Pre/Post Outage Thermal Performance Evaluation
by Sanjeev Jolly, P.E. – Senior Engineer, Engineering Services
NAES has developed pre/post outage thermal performance guidelines to assist plants in quantifying changes in combustion turbine (CT) performance because of upgrades, repairs and modifications performed during outages. ASME codes provide guidelines to conducting performance tests for various power plant equipment; but meeting these requirements often require resources that may be difficult to justify in today’s power markets. A simplified version of the test guidelines based on using existing plant instrumentation can help plants determine if repairs/modifications/upgrades made during an outage had any quantifiable effect on performance. Common tasks performed during the outage that impact the performance are – HRSG tube cleaning, inlet air filter replacement, hot gas path inspection, replacement of compressor or turbine blades, turbine blade ring modifications, compressor waterwashes, modifications in exhaust temperature control curve, expansion joint repairs, etc.
NAES’s pre/post outage thermal performance guidelines apply corrections to the pre-outage data to predict expected post-outage performance. The predicted values are directly compared to the post-outage values; the difference is the change in performance. For initial set-up, CT correction curves and reference conditions are required. Both are supplied by the OEM as part of deliverables when the CTs were installed. Another source is the original performance test procedure provided to conduct the performance test after the plant was commissioned.
It is required to run the CT in base-load operation on exhaust temperature control curve for performance evaluation. Pre-test stabilization criteria, test duration and mode of operation are specified in the guidelines. The recommended test duration is 60 to 90 minutes after the pre-test stabilization criteria has been met.
Performance is typically evaluated for power output (CT, ST & Total), heat rate, inlet and outlet exhaust gas pressure drop improvement, compressor efficiency, overall plant thermal efficiency and other parameters as required.
The following two tables are examples of evaluation done for key performance indicators – power output and heat rate. The pre-outage corrected output is the predicted output from pre-outage test corrected to post-outage test conditions.
The results are presented in a report that details the calculation procedure, corrections applied, and quantified performance in summarized tables and charts and include direct pre and post outage comparison of compressor and thermal efficiencies, output, heat rate, CT inlet air and exhaust gas pressure drops.
It is critical that the pre/post outage thermal performance guidelines be followed to ensure an accurate and cost-effective thermal evaluation process. These guidelines are applicable to all simple and combined cycle natural gas-based power plants. If interested in having a thermal evaluation done at your facility, please contact Sales here.