Measuring and testing – Test stand – For engine
Reexamination Certificate
2000-12-27
2003-05-27
McCall, Eric S. (Department: 2856)
Measuring and testing
Test stand
For engine
C073S117020, C073S822000
Reexamination Certificate
active
06568254
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for monitoring the creep behavior of rotating components of a compressor stage or turbine stage, such as are used, for example, in gas turbines.
BACKGROUND OF THE INVENTION
The rotors and moving blades of stages of this type are exposed to high mechanical load by centrifugal forces and to a very high temperature. The temperatures are, as a rule, above the transition temperature of the materials, so that time-dependent plastic expansion, so-called creeping, constitutes an essential factor limiting the useful life of the respective components.
It is therefore an important task, in the operation of a plant, to determine the creep behavior or the remaining useful life of costly components of a turbine, such as rotors and moving blades. In this context, on the one hand, safety aspects, and, on the other hand, financial aspects play an important part. Thus, a very late exchange of the components leads to a high safety risk within the plant, while too early an exchange of the components entails unnecessary costs. It is therefore very important, during the operation of a plant of this type, to monitor and estimate the creep behavior of rotating components in compressor stages and turbine stages and to estimate correctly the remaining useful life of these components.
At the present time, the creep life of turbine components is determined, as a rule, by means of the Finite Element Method (FEM), using viscoplastic material models. However, models of this type require an accurate knowledge of the material constants, boundary conditions and operating conditions, to which the components are subjected during operation. The accuracy of prognosis of these computational models is very limited because of the uncertainties in the specification of these parameters. Thus, the external boundary conditions, in particular the material temperatures during operation, cannot always be specified with sufficient accuracy. On the other hand, in particular, the material temperature has an appreciable influence on the results. Another uncertainty factor is found in the available data on the creep behavior of the material. The material composition in the production of the components is subject to variations which may, in turn, lead to different creep behaviors of the material of the component. It is not possible, because of these variations, to know the exact data relating to the creep behavior of the material of the very component to be monitored.
Finally, the development in time of creep damage is also dependent on the respectively preceding development in time of the force load and temperatures, that is to say on the previous history of the component. An exact prognosis of the creep life of a component used during operation would therefore require a simulation of the actual operating cycles of the plant. This is virtually impossible, however, because of the multiplicity of influences involved, such as hot or cold ambient conditions, transient operation, etc. The purely computational prognosis of the creep life of a component therefore does not lead to satisfactory results.
An improvement in prognosis can be achieved by the prognosis being checked by means of concrete measurements of the creep damage of the monitored component after various operation periods and, if appropriate, being corrected by adaption of the parameters. This makes it necessary, however, to determine the creep behavior or creep damage of the component by means of nondestructive test methods.
At the present time, however, there are no nondestructive test methods available which could provide reliable evidence on the creep damage of a component at an early operational stage.
Thus, admittedly, the so-called replica technique is known, in which conclusions as to microstructural damage can be drawn from an impression of the component surface or of parts thereof. However, this damage is often detected only in the so-called tertiary creep stage, that is to say at a time when the turbine components should have long since been taken out of service.
Another technique often used for determining the creep behavior of a component is the measurement of the dimensions of the component. This dimension changes due to the accumulation of inelastic expansions during the operating period. By comparing the dimensional changes with the computational results, corrections can likewise be made to the computing parameters in this way. One disadvantage of this technique, however, is that only overall creep damage of relatively large components can be detected with it, since the dimensional changes must have a measurable magnitude. This necessitates either large accumulated creep expansion or a correspondingly large length or dimension of the component.
In a further technique, test material is extracted from the component after a specific operating period and is investigated. However, this test material can be extracted only from places which have no influence on the further operating behavior of the component. These are regions with low stresses or without appreciable stresses, so that, although this test material has been exposed to the same temperatures as the component, it has not been subjected to any loads which are relevant to useful life. It is therefore scarcely possible to demonstrate creep damage on such testpieces, with the result that this technique supplies information for correcting the computational models to only a very limited extent.
Other nondestructive test methods, such as are used in other technical sectors, also do not at the present time supply any satisfactory results for evaluating creep damage. Thus, discontinuities in the material can be detected by ultrasound or magnetic methods. Even here, however, evidence of creep damage or of further behavior again requires the use of new computer models, such as are proposed, for example, in U.S. Pat. No. 5,140,528.
In summary, there have hitherto been no satisfactory methods for either monitoring or determining the remaining creep life of a rotating component of a turbine stage or compressor stage.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for monitoring the creep behavior of rotating components of a compressor stage or turbine stage, by means of which the creep behavior or creep damage of the component to be monitored can be detected with higher accuracy than hitherto. The method is to make it possible, in particular, to detect the creep damage at various times during the useful life of the component.
The object is achieved by the method according to the invention, wherein at least one test element is mounted in a region of the compressor stage or turbine stage in which said test element is exposed to an operating load comparable to that of the component to be monitored. By operating load are to be meant here, in particular, the forces acting on the component, for example the centrifugal force, and the temperatures to which the component is exposed during operation. After a predeterminable operating period of the component to be monitored, the creep behavior of the test element is investigated. The creep behavior or creep damage of the component to be monitored is derived from the creep behavior or creep damage of the test element determined in this way. These data may subsequently be compared with a computational model, so that the parameters of the model can be corrected accordingly.
It goes without saying that the test element is mounted in the turbine stage or compressor stage in such a way that said test element does not impair the operation and functioning of the latter. For this purpose, it is necessary, on the one hand, for the test element to have a correspondingly low weight and, on the other hand, to be provided at a point where it does not disturb the flow conditions within the turbine stage or compressor stage.
The one or more test elements are exposed to the same operating cycles as the component itself while the component is in operation. At the same time, the test elements are provi
Alstom (Switzerland Ltd
Burns Doane Swecker & Mathis L.L.P.
McCall Eric S.
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