Session: 21-08 Field Service and Modernization

Paper Number: 102664

102664 - Rework and Repair Options for Steam Turbine Components Subject to Flexible Operation. 

The increase in renewable energy penetration on the grid has accelerated the need to transition conventional fossil-based energy sources from their traditional base load operation to more flexible operational regimes.  This transformation is becoming far more common and for numerous plants, the normal mode of operation.  Conventional plant operations have dramatically changed in terms of the numbers of starts, operating hours per annum and variation in load level.  The effect of this change requires faster start-ups, more frequent lower part-load operation, and longer and more frequent unplanned downtime.  This energy transition and these associated operational impacts result in greater thermal transients on operational equipment leading to an increase in Low Cycle Fatigue (LCF) damage to steam turbine components, as well as potential consequences of standby operation with limited or no preservation measures.

To ensure the continued integrity of the steam turbine components, it is essential to assess the lifetime status.  This can be achieved by applying residual lifetime analysis (RLA) methods.  The key inputs for these methods are plant operational regime, geometries, and materials.  This approach covers steam turbine components operating in the high temperature regime (>660°F / 350°C) supplemented also by condition assessment with Non-Destructive Testing (NDT) during inspections.  Depending on the amount of lifetime consumption and the extend of potential crack findings, different component repair options are possible. 

The rework or repair options for steam turbine components can be divided into two main groups, namely cold- and hot-rework.  These two options can also be carried out consecutively.  After a thorough evaluation of the current condition, usually in the context of a Root Cause Analysis (RCA), rework options can be identified.  Very often, extensive investigations, including mechanical integrity (MI) evaluations using the Finite Element Method (FEM) are required to decide which rework approaches can be made. 

All rework or repair options, whether cold- or hot-rework, provide the opportunity to improve the application of a component by applying profiling with improved stress fields and even superior materials, in the case of hot rework.  Previous service projects will be used to present examples such as improving the thermal stress level on a steam turbine rotor showing end of service life or the weld repair of a valve chest after successive water hammering damage.

In many cases, cold rework may be appropriate, which aims to eliminate all damages solely by machining.  The reworked component stress margin must be assured for a safe operation in the future, especially for components operating in the creep regime.  The surface finish can be improved by various processes, e.g., emery stone polishing, grinding to achieve smooth surfaces or cold rolling and shot peening to increase material resistance to HCF crack initiation. 

In case some damages are more severe, a rework recommendation from the second group, hot rework, may need to be applied.  Weld repair must be applied, for example, when safe future operation cannot be demonstrated with cold rework alone.  For hot rework, machining must also be performed to remove possible damages, but also to create a geometry that permits the application of welding.  Superior weld materials with increased resistance to fatigue- and creep-damage as well as corrosion can be utilized for the hot rework. 

Beyond a mere rework, it is possible to recondition a steam turbine.  For example, this method can be used to install new blades with higher efficiency, new seals, or refurbish bearing areas.  Nevertheless, use of such engineering measures requires demonstration of viability considering lead-times for parts and outage durations.  A temporary solution to bridge the longer lead-time for more comprehensive solutions may in some cases be an option.

The aim of the rework / re-conditioning is to ensure that the steam turbine component is suitable for future operation.  This ensures that plants are well placed to deliver more flexible operation in the energy industry through carefully tailored refurbishments and reworks.

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Presenting Author: Frank Biesinger GE Power GmbH

Presenting Author Biography: - Study of Mechanical Engineering, University of Karlsruhe (TH), Germany
- Diploma thesis, University of Colorado in Boulder/Colorado, USA
- Calculation Engineer, Alstom Power GmbH, Mannheim, Germany
- Dr.-Ing. in Mechanical Engineering, University of Karlsruhe (TH), Germany
- People Leader Lifetime Assessment, Alstom Power GmbH, Mannheim Germany
- People Leader Lifetime Assessment, GE Power GmbH, Mannheim, Germany
- Principal Engineer Component and Lifetime, GE Power GmbH, Mannheim, Germany

Authors:

Frank Biesinger GE Power GmbH
Huascar Lorini GE Power GmbH
Ritesh Shah GE Power