POWER April 2017 - 28

OPERATIONS & MAINTENANCE
Reducing Cycling Damage to
Combined Cycle Steam Turbines
Source: POWER/Tom Overton
Cycling a combined cycle plant places additional stresses on all equipment, but the
impacts extend beyond the gas turbine and heat recovery steam generator. Plant
owners and managers are beginning to see cycling-related damage on steam turbines
too. A recent study by the Electric Power Research Institute assessed the ways
plants are finding to deal with it.
Merrill Quintrell
M
28
any combined cycle gas turbine
(CCGT)-based power plants
have seen a significant change in
their operating profiles over the past two decades.
In many cases, plants that were originally
designed for baseload or hours-based
operation are instead realizing operating profiles
that experience high starts, lower-thanexpected
capacity factors, and a significant
amount of load-following throughout the
course of the day.
Over the years, original equipment manufacturers
(OEMs) and owners have investigated
methods to improve the operational
flexibility of CCGT facilities, including enhanced
turndown capability and increased
ramp rates-all in order to improve their
market presence. However, much of this
work has been focused on the combustion
turbine generator (CTG) side of the facility.
Very little regard has been given to what
shorter start times, lower turndowns, and
other modifications to the gas turbine side
of the plant may have on the steam side of
the facility, with particular emphasis on the
steam turbine generator (STG, Figure 1).
A recent study by the Electric Power
Research Institute (EPRI) investigated the
impact of increased cycling on the STG.
The study and its findings are described in
an EPRI report entitled Operational and
Control Strategies for Reducing Steam Turbine
Damage from Cycling Combined Cycle
Plants (EPRI report 3002006026).
Study Methodology
The EPRI study involved telephone interviews
with representatives at a broad range
of plants, followed by site visits to selected
facilities. Participants were culled from a
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cross-section of the industry, representing
predominantly F-class CCGT facilities, but
also including E-class units and aero-derivatives.
In
all, telephone interviews were conducted
with plant staff at nearly 50 CCGT power
facilities in the U.S., Japan, England, and Italy.
Interview questions encompassed general
inquiries regarding age of the facility, basic
design configuration, dispatch operations,
plant procedures, and operations and maintenance
history of the steam systems in the
plant, including the heat recovery steam generator
(HRSG), main steam piping, attemperators,
and in particular the STG. With regards
to the STG, detailed discussions were held
regarding maintenance histories, discoveries,
inspection frequencies, and lessons learned,
all of which also included all STG valves and
ancillary equipment. As part of these discusPOWER
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POWER April 2017

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