Nearly every combined-cycle operator recognizes that cycling reduces the life expectancy of hot-gas-path components in combustion turbines. Often overlooked, however, is that the same phenomenon affects the heat-recovery steam generator (HRSG). In fact, the greater physical size, elevated gas temperature, and higher steam conditions in today’s HRSGs have disproportionately increased thermal stresses and fatigue damage, particularly in the superheater section. Not surprisingly, improvements made in the design stage offer the greatest benefits in extending cyclic life.
However, existing plants also can reduce fatigue through equipment modifications, such as relatively minor changes to drain piping and drain valves, and by changing start-up and shutdown procedures. Following are some specific improvements that you can make at your plant.
Drain Piping and Valve Improvements
Carefully Size Drains. Generously size the HRSG drains to permit removal, at high temperature and pressure, of the large quantities of condensate that collect in the lower headers of superheaters during coast down and prestart purge. If condensate at saturation temperature is not removed prior to reestablishing high-pressure (HP) steam flow on a hot HRSG, a condensate quench occurs in outlet headers and steam pipes. Restarts shortly after a combustion turbine trip are a particularly nasty source of quench damage. Designers and operators often overlook the damaging fatigue effects of condensate quenching.
Superheater drains at some installations are not designed for, and cannot be used during, shutdowns and hot restarts. At other installations, the drains are available, but operators fail to use them correctly. At still other plants, the drains are inadequately sized to handle the substantial quantities of condensate. Regardless of the reason, failure to remove condensate from superheater headers obstructs steam flow during the next start-up. This produces a significant temperature differential between adjacent tubes and raises thermal stresses. Failures caused by this condensate-quench phenomenon have occurred after only 300 cycles.
Also, the substantial quantities of condensate in the superheater tubes should be drained during the combustion turbine coast down to further minimize thermal stresses. Superheater drain valves should be motorized to permit convenient, remote operation. In addition, where automated sequence controls and interlocks are available, the motorized drain valves should be automated.
Install Stack Dampers. Install dampers in the HRSG exhaust to restrict convection flow through the unit and to maintain HP steam pressure as high as possible. HRSGs without exhaust or stack dampers depressurize within a few hours after shutdown; thus almost every restart is from cold conditions. On a P91 header, a cold start will cause about 20 times more fatigue damage than a well-designed hot start; on a P22 header, cold-start fatigue damage may be 30 to 40 times that caused by a hot start.
Include Inspection Access. Provide convenient access for internal inspection. Fatigue cracks usually initiate inside tubes and headers. Some HRSGs have no facility or space to inspect header internals; thus, fatigue damage will not be evident until a crack propagates all the way through the header wall. If internal cracks are detected early, their growth rate can be monitored and replacement components procured in advance, keeping outage time to a minimum.
Change Your Shutdown Procedures. Many combined-cycle stations base operating procedures on the ideal requirements for the combustion or steam turbine. Operators of existing HRSGs may substantially lower HRSG thermal stresses simply by changing these procedures. Unfortunately, these procedures often cause severe fatigue damage in the HRSG.
Most operators believe that rapid start-ups are what damage HRSG components, but shutdowns — both routine and emergency — can be more damaging. The shutdown procedure is usually intended to keep superheater-outlet steam temperature as high as possible to permit fast reloading of the steam turbine after an overnight or weekend shutdown. In this procedure, combustion turbine exhaust-gas temperature and steam flow are both reduced rapidly during unloading, so that when combustion turbine firing stops, only moderate reduction in superheater-outlet steam temperature has occurred and the majority of the header remains near maximum steam temperature. However, as cooler air is delivered from the combustion turbine compressor during coast down and HRSG purging, condensate rapidly develops in the superheater tubes and then runs down into hotter headers, causing these sections to quench to saturation temperature. This leads to substantial thermal stresses at the inner surface of the headers.
A better procedure for normal shutdown is to ramp down the superheater outlet steam temperature during gas turbine unloading, prior to tripping the combustion turbine. One original equipment manufacturer recommends a ramp rate of 14F/min to a steam temperature of about 700F. By the time condensate begins to collect in the header, the bulk temperature of the header has been reduced further to about 60F above saturation temperature. This may extend the subsequent hot restart time after an overnight shutdown from a typical 60 minutes to 75 minutes, but it reduces fatigue damage by as much as 50%.
Rewrite Start-up Procedures, Too. A procedure sometimes used for hot starts deliberately lowers HP steam pressure prior to the restart. This practice is intended to reduce throttling at the steam turbine during start-up, and thereby shorten the start-up time. Nevertheless, by lowering HP steam pressure, the superheater is cooled to a lower saturation temperature, which results in a more damaging step increase — from saturation temperature up to combustion turbine exhaust temperature — immediately after steam flow is established.
To reduce thermal stresses in the superheater during hot starts, the step change should be minimized. This can be accomplished by maintaining pressure, thus saturation temperature, as high as possible in the superheater. In addition, combustion turbine exhaust temperature should be kept as low as possible when steam flow is first established. Note that combustion turbines equipped with inlet guide vanes (IGVs) tend to produce higher gas temperatures during start-up than those without IGVs. After steam flow is established, subsequent combustion-turbine ramp-up should be slow enough to maintain the temperature gradients induced by the initial step change.
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