Table of Contents
4.3.3 The operator is not always to blame! Failure favorable influences due to production and assembly
Once again, the car owner should serve as an example. To be sure, we have a bad conscience when there are damages on the motor, because the oil change was long since necessary or the level of cooling water had been critical.
Thus, we are often prepared to take the onus of the blame for the failure upon ourselves. However, a detailed observation may show, under certain circumstances, that the cause of failure lies elsewhere, e.g., in a defective part. It is typical that the first damage arises after a period of operation, whereby the relation to a new part cannot be automatically established.
It is also imperative for the operator of a gas turbine to recognize typical failure inducing influences for which he is not responsible. These are, especially, problems of production and assembly.
To identify and prove such influences, expertise to which we can have confidence is needed. We find this case specific and correspondent to the interests at the
- OEM,
- Insurance company,
- independent expert/consultant,
- own specialists.
It is important that a comprehensible and sufficient documented problem analysis is made. Fatigue cracks always initiate from a locally weak spot such as score, notch, pore or a material damage. Whether the weak spot is within the specified limit and if the requirement corresponds to the best available technology is decisive. If the features are outside the specification /drawing/design which follows the best available technology (state of the art) we speak about a causative failure. In such a case we can trace it to the manufacturer. Is the feature traced inside the specifications we call it a weak spot. In this case normally the operation loads are responsible for the operation failure (main cause failure, root cause failure). Thus, the operator is suspect. Weak spots within the specifications are, e.g., material typical pores in castings such as turbine blades. In contrast, e.g., grinding cracks in the blade root probably lie outside the specification.
Regarding failures on hot parts through creep or thermal fatigue, it must not always be the over temperatures set by the operator that is the cause, e.g., because of faulty operation. Blockages, and reactions in the cooling air holes through molds, coating auxiliary materials or reactive shot peening media are only provable after an exact examination, but they also lie outside the area of responsibility of the operator.
Extraordinary crack initiation through thermal fatigue in the air foils of turbine blades can, e.g., also be attributed to thick diffusion coatings.
Violation of the best available technology (failure mechanisms that the expert should know) are possibly more seldom than is thought. Gas turbines take a special position here, derived from flight engines. On the one hand, they profit from the complicated development and series experience. On the other hand, when weak spots are recognized, immediate remedy is accomplished, often only accounted for after long service life in industry gas turbines. This is due to the comparatively long overhaul time intervals, not allowing immediate access, solely for technical reasons and organizational separation between aero engine- and industrial gas turbine manufacturer.
An assembling process can be also the cause of failures. So the wrong lubrication/grease can cause a bolt fracture ( "Ill. 4.2.3.1-1"). Careless handling creates scratches, notches or deformations with the risk of a later fracture of the part ( "Example 4.3-3"). Leaved foreigen objects (e.g., tools) and contaminations in the engine can trigger e.g., compressor failures and bearing failures.
Those examples are naturally not all the possibilities of failures for which the operator has not the reponsibility. In the case of a suspicion the operator/owner should think about a neutral examination.
Literature of chapter 4.3
4.3-1 J.F.Rudy, Beitrag zur Asian Aircraft Engineering and Maintenance, 1986. Conference Proceedings Page128.
4.3-2 P.H. Wulff, „Optimierung der Unterhaltskosten von Gasturbinen“, aus „Gasturbinen in Praxis und Entwicklung“, VDI-Gesellschaft Energietechnik, VDI-Berichte 1721, ISBN 3-18-091721- 0, page 157 up to 168.