POWER October 2014 - 40

OPERATIONS & MAINTENANCE
Prepare Your Gas Plant for Cold
Weather Operations
Cold weather operations are challenging for plant designers and operators
alike, particularly where severe winter weather is rare. However, another
cold winter is expected, so now is the time to review the lessons learned
by many plants after the February 2011 Southwest freeze.
Dr. Robert Peltier, PE
A
sustained Arctic blast composed of
freezing rain, record snowfalls, and
high winds hit 22 states, from Mexico
to New England in early February 2011. The
extreme weather severely affected the power
generation and delivery systems in the southwestern
U.S. beginning on Tuesday, Feb. 1,
finally diminishing the following Saturday.
El Paso, for example, experienced the second-worst
winter weather in a century, and
the worst in 50 years, with 60 consecutive
hours of temperatures below 20F. The temperatures
in the Electric Reliability Council
of Texas (ERCOT) region during the systemwide
emergency ranged from 11F to 35F,
equivalent to -5F to the low teens when the
wind chill factor is considered. In Austin,
temperatures remained below freezing for
over 70 consecutive hours.
In Texas, the deep freeze affected every
generating company. A reported 225 individual
units, including several recently
constructed plants, experienced a trip, were
derated, or failed to start, reflecting 14,855
MW of unplanned unavailable capacity.
Coupled with 12,413 MW of prescheduled
generation outages, ERCOT was unable to
serve electricity demand, and the results were
widespread outages and rolling blackouts
caused by frozen equipment (Figure 1).
The Federal Energy Regulatory Commission
and North American Electric
Reliability Corp. (NERC) subsequently
released their report " Outages and Curtailments
During the Southwest Cold Weather
Event of February 1-5, 2011 " in August
2011. That report describes the cause of
the Texas extreme weather event as well
as several proactive measures that would
improve plant reliability in the future. According
to the report, while " many generators
were proactive in their approach
to winterization and preparedness, others
were not. " A key finding of the report was
that " the lack of any state, regional or Reliability
Standards that directly require generators
to perform winterization steps has
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POWER | October 2014
left winter-readiness dependent on plant or
corporate choices. "
Re-Learned Lessons
Plant outages caused by extreme cold weather
events (as well as impacts on natural gas
pipelines that supply the rising number of
gas-fired combined cycle plants now operating)
appear to be more problematic in the
southern region of the U.S. Natural gas-fired
plants in the northern states continued to operate
reliably because they were designed
to do so from the outset. For example, Minnesota
Municipal Power Agency's Faribault
Energy Park, located in Faribault, Minn.,
routinely operates for months at a time without
a problem when the outside temperature
is below zero (see " How to Make a Power
Plant a Welcome Neighbor, " March 2008).
In contrast, plants built in the southern
states, from Florida to Arizona, are often
of an outdoor design that makes retrofit an
expensive proposition. Regulators in some
NERC regions, such as ERCOT, have since
become more proactive with plant inspections
and third-party design reviews of plants
vulnerable to extreme cold and are ordering
temperature-related reliability upgrades.
Our survey of several of the lessonslearned
reports prepared in the wake of the
February 2011 freeze found a number of
common failure modes, many of which have
since been the subject of reliability upgrades
at many affected plants.
Instrument and Sensing Lines. Many
plants reported outages caused by frozen
boiler airflow, deaerator level, and boiler
drum instrumentation and transmitters. A
commonly reported problem was a unit trip
from a low drum level signal caused by frozen
instrumentation and sensing lines.
According to Black & Veatch, which analyzed
the impact of the event on one Texas
utility, " The frozen sensing lines prevented
the automatic drum level control system from
operating to maintain the water level in the
1. Frozen solid. Many combined cycle plants located in southern states were unable to reliably
start or run during the freeze that occurred in early February 2011 because important equipment
was not cold-weather protected, as can be seen with this heat-recovery steam generator
steam desuperheating station. Source: POWER

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POWER October 2014

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