EtaPRO™ Perfomance & Condition Monitoring
Rankine Cycle Plant Case Study
The case study described below is based on an actual event. All graphs are derived from actual EtaPRO data.
Improved Power Plant Performance Through On-Line Heat Rate Monitoring
A mid-western US customer installed GP Strategies’ EtaPRO Plant Performance and Condition Monitoring System on three coal-fired generating units. The system was configured for monitoring operator controllable losses, as well as performance of the boiler, steam turbine, condenser, and feedwater heaters. Data gathered over nine months of operation were examined to quantify the heat rate improvement attributable to greater operational control of cycle parameters, such as throttle temperature and pressure, boiler excess oxygen, and exit gas temperature. These “operator controllable” parameters are defined as those conditions over which the power plant operator has some degree of control, either directly (through a control setpoint) or indirectly (i.e. through sootblowing, level control, etc.).
Figure 1 - Net Unit Heat Rate
Direct control parameters include turbine throttle pressure and temperature, while indirect control parameters include condenser backpressure and boiler exit gas temperature.
Examination of turbine performance data during the nine month period yielded a curious, but undeniable trend: net unit heat rate had increased (see Figure 1).
Despite an improvement in everyday operational control and the resultant reduced costs, unit performance had indeed declined. The questions being asked by plant management were how much and why? EtaPRO proved invaluable in providing answers to these questions.
Figure 2 - HP Turbine Efficiency
Causes of the Heat Rate Increase
In deducing why NUHR increased, the boiler efficiency and auxiliary load were considered first. Boiler efficiency was found to have improved substantially by 0.8%, while auxiliary power consumption decreased.
The trend in these parameters contributed to a better heat rate, therefore, the problem was isolated in the turbine cycle.
Turbine cycle heat rate was plotted and found to have the same trend as net unit heat rate: a significant increase over the nine month period. Checks of turbine cycle conditions detected no significant anomalies expected to increases heat rate (steam conditions normal or better, feedwater heaters operating normally, exhaust pressure improved), therefore the investigation focused on the internal condition of the turbine steam path. Indeed, a plot of HP turbine efficiency revealed a dramatic 12% decrease in enthalpy-drop efficiency (see Figure 2).
The turbine had been overhauled, therefore a large decrease in turbine section efficiency was sudden, unexpected, and suspect.
Figure 3 - Extraction Temperatures
Nonetheless, a plot of generator load and turbine extraction temperatures provided important corroborating evidence and revealed the nature and date of the event: a turbine trip at the end of the follwoing month.
Figure 3 shows a sudden and large (200°F) increase in the first extraction temperature (teal) upon return from a turbine shutdown.
Also note the lesser, but noticeable, increases in downstream extraction temperatures (yellow, pink and green). The temperature increase in the first extraction is too large in magnitude to be the sole result of mechanical damage to the turbine blading upstream of the extraction.
The analysis concluded that the turbine trip resulted in rubbed packings on the governor-end shaft seal, causing high-energy steam to be directed to the first extraction line.
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