Efficient runner safety assessment during early design phase and root cause analysis

Fatigue related problems in Francis turbines, especially high head Francis turbines, have been published several times in the last years. During operation the runner is exposed to various steady and unsteady hydraulic loads. Therefore the analysis of forced response of the runner structure requires a combined approach of fluid dynamics and structural dynamics.Due to the high complexity of the phenomena and due to the limitation of computer power, the numerical prediction was in the past too expensive and not feasible for the use as standard design tool. However, due to continuous improvement of the knowledge and the simulation tools such complex analysis has become part of the design procedure in ANDRITZ HYDRO. This article describes the application of most advanced analysis techniques in runner safety check (RSC), including steady state CFD analysis, transient CFD analysis considering rotor stator interaction (RSI), static FE analysis and modal analysis in water considering the added mass effect, in the early design phase. This procedure allows a very efficient interaction between the hydraulic designer and the mechanical designer during the design phase, such that a risk of failure can be detected and avoided in an early design stage.The RSC procedure can also be applied to a root cause analysis (RCA) both to find out the cause of failure and to quickly define a technical solution to meet the safety criteria. An efficient application to a RCA of cracks in a Francis runner is quoted in this article as an example. The results of the RCA are presented together with an efficient and inexpensive solution whose effectiveness could be proven again by applying the described RSC technics. It is shown that, with the RSC procedure developed and applied as standard procedure in ANDRITZ HYDRO such a failure is excluded in an early design phase. Moreover, the RSC procedure is compatible with different commercial and open source codes and can be easily adapted to apply for other types of turbines, such as pump turbines and Pelton runners.

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

Ivan William McBean, Benjamin Megerle, Peter Ott

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

Asme2013

Analyse de l'état de contrainte et de l'endommagement d'assemblages sertis en matériau mixte métal-composite sollicités en traction ou en flexion

Alain Michel Prenleloup

The constant improvement of polymer composite materials has allowed their integration in many industrial fields where their mechanical and chemical properties are apparently attractive. Mixed metal-composite insulators are now increasingly used for high voltage networks instead of conventional porcelain structures because of their high strength and reduced maintenance requirements due to their strong resistance against chemical degradation and dynamic loading. The composite insulators investigated in this study are made of an epoxy rod reinforced with glass fibres, two steel fittings being strongly crimped to both ends of the rod to transfer loads between the suspended electrical components and the fixation points. Due to the plastic deformation of the steel induced by the crimping process, a purely mechanical link results from the residual radial stresses at the interface between the composite rod and the metallic fitting. The strength of the joints plays an important role during the tensile and bending loading of the assemblies (clamping of the high-voltage line on the electrical tower and guiding of the electrical transformation substation). Experimental and numerical studies on the crimped joints are performed simultaneously in order to determine the strength of the structures and to characterise the damage and fracture mechanisms. The traction and bending tests are achieved to highlight the failure modes appearing for very high loads. In parallel, the numerical analysis provides complementary information on the stress state and the sensitivity of some influent parameters. The proposed dual approach allows the identification of the critical loads corresponding to the failure initiation and the possible mechanisms associated. The results for the tensile tests performed on structures subjected to a crimping load between 290 and 460 kN exhibit a nonlinear increase from 110 to 160 kN of the maximum pull-out load, and a transition of the failure mode of the link from a sliding of the rod out of the end-fitting for weakly crimped insulators and a delamination of the composite for strongly crimped ones. For still higher crimping conditions, the maximum tensile load decreases due to the damage introduced before tensile loading. In parallel to the experimental study, the numerical model developed for the crimping and traction steps provides the stress state in the joint during the solicitations. The comparison between the numerical predictions and the experimental observations highlights the strength limit of the bond by delamination of the outer layers of the composite. The use in the numerical model of a tensor damage factor depending on the stress state in the composite provides additional information on the failure initiation resulting from the stress combination and allows the characterisation of the critical crimping and loading conditions. The analysis of the crimped joints under tensile loading is extended to the study of insulators subjected to bending loading. Experimental tests are performed for two different sizes of insulators showing different final failure modes. For 51mm-thick insulators, a collapse of the compressive part of the rod occurs while for the 63mm-thick assemblies a shear crack propagation on the mid-plane of the rod can be observed. In addition to the fracture behaviour, the tests show a progressive embrittlement of the joint due to the damage onset appearing at 60 and 50% of the maximum mechanical loading of 15.7 and 21 kN found for the 51mm- and 63mm-thick rods respectively. Moreover, the numerical simulations contribute to the understanding of the failure zones and allow the identification of the critical loads and the stress state associated with the damage observed. Owing to the numerical and experimental analysis, structural modifications are proposed for improving the current bearing capacities of the crimped structures in order to shift the damage initiation and to increase the maximal strength of these assemblies under industrial use. In the tensile loading case, an optimal contact area between the rod and the end-fitting ensures an increase of 50% of the maximum mechanical load for the 18.7mm-thick insulator. For the bending case, the damage initiation at loading rates which are much lower than the bearing capacity can be postponed by changing the conical form of the metal end-fitting. However, the maximum bending load cannot be improved since it is directly limited by the diameter of the rod.

EPFL2008

Geological characterization of cataclastic rock samples using medical X-ray computerized tomography

Pierre Guillaume Christe

Technical difficulties associated with tunnelling operations in tectonized geological settings are frequently encountered. They may include instantaneous and delayed cavity convergence, sudden collapse of walls or roof of a gallery, outpouring of fault-filling materials and water inflows. These phenomena may negatively affect economical and safety aspects of construction sites. The present study refers to two previous research projects conducted at the EPFL addressing the problem of an improved geological and geomechanical characterization of weak cataclastic rocks in underground excavation works (Cataclastic fault rocks in underground excavations, a geological perspective, Bürgi, 1999; Caractérisation géomécanique de roches cataclastiques rencontrées dans des ouvrages souterrains alpins, Habimana, 1999). Cataclasis is an evolutionary rock degradation process in time and magnitude related to fault zone activity in the Earth's crust. A conceptual model describing the occurrence and variability of cataclastic rocks at shallow crustal conditions is proposed in this research. It describes environment of fault zones based on both theoretical considerations and field experiences collected during self-realized short-drilling operations performed in contrasted petrological settings. The model considers cataclasis from a regional perspective (macroscopic scale) down to its microscopic manifestations on rock materials. Cataclastic rock characteristics are strongly influenced by the degree of tectonic solicitation and the initial rock composition. Guidelines about specific damage microstructures diagnostic of the cataclastic intensity and function of the initial rock protolith can meaningfully assist the interpretation of rock specimens recovered from reconnaissance drilling operations. Despite the poor mechanical quality often observed for such samples and their readiness to collapse, this study demonstrates that the in situ extraction of cataclastic specimens can be largely improved with the use of a high-quality drilling equipment. Accordingly, with the availability of representative materials for subsequent studies, the uncertainties affecting the geological prognosis can be strongly limited at a preliminary stage of geotechnical studies. It is shown that the geotechnical characterisation of cataclastic rock cores by means of laboratory investigations requires combining information about rock microstructure and mineralogy with strain-stress relationship derived from triaxial compression tests. Such a framework makes it possible to interpret and explain the specific mechanical behavior of heterogeneous-anisotropic rock materials in a much more coherent manner. With this regard, the approach followed by Bürgi (1999) with a mineralo-structural index (MSI) determined on 2D thin sections and proposed to predict cataclastic rock strength based on geological evidences, is found to have potential for geotechnical studies. However, Bürgi's approach still faces heavy criticism in the light of the practical application. Its limitations are detailed and discussed in this study. Accounting for both aspects of the rock characterization procedure, an operational methodology is therefore proposed aiming at its application in geotechnical studies. The medical X-ray computerized tomography (XRCT) has been specifically implemented in this context for the analysis of geotechnical rock cores, as a non-destructive technique which allows a rapid indirect imaging of the rock core structure before any further analytical manipulations are conducted. The technique is found facilitating to a great extent the study of cataclastic materials. An irradiation protocol has been defined and an acquisition strategy minimalizing inherent artifacts of the XRCT technique has been tested. Obtained results are highly encouraging. Moreover, an image analysis tool has been developed in order to improve the segmentation of geological damage features from indirectly recorded X-ray models. As discussed in details within this study, a critical analysis of 3D segmented models corresponding to sample structures before and after mechanical tests is thought to be capable to improve the triaxial test interpretation. The XRCT is therefore envisaged as a bridge technique permitting an improved correlation between specific properties of cataclastic rocks based on indirect evidences. With it, the geological and mechanical characterizations can be performed on the exactly same samples, reducing uncertainty in the characterization procedure. This step goes towards the development of a precise geotechnical classification for cataclastic rocks. Based on the XRCT technique, it is also discussed how Bürgi's approach could be translated in a three-dimensional form to reach a wider agreement. Based on a database of carbonatic cataclasites, it is evidenced that cataclastic strength and deformation properties are strongly influenced by textural and structural characteristics of pre-existing damage microstructures. Strength data obtained in this research show that in the same fault zone environment, different mode of deformations affect cataclastic rocks and that peak triaxial strength can vary significantly. Heterogeneity and material anisotropy are prime factors affecting specific behaviors of cataclastic rocks. Therefore, it is suggested to incorporate geological features into constitutive frameworks which describe mechanical behavior of such materials. This last aspect constitutes a very stimulating research field where further work is needed today.

EPFL2009