POWER June 2010 - 37

combined cycles
can be broken down into which inspection
category it is in for each outage. Because the
component in question has a large amount
of life left, only a visual inspection is required.
Inspections of this nature would include
looking for the following:
■ Tube bowing
■ Drum internal cracking
■ Tube or fin discoloration
■ Signs of leaks
■ Heavy deposits on the tubes in hot areas
■ Changes in performance
Focused Inspection: DF = 0.5-0.7.
Critical components with a damage fraction
between 0.5 and 0.7 fall into the focused inspection
category. Components that have a
damage fraction less than 0.5 and that were
found to have issues in the general inspection
can also fall into this category. In this category,
the component has used up more than
half of its life and needs to be inspected more
thoroughly. In addition to a visual inspection,
inspection using nondestructive evaluation
(NDE) methods such as UT testing is also
recommended.
Detailed Inspection: DF = 0.7-0.9.
Critical components with a damage fraction
between 0.7 and 0.9 fall into the detailed inspection
category. Components that have a
damage fraction less than 0.7 and that were
found to have issues in the focused inspection
can also fall into this category. In this
category, the component is reaching its end
of life. It is recommended to thoroughly inspect
the component using NDE methodologies
to look for cracks.
Possible Replacement: DF = 0.9+.
Critical components with a damage fraction
above 0.9 fall into the possible replacement
category. Components that have a damage
fraction less than 0.9 and that were found
to have issues in the detailed inspection can
also fall into this category. In this category,
the component has reached its end of life and
is nearing when it should be repaired or replaced.
The component should be subjected
to a detailed inspection, with NDE methods.
Never Assume
The differences between the conventional
LCA inspection program using the assumed
operations as specified during the design
phase (Figure 8) and one based on actual
operational data (Figure 9) is remarkable.
The more conventional approach to scheduling
inspections results in uniform and much
relaxed inspection requirements, indicating
that serious damage may not happen soon.
However, when actual plant operations
and the severity of those operations are
considered, there is the distinct possibilJune
2010 | POWER
9. A better answer. A revised HRSG inspection schedule based on actual consumption as
estimated by LCA software based on the ODB methodology. The inspection schedule is based
on a June 2008 analysis date. Crack initiation is expected when DF = 1.0. Source: Vogt Power
International Inc.
Drum
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
High-pressure superheater
High-pressure reheater
Possible repairs
Detailed inspection
Focused inspection
Year
Notes: Drum = High-pressure drum, HPSHTR = High-pressure superheater 1, RHTR = Higher-pressure reheater 1.
10. Pay me now or pay me later. A comprehensive economic analysis of faster starts
that takes into consideration component remaining life computations allows the plant owner
to fully optimize the plant's operating economics. In this case study, shorter start-up times
increase revenue, but at the expense of increased O&M costs. Longer start-up times reduce
plant O&M expenses at the expense of reduced revenue. The economic " sweet spot " is to
start up the HRSG in approximately 60 minutes to produce the optimum electricity sales revenue.
Source: Vogt Power International Inc.
12
10
8
6
4
2
-2
-4
90
80
70
60
ity that some components may be deteriorating
faster and some sort of repairs
or replacements are needed much sooner
than expected. For example, an inspection
schedule based on actual life consumption
(Figure 9) shows that the high-pressure
superheater is reaching possible repair
or replacement in 2017. An inspection
schedule based on assumed operation incorrectly
tells the plant owner that only a
focused inspection is required. This early
indication assists the owner and operators
www.powermag.com
50
HRSG start-up time (min.)
in avoiding a future forced outage and
gives the owner time to adjust plant operations
to prolong the life of the HRSG.
The model can also be used to perform
optimization studies to fully understand
the future cost consequences of decisions
made today (Figure 10). ■
-Darryl Taylor (dtaylor@vogtpower
.com) is manager of central engineering,
and Akber Pasha is technical director,
Vogt Power International Inc., a Babcock
Power Inc. company.
37
40
30
20
Additional revenue due
to faster starts
Estimated life (years)
Maintenance/
replacement cost
Net additional
revenue
35
30
25
20
15
10
5
-5
Revenue/cost (milion$/year)
Damage fraction
2000
2002
2004
2006
2008
2010
2012
2014
2016
2018
2020
2022
2024
2026
2028
2020
2032
Estimated HRSG life (years)

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POWER June 2010

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