisions
CO2 released into an inadequately designed
system or a leaky enclosure may not extinguish
the fire.
CO2
is still the prevalent fire protection
agent used for air-cooled hydroelectric generators
due to its nonconductive properties,
relatively low agent cost, and lack of residue.
The downside of releasing CO2
in an enclosed
space, such as a generator enclosure, is the
danger it poses to personnel (Figure 2).
Personnel safety is the most important
aspect of using CO2
a space that results in a CO2
. Any CO2 release into
concentration
exceeding 7% is dangerous to humans. For
comparison, the minimum concentration
of CO2
required to extinguish a flame is
usually between 34% and 50%. This high
concentration of CO2
has many owners and
operators concerned, and for good reason.
There have been 119 documented fatalities
involving total-flooding CO2
systems,
many of which involved marine applications.
According to some estimates, the actual
number of fatalities exceeds 300. An
additional concern: CO2
gas could migrate
to lower areas of the powerhouse and form
dangerous " pockets " in unexpected spaces
that can remain a personnel danger even
after the fire is extinguished.
These and other personnel protection concerns
are addressed by the current edition of
NFPA 12, Carbon Dioxide Fire Extinguishing
Systems, which contains a number of new
retroactive personnel safety requirements,
including ANSI-compliant three-panel warning
signs, manual lockout valves, and audible
and visible alarms. If the system is not provided
with a discharge delay, a formal procedure
must exist that ensures that the CO2
system is disabled anytime someone enters a
protected space. This procedure should also
ensure that the system is reactivated once
personnel have exited the enclosure. Other
design and operating considerations include
those that follow.
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POWER April 2011

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