POWER October 2014 - 42

OPERATIONS & MAINTENANCE
time to freezing must be determined based on
multiple factors: design low temperature, coincident
wind velocity, assumed temperature
of the water within the equipment (usually the
heat tracing design criteria), and the existing
insulation of the equipment. The plant staff response
time to instigate additional freeze protection
actions (typically 24 hours) can then be
compared to the time to freeze for the HRSG
to determine if the HRSG must be drained to
prevent water freezing and tube ruptures.
Sub-freezing weather can cause harmful
problems with forced-draft cooling tower operation,
particularly when there is freezing within
the tower fill media. In one instance, operators
observed ice forming on the tower and shut
down the circulating water pumps and fans to
one or more cells. Ice formation on the cooling
tower should be avoided, because it could be a
risk to the circulating and other cooling water
pumps that take suction from the tower basin.
One plant noted that it quickly shut down the
cooling tower to prevent icing when the plant
unexpectedly tripped (Figure 4).
Plant demineralized water systems should
be freeze protected and capable of delivering
properly treated water during a cold weather
event. Also, ensure that there is sufficient
chemical inventory, and/or have a plan in
place for delivery of critical chemicals and
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gases (such as hydrogen) during extreme
weather. Some plants experienced freezing
of water treatment chemicals used for demineralizer
regeneration and have since installed
reverse osmosis water treatment plants to
eliminate the problem. Also, if vendors can't
get to the plant during extreme cold weather,
critical material deliveries will be delayed.
Make sure offsite support personnel for critical
systems, such as the substation, are identified
and can be contacted in an emergency.
The combustion turbine (CT) main fuel
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gas regulator and fuel gas stop valve must be
well insulated or they can freeze and cause a
trip. Also, icing of inlet air ducts and filters
can cause a spike in the differential pressure,
causing a unit trip or derate. The worst case
is a frozen inlet filter causing the CT compressor
to stall, which can in turn cause significant
damage to the turbine internals. Inlet
heating systems will keep the inlet filter icefree.
Finally, carefully inspect the compressor
water wash and evaporative cooling systems
for leaks that can form ice (Figure 5).
CTs must also be properly tuned for cold
weather operation. As ambient temperatures
drop, power output increases. This means that at
very low temperatures the power output is maximized
but so is mass flow of emissions and the
possibility of permit exceedances. The turbine
control system should be tuned so that permit
limits are not exceeded at low temperatures.
Transformer insulators and arrestors
should be thoroughly cleaned prior to the
cold weather season in order to minimize the
risk of flashover. This is very important for
those plants where the prevailing winds push
cooling tower drift toward the switchyard.
Corona discharge can damage insulators and
other substation hardware and is a sure way
to force a unit offline.
Setting Priorities
Chances are your list of low-temperature operation
upgrades is extensive and expensive. You
42
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POWER | October 2014
4. Towering problem. Ice forming on a
forced-draft cooling tower during freezing conditions
can damage the tower internals. Ice
forming within the basin can reduce flow and
damage the cooling tower pumps. Source:
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POWER October 2014

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