In General Electric designed MS6001B (Frame 6B) gas turbines, the compressor discharge casing is a complex cast iron structure that locates the combustion hardware (e.g. fuel nozzle, combustion liner and transition pieces) between the compressor exit and the turbine inlet. In the Frame 6B design, the compressor discharge casing is prone to cracking at certain locations. Indeed, units which have undergone more than a few hundred starts are likely to have such cracks present. However, there is little information available about the effects of such cracks on the long term reliability of Frame 6B units. This project aims to assess, quantify and predict crack growth behaviour in the compressor discharge casing in order to develop best practise guidelines for the management and monitoring of cracks in the compressor discharge casing. 

The regular maintenance and inspection outages of heavy duty industrial gas turbines are used to assess the condition of the critical components of the turbine. In the short and medium term, the components of interest are generally the hot gas path components (i.e. the combustion and turbine components). In the long term, consideration of the condition of the turbine rotor must be undertaken, as the rotor is both expensive and difficult to replace. Inspection for long-term damage mechanisms on the rotor, such as thermo-mechanical fatigue and creep cracking needs to be optimised for the long term reliable operation of the turbine up to design life, and as part of any life-extension program. This project covers the ongoing development of custom tooling and techniques for the inspection of critical regions of the turbine rotor to increase certainty in the risks of continued operation of life extended rotors.

General Electric turbines accommodate firing temperatures above baseline (I.e. peak firing) within their maintenance interval algorithms. The project analysed the effect of a higher peak firing temperature on GE E-Class peak firing units to take advantage of a starts-based maintenance schedule, i.e. convert the unused factored hours within the starts-based schedule into a higher peak firing temperature, and hence higher peak output. A desktop analysis of the GE maintenance algorithms and the effect of increases in firing temperature on hot gas path materials was undertaken. Based on actual material properties, a robust and conservative relationship between allowable peak firing temperature increases and running times was calculated.

The repair of hot gas path components is a path for efficient use over the lifetime of these expensive parts. Repair procedures for E-class turbines are offered by a number of sources, including the OEM, approved vendors and independent 3rd parties. The application of a consistent specification for repair processes and quality assurance increases the likelihood of consistent and reliable repairs. A suite of specifications have been produced which cover most Frame 6B hot gas path parts (e.g. combustion liners, transition piece, buckets, vanes and shroud blocks). 

Gas turbine lifing provides owners usable insight into the extension of operational life.  Data from operation and condition assessments can be combined with sophisticated modelling techniques to understand locations and damage mechanisms that may result from during extended service.  The modelling techniques are applied to the two bearing, the bolted Frame 6B rotor to highlight critical regions of the rotor.  This was achieved through computational fluid dynamics (CFD), with stress and thermal loading applied to both steady and transient operational modes.  Fracture mechanics methodology is applied to derive an inspection interval.  Further work on this project includes development on inspection technique and tools for the Frame 6B rotors.

In parallel an ultrasonic Non-Destructive Inspection (NDI) technique was developed for the detection and accurate length sizing of cracks.  The development involved a novel approach to the scanning controlled with very limited moving parts to achieve scanning along the high risk portion of the blade.  The development and proof of the method was aided by the use of ex-service blades constructed with artificial defects at the anticipated crack initiation sites.

The practicality, speed and accuracy of the ultrasonic method were demonstrated on an in-service machine with 32 blades inspected in one day in-situ. Each blade is inspected ultrasonically via access through the guide vanes. The combination of FEM, fracture mechanics and the NDI procedure provides a practical fatigue life management solution for the Row 1 compressor blade.  

By combining the latest modelling techniques, effective life management of gas turbines provides the owners a powerful tool to extend rotor life.  Modelling techniques employed are finite element analysis and computational fluid dynamics and these are coupled with operation and condition assessment data.  The analysis involved modelling the two bearing, three bearing and the through bolted Frame 9E rotor to identify the critical regions within the rotor.  Fracture mechanics methodology is then applied to calculate appropriate inspection intervals.  The project is nearing completion.

General Electric turbines accommodate firing temperatures above baseline (I.e. peak firing) within their maintenance interval algorithms. The project analysed the effect of a higher peak firing temperature on GE E-Class peak firing units to take advantage of a starts-based maintenance schedule, i.e. convert the unused factored hours within the starts-based schedule into a higher peak firing temperature, and hence higher peak output. A desktop analysis of the GE maintenance algorithms and the effect of increases in firing temperature on hot gas path materials was undertaken. Based on actual material properties, a robust and conservative relationship between allowable peak firing temperature increases and running times was calculated.

Effective life management of gas turbines gives owners the scope to extend rotor life. Operation and condition assessment data can be combined with state of art modelling techniques to identify location and type of damage mechanisms which may occur during extended service . This project involved analytical modelling of the two bearing, fully welded GT13E2 rotor to identify the critical regions within the rotor. Using computational fluid dynamics, with stress and thermal loading applied to both steady state and transient operation models enabled the identification of the critical regions. Fracture mechanics can them be applied to calculate appropriate inspection intervals.

Effective life management of gas turbines gives owners the scope to extend rotor life. Operation and condition assessment data can be combined with state of art modelling techniques to identify location and type of damage mechanisms which may occur during extended service . This project was similar to the general project completed for Rotor Life Management of the GT13E2, but with upgraded boundary conditions and geometry changes for the higher firing temperature MXL variant. The project involved analytical modelling of the two bearing, fully welded GT13E2 MXL rotor to identify the critical regions within the rotor. Using computational fluid dynamics, with stress and thermal loading applied to both steady state and transient operation models enabled the identification of the critical regions. Fracture mechanics can them be applied to calculate appropriate inspection intervals.