Industrial Gas Turbines

The disks of the high pressure turbine ( "Ill. 3.3-1") are subject to a special load combination. These are the centrifugal forces of the high speed HPT rotor. Here, the absolute peak of the temperatures, the temperature differences, the gradients and corresponding thermal stresses ( "Ill. 3.3-5"), each attain the maximum of all disks in the engine. Hence, at this point, one should speak about the origin of the loads, their typical distribution in the component and their effect on the life, as representative for all other disks (compressor and LPT).

The centrifugal force leads to an expansion of the disk. Here, the inner disk regions are especially highly tensile loaded through tangential stresses, as far as they are not relieved by design measures. This load reduces clearly towards the periphery. The height of the radial stress lies at the outer diameter (rim) in the area of the tangential stress (hoop stress). We know from experience (e.g., carousel ) that the masses of the outer zones of a rotating disc (rim, "Ill. 3.3-1"), or a rod, experience clearly higher centrifugal forces than the inner zones. The fact that the centrifugal forces stress the hub more than the rim area is not immediately plausible through superficial observation. Though the masses experience higher centrifugal forces at the periphery, they must be held (radial towards the interior) by the neighboring parts. Thus, the centrifugal forces of the outer zones are transmitted onto the hub region, stressing it intensely; although contributing sparsely to the centrifugal force itself. For this reason, rotor disks are much thicker in the hub area than in the periphery zones ( "Ill. 3.3-1"). Despite that, cyclical plastic deformations in the hub region appear in modern engines. A life time determining LCF load is present.

Added to the load on the disk is the hindered thermal expansion as a consequence of the temperature gradients inducing certain thermal stresses, superimposing those stresses produced by the centrifugal forces ( "Ill. 3.3-5"). Because the thermal stresses are dependent on the temperature gradients, they change with operation conditions.

The gas temperature increases at the start. The rotor absorbs much heat through the big gas loaded airfoil of the blades. The relatively thin outer disk cross section heats up consequently ( "Ill. 2.2-3" and "Ill. 3.3-5"). The thick hub is very much slower in its heating up behavior and is, therefore, comparatively cold. Very high temperature gradients between rim and hub originate during start. The hot rim wants to expand but is prevented by the cold massive hub region of the disk. High compression stresses form at the rim (up to the plastic region). The hub area, which takes care of the stress balance, is correspondingly highly tension loaded. If one adds the centrifugal load to it, the rim region is relieved, while the hub region is additionally loaded. The start is the most critical stress condition of discs and correspondingly influences the intended cyclic life. This is therefore often specified as maximum acceptable number of start-stop-cycles ( "Ill. 3.3-5") and not as operation time.

The disk attains a stable temperature gradient, depending on the design and cooling, only after several minutes of steady state operation. The occurrence of the highest gradient and the highest stress can lie in this interval of time (transient condition, "Ill. 2.2-4"). The operator should be aware that the start procedure, up to full load extraction, is of great importance for the expected life of the turbine discs and the engine ( "Ill. 2.2-5").

During shut down of the engine, the gas temperature along with the centrifugal force drops quickly, which is a double unloading of the hub. The cooling of the rim region ( "Ill. 3.3-11") through the cold gas flow leads to a big temperature difference, in contrast to the still hot hub. The rim contracts and high tensile stress are produced, reaching into the plastic region (thermal fatigue, "Ill. 3.3-16"). Corresponding compression stresses relieve the hot hub region. Thus, in the hub region, through the start and shut down of the engine, LCF stress ( "Ill. 3.1.2.1-0") of low frequency is initiated with local plastic deformations and relatively few load changes (ca. 104 ). The fatigue limit is decisively determined through this load. Usually, a number of cycles is chosen for a safe life limit, ensuring that there is a sufficient margin leading statistically to a technical crack (elliptical crack with about 0.8 mm length). This incubation time is relatively strongly scattered. The remaining life can be clearly longer than the initiation period, because of a stable crack propagation up to the critical crack length ( "Ill. 3.3-17"). Especially under thermal fatigue, this time can be significant longer than the incubation time. This has not been useful up to now, as a sufficiently secure, non destructive crack inspection is not yet possible. At this the retirement for cause concept falls through ( "Ill. 5.3-2"). Such assessments are thoroughly useful in the context of failure cases,where the remaining risk for other engines of the same type can be evaluated.

The dependence of the life on the special stresses of the disks in the individual operation phases can be used to exchange the parts on grounds of actual emerging loads. Here, it is necessary to know the engine specific component loads, known, usually, solely to the manufacturer (OEM). Premise is to put all necessary data continuously on record pertaining to the entire operation. This includes, e.g., starts, rpm, acceleration cycles (mini cycles), gas temperatures and the performance output. The more the actual life of the part is used, the more precise the data should be. There follows a calculation in reference cycles, i.e., in life that a normal start cycle consumes. For this the dimensioning must be known and used in special algorithms. In the end such estimations are not possible without data of the OEM. The continued tracing of the residual lifetime, respective the consumed life resumes, the so called “life monitoring“ (Chapter 5.3).

It is to be noticed that the casings of the engine are designed so as to include a blade fracture, i.e., it can be contained. This capacity of containment is not generally given in a disk fracture, at least in derivates. Through especial measures (e.g. containment ring ), disk fracture bits can be contained. Also for gas turbines of the hevy frame type ( "Ill. 2.1-7") a containment is not save if catastrophic failures occur (Lit. 2-15).

Since the HP turbine blades must be cooled, the necessary cooling air is to be guided through the disc to the blade root ( "Ill. 3.3-11"). In order to guarantee a cooling air film ( "Ill. 3.3-3"), also against the high pressures in front of the turbine, the cooling air is additionally compressed, often through a kind of radial compressor ( cover plate ) in front of the HPT disc. In some types, this job can also be done by a double walled conical shaft from the rear. The air duct system to the HPT rotor blades is very complex in its mechanical influence. Depending on the condition of the seals belonging to it, gas oscillation in this system can occur influencing the rotor again. Thus, the inexplicable, perhaps, ‘disturbed run’ of a gas turbine has its origin here in certain phases of operation.