Most US combined cycles are unregulated assets and operate in challenging merchant power markets. Competition is keen and a plant's pedigree must be characterized by high efficiency, availability, and starting reliability to meet pro forma expectations.
In this type of environment, you can't manage a generating station by the seat of your pants and expect to have any measure of success. You need top-notch personnel and sophisticated analytical tools to make your numbers.
At the center of the combined-cycle universe is the gas turbine (GT). No surprise there. The steam turbine is respected for its reliability, having more than a half century of operational experience with the same working fluid at comparable pressures and temperatures. The heat-recovery steam generator (HRSG) gets the least respect and sometimes is thoughtlessly referred to as "the dumb hunk of steel" between the gas and steam turbines.
GT operation is carefully monitored onsite and at remote M&D (monitoring and diagnostic) centers, data are recorded and trended, the remaining lifetime of critical hot-gas-path components (in particular) is assessed online using the latest software, etc. Attention befitting a beauty queen.
Steam turbines are far more robust than the GTs. If water/steam chemistry and steam quality are held within specified limits, lube/control oil is maintained in good condition, and proper warmup, ramp, and shutdown procedures are followed, the unit should be able to operate for its design lifetime with a minimum of issues.
What about the HRSG? Although the health of rotating equipment is actively monitored, and the lifetime remaining in critical parts continually updated, little comparable intelligence is available for the HRSG. Stack temperature traditionally has been used as an indicator of deterioration and fouling of heat-transfer surfaces, steam temperature as an indicator of proper spray-valve operation, makeup flow as an indicator of tube leakage, water analysis as an indicator of waterside metal health, etc. These are qualitative assessments, not quantitative.
Inspections of the HRSG on the gas side, and on the water side to the extent possible, are conducted during plant outages and that information traditionally has been used to guide repairs and replacement of pressure parts, such as module harps.
Prior to the introduction of LCAMP™ (Lcamp, for Life Consumption Assessment and Monitoring Program) by Vogt Power International Inc, Louisville, there was no commercially available monitoring tool with the ability to track the life remaining in critical boiler parts-at least not one known to the editors. The proprietary software package was developed over several years by a passionate engineering team headed by Technical Director Akber Pasha and supported by Dr Peter Sun and Lin Shian Tsai.
Lcamp imports and analyzes information gathered by a plant's data acquisition system and calculates the estimated life consumption of the specified boiler component. Having an accurate estimate of end of life allows timely ordering of replacement parts and proper planning of the outage required to install the new components. Operating until a pressure part fails generally is unacceptable for a merchant plant because of the cost and outage time incurred for repairs and extensive lead times for replacements.
The software also can be used for predicting crack initiation and for measuring the effects of various boiler operating modes on the lifetimes of critical pressure parts. The latter enables better operational decisions to keep maintenance expenses within budget. Lcamp also suggests when and where to conduct detailed inspections.
Beta testing by The Southern Company, which worked closely with Vogt during Lcamp's development, verified that the program is relatively easy to install and use and provides meaningful operational intelligence.
The editors visited with Jimmy McCallum, manager of CCP and CT technical support for Southern Company Generation, and two of his senior engineers-Andre Prewitt and Dean Sheffield-in Birmingham last spring to learn more. Not present was McCallum's engineering analyst, David Jenkins, who wrote the code to merge Lcamp and Southern's OIS software (Operational Information System from AspenTech). Vogt engineers participated in this activity as well.
Obvious from the discussion was that McCallum and the two engineers had intimate knowledge of the software and were comfortable using it. Input data for calculations comes from instrumentation typically installed on the HRSG (pressure and temperature), plus drum-metal thermocouples. Drum TCs must be installed if not supplied with the unit. At Southern, Lcamp extracts the data required from the DCS via the company-standard OIS, validates that information, and prepares the input for its use.
Important to note is that no subjective interpretation or statistical inferences are required to use Lcamp, neither is experience with stress analysis. Output can be tailored to the specific needs of any plant and displayed using standard commercial software-such as Excel. Lcamp works for any OEM's HRSG.
Southern's applications thus far have not run Lcamp in real time. Plant McDonough, which is scheduled for commercial operation in mid 2011, will be the company's first real-time application.
The online version will have the ability to calculate the cost of a startup-incorporating fuel price, the sale of power to the grid, and impact on the life cycle of critical components-as the unit is being brought online. This allows the operator to increase or decrease startup time based on current conditions. The present version of Lcamp calculates startup cost after the cycle is completed.
McCallum said analytical tools like Lcamp are particularly valuable to owner/operators today because fuel costs and electricity prices continually impact dispatch schedules. Citing the boiler as an example, he said that market conditions can cause the HRSG's operating profile to deviate significantly from that assumed by designers.
Consequently, some boiler components may deteriorate faster than others and may require repair or replacement much sooner than anticipated by the design team. A reduction in expected life caused by unforeseen operating conditions would not be bad, McCallum added, were it static and fixed in time at a certain stage. However, life-expectancy estimates change as operations progress and operating requirements change in response to market requirements.
A core value of Lcamp is that it assesses the impact of changing operating conditions on the lives of critical components and continuously updates the life consumed so plant personnel can schedule inspections and repairs or replacements before a failure occurs.
Information provided by the Vogt software program includes HP drum and superheater cycles consumed based on fatigue calculations, and HP superheater and reheater hours consumed based on creep calculations.
McCallum and his team ran through a series of "results" charts developed for one of its plants with two 2 × 1 F-class combined cycles that had been in service since 2000. Life consumption was estimated from 2000 through 2007, before LCamp was installed.
Life-consumption estimates compiled at the central engineering center on Inverness Center Parkway are shared with the plants on a quarterly basis to avoid overwhelming station personnel with data. Reports generated include identification of "significant" events, recommendations for changes in operating procedures to extend component lifetimes, and a suggested inspection plan for critical components. Interestingly, the data gathered also can be used as a shift "report card" and point to deficiencies that can be corrected through focused training.
To illustrate: A data set for the start of one unit indicated that if the same conditions and actions were repeated 162 times, the HP-drum downcomer nozzles would be at end of life-as estimated by Lcamp. If units were cycling upwards of 200 times annually, a number like this would get attention quickly and deficiencies would be addressed. Data for another start revealed ideal conditions and a projected lifetime of nearly 9000 starts.
Summary charts for management are easy to read at a glance. Detailed curves of temperatures and pressures over time also are produced to help operations managers and staff engineers assess the root cause of off-spec data, which might be something as simple as a defective instrument.
The summary graphics include the cumulative charts from 2000 through the current quarter for component life consumption (Fig A) and for cycles by type (Fig B). A dollar value for life consumed is displayed in another chart. Plus, the same data are presented on a series of charts for the current quarter.
The numbers in parentheses in the charts show the range in values across the four HRSGs serving the two combined cycles at this plant. The HRSG data presented in Fig A was the best of the four units. The reason is evident in Fig B, which profiles the same HRSG. It had the fewest starts, fewest cold starts, and the most hot starts among the site's four boilers.
To illustrate how dramatically the dispatch schedule can change over time, the Southern engineers compared starts data for one unit during 3Q/2008 with data for 1Q/2009. There were more than 60 starts in first period, only five during the second. Reason: Low gas prices made GT-based power less expensive than that from coal-fired units and the combined cycles were operating many more hours on a per-start basis.
The Lcamp experience has helped The Southern Company improve some of its traditional design and O&M practices. A few lessons learned:
- Specify HP drums for future HRSGs with thermocouple wells to accurately and reliably measure drum center and skin metal temperature. Southern engineers were reluctant to drill existing drums to install TC wells and used other methods for TC attachment.
- Lcamp suggested that the thick nozzle welds on the HP drum's two downcomers would fail earlier than predicted because of cold-condensate impingement attributed to daily weekday cycling. That prompted plants to remove insulation from downcomers prior to boiler inspections.Inspectors found cracks where Lcamp predicted they would be. Those identified soon after cracking started could be ground out and re-welded relatively easily.
- One of Southern's "Golden Rules" is to keep HP drum skin temperature above 250F to protect against material damage. The company engineers now assure that new plants have a reliable backup source of steam to keep drums hot during unit shutdowns.
