POWER June 2010 - 36

combined cycles
7. Calculation methods. Operating data-based (ODB) methodology damage estimating
system architecture and components. Source: Vogt Power International Inc.
Plant
DCS
Plant
DCS
Operations
information
Real time
Historian
Cycle entry
SQL
database
Post-calc results
Global data access /ODBC/OLE DB/.NET CRL
Tag mapping/configuration
Cycle identification
Data conditioning/filtering
Cycle info pre-processing
LCAMP engine damage
calculation
Post-processing/validation
Excel application
report generator
8. Good guess. Inspection and maintenance schedule based on a conventional LCA using
owner-supplied operational requirements. Crack initiation is expected when DF = 1.0. Source:
Vogt Power International Inc.
High-pressure drum
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
Cycle explorer
summary/charts
Real-time damage
assessment
Online trend prediction
ing conditions. The result is a priority list
of the highest risk components. Once the
critical components are identified, a finite
element analysis (FEA) is used to define
the stresses present in those components.
The allowable cycles and allowable hours
deduced from the detailed FEA analyses are
used to develop damage factor calculation
procedures.
Historical data for individual components
are extracted from the plant distributed control
system (DCS) to determine various inputs
for each cycle such as the drum wall
temperature difference, pressure ramp rates,
superheater and reheater metal temperature
gradients, and the like. The program uses
a cycle-counting methodology that determines
when a cycle starts and stops. These
procedures, including data validation and
smoothing processes, are codified in the
dynamic software that is unique to a given
boiler (Figure 7).
The start-up temperature ramp rates,
operating pressures, temperatures, and
shutdown ramp rates are calculated by appropriate
programs or are retrieved from
various data tables. These values are used as
input to a unit-specific program that calculates
the estimated fatigue, creep, and total
DFs for the critical components for a given
cycle/scenario. These data are then used to
calculate the estimated life consumption of
the HRSG's critical components.
The output can be combined into a single
parameter: damage fraction of the component.
A DF of 1.0 would not indicate component
failure but rather the possibility of crack
initiation. The monitoring of DFs for various
critical components indicates how the unit
life is being consumed on a component level
rather than the whole unit level.
Periodic general inspections
Years in operation
Notes: HPSH1 = High-pressure superheater 1, RHTR1 = Higher-pressure reheater 1.
the particular unit to increase accuracy and
provide meaningful results. Calculations in
the program should be based on a uniform set
of codes (for example, all American codes:
ASME Section I and VIII, AWS, API, and
other common design codes) rather than a
36
mix of codes with underlying conflicting or
different assumptions.
The first step of the ODB methodology
is to perform a dynamic simulation of the
boiler to determine the transient behavior of
various components under varying operatwww.powermag.com
Develop
an Inspection Program
Once the DF has been determined for each
of the critical components, predictions can
be made as to when each specific component
needs to be inspected and how thoroughly.
The DF will also help to determine
the frequency of inspections based on how
quickly it is increasing with each start-up
and shutdown.
Inspections for a given component can be
broken down into several different classifications
based on the DF, as detailed below.
As the damage fraction increases, a more detailed
inspection will be required.
General Inspection: DF = 0.3-0.5.
Critical components with a damage fraction
between 0.3 and 0.5 fall into the general
inspection category. Based on the rates of
life consumption calculated by the dynamic
life-cycle program, an inspection schedule
can be developed. Each critical component
POWER | June 2010
Damage fraction
2000
2002
2004
2006
2008
2010
2012
2014
2016
2018
2020
2022
2024
2026
2028
2020
2032

http://www.powermag.com

POWER June 2010

Table of Contents for the Digital Edition of POWER June 2010