Table of Contents
5.3.1 Lifetime estimation from operation data
‘Big brother’ in terms of electronics monitors also the car. It registers worn brakes, a slowly flatening tire, a reluctant catalytic converter and creeping aberrances of the motor. That raises the safety. Now we can initialise a repair before a final break down, which as we know, always happens at the most adverse moment. But there also emerges the feeling to be fully at someones mercy, crawls into our consciousness. Does the shop try to exchange a seviceable component for a profit maximisation? Is it really necessary to replace the whole expensive arrangement, or could it be possible to do the repair with a single hand movement? Should we accept the replacement with a new, improved version or must we then reckon with an other mode of failure which shoots the promised benefit down? Here the confidence in the shop is the basis for success and the satisfaction of both partners.
In a similar situation are the operator of a gas turbine and the OEM, respectively the caring overhaul shop. Therefore it just here means: „…check before you oblige yourself for ever…“. Also a change in the alliance of trust can be later a painful break.
What today is in test or already in service in areo engines, will come fairly soon into use for industrial engines. This is especially valid for procedures that make possible a long, risk free use of the engine and its components. Obviously components of a gas turbine, corresponding to their operation demands, are so equipped that they have sufficiently long life. This lifing, (design, method, release of safe life) succeeds more precisely, the more experiences (measurements) we have of comparative engines and operation conditions.
If from the operation data (e.g., time dependent behavior of the revolutionary speed and temperature cycles) that is continually measured on an engine, the used life and with it also the rest life of every individual life limited component is evaluated, one speaks of life monitoring. A pre requisite is the adequate exact knowledge of the individual component strain and the algorithmic belonging to it as well as continual acquisition of necessary data, to calculate the life consumption. A calibration of the calculations on the actual life consumption is possible by sampling, through the follow-up examination of the used parts. Thus, e.g., in the aero engine design used rotor parts (disks, spacer) of the engine are removed and through selective loads (cyclical centrifuging tests) the rest life is assessed, which is then compared to the calculations. It must be, thus, possible, at least in flight turbine derivates on whose engine version such examinations are carried out, to make exact statements on weak areas and the life of the components considering the expected operation data.
Since in the interim, many civil operators of flight engines carry out life monitoring and life management on line, centrally, through radio, a corresponding service may be offered from the manufacturer or independent institutions also for industry gas turbines in the near future (Lit. 5.3-1).
Lifing and life monitoring are the requirements for a successful life management. Included in this is the individual life monitoring of components, the determination of momentary conditions and the provision of exchange intervals. The exchange of parts with finished life is placed under the term “on condition” (Lit. 5.3-2) exchange. Naturally, such a procedure is only practicable, if this exchange takes place with meaningful overhaul intervals and the effort remains moderate.
These kind of components are fault free in the majority of cases and would make possible many more overhaul intervals according to the demands of fail-safe. That is why one has been long deliberating on how one can introduce these parts, with acceptable reliability, to be used again. This is then feasible, when, with enough safety, non-destructively, technical cracks (0,4 mm long) can be found and, therefore, one can utilize an adequately long, (fracture mechanically evaluated) crack growth. A pre requisite is also adequate trustworthy and sensitive inspection methods. This is at present not available for practical use. It is, therefore, dependent on further development of the inspection method, whether this highly cost saving principle (retirement for cause, "Ill. 5.3-2") can be implemented.
"Illustration 5.3-1": The trend to always higher performance and efficiency often includes an increase of component strain. In a conservative estimate of life rating through cyclically stressed parts such as rotor disks, one calculates from the time up to the fatigue crack initiation (technical crack = half elliptical surface crack with 0,8 mm length). The higher the stress, the smaller the damage or weak area in the interior or on the surface, is necessary, from where a fatigue crack can grow with every load change. In particularly highly loaded components of modern engines with a sufficiently large vulnerable area, one can expect a crack growth in the micro region already from the first load cycle onwards. This can lead to the fact that the useful cyclic life is relatively short up to the detection limit of the crack.
As there is no material, respectively, part, where one can be certain that there is no fault in the region of the detection limit of the inspection ( "Ill. 5.3-3") method, one must assume for safety purposes that, indeed, a crack initiating defect is present in a highly stressed zone. The safe life, relative to a hypothetical crack progression, is so established that enough safety up to a certain crack length (critical crack length) leading to the spontaneous fracture of the component (unstable crack growth) is provided.
A new approach for the life rating of a component could lengthen the permitted life about a portion of crack progression up to the technical crack. Thus, one would be still suffitienly away from a spontaneous failure. Such a procedure is only known during risk assessments in the context of failures so far. For a decision in already assembled parts regarding special actions (e.g., the immediate disassembly of the engine) such an observation can be most helpful.
"Illustration 5.3-2": The „retirement for cause“ concept (replacement when necessary, Lit. 3.3-4) uses the saving potential of the individual cyclical fatigue life limited components. It should remain in use for the maximum possible life limit up to its unacceptable damage (Lit. 3.3-3). This is especially valid for life limited rotor components like disks of peak loaded engines. On the grounds of statistical security, it may be required to exchange a large number of the disks (in the picture, below, left) that have not reached (in the picture above) their fatigue life limit (marked through points on the failure) around the MTBF (mean time between failure. Lit. 0-4). A pre requisite for the retirement for cause concept is a sufficiently sure, non destructive examination of the components which is practicable. The standard available procedures today, such as ultrasonic inspection, X ray and eddy current inspection, are apparently not so certain ( picture, below right), so as to have established themselves.
"Illustration 5.3-3": (Lit. 5.3-3 and Lit. 5.3-4): The diagrams show the trends of the detecting likelihood (POD = probability of detection) of surface cracks. This allows the first classification of the reliability of the most frequent non destructive testing (NDT) in series application. This concerns test results on aluminium samples with small artificial fatigue cracks. Generally counts that the crack size respectively crack length and crack depth affect the behavior different. Obvious a POD of 100% can only be expected in very fortunate cases. That means that for very high loaded components dangerous failures in the millimeter region can no more absolutely certain suspended by non destructive testing (upper diagram). Only by a combination of measures like some different non destructive testings the claimed high safety can be expected.
In this particular case was most unreliable the POD of X-ray for small surface cracks (diagramm below left). The best POD for small surface cracks showed the ultrasonic inspection. Because the presented literature informations are a little anterior we can assume that ultrasonic testing and eddy current testing meanwhile experienced improvements in technique and interpretation. Especially the application of computers for the interpretation may have improved in some cases the POD markedly (ultrasonic testing, eddy current testing).
Anyway, also here we can not act on the assumption that parts without findings are free from failures.
Literature of chapter 5.3
5.3-1 Land Instruments International, „Combustion Turbine Blade Temperature Analysis“, www.landinst.com, 21.Sept. 2006.
5.3-2 M.P.Boyce, C.B. Mehr-Homji, B. Wooldridge, „Condition Monitoring of Aeroderivative Gasturbines“, ASME Paper 89-GT-36.
5.3-3 R.E.Green Jr, „Non-Destructive Methods for the Early Detection of Fatigue Damage in Aircraft Components“, AGARD Lecture Series No. 103 „Non-Destructive Inspection Methods for Propulsion, System and Components“, 23-24 April 1979, London, UK, und 26-27 April Milan, Italy, Page 6.1-6.31.
5.3-4 W.D.Rummel, P.H.Todd Jr.,R.A.Rathke, W.L.Castner , „The Detection of Fatigue Cracks by Nondestructive Test Methods“, Zeitschrift „Materials Evaluation“., 1974, No. 32, Page 205-212.