Steam Turbine Reliability Improvement: Critical Features Evaluation with ProTRAX

OVERVIEW

In considering future process changes, a U.S. utility contracted TRAX to analyze a combined cycle power plant’s dynamics during startup, shutdown and load maneuvering events. This client was considering adding the GE Steam Turbine Agility system with the intent to improve the plant’s startup time and reduce fuel consumption. Using modeling, TRAX can help choose process and controls changes to optimize plant operation and minimize cost before these investments are made.

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MODELING THE EQUIPMENT

TRAX built a high-fidelity simulation of the combined cycle power plant heat recovery steam generator (HRSG), balance of plant and steam turbine (ST) systems. The simulation model was validated by comparing plant historian data against model data during:

• A cold startup
• A hot startup
• Plant shutdown

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Conducting an Analysis

Once validated against plant historian data, TRAX tested incrementalchanges in the model to predict the behavior and evaluate the benefits anddrawbacks of the client’s proposed changes. Using the modeled solution, TRAXtested changes intended to improve the client’s combined cycle startup time andreduce fuel consumption. Our finding included:

• The client could add a thermal blanket to the steam turbine to heat the turbine shell for startup as well as to keep it warm while offline in standby mode.
• The client requested two reference startup conditions be tested to demonstrate rotor stress. In the model, TRAX was able to demonstrate the effects of increasing the steam turbine loading speed by up to four times without over stressing the turbine.
• TRAX tested startup and turbine loading under different configurations of the thermal blanket and different components of the GE Agility system to show the client a side-by-side comparison of the startup times and stresses that each condition presented.
• TRAX also tested a hot startup and shutdown condition.

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This testing included several important data points for the client:

• With the thermal blanket enabled in the model, the Gas Turbine (GT) Inlet Guide Vane did not need be closed to the minimum full speed position, prior to loading up the GT. (This can lead to the GT gas turbine exhaust flow and temperature profiles changing.)
• TRAX tested loading rates and time for the turbines and their relationship to rotor stress calculations. The side-by-side testing exactly quantified the time cost of each startup condition, the benefit of the thermal blanket and the steam turbine agility maneuvering.

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TRAX’s modeling confirmed the startup time can be reduced by 100 minutes when using the thermal blanket as shown below.
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TRAX was also able to show rapid improvement in other cases. The first image shows a comparison of steam turbine load, where the turbine reaches base load in IPC mode 80 minutes faster than the unmodified condition. The second image shows a similar comparison of the combined cycle total load with and without ST improvements.
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Areas of Concern

If any event causes a plant trip or other major upset during the evaluation of the simulation, TRAX tests a variety of corrective actions. In this case, TRAX identified several areas of change to the client, allowing them to train for specific conditions and concerns:

• Loading the gas turbine too fast may lead to higher HP pressures (discharge through relief values or ruptured discs).
• The new configuration showed more swings in the drum levels.
• These changes would lead to additional thermal and mechanical stress in the HRSG.

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TRAX highlighted this area as a possible concern to the client. The high pressure observed in the model indicates an operator may need to take preventative actions to relieve the high drum pressure.
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