Table of Contents
2.5 Changes of the operation performance with the running time
Low data of the fuel consumption indicated in prospects of a new car, usually are achieved not until some thousand run-in kilometers. At running times from about 100 000 km the motor power can drop noticeable. Fuel and oil consumption rise, also the temperature niveau of the motor, respectively the cooling water. The exhaust may show some clouds of burned oil and/or root from fuel, dependent on the motor type. The reason for those changes at the beginning is a run-in effect with a drop of internal friction. The later aging symptoms are essentially due to abrasive wear (piston rings, valve train and fuel injection nozzles) and deposits (motor, oil system). Gas turbines also change their behavior during their operation life.
How this happens can be an important argument for purchasing a special type of engine.
In contrast to piston engines, gas turbines do not, however, experience an improvement during the „running in“. On the contrary, with the first start/shut down cycles, one observes a relatively quick decrease in the level of efficiency (deterioration), which slows down, and, in peak load engines, remains constant over thousands of cycles ( ( "Ill. 1.1-3" and "Ill. 2.5-2"). Irrespective of that, we can save money and lay the ground stone for long time contentment with our gas turbine, if we follow instructions and have a possibly careful „ run in“ phase. Heavy rub in events when there a big gap coverage with corresponding intensive abrasion needless occurs, cause local heating on the seals (labyrinths). The consequence are elevated gap losses over the whole further life time. They act at the expense of the efficiency ( "Ill. 2.5-2") and/or the life of the hot parts. A shortening results from higher gas temperatures ( "Ill. 2.3-2") when a possible power drop must be compensated ( "Ill. 3.3.3-2" and "Ill. 3.3.3-5"). A further danger develops during severe rub in when hot tears/heat cracks ( "Ill. 3.1.2.4-7.1") develop at contact surfaces like labyrinth tips. If they can grow under normal LCF-loads the usable component life can be shortened.
A further serious impact at the efficiency develops by contamination (fouling) of the compressor ( "Ill. 3.1.1-2" and "Ill. 4.2-1.2") and the turbine. Against it helps cleaning of the blade/vane surfaces, e.g. washing of the compressor ( "Ill. 4.2-1.1").
Not only a decrease in efficiency with it’s typical consequences over a long period of time is observable. After a long operation time other changes as result of abrasive wear can develop.This includes axial loads on the main bearings ( "Ill. 2.5-1") and vibrations. If those influences are in a normal range, they may not directly strain the purse, can however herald considerable damage and therefore become very expensive.
It is worth therefore, to observe our gas turbines over a long period of time and to look at the causes of the changes.
Meanwhile a multiplicity of, mostly electronic monitoring systems is offered in the market. They notice, record and evaluate ( "Ill. 5.1-3") measurements of temperature ( "Ill. 3.3.3-1", "Ill. 3.3.3-2", "Ill. 3.3.3-5" and "Ill. 3.6.2-3"), pressure and vibation ( "Ill. 2.5-3" and "Ill. 5.1.2-1"). If failure relevant variations are recorded ( "Ill. 2.5-4") a warning occurres.
"Illustration 2.5-1": The condition of the seals influence the air and gas pressures in the different engine areas. The compressor builds up the air pressure towards the rear. Without the seal at the compressor end, it would experience, in addition to the forces effective towards the fore, corresponding to the outlet cross section and the entire pressure relationship, a piston force towards the front upon the disc at the compressor end. In the turbine, the pressure falls in the direction of the gasstream. As a result, the turbine experiences a high axial force towards the rear. These forces between compressor and turbine are compensated via the shafts, by the designer, as far as possible. The average of the labyrinth seals on the compressor outlet and the turbine inlet are so tuned to each other that, during the entire time of operation, perhaps, a good controllable axial, bearing load works in one direction. A too little bearing load, or a change in the direction of the bearing load, depending on the state of operation is undesired (e.g., because skidding of the bearings). If the labyrinth clearances change in the course of operation, e.g., through wear or erosion, the leaks and with them the pressures and the bearing loads alter, as a consequence.
By unusual big labyrinth wear, the bearing can also be remarkably affected.