Users have learned—often, the hard way—that daily cycling reduces the life expectancy of hot components in gas turbines. For years, manufacturers of turbines have designed them to withstand the cumulative wear and damage caused by frequent starts and stops. However, until recently not much attention was paid to designing heat-recovery steam generators (HRSGs) for cycling service.
One reason for this lack of attention has been deficiencies in some codes for designing and manufacturing water-tube boilers and steam generators. For the most part, the ASME's Section I Power Boiler code and the British standard BS 1113 provide little guidance on designing HRSGs for cycling service. Other European design codes, such as EN 12952 and Germany's TRD 301, do include practical methods for evaluating boiler and HRSG components for both creep and fatigue. Such information is essential to operating pressure vessels in ways that will extend their useful life.
Over the past decade, vendors and users alike have not only learned plenty about HRSGs' sensitivity to cycling but also what to do about it. This article discusses several aspects of design and operation that users should be intimately familiar with when specifying a new HRSG for cycling service or when switching one from baseload operation to a daily diet of starts and stops.
Coil flexibility
Before cycling combined-cycle plants became prevalent, it was unnecessary to make HRSG coils flexible in the right places to eliminate or at least minimize low-cycle thermal fatigue. But now it is essential to maximizing HRSG longevity. When older units are switched from baseload to cycling mode, low-cycle fatigue or creep-fatigue often develops rapidly.
Exacerbating the problem, codes such as EN 12952 and TRD 301 provide rules for designing boilers for high-cycle fatigue but none for preventing premature failures as a result of low-cycle creep and fatigue. For HRSGs, low-cycle fatigue is almost always due to unresolved thermal expansion. Non-corrosion-related failures of HRSG tubes, pipes, and headers are typically caused by low-cycle thermal fatigue.
There are two important aspects of coil flexibility to consider: tube-to-tube temperature differentials and superheater/reheater interconnecting piping.
Temperature differentials. In all high-temperature superheaters and reheaters, differences in tube metal temperatures arise as steam is heated from inlet to outlet. In most HRSGs, the rows of tubes closest to the gas turbine (GT) will be the hottest and those nearest the stack the coldest. Tubes at different temperatures expand at different rates. These differences in temperatures and expansion rates are greatest at unit start-up and narrow as full steam flow is established.
There are two options for configuring coils to deal with row-to-row temperature differences. Figure 1 depicts one of them. Here, steam enters the inlet header and is heated by exhaust gas. In the configuration shown, the inlet header at the top of Row #4 is fixed to provide support while the lower headers are allowed to move vertically unrestrained.
1. Beating the heat. This diagram shows a heat-recovery steam generator (HRSG) superheater/reheater coil configuration with one upper header spring-supported. Source: Nooter/Eriksen Inc.
All row-to-row temperature differentials must be absorbed within the coil—by header rotation, tube flexing, and/or axial compression or tension of the tubes. Under transient conditions (such as during unit start-up and shutdown), the mechanical stresses created by the temperature differentials are the highest and are sufficient to produce thermal fatigue. Accordingly, any HRSG whose structural configuration restrains both upper headers from moving vertically would be damaged slightly each time it is cycled. However, the simple addition of a spring to either header for support would enable the tube row to which it is attached to move vertically, decreasing thermally induced stresses by an order of magnitude.
The second superheater/reheater coil-configuration option (Figure 2, p. 40) is one commonly seen in the field. Here, each tube row is supported from above by its own fixed header, and link pipes connect the lower headers to a collector manifold. In this configuration, the maximum thermal stresses are at the bends in the link pipes. This layout does not lend itself well to cycled HRSG operation because components cannot move freely relative to each other. Absorption of row-to-row temperature differentials depends entirely on the flexibility of the coils and the link pipes and rotation of the manifold.
2. Too reliant on flexibility. This superheater/reheater coil configuration has fixed upper and lower headers.Source: Nooter/Eriksen Inc.
To wrap up this discussion, Figure 3 illustrates some bad and good superheater/reheater coil configurations. None of the three layouts shown in Figure 3a can cope with row-to-row differentials in the magnitude or rate of thermal expansion. In all three cases, the tube rows cannot move freely relative to each other because they are tied together, either by upper and lower headers or a manifold. It's worth noting that, although these layouts work well in evaporators (where row-to-row temperature differentials are very small), they leave superheater and reheater tubes vulnerable to cycling-induced thermal fatigue.
3. Bad/not bad. At left (3a) are three layouts that constrain free relative movement of tube rows. The three configurations on the right (3b) either facilitate free relative tube movement or maximize row-to-row flexibility.Source: Nooter/Eriksen Inc.
For contrast, note how the three configurations in Figure 3b either facilitate free relative tube movement or maximize row-to-row flexibility. In the example on the left, the outlet header is supported by a spring, allowing the header to move up or down depending on the temperature difference between the two rows. The other two examples feature long horizontal pipe runs that allow the lower headers to move easily relative to one another.
Interconnecting piping. During HRSG start-up, it is not uncommon for the pipes not heated by gas flow that interconnect superheaters and reheaters to be hundreds of degrees (Fahrenheit) cooler than the coils to which they are attached. In normal operation, the temperature differential between the piping is much smaller and might be accommodated by the piping's flexibility. Nonetheless, it is important that the layout of interconnecting piping consider the start-up differences. Figure 4 shows a configuration that connects the top of the superheater coil on the right to the bottom of the coil on the left.
4. T spells trouble. Because it is unheated by gas flow, this pipe connecting two superheaters, top to bottom, could be hundreds of degrees cooler than the coils to which it is attached. Such a layout could create higher-than-expected thermal stresses in the pipe, shortening its service life.Source: Nooter/Eriksen Inc.
Similar arrangements are used for HRSG components such as evaporators and economizers. But these components exhibit fewer thermal-transient problems because the water they contain absorbs thermal shocks, keeping parts at a more constant temperature. Evaporators, however, have been the subject of some concern. During start-up, the tube rows closest to the GT will heat up somewhat faster than the rows further from it. In addition, the entire coil will heat up faster than the downcomer. These temperature differences, typically around 100F, are the concern. Interconnecting piping should be designed with sufficient flexibility to handle the force created by differential thermal expansion.
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