Using Simulations to Evaluate HRSG Stress from Thermal Expansion
The effect of thermal expansion on operating stresses is a critical factor for assessing the impact of cycling operation on HRSG components. High cyclic stresses can lead to fatigue failures in HRSG components after a relatively small number of starts.
The high-temperature tube bundles at one plant were considered to be at risk from thermal stresses. The tubes at different temperatures entering a single header will tend to expand by a different amount. This differential thermal expansion will raise stresses at the tube to header joints. Simulations with the boiler software can be used to generate temperature profiles as inputs to a finite-element analysis (FEA) that will calculate peak stresses in components. For the primary superheater, the following average temperatures were calculated for tube rows 1 to 3 (coolest to hottest) at full design load:
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Row 1: T12 651F (344C)
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Row 2: T22 774F (412C)
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Row 3: T22 788F (420C)
Figure 4 shows the three rows (row 1 at left) and the calculated displacements. Rows 2 and 3 are expanding farther downward, causing a theoretically uneven displacement. This will produce a crack opening stress, as shown in Figure 5. The stressed region starts in the weld between the header and the tube and stretches up to the middle of the bend in the tube.
Combined thermal performance and FEA simulations on the other HRSG modules showed that there were several more high-stress areas caused by temperature differences across the tube rows.
4. Checking for fatigue failures. Displacement of SH1 tubes at lower header. Courtesy: Tetra Engineering Europe
5. All stressed out. An area of high stress on row 1 tube-to-header junction. Courtesy: Tetra Engineering Europe
Simulating the Dynamic Response of Once-Through HRSG
A dynamic simulation is useful for analyzing the effect of various operating transients on process conditions. In a once-through HRSG (OT HRSG), the dynamic response to changes in input conditions is a critical consideration. Because of their small water inventory, any change in heat input will immediately affect the system.
Many other OT HRSG units are used to generate saturated steam for enhanced oil recovery operations. A key consideration in these units is that the steam fraction does not exceed about 80% to avoid dryout and excessive deposition of contaminants on the boiler tube walls.
Figure 6 shows the output from a simulation for an OT HRSG where the duct burner is shut down and then restarted. The outlet steam fraction is set to 80% by controlling feedwater inlet flow. Shutoff of the duct burner has a significant effect on steam production, but the other parameters — final evaporator fin tip temperature and entering steam fraction — are stable.
6. Simulating changes in input conditions. This graph shows a once-through HRSG’s dynamic response to duct burner shutdown and restart. Courtesy: Tetra Engineering Europe
Simulation’s Benefits
Boiler simulation can be a useful tool for in-depth investigation of various operating issues in an HRSG. The examples presented here used relatively straightforward steady-state models to look into some common operating issues. More complex models can be used to look at other issues of interest, such as checking if temperature and pressure values remain within OEM ramp limits on start-up or shutdown, or testing the effect of nonhomogeneous (stratified) gas temperature and flow distributions on HRSG performance.
—Christian Daublebsky von Eichhain (chistian@voneichhain.de) is the chief engineer for KED GmbH, Rodenbach. James W. Malloy (jmalloy@tetra-eng.com) is the technical director for Tetra Engineering Europe, and Mark J. Taylor (mtaylor@tetra-eng.com) is the senior consulting engineer for Tetra Engineering Europe.
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