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DIESEL ENGINE OR ELECTRIC MOTOR
FOR YOUR FIRE PUMP DRIVER?
What are advantages and disadvantages of diesel-engine and electric motor
driven fire pumps, and when is each the appropriate choice?
Gayle Pennel, P.E.
Schirmer Engineering Corp.
ASSUMPTIONS
For this study, it is assumed that the appropriate
pump size is known. Cost of fire pump maintenance
and operation is factored into this analysis, while
requirements for fire pump maintenance are beyond
its scope. Discussions of reliability are based on the
following assumptions:
• Fire pumps are installed according to NFPA 20,
Standard for the Installation of Stationary Pumps
for Fire Protection.
• Fire pumps are maintained according to NFPA 25,
Standard for the Inspection, Testing, and Maintenance
of Water-Based Fire Protection Systems.
• Emergency generators are installed according to
NFPA 110, Standard for Emergency and Standby
Power Systems.
SELECTION
Primary considerations in selecting a fire pump
driver are:
• Reliability
• Initial cost
• Maintenance cost
• Operating cost
• Design
• Safety
RELIABILITY
Causes of most fire protection system failures can
be eliminated by installing fire protection systems
and components according to NFPA standards,
maintaining fire protection systems according to
NFPA 25, and providing remote supervision
according to NFPA 72, National Fire Alarm Code.
Reliability of Fire Pump Diesel Engine Drivers
Diesel-engine-driven fire pumps are not affected by
power outages, and are designed to be more reliable
than automotive, truck or standby diesel generator
set engines. Most of the reasons that engines fail
to start are minimized or eliminated in a fire pump
installation. The fire pump diesel engine isn’t
subjected to cold temperature, backup batteries are
part of every fire pump installation, batteries are
monitored, and the engine is tested weekly for 30
minutes. The fuel level is supervised.
Reliability of Single-Source Electric Power
for an Electric Motor
NFPA 20 requires power supply to electric-motordriven fire pumps be reliable, but doesn’t define
reliable. Reliability of electric power is affected by
many things including natural disasters, transformer
or substation failure, utility grid maintenance, etc. In
August 2003, much of the northeastern grid covering
parts of New York, New Jersey, New England,
Pennsylvania, Ohio, Michigan and several Canadian
provinces was blacked out up to several days.
Hurricane Isabel caused massive blackouts the
following month, up and down the East Coast. Since
1999, the utility serving Chicago, IL, experienced
various outages, some lasting 3 to 4 days. Downtown
Chicago recently experienced outages involving
multiple substations.
Loss of power and the start of a fire cannot be viewed
as entirely independent events, i.e. there is a higher
likelihood of fire during an outage. Storms, lightning,
tornados, hurricanes and terrorism may cause both a
power outage and fire. Reaction to a power outage
may result in a fire, too, as when using candles.
Reliability of Electric Power from Two
Substations for an Electric Motor
Providing backup power can increase reliability of
electric-motor-driven pumps. Backup power can be
from a different substation or an onsite generator.
Theoretically, if one power source is 99% reliable,
two totally independent power sources would be
99.99% reliable. However, while backup power from
a second substation improves reliability, substations
aren’t totally independent. Events causing loss of
power at one substation may cause loss of power
at other substations, as with the northeast outage.
Reliability of Electric Power from a Utility
and an On-Site Emergency Generator for
an Electric Motor
Properly installed and maintained emergency
generators increase reliability of an electric-motordriven fire pump. However, there are significant
differences between NFPA 20 diesel engine fire pump
drivers and NFPA 110 emergency generator drivers.
Then too, differences in installation and testing
requirements make diesel engine fire pumps more
reliable than emergency generators.
Failure Probabilities and Expected Losses With Different Pump Drivers
Item
Power Supply Reliability
Power from Single Utility
Power from Two Substations
Emergency Generator
Pump Reliability
Pump Driver Reliability
Diesel Engine Fire Pump Driver
Electric Motor Fire Pump Driver
Electric Motor Driven Fire Pump
Single Utility
Two Substations
Utility and Emergency Generator
Diesel Engine Driven Fire Pump
Dual Electric Motor Driven Pumps
With emergency generator backup
One Diesel and One Electric Pump
Without emergency generator backup
Dual Diesel Engine Driven Pumps
Failure Probability
Reliability
Expected Loss
with $10,000,000 Exposure 2
1.0%1
0.5%1
1.0%1
0.1%1
99.00%
99.50%
99.00%
99.90%
0.10%1
0.010%1
99.90%
99.99%
1.1089%
0.6094%
0.1200%
0.1999%
98.8911%
99.3906%
99.8800%
99.8001%
$110,889
$60,944
$11,998
$19,990
0.0101%
99.9899%
$1,012
0.0022%
0.0004%
99.9978%
99.9996%
$222
$40
POWER SUPPLY
When the site’s transformer is installed and owned
by the utility, the cost is built into the rates charged.
If the owner must pay for the transformer, it’s likely
that a diesel engine fire pump will cost less than
the total cost of the electric-motor-driven fire pump
with transformer.
DIESEL VS. ELECTRIC
Diesel-engine-driven fire pumps are more reliable
than electric-motor-driven fire pumps with power
from a single utility, or two substations. The reliability
of a diesel-engine-driven fire pump is slightly less
than an electric-motor-driven fire pump with power
available from both a utility and an emergency
generator. The simplified expected loss shown in the
accompanying table provides a way of evaluating
reliability. When exposed values exceed $10 million,
increased reliability (diesel-engine-driven fire pump
or electric-motor-driven pump with emergency
generator backup) is clearly justified. A more
sophisticated analysis is included in the complete
white paper to compare cost and reliability.
1. In general, when the transformer is supplied by the
utility and the feeder is not too long, electric-motordriven pumps with a single utility power source and
utility-furnished transformer can be installed for less
than diesel-engine-driven pumps. The cost differential
is less when a reduced voltage starter is required for
the electric-motor-driven fire pump. The other
significant variable in the total installed cost of an
electric-motor-driven fire pump is the length of the
electric feed from the transformer to the electric
motor. This can quickly make the electric-motordriven fire pump more expensive than the dieseldriven fire pump. When the exposed value is low
(less than $10 million) the decreased reliability of
an electric-motor-driven fire pump with power from
a single utility may be acceptable (expected loss
increases by $90,899).
2. Diesel-engine-driven pumps provide better
value when considering installed cost and expected
loss when exposed value is moderate ($10 million
to $100 million).
1. Failure Probability based on author’s best estimate.
2. Expected loss for a single fire, assuming a negligible loss if the fire pump operates and a total loss if the fire pump fails to operate.
The expected loss is presented to aid in evaluating reliability issues. The exposed value includes the value of the building and
contents, but also business interruption potential and liability exposure.
3. Installed cost of diesel-engine-driven pumps
is less than for electric fire pumps with a dual
power source (i.e. an emergency generator or two
substations), and provides nearly the equivalent
reliability to a system with an emergency generator
and greater reliability to two substations.
4. For high exposed value (>$100 million), dual
pumps (two diesel-engine-driven or one dieselengine-driven and one electric-motor-driven) with
independent water supplies are needed to provide
suitable reliability.
5. It should be noted that some events that cause a
power outage, may also cause a fire (ex. lightning,
tornados, hurricanes). This effect is not factored
into this analysis. In areas that may be subjected to
prolonged power outages, increased reliability may
be justified at significantly lower exposed values
than identified in this article.
6. Increased reliability should also be provided whenever there are significant life safety considerations.
Many health care facilities, i.e. nursing homes and
assisted living facilities, have immobile patients who
are difficult to evacuate. Assembly occupancies have
high population densities where crowd control and
evacuation issues necessitate increased reliability,
Lastly, electricity demand charges have significant
impact on yearly maintenance and present value cost
for electric fire pumps. In general, in areas with
significant demand charges for electricity, presentvalue cost for a diesel engine is less than for an
electric-motor-driven fire pump when the pump’s
electrical demand exceeds electrical demand for other
uses, and when weekly pump testing is conducted
during peak demand hours.
This study is adapted from the Schirmer
Engineering White Paper on Diesel vs.
Electric. To request a copy of the complete
white paper, call 614.764.1224 or
e-mail diesel@clarkesystem.com.
When
Electric
is out...
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Diesels Work.
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of downed or cut power lines, power brownouts or blackouts –
CLARKE DIESEL ENGINE DRIVEN FIRE PUMPS WORK.
Let a Clarke diesel work for you.
Call Clarke at 1.800.513.9591, or 513.771.2200 today.
Visit us online at www.clarkefire.com.
TM
Fire Protection Products, Inc.