Calculating True Cost of Cycling at
Combined Cycle Power Plants
Background on Cycling Effects
During the course of hundreds of engineering investigations and analyses, Intertek
APTECH has found that equipment wear and tear due to power plant on/off and load
cycling is a very significant production cost component. This component needs to be
unbundled and quantified. Furthermore, it has been Intertek APTECH's experience that
this component is not well understood or accounted for by most power plant owners.
This brochure introduces the general concepts of cycling effects and describes how we
work with plant owners to determine the cost of cycling. These costs enable the utility
to better optimize dispatching by taking into account the damage and reliability
impacts of cycling and, ultimately, to increase corporate profits. Such investigations
also result in recommendations reducing cycling costs through operational or design
changes.
Operational Cycles Lead to Damage
Every time a combined cycle power station is brought on-line, shutdown, or used for
load following, the gas turbines (GT), steam turbines, heat recovery steam generators
(HRSG), steam piping, and auxiliary components go through large thermal and pressure
cycles. Expansion and contraction of complex GT and HRSG components leads to
thermal and mechanical fatigue damage. Other phenomena, such as corrosion fatigue
and water chemistry transients, creep, creep/fatigue interaction, erosion, wear, and
vibration also contribute to accelerated life expenditure of critical components.
Figure 1 shows three starts executed over a 24-hour period by a client's large GT.
Exhaust gas temperature, power output, and exhaust temperature ramp rates are
plotted. The ramp rates, expressed as °F change per hour, are important drivers of
fatigue and creep/fatigue interaction effects. The GT is dispatched to about the same
load each time. However, after each shutdown it is restarted before reaching a cold
Intertek APTECH
601 West California Avenue, Sunnyvale, CA 94086-4831
Phone: 408.745.7000 Fax: 408.734.0445
Websites: www.intertek.com/aptech
16100 Cairnway Drive, Houston, TX 77084
Phone: 832.593.0550 Fax: 832.593.0551
E-mail: [email protected]
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condition, causing different startup ramp rates. The initial cold start exhibits a ramp rate of 2780°F/hr in
exhaust gas temperature transient. The second (or warm) start exhibits a lower ramp rate at 1955°F/hr. The
third (hot) start exhibits a ramp rate of nearly one half the level of a cold start at 1452°F/hr. Intertek APTECH
has found that such differences in ramp rates between various cycles are of extreme importance when
determining the stress levels and amounts of cyclic thermal fatigue damage accumulated by both GT and
HRSG components.
A similar situation exists in the HRSG when the duct burner is activated or shut off, or when the gas turbine
is dispatched in a load following mode. Subtle changes in temperature occur among the individual tube
circuits causing additional thermal/mechanical fatigue cycles. Since HRSG components are densely packed
and rigidly supported to optimize the amount of heat recovered from the GT exhaust, flexing of tube-toheader connections and transfer piping nozzles impose huge stresses on connections, ultimately resulting in
cracking and tube failures. In addition, steaming of water-cooled components and chemical attack of
exposed metal beneath internal deposits can lead to further damage.
Equipment Susceptibility is Important
Through Intertek APTECH's many cost-of-cycling projects, we have determined that some units are better
designed for cycling than others. Some designs are so bad or the service duty/ramp rates are so severe that
the HRSGs experience forced outages within a hundred on/off cycles. Other units can cycle on a daily basis
without major problems, enduring thousands of on/off cycles without failure. Modern power plant designs
have tended to compromise physical robustness to cycling stresses for higher efficiencies and lower installed
costs.
Another factor in cycling susceptibility relates to the method of chemistry control. Some control schemes
simply are not good for cycling units, as they lead to a high probability of chemistry upsets and off-normal
conditions during cycling that cause considerable damage.
Intertek APTECH uses its extensive experience in analyzing failures, including the use of calibrated damage
and life expenditure models, to assess how susceptible an HRSG or GT is to cycling damage.
Cycling Increases Heat Rates
Another added cost that is not fully accounted for by plant owners is the extra fuel needed (i.e., higher heat
rate) due to cycling. Most people use incremental heat rate data based on static tests at specified set points
to determine the heat rate at a certain load level. This does not account for the loss of efficiency due to
variable load operation impacting the heat rate 2% to 20%. Intertek APTECH determines the overall heat
rate effects of each type of cycle (hot starts, warm starts, cold starts, and load follows) under varying load
levels and ramp rates.
How Does Intertek APTECH Estimate Cycling Costs?
Intertek APTECH has developed and used a methodology that employs a combination of “Top-Down”
models and “Bottom-Up” methods. Our Top-Down models are based on the use of past histories of
operations, forced outages, and costs. We have successfully used this method on hundreds of generation
units throughout the world. Bottom-Up methods rely on calculations of operational stresses (e.g.,
temperature, pressure, and chemistry), life expenditure of critical components, and analysis of historical work
order costs. The flowchart shown in Figure 2 depicts this combination of methods. For new combined cycle
units, the Top-Down methods would not apply due to inadequate operating and cost history. Thus, for
these units, Intertek APTECH would apply its Bottom-Up methods and industry experience on similar units to
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derive cycling cost estimates. The heat rate
effects of cycling can be determined with only
a few months of hourly megawatt and gas
consumption data.
Figure 3 shows a break down of cycling costs
for hot, warm, and cold starts, and low load
follow for a typical unit.
How Can the Results of Intertek
APTECH’s Work be Used?
Past clients have used Intertek APTECH's
cycling cost results in the following ways:
To provide proper unbundling of variable
costs to better:
Negotiate long-term contracts
Bid into power markets
To recover costs of ancillary services
(e.g., AGC)
To develop better Information for
maintenance budgeting
To Enhance least cost system dispatch
(including Intertek APTECH's CyclingvAdvisorTM)
To provide improved cycling operations (including Intertek APTECH's COSTCOM®)
Clients have saved millions of dollars annually in reduced overall production costs, with increased profits.
Intertek APTECH's Technical Paper TP054 can be requested for more information.
What is CyclingvAdvisor and COSTCOM?
These software products were developed by Intertek APTECH to allow plant owners to use the cost of cycling
results in real-time. COSTCOM uses plant data to track cycling costs. COSTCOM can be used as a training
tool for operators to better control damage costs related to cycling. It provides actual unit damage costs
while the unit is running, as shown in Figure 4. It can
include fuel costs, dynamic heat rate effects, and
Cycling Cost Breakdown
emissions credits/costs thus giving total unit production
Primary and secondary
costs in real time.
120
startup fuel costs
100
Once cycling costs are determined, the CyclingvAdvisor
is used on a generation system level to optimize
dispatch schedules of the plants. It is unique in the
industry because of its rigorous accounting of damage,
costs as well as fuel costs. It can also include emission
costs in order to minimize total production costs of an
entire utility system.
80
Extra fuel costs from
low- and variable-load
operation
60
40
Future forced outage
replacement power cost
20
0
Cold Start
Warm
Start
Figure3
Hot Start
Load
Follow
Future maintenance,
operation and capital
cost
Figure 3
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Other Combined Cycle Support
Services Offered by Intertek
APTECH
Intertek APTECH also provides
following services to our clients:
the
Acceptance testing
Performance testing, prediction,
and improvement
Failure investigation, metallurgical
examination, root cause analysis
Inspection and maintenance
planning (strategic and fast track
turn around)
Condition monitoring and trend
analysis
Emissions system evaluation
EP&C contract drafting, review, and monitoring as Owner's Engineer or Financing Institution Engineer
Applications engineering for power plant modifications and scope of supply
O&M audit of complete combined cycle systems
Asset valuation and due diligence evaluation
Operating and engineering staff training
Litigation and expert witness support, including warranty issues
The following is a list of tasks APTECH would typically
undertake to determine cycling costs of a combined cycle:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Project Kickoff and Data Gathering
Review of Plant Signature Data
Engineering Assessment and Operations Review
Survey of Selected Plant Personnel
Damage Modeling
Statistical Regression
Component Life Expenditure/Cost
Heat Rate Impact Modeling
Total Cost Estimation
Evaluate Measures to Mitigate Cycling Costs
Report and Optional Training
Intertek APTECH is committed to assisting its clients with the operation and
maintenance of their power plant equipment. Intertek APTECH will help you
run your equipment as long as is practical, safe, reliable, and economical.
Intertek APTECH's goal is to work with its clients to:
REDUCE POWER PRODUCTION COSTS
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