Power: Business And Technology For The Global Generation Industry

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 compo-
nents, but shutdowns—both routine and
emergency—can be more damaging. The
shutdown procedure is usually intended
to keep superheater-outlet steam tem-
perature 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. How-
ever, 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 satu-
ration temperature. This leads to substan-
tial thermal stresses at the inner surface
of the headers.

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