All gas turbines deteriorate in performance during operation. The impact of performance deterioration results in loss in power output and increases fuel consumption. Loss in power output can have an impact on revenue because production can be reduced. Increase in fuel consumption increases operating costs. Both these factors increase equipment life cycle cost. Performance deterioration also results in higher firing temperatures, resulting in increased turbine creep life used for a given power demand. In power generation, even a 1% increase in fuel consumption and a 3% loss in power can result in additional operational costs which could exceed £1m.

How performance deterioration occurs

Gas turbine performance is determined by the interaction of the engine components, namely the compressor, turbine and the combustor. When these component take damage resulting in the alteration of these component characteristics, then the interaction of these revised or damaged characteristics result in loss in power output and increased heat rate, giving rise to increased life cycle costs.

Typical compressor characteristic

Causes of performance deterioration:

• Fouling
• Hot End Damage
• Tip Rubs
• Vibration
• Seal Wear & Damage
• Start-up Problems
• Variable Guide Vane Schedule
• Foreign object damage (FOD) & DOD
• Erosion
• Corrosion

The interaction of components determines the measured parameters such as pressures, temperatures, speeds and fuel flow. When components suffer damage resulting in change in their component characteristics, these measured parameters alter. By comparing the measured parameters with their expected values, component characteristic changes can be detected. These changes in component characteristics are called FAULT INDICES.

The above figure illustrates the methodology employed in determining the Fault Indices of engine components.

Diagnostics

By trending Fault Indices diagnostics can be performed. Faults such as fouling, seal wear, rotor-casing clearance changes, and hot end damage can be determined.

An example of the application of Fault Indices

Figure below shows the measured and derived engine parameters using gas path analysis techniques when faults are present (Actual Performance) and when no fault are present (Design Performance). If no faults are present in the engine then the actual performance will correspond to the design performance. Clearly the actual performance does not match the design performance. The question then arises as to the cause of the performance deviation.

By comparing these values it difficult to determine the cause of the fault. We can plot the operating points on the compressor characteristics, for the actual (Cross) and design (Circle) cases (See figure below).

If no faults exist then the two points will be coincident. Since this is not the case, again confirming that performance deterioration has occurred. Examining the data on the compressor characteristics do not indicate what the fault is. The engine being considered is a three shaft engine and there is a unique relationship between the non dimensional speeds of the LP and HP compressors. Figure below shows this relationship and the operating points. It is still difficult to determine the fault.

Plotting the deviations between the actual and design measured parameters (figure below), does not indicate the fault either.

By calculation of the Fault Indices, the components that have suffered damage are clearly shown (figure below).

LP Compressor fouling and HP turbine hot end damage has been detected. A compressor wash should overcome the fouling problem. If the fouling problem persists after a wash then the LP compressor rotor - casing clearance has increased significantly or the Variable Guide Vane Schedule is incorrect. Damage to the HP turbine is more permanent and the Nozzle Guide Vane trailing edges may have been burnt resulting in an increased flow area. The damage has also resulted in a 6% loss in power output and a 2% loss in thermal efficiency.

Note the quantitative and qualitative nature of Fault Indices. If these problems become worse then the Fault Indices will also become worse and alarms can be raised when the these Indices exceed alarm levels. Fault Indices are a invariant of operating conditions; the value of these Indices do not change with operating conditions and therefore reflect the performance health of these components.

Measurements required for the calculation of Fault Indices for a typical two shaft gas turbine are as follows:

• Compressor inlet temperature
• Compressor inlet pressure
• Compressor exit temperature
• Compressor exit pressure
• Gas Generator Exit Temperature EGT/TGT
• GG turbine exhaust pressure
• Power turbine exit temperature
• Power turbine exit pressure
• Gas generator speed
• Power turbine speed
• Fuel flow Rate
• Lower Calorific Value of the Fuel (LCV)