POWER June 2010 - 30

COMBINED CYCLES
Economic Operation of
Fast-Starting HRSGs
Fast-starting combined-cycle plants are designed
for a certain operating life based on
a customer-specified set of operating scenarios.
During that design phase, periodic
inspection and maintenance procedures
to benchmark equipment actual wear and
tear should be developed, but seldom are.
Without an accurate assessment of remaining
equipment life for components
subjected to fast and frequent start-ups
and shutdowns, the real operation and
maintenance cost is only a guess.
By Darryl Taylor and Akber Pasha, Vogt Power International Inc.
C
ombined-cycle plants installed in the
mid-1990s to the early 2000s were
largely designed to operate at baseload,
with relatively few planned starts each
year. However, deregulation of the electricity
markets and the rising cost of natural gas
relegated many of those plants to daily cycling
to shave load peaks. Some owners were
even " two-cycling " their plants: covering the
weekday morning peak, shutting down, and
restarting to cover an afternoon or evening
peak. These unplanned operational modes
caused much anxiety among plant owners and
operators because daily cycling increased the
wear and tear on components, further driving
up their cost to generate electricity. Few plant
owners have quantified the effect of increased
cycling on the remaining life of components
and the " hidden " economic penalties.
The design of every heat-recovery steam
generator (HRSG) begins with a life-cycle
analysis (LCA) using very specific plant operating
procedures and owner-specified operating
profiles that include number of starts,
start-up ramp rates, number of load swings
and shutdowns, and operating pressures (Table
1). In actuality, few owners are capable
of accurately projecting these statistics over
30
Table 1. The customer specifies the plant operating profile. The heat-recovery
steam generator (HRSG) purchaser will normally specify the operating profile of the equipment
purchased. A typical operating profile of a 2 x 2 combined-cycle plant with supplemental
HRSG firing is illustrated. Source: Vogt Power International Inc.
Start-up profile
Start-up type
Cold-cold
Cold
Warm
Hot
Load changes
Trips
Plant operating configuration
2 x 2 unfired
2 x 2 fired
1 x 1 unfired base load
1 x 1 unfired part load
Initial HRSG pressure
200
400
800
1,200
2,200
Operating profile
Operating pressure (psi)
2,250
1,835
1,050
900
a 30-plus-year life cycle. The assumed inspection
and maintenance programs in the
conventional LCA for the typical combinedcycle
plant are based on fixed periodic inwww.powermag.com
Number
of operating hours
per year
2,400
3,600
200
200
tervals and statistical information derived
from baseloaded units. But an inspection and
maintenance program for any unit should
take into account the actual operation of the
POWER | June 2010
Number of cycles per year
5
30
100
200
10
10
Progress Energy Florida's Bartow Plant. Courtesy: Vogt Power International Inc.

http://www.powermag.com

POWER June 2010

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