Industrial Gas Turbines

"Illustration 5.1-1": (Lit. 5.1-8): ‘Continuous’ engine monitoring was probably initially introduced in large-scale for aero engines. In this early example, relatively little data over a long operation time are concerned. The positive experiences with this technology could be used in short time also for the stationary application. This picture contains typical examples of a two shaft (JT-8D, Lit 5.1-1, similar to the assessments of other aero engine types, Lit 5.1-4).

At this presentation the operation parameters are vertical arranged. The operation time is on the abszissa. If certain parameters of the engine change during operation, one can attribute this to possible failure areas and types of failures.

Leaks in the high pressure compressor: Examples are failures on air removal tubes and bleed valves. In steady state operation such a leakage leads to the drop of the compressor end pressure. More fuel must be supplied for performance maintenance, in order to raise the turbine inlet temperature and the corresponding performance. This happens on condition that the speed of the compressor is increased and the demanded end pressure is built up again. A compressor leak has a similar effect as the deterioration of compressor efficiency ( "Ill. 3.1.1-2"). All the described influences lead to a remarkable alteration of the monitored parameter

Combustor failure (Fractures of the combustor walls "Ill. 3.2.3-1", failures on the injection nozzles): These failures are difficult to identify from the monitoring data. Often, secondary damages in the turbine make recognition possible. These are typical parameter changes that are also to be expected in turbine failures: drop of speed, rise of turbine inlet temperature, as a consequence of increased fuel addition, in order to guarantee the demanded performance.

Failure on HPT guide vanes (see example in "Ill. 3.3-9"): if a larger share of the vanes is affected, the speed falls significantly. Turbine inlet temperature and fuel consumption increases, similar to the combustor failures. To be noticed is the clearly higher vibration measurements not explained in the literature. To be considered is, however, a vibration excitation, through flow irregularity at the periphery in the turbine area.

Failure on the HPC bearing: the hpc speed recedes as expected. Entirely surprising is, however, the clear reduction of the vibration level, observable in the rear engine area. This clearly shows how much expertise is imperative for the correct evaluation of the monitoring parameter.

"Illustration 5.1-2": (Lit. 5.1-7): Such an on line, real time software simulator corresponds the present state of the art. It concerns a computer supported system. Measurement data will be analysed and evaluated on-site or with a central monitoring, using algorithms. The results can be transferred and documented/stored. Important activities are:

• The current efficiency of the engine.
• Influences at the hot parts lifetime (creep, "Ill. 2.3-3" and "Ill. 2.3-4").
• Influence at the emissions (exhaust).
• Intake losses and exhaust losses.
• Cost minimisation (exchange, repair, overhaul, maintenance).
• Logistics as well as planning of maintenance (e.g., compressor wash) and overhaul.
• Damage prevention.

With the computer model of the engine (simulator) it is possible to evaluate also typical effects and their consequences:

• Environmental influences like temperature, pressure and humidity.
• Efficiency drop (deterioration).
• Actions to increase the performance like an inlet cooler, water injection and optimising of the plant.
• Control system: (rotation) speeds, performance limits.
• Trends which point at changes, especially failures.
• Fuel aberrations/-changes.

Above this it is possible to investigate the reaction of the plant at operation data which can mean during hard ware test runs a high failure risk. To this belongs the overheating of hot parts by extreme gas temperatures. This picture largely correlates the display of a so called gas turbine simulator. Own blocks are assigned to the measured parameters and from those determined operation data like efficiencies, costs, malfunctions, emissions as well as dedicated alarms. In the real implementation the display shows data and numbers with dimensions. Here they are disclaimed. Additionally, in many cases it is possible, to select further displays with detail informations.

"Illustration 5.1-3" and "Illustration 5.1-4": (Lit. 5.1-7): Software simulators can be also used for combined plants like ‘combined heat and power systems’ (= CHPsystems, cogeneration systems, "Ill. 2.1-3.2"). The heat recovery raises the efficiency enormous. That reduces the emissions. For a maximum effect the plants, respectively the operation, must be appropriate adapted. That is also true when there are changes in influences like the surroundings/ environment, fuel, performance/heat quantity. Shown is a site with two gas turbines, whose exhaust heat is used in a single steam generator

"Illustration 5.1-5": (Lit. 5.1-7): An important role plays the coaction of the components (matching). If failures occur, they alter the chracteristics of the components and so the measuremet data. Out of it the gas stream analysis determines characteristics of the deviations (fault indices). With the assistance of the gas stream analysis there can also faults of sensors/probes ( "Ill. 3.6.2-1") of the data generation (instrument error) be recognised. For this purpose the computer program utilises logical examinations (plausibility). An example are superior values of a compressor (efficiency, airflow) than the design respectively the new condition.

The gas stream analysis of a two shaft engine requires the following typical data, respectively parameters (sketch at the right):

• Compressor
  • temperature at the intake,
  • intake pressure,
  • exit pressure,
  • temperature at the exit.
• Gas Generator (=GG):
  • temperature at the exit (EGT/TGT),
  • exit pressure
  • (rotation) speed.
• Power Turbine (= PT)
  • temperature at the exit,
  • exit pressure,
  • (rotation) speed.
• Fuel
  • flow rate,
  • lower caloric value of the fuel (= LCV).

With these data a contnuous monitoring of the engine is possible ( "Ill. 5.1-2"). From this, impor-

"Illustration 5.1-6": (Lit. 5.1-7): This chart shows the trend of a compressor contamination (fouling) during some weeks as efficiency drop (deterioration) compared with the clean condition. So the optimum point in time for a compressor wash can be defined ( "Ill. 4.2-1.2").

"Illustration 5.1-7": (Lit. 5.1-7): With help of the gas stream analysis the compressor characteristic diagram ( "Ill. 3.1.1-1" and "Ill. 3.1.1-2") with the position of the up-to-date operating point can be established. Its deviation from the design will be evaluated and is distinguishable within one percent. From this it is possible to suggest the condition of the compressor and possible measures:

• Maintenance like washing of the compressor ( "Ill. 4.2-1.1" and "Ill. 4.2-1.2").
• Overhaul, e.g., for the purpose of minimising the tip gaps ( "Ill. 3.1.2.4-1") or due to exhausted lifetime of the hot parts ( "Ill. 2.2-5" and "Ill. 2.3-3")
• Logistics e.g., purchasing/furnishing of parts/ components.