Reducing the minimum load at which a unit can reliably operate is one method to manage changes in market demands and avoid inherent concerns over frequent on and off cycling. For this reason, it is now becoming common practice for plants to develop new lower minimum load levels that are well below conventional targets provided when the unit was first commissioned. For many plants, the criteria for successful operation were not based on optimizing minimum load levels. In fact, most conventional steam plants were commissioned during an era when full base load operation was expected throughout the life of the plant. Base load availability was the key driver not parameters that promoted unit flexibility. As a result, there are opportunities for plants to lower minimum load levels, but it is important for owners to understand the trade-offs and risks that come with such operation.

TG Advisers (Turbine - Generator) and Tetra Engineering (Boiler) partnered on an analytical assessment and process simulation for a US site with four vintage boilers and steam turbines, the boilers having been converted from coal to gas-firing some years earlier. The boilers were modeled at different load points using boiler and power plant process simulation 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 boiler 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 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 the current practice of 65 MW to 31 MW.

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