Rankine Cycle Plant Case Study

Improved Power Plant Performance Through On-Line Heat Rate Monitoring

The case study described below is based on an actual event. All graphs are derived from actual EtaPRO data.

A mid-western US customer installed GP's EtaPRO Plant Performance Monitoring System on three coal-fired generating units in June 1998. 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.).

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).

Figure 1 - Net Unit Heat Rate

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.

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).

Figure 2 - HP Turbine Efficiency

Net unit heat rate (NUHR) is defined as:

NUHR = Q_tc/h_blr x Net kWh

where,

Q_tc: Heat absorbed in boiler steam and water circuits

h_blr: Q_tc in terms of percent boiler fuel heat input

Net kWh: Useful power output (Gross kWh - Auxiliary kWh)

The turbine had been overhauled in September 1998; therefore, a large decrease in turbine section efficiency was sudden, unexpected, and suspect. 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 on October 30, 1998.

Figure 3 shows a sudden and large (200°F) increase in the first extraction temperature (teal) upon return from a turbine shutdown.

Figure 3 - Extraction Temperatures

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.