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Protecting Steam Cycle Components During Low-Load Operation of Combined Cycle Gas Turbine Plants

How low can you go? That’s the question owners of gas turbine combined cycle plants are asking these days as they are being called upon to operate those units for rapid response in markets where load following is becoming the norm. The resulting cyclic operation introduces challenges that can result in damage to steam cycle components if you aren’t careful.

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Thermal Shock / Impingement Assessment

Many combined cycle power plants have experienced thermal shock damage to bypass power piping, especially in the HP steam and Hot Reheat systems. Severe shocks occur when inadequate drainage exists and liquid water accumulates in bypass piping, for example during periods of operation when the bypass is closed or during shutdowns.   Startups where the bypass is opened then allows the accumulated liquid to be picked up and carried at high velocity downstream where it impacts on the internal surface at a change in piping direction, imparting a significant load and associated stress.  Low load operation is another means by which very high attemperator spray can fail to be absorbed by the steam and consequently come in contact (in liquid form) with the pipe wall.

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Understanding variations in Flow-Accelerated Corrosion Wear Rates in HRSG Evaporator Tubes

Very large differences in tube wall thinning rates by flow-accelerated corrosion (FAC), or in some cases fluid erosion, are observed in the low-pressure (LP) evaporator tubes of certain HRSG designs.  The tubes located near the duct wall and occasionally near the gaps between module bundles have more rapid thinning.  Tubes in a given row nominally should have very similar process conditions, both on the gas-side and on the waterside.  Different wall thinning rates mean that process conditions differ across the tube row. 

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T23 Tube Failure

During the startup of the triple-pressure HRSGs, plant staff noticed steam coming from the stack and a discrepancy between feedwater and steam flows.  On shutting down the unit a T23 superheater tube was found with a complete circumferential failure.  The failed tube was removed and the bore was plugged with some difficulty due to poor access and the uncommon material.

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HRSG Condition Assessments Identify CAPEX, Maintenance Priorities

As combined cycle gas turbine plants are called upon to play a larger and more flexible role in the generation mix, it’s important to schedule a comprehensive assessment of major components at key intervals to ensure reliable operation.

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