POWER June 2014 - 40

GAS-FIRED GENERATION
nents such as turbine blades, vanes, and other
hot-section components, is caused by thermal
expansion and contraction, reinforced by
mechanical strains associated with centrifugal
and torsional loads.
Differential Thermal Expansion. This
damage occurs when components undergo
high thermal growth relative to surrounding
components. This mechanism can affect
combustor cans, boilers, and HRSGs.
Corrosion. Two-shifting or any other operation
that challenges the ability of the plant
to maintain water chemistry can lead to increased
corrosion and accelerated component
failure. This mechanism can manifest itself
as increased problems with corrosion-fatigue
of economizer tubing and stress corrosion in
steam turbines (STs).
Impaired Performance of Environmental
Control Equipment. Load following
and other modes of flexible operation can
affect the performance of selective catalytic
reduction (SCR) systems.
Figure 1 shows the types of potential cycling-related
damage that might be expected
in different areas of a CCGT plant.
What are the consequences of this damage?
Recent investigations found that a change
from baseload operation to operating under
cycling conditions can result in increased
capital spending for component replacement,
increased routine O&M costs due to equipment
wear-and-tear, lower availability due
to higher failure rates and outage times, and
higher fuel consumption due to operating in
less than an optimal manner (more stops and
starts and more load changes), negatively affecting
unit heat rate. Also, when a unit is
subjected to cyclic operation, reliability can
suffer. These consequences can lead to a unit
becoming less reliable and more expensive to
operate, resulting in a lower dispatch order
and increased need for additional flexibility,
effectively creating a spiral of cycling operation
leading to more cyclic operation.
In 2013, an EPRI study investigated the
impact of cycling on the O&M costs of
CCGTs with capacities of 110 MW to 492
MW. The results showed that the strongest
indicator of annual O&M costs was the
number of equivalent hot starts (EHS) that a
unit performs. The study assumed a hot start
equals 1 EHS, a warm start equals 3 EHS,
and a cold start equals 5 EHS.
The same study explored the equivalent
forced outage factor (EFOF) for CCGT plants
operating in both baseload and cycling modes.
EFOF is the fraction of a given operation period
in which a unit or a train is not available
due to forced outages. This particular parameter
is very useful in measuring forced outages
in cycling power plants, because it takes into
account the derating hours.
40
2. More starts, more outages. This chart shows the average equivalent forced outage
factor v. lifetime equivalent hot starts for combined cycle gas turbine (CCGT) plants operating in
cycling regimes. Source: EPRI
Mean
18
15
12
9
6
3
600
1,200
1,800
Findings showed that the average EFOF
value for CCGT plants operating in the cycling
regime is about 3% higher than the
plants operating in the baseload mode in the
first six years of operation and about 1.5%
higher between six to 20 years of operation.
EFOF for cycling plants increases much
more abruptly between 20 and 30 years of
operation compared with baseload plants.
Figure 2 shows the average EFOF versus
lifetime EHS for CCGT plants operating in
cycling regimes.
The study also looked at the equivalent
planned outage factor (EPOF) for CCGT
plants operating in baseload and cycling
modes. Planned outages normally refer to
the removal of a unit from service to perform
work on specific components that is scheduled
well in advance and has a predetermined
duration, such as annual overhaul, inspection,
and component testing.
In general, increased routine maintenance
is required due to increased levels of wearand-tear
when a plant moves from baseload
Mean
18
15
12
9
6
3
500
1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500
Equivalent hot starts
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POWER | June 2014
Upper limit
Lower limit
2,400
Equivalent hot starts
operation to cyclic mode. Results showed
the planned outage levels for cycling CCGT
plants are within about 6% to 9% during the
first six years of operation and within about
4% to 6% for the next 14 years of operation.
The EPOF achieves its minimum level between
10 and 14 years. During the " major
component wear-out period, " which is near
the end of life (assuming major components
at or near end-of-life have not been replaced),
the EPOF value for cycling CCGT plants increases
to about 15% to 18%. Figure 3 shows
the average EPOF versus lifetime EHS for
CCGT plants operating in cycling regimes.
Another recent EPRI study documented
23 cases in which major HRSG components
unexpectedly reached end-of-life. Many of
these failures can be attributed to more frequent
cyclic operation than originally anticipated
in the plant design. For components to
have a full design life, the factors anticipated
by the designer need to be similar to those actually
experienced by the plant components
in service. These factors include the operat3.
Planned outages increase. This chart shows average equivalent planned outage
factor vs. lifetime equivalent hot starts for CCGT plants operating in cycling regimes. Source:
EPRI
3,000
3,600
4,200
Upper limit
Lower limit
Equivalent planned outage factor (%)
Equivalent forced outage factor (%)

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POWER June 2014

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