Reducing the minimum load at which a unit can reliably operate is one method to increase revenue potential during lower demand periods. For this reason, it is not unusual to see merchant plants operating at super minimum load levels. However, these units are operating well outside the original OEM design basis. It is important for owners to understand the trade-offs and risks that come with such operation.  

As an introduction, this paper will provide an overview of the key boiler and turbine failure modes which have increased susceptibility while operating at low loads.  Industry best practice mitigating strategies and unit supervisory instrumentation to monitor the susceptibility of the failure modes will be provided. 

TG Advisers (Turbine - Generator) and Tetra Engineering (Boiler) will provide details of an analytical assessment and simulation completed for a US site with four vintage boilers and steam turbines.   The boilers were modeled at different load points using Power Plant Simulator & Designer (PPSD) software.  Key issues analyzed were superheat steam temperature, stability of natural circulation, and maintenance of minimum flow velocities.  Secondary factors included cold end condensation and the potential for accumulation of dissolved solids in the circuit. 

Utilizing the results of Tetra’s model, TGA completed off-design modeling and calculations for the steam turbine and balance of plant equipment.  Examples of primary interest was the impact of the predicted steam conditions and superheat, resulting thermal transient cycles, and LP blading concerns influenced by moisture content and back pressure control.  Finally, balance of plant equipment was reviewed to ensure acceptable operating points for key equipment such as boiler feed pumps, feedwater heaters, and hood spray systems.

Following computer simulations, a plant testing plan was developed, and plant testing was completed.  The paper will review comparisons of analytical predictions and actual plant testing as well as overall lessons learned from the project.  Through these analytical and testing efforts the minimum load was reduced from 65 MW to 31 MW.  


The Change in Boiler and Steam Turbine Failure Modes With Minimum Load Operation: Using Modeling to Predict Susceptibility With Validation Through Plant Testing

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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