POWER May 2010 - 56

Plant safety
to determine if a dust explosion had occurred
at Station 2. The observed hang-up inside the
head chute consisted mainly of fines (fine coal
particles). A slug of burning material could have
raised this dust into suspension and provided an
ignition source. The investigation team found
there were many observations supporting this
theory. For example, they found torn siding and
gaps in the siding. They also observed burn patterns
on the ceiling and north wall.
On the flip side, the investigation team discovered
much evidence that contradicts this
theory. Melted siding shows that fire burned up
against the south wall for a period of time. Siding
on the south wall was not blown away from
the building. Only one explosion was heard.
No security cameras were positioned to
view these areas. Additionally, no witnesses
could detail the beginning of events at Station
2. The investigation team concluded that without
an eyewitness account, it is impossible to
say exactly what the sequence of events was
at Station 2. Fortunately, in spite of recent
cold weather, Station 2 was still being washed
down on a regular basis. Those good housekeeping
practices limited damage and potentially
prevented a secondary explosion.
Proposed Corrective Actions. The investigation
team came up with the following
recommendation to prevent future fires from
happening at the Cook Coal Terminal and to
more safely handle a fire should one occur:
■ Continue to fund replacement of the fire
detection systems.
■ Install Class A foam proportioning capability
if the existing baghouses remain in
service.
■ Install CO monitors.
■ Develop a fire emergency handling procedure.
■
Evaluate the hydraulic design of the existing
deluge systems due to concern about
all sprinklers in the transfer houses being
left open.
■ Investigate/remedy the cause of coal hangups
in the Station 2 chutework.
■ Limit the use of combustible construction
in coal-handling areas. If using noncombustible
materials is not an option, provide extra
attention to these areas during washdown.
■ Keep the maintenance doors closed and
secured when not in use
The Failure of a Forced Draft Fan
John Amoo-Otoo, PE, senior electrical engineer
at Exelon Nuclear, presented an overview
of the failure of the Unit 2 forced draft
(FD) fan at an undisclosed U.S. coal-fired
power plant at the 2009 ELECTRIC POWER
Conference. The following profile of the incident
is based upon his presentation.
The coal-fired power plant where the draft
56
fan's motor failed consists of three steam turbines
with a net generating rating of 750 MW.
The units began commercial operation in 1960,
1965, and 1970. Coal is received by rail and
limestone by rail or truck. Plant personnel unload
the rail cars into a rotary car dumper at a
rate of 20 to 25 cars per hour. A 30-day supply of
coal is maintained in the on-site storage area.
Overview of the Equipment Failure. On
June 6, 2004, at 10.30 a.m., the Unit 2 FD fan
motor at the facility tripped. There was a double
phase to ground fault indication of the electromechanical
relay. The fan motor is protected by
an electromechanical relay, which has a disadvantage
in respect to its capability of being used
for temperature monitoring. It cannot monitor
the temperature of the windings compared to
multifunction relays. When the Unit 2 FD fan
inadvertently failed, the fan's motor erupted
into flames. The fire caused damage to the insulation,
ground wall, and the coil of the FD fan
motor (Figure 10 and 11)
Investigation of the Incident. Plant personnel
formed an investigation team to determine
the cause of the malfunction of the
Unit 2 FD fan and to implement correction
actions to prevent similar incidents from occurring
in the future. The team decided to use
a " root cause investigation " approach. This
investigation method is highlighted in the
book TapRoot: Root Cause Analysis, Problem
Investigation, Proactive Improvement by
Mark Paradies and Linda Unger.
Early in their investigation, the team concluded
that the FD fan 2B inadvertently failed
due to the destruction of a section of both the
winding and the ground wall insulation of the
motor, which set the motor on fire. Thermal
aging of the winding insulation of high-voltage
motors has prompted the team to consider
ways to monitor the condition of the insulation
of the winding of its 4,160-V critical motors.
The electrical insulation used in stator
windings as well as the stator core laminations
has a major impact on the reliability of
large motors. Failure of the insulation directly
or indirectly will result in the failure of the
machine, which in turn will cause a forced
outage or derating of a unit. Many insulation
failures originate with aging that occurs
over many years. To operate cost effectively,
means to periodically assess the condition of
insulation of the winding are needed.
The investigation team decided to take the
following actions:
■ Conduct a root cause analysis of the inadvertent
failure of Unit 2B FD fan.
■ Determine the steps that needed to be taken
to correct the failure.
■ Implement the corrective actions.
Six main areas were determined to be the
www.powermag.com
10. Gone up in smoke. When the forced
draft fan inadvertently failed, the fan's motor
erupted into flames. The fire caused damage to
the insulation, the ground wall, and the coil of
the fan's motor. Courtesy: John Amoo-Otoo
11. Cooked coil. This photo shows the
damaged coil of the forced draft fan motor after
it caught on fire. Courtesy: John Amoo-Otoo
root cause of the Unit 2 FD fan's failure: excessive
heat that can lead to thermal aging
and deterioration, inadequate impregnation
of the ground wall, voltage surges, contamination,
abrasive particles, and inadequate end
winding spacing. The investigation team's
analysis showed that out of the six potential
causes, three of them were the most likely
causes: thermal aging, operator's failure to
follow the manufacturer's hot start and cold
start timing sequence, and excessive heat due
to poor maintenance work on cleaning and
changing the filters.
Implementation of Corrective Actions.
After the incident, plant personnel immediately
repaired the badly damaged insulation
and winding of the motor. They later sent
the damaged motor to Joliet Motors to be rewound
(Figure 12).
Based upon the findings they obtained
from their forensic engineering analysis, the
investigation team came up with a number
of corrective actions that they recommended
the plant personnel implement in order to
prevent future failures associated with Unit
1, 2, and 3 large motors. They suggested that
these proposed corrective actions be coordinated
with pre-outage and outage work. They
also recommended that online testing be performed
during pre-outage periods and that
off-line testing be conducted during outage
periods.
Plant personnel carried out the following
corrective actions that were recommended by
the investigation team:
POWER | May 2010
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POWER May 2010

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