Tetra Engineering is presenting two papers at the 2020 ASME POWER Conference which was scheduled to take place at the Disneyland Hotel in Anaheim, California August 3-6 but will now be a virtual conference online this year. The second paper has been written in collaboration with our partners at TG Advisers, Inc.
This paper presents key lessons learned from hundreds of field missions for assessing the condition of HRSGs at sites around the world. These involved field inspections, root cause of failure analyses and performance assessments on HRSGs of nearly every design, fuel type and operating mode. Statistics on degradation and failure risk areas are presented. Analysis techniques used in both preparing for field work and in assessing field examination results are also discussed. Trends in the evolution of the principal damage mechanisms over the years are examined, and predictions of future problem areas are considered with a view to guiding asset management actions. The impact on HRSG integrity of changes in unit designs, such as the move to flexible operation in response to the increasing contribution from renewables, is also assessed, and likely future trends are outlined. The use of qualitative and quantitative risk-based methods is also discussed and compared to the use of condition-based inspection methods as a basis for a fleet wide management strategy. A case study including a large IPP with several generating assets in its portfolio is presented and the most cost-effective method with respect to current corporate strategy is discussed. Recommendations for fleet management strategies are made.
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.
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