Obtaining accurate data about the performance of a plant’s heat-recovery steam generator is crucial to ensuring the smooth operation and maintenance of the equipment. Software designed to model and simulate HRSG operations can provide valuable information about corrosion and other operational problems.
Power plant staff need reliable information on process conditions in their heat-recovery steam generators (HRSG) to ensure the equipment’s dependable operation and optimum performance. Current and historical operating data from the distributed control system (DCS) can be used to determine the root cause of failures or to predict the degree of wear from a variety of mechanisms. These include flow-accelerated corrosion (FAC), corrosion fatigue, and creep and fouling of the cold-end tubes. Unfortunately, data available from a DCS are at times insufficient for in-depth analysis because they do not provide a complete picture of process conditions in the region of interest. This may be due to insufficient online instrumentation, scanning rates that are too low, or limitations on the tags stored in the DCS history database.
Software Simulations Support HRSG Analysis
The modeling and simulation of HRSG operation in software is a way of overcoming the difficulties related to insufficient DCS data. If such simulation yields accurate predictions of the process conditions at any point in the HRSG, it can complement or validate information available from the DCS for root cause and unit life assessments. In addition, it permits the prediction of HRSG performance with any given set of operating conditions. This is useful for generating "what-if" scenarios, as a desktop simulator to train operations staff, or for checking the impact of design modifications and repairs.
This article presents the results of simulations performed with boiler-modeling software to support the analysis of actual HRSG operating issues at a number of power plants.
For maximum value, modeling and simulation software should have certain essential features:
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Accurate modeling of single- and two-phase flow regions in both natural and forced circulation regimes with pressure drops and fluid velocities calculated at the component level.
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Capabilities for dividing heat exchanger elements into zones and gas/water/steam flow into separate streams to better calculate temperature and mass flow distributions.
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Predictions of tube and fin metal temperatures in all heat transfer regimes.
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Heat transfer algorithms that include the effect of thermal radiation from gases on boiler components with component thermal conductivities that are material- and temperature-dependent. This is extremely important for assessing accurate heat loads on tube panels downstream of duct burners.
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Dynamic simulation capability to permit the evaluation of process conditions during transients such as start-ups, shutdowns, or system upsets.
Modeling the HRSG
The first step in doing a simulation of HRSG operation is to build a software model using the mechanical design data for the unit in question. The level of detail and type of analysis will drive the complexity of the HRSG model. Of particular importance is determining if the simulation is to be a steady-state or dynamic one.
Components in the surrounding balance of plant are incorporated when needed for the particular application. Typically, these consist of vessels, pumps, and the steam turbine. Other systems that are tied to the steam cycle, such as gas turbine (GT) coolers and auxiliary boilers, can be added as well. The GT is modeled as a heat source with mass flows, temperatures, and gas composition derived from the original equipment manufacturer’s (OEM) performance data. Proportional integral derivative control elements are added to provide automated control of boiler elements such as valves, attemperator sprays, and duct burner fuel flow for a dynamic simulation.
To simplify the modeling process, the gas-side (Figure 1) and water-side (Figure 2) streams are laid out separately while sharing the same components. Initially, the model is constructed using OEM design data. Running this "clean" model should yield results that closely match those specified in the OEM design cases. The "clean" model can then be adapted to investigate various operations and maintenance (O&M) issues, as shown in the following examples taken from recent projects at a number of power plants.
1. Triple play. This is an example of a gas path model for triple-pressure reheat HRSG. Courtesy: Tetra Engineering Europe
2. Following the flow. This is an example of a water path model for triple-pressure reheat HRSG. Courtesy: Tetra Engineering Europe
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