Table of Contents
2.3 Steady state operation
A continuously ride on the freeway, without jams and with moderate speed pleases the driver. This means stress free driving and low fuel consumption in comparison to the typical start-stopoperation in the town. It is noticed particularly from the low costs at the gas station. But also the technique is preserved and adds to the cost minimization. The deterioration of the motor, brakes, tires and clutch is refered to the driving distance comparable low. A typical example is the high, damage free kilometrage of a passenger car that is used by a field crew. Also the environment benefits are limited, because the emissions at optimal motor operation during start and power output. A quite comparable situation is the operation of a gas turbine.
It is precisely through steady state operation of the gas turbine that you profit from gas fuel. For one, the rules limiting the emissions of nitrogen oxide (low NOX) are getting constantly stricter; these demands are accommodated with modern combustor designs, without water or steam mixtures (dry low NOX). Another advantage of comparatively pure natural gas (Chapter 2.6) is the statistically proved, lower maintenance effort involved ( "Ill. 2.3-1"). An explanation for this is the lower radiation energy of an non sooting flame and the lower contaminations in the hot gas. The lower radiation energy is the cause of lower component temperatures. Less sulfur decreases the danger of high temperature corrosion (‘sulfidation’) at the hot components ( "Ill. 3.4-2" and "Ill. 3.4-3"). Despite all these advantages there are risks, irrespective of the kind of fuel used. An outline to this effect is given below.
Failure mechanism in steady state operation is material creep ( "Ill. 2.3-1"). Here, the temperature niveau played a decisive role ( "Ill. 2.3-2").
Steady state operation characterizes itself in that no significant changes in the operation state takes place and, therefore, the components of the gas turbine have reached a long time state of balance, (temperature distribution), that despite the power output for many components, e.g., rotor discs, means relatively low mechanical loads, e.g., in comparison with the start phase ( "Ill. 2.2-3"). The lifetime ( "Ill. 2.3-3") correlates in this case especially the base load operation. The overall life expectancy of a modern engine is 50,000 hours.
In steady state operation, however, other long, time dependent influences become important. For this reason Chapter 3.4 covers long time influences like corrosion, erosion, fretting or material fatigue. These are known to the manufacturer of your gas turbine and he has already considered them in the layout and design for „normal operation“. The task of the operator is, therefore, to guarantee this „normal operation“. In this regard, a „feeling of security“ based on ignorance is hardly helpful, instead, active care in the area of operation and maintenance is imperative for a knowledge of the problems ( "Ill. 3.2.1-4"). For this reason, such long time influences like corrosion, erosion, abrasive wear or material fatigue will be covered in Chapter 3.4. Very helpful is the monitoring as technical machine surveillance with the so called gas stream analysis (Chapter 5.1).
"Illustration 2.3-1": Metallic materials under static load undergo plastic deformation during the time of load when subject to higher temperatures (creep) and fail on reaching creep rupture elongation. This procedure progresses according to the creep or creep rupture curve, stable temperature and static tensile load, represented here in typical form. The curve can clearly alter, depending on the material stress value and temperature level, in that the three typical stages of progress can be very differently distinctive.
The course of the curve is characteristic for a process by which two contra effects operate. In the case at hand, the strengthening increase is represented as a black arrow and the strength release as a white arrow. Depending on the stress phase, the strengthening is preponderant (primary creep area „I“), if they are approximately of the same size (secondary creep area „II“) or if the loss of strength prevails (tertiary creep area „III“) shortly before the final failure through fracture. Technically used are the areas „I“ and „II“ ,which also determine the design of the components.
"Illustration 2.3-2": Basically the expensive materials of the hot parts, especially super alloy, are used near the limit range of the maximal possible application temperature. Therefore the following consideration can be seen as generally. In this diagram, creep life of hot parts, the tensile load is at the ordinate and the life is the abscissa. The material behavior is represented through curves for particular operation temperatures. One can easily recognize (see also "Ill. 2.3-1") the temperature influence on the (creep) life for a typical blade material (see also "Ill. 3.3-4" ), Nibase casting alloy, (see Fig. 3.3.1.1-3): If one assumes a typical tensile load of a turbine rotor blade in the carrying cross section of approximately 90 MN/m,(point „1“), one must reckon, according to the curve, with a constant operation temperature of 1030°C, (point „2“), with a lifetime of about 2000 operation hours (point „L/2“). For an approximately 15°C lower operation temperature by 1050°C, one has an approximately double so high creep life „3“ (point „L“ ). An increase of the temperature niveau of about 15 °C leads to a besection of the creep life. This is an exponential process. That means 30 °C shorten the life to a quarter. 45 °C even act with the Factor 10. This connection is of decisive importance. It explains why it is thoroughly advantageous to have a bigger engine by relatively lower hot gas temperatures, derated, to operate as a small engine at full load (see "Ill. 2.2-5").
"Illustration 2.3-3": (Lit 2-14): The life time of many hot parts during stationary operation is determined by creep ( "Ill. 2.3-1"). Thereby the connection between life time and operation time is nearly linear. This can be applied for:
- Actual life time at typical machine operation phases. It is assigned by computer-aided monitoring (Chapter 5.1).
- That is valid when the engine would be always in a ‘clean condition’.
- The operating life of the real engine aberrates significantly from those data. This pretends dangerously a low life consumption. So the danger of surprising failures/damages arises. The consequences are high costs (secondary failures, outage times) and logistic problems (acquisition of spare parts). Normally the life time is limited due to the design at 2 % creep elongation (plastic deformation). Corresponding with the design, the life time of derivates lays typically in the region of 20 000 operation hours at 100% load .
An increased life time consumption is due to a temperature rise to compensate the power decrease by deterioration (decline of the efficiency, chapter 2.5.1.1). There fouling of the compressor acts an important part. Besides the rise of the temperature the rotor speed can also increase and with this the mechanical stress (centrifugal force) in the rotor components. It is to note that for cooled turbine blades and vanes, typically for modern gas turbines, frequently thermal fatigue (TF, TMF, "Ill. 3.3-16") is the dominating factor. This depends especiallx from type and number of the start/stop cycles and possible load cycles ( "Ill. 2.2-5"). High thermal stresses exist also during the stationary operation with corresponding temperature gradients.
"Illustration 2.3-4": (Lit 2-14): The delivered power of a gas turbine gravely affects the life time of the hot parts. This is especially true for the combustion chamber and the blads and vanes that are in contact with the hot gas. Because of the cooling, changes in gas temperature are not to equalize with the live time relevant material temperatures but are normally slightly lower. Responsible are the cooling air veils between the hot gas stream and the suface of the parts (combustion chamber, HPT-blades and vanes).
