Iacopo Giovannetti

The Dynamic Influence of Crystal Orientation on a Second Generation Single Crystal Material for Turbine Buckets

High cycle fatigue is a factor that influence gas turbine buckets lifetime and it’s due to high frequency vibrations during service. Rotation and fluid flow around the blades cause static and dynamic stresses on the buckets row. For this reason the natural frequencies and HCF resistance evaluation are fundamental in the design phase of gas turbine engines in order to avoid resonance problems during service. Single crystal and directionally solidified superalloys shows anisotropic material properties, in particular single crystal can be modeled as orthotropic material in lattice directions for FEM simulations purposes. In this paper the influence of the lattice growth orientation, identified by two angles, on the natural frequencies of first stage bucket has been investigated. Six-sigma analysis has been performed in order to obtain a transfer function between lattice orientation and bucket vibration. The Design of Experiment (DoE) has been performed using FEA modal results on ten different vibration modes. The results obtained by FEA are verified by an experimental test on the real Heavy Duty MS5002 buckets.Copyright

New high-temperature seal system for increased efficiency of gas turbines

With increasing fuel prices and significant pressure to reduce emissions, the efficient use of fuels in gas turbines is gaining ever more attention. Abradable seals in gas turbine compressors have contributed to improved efficiencies for quite some time now. To further enhance efficiency, current efforts focus on the seals in one of the hottest sections of the engine, the first turbine stage. Sulzer Metco and GE have designed and tested a new high-pressure turbine abradable seal system in the framework of a European consortium project led by GE Oil & Gas. In the scope of this project, Sulzer Metco developed a novel ceramic coating for application by atmospheric plasma spraying (APS). An engine test verified performance and economic advantages of the new seal system. The measured improvements in engine performance exceed the expected gains as predicted by model calculations.

Clearance Reduction and Performance Gain Using Abradable Material in Gas Turbines

An improvement in the energy efficiency of industrial gas turbines can be accomplished by developing abradable seals to reduce the stator/rotor gap to decrease the tip leakage flow of gases in the hot gas components of the turbine. “ABRANEW” is a project funded by the European Commission aimed at developing a high temperature abradable material capable of controlled abrasion and resistant to erosion and oxidation. In order to define the basic parameters such as the component shape, the existing gap, the expected gap reduction, the seal thickness and other geometric parameters, a comprehensive review of the design of the blade/shroud/casing system was performed.Copyright

Novel Ceramic Abradable Coatings With Enhanced Performance

Abradable coatings are employed in modern gas turbine engines to minimize clearance and reduce over-tip leakage by allowing blade tips to cut into the coating. While such clearance control coatings have been used in the compressor modules of jet engines and stationary gas turbines for many years, their use in the High Pressure (HP) section of turbine modules is relatively new. Because of the high temperatures encountered there, ceramic materials must be used to provide a durable seal. In this paper the performance of two novel ceramic abradable seal coatings is reviewed and compared to a baseline system. Emphasis is placed on the resistance to thermal shock and erosion. The abradability of coatings by hard tipped blades is reviewed using a wear map consisting of five distinctly different rub test conditions. Among the coatings considered, an APS sprayed dysprosia stabilized zirconia ceramic abradable offers a potentially superior combination of thermal shock resistance, abradability and erosion resistance for high temperature turbine seal applications.© 2006 ASME

Advanced Design of Packing and Cylinders for Hyper-Compressors for LDPE Production

The optimization of packing cup geometry is important for improving the reliability of hyper-compressors. The complex arrangement and the transmission of loads through the various components requires the use of FEA to properly determine the stress level. The need to reduce the sensitivity to external factors requires an investigation with more advanced simulations. The entire cylinder and associated components such as head flanges, cylinder chambers and tie rods, were modeled. This provided the characteristics of the interfaces and the boundary conditions necessary to simulate the packing cups. Considering the thickness, ring housing profile, lube-oil holes, mating surfaces, interference-fit and diameter make it possible to obtain the stresses at the most critical points. This new simulation approach shows the effect of the parameters that influence the stress level at critical points, thus optimizing the packing cup profile and its relevant features.Copyright

Evaluation of the material’s damage in gas turbine rotors by instrumented spherical indentation

Experimental indentations are carried out on items of two different materials, taken in several location of various components from high pressure gas turbine rotor which have seen an extensive service. The components object of investigation consisted in 1st and 2nd high pressure turbine wheels made in nickel-base superalloy (Inconel 718), the spacer ring (Inconel 718) and the compressor shaft made in CrMoV low alloy steel (ASTM A471 type10). Aim of the work is to set up the capability of the instrumented spherical indentation testing system to evaluate variations in the material properties due to damage, resulting from temperature field and stresses acting on components during service. To perform this task load-indentation depth curves will be acquired in various zones of the above mentioned components. The analysis of the results has allowed to identify an energy parameter which shows a linear evolution with the mean temperature acting on the components.

Development of New Abradable/Abrasive Sealing Systems for Clearance Control in Gas Turbines

Abradable/abrasive sealing systems are currently used in gas turbines to reduce the blade tip gas leakage and consequently improve the turbine efficiency. The coatings selection is directly related to the section in which they are used. Seal systems for hot gas paths are primarily required to withstand high temperature. The abradable coating should be easily removed by the tip blade without causing significant blade wear, whereas the blades should have sufficient cutting capabilities. Durability properties, such as erosion resistance, are also required.Owing to their temperature capabilities, porous ceramic coatings are successfully used as abradable coatings. Although they are characterized by good abradability properties, their resistance to environmental attacks, such as solid particle erosion, is limited by the porous microstructure which negatively affects their service life.It is apparent that durability and abradability are the main targets to be simultaneously achieved for ensuring longer service life and improved efficiency. The present work is aimed at developing new abradable/abrasive coatings pairs able to ensure both the durability performances of the coatings and good abradability properties.Three ceramic abradable coatings with DVC and porous microstructure have been studied. The down-selection process has been carried out by considering the microstructure, the hardness, the tensile adhesion strength, the erosion resistance, and the furnace cycle test resistance.A composite coating made by NiCoCrAlY matrix containing abrasive grits applied by electrolytic process was selected as abrasive material system. The abrasive grits (patent application in process by GE Oil&Gas) consists of a mixture of ceramic particles. These grits ensure both short-term cutting capability and thermal stability, assuring the clearance maintenance over time.The abradability of the seal system was assessed by a properly designed test, namely Rub Rig test, which simulates the blade incursion in the abradable coating. Surface patterns on abradable coating were also considered to further enhance the abradability. Engine tests are foreseen for assessing the service behavior of this seal system.Copyright

Damage Evolution and Failure Mechanisms for APS-TBCS

Thermal barrier coatings (TBC-s) have been utilized in gas turbine engines for over two decades, primarily to protect the existing materials under the demands for higher temperatures and greater engine efficiency. Atmospheric Plasma Sprayed TBC, commonly used for hot combustion chamber components of advanced gas turbines, are exposed to thermo-mechanical loads, which may lead to failure in form of macroscopic spallations from the metallic component. The durability of TBC is limited by the interaction of different processes and parameters, such as bond coat oxidation, cyclic strains, visco-plastic and relaxation properties, interface roughness and others. In this work, the spallation failure mechanisms and damage evolution of APS-TBC system are investigated on samples aged by isothermal and thermal cycle tests using different time and temperatures exposures. Several parameters have been analyzed by SEM and a life prediction model approach for APS-TBC is being developed focusing on oxidation kinetics, identifying the parameters such as rumpling, bond coat oxidation, TGO thickness and interdiffusion of base metal elements which drive the oxide formation and TBC spallation mechanisms.Copyright

Increased liquid droplet erosion resistance of steam turbine blades

Abstract Alloys used for the blades of steam turbines usually do not show a satisfactory resistance to erosion and require surface protection with suitable coatings. Starting from the results obtained with Stellite 6 coatings on AISI 420 stainless steel blades, this research has been developed with the aim to increase erosion resistance by modifying the chemical composition of the coatings. Several samples have been coated by laser cladding and exposed to liquid erosion tests. In particular, concentrations of carbide forming and solid solution strengthening elements have been varied in order to establish their effects against liquid droplet erosion. Results of the tests and EDX analyses on the samples are reported, with a discussion on the various effects on erosion resistance. Two directions have appeared as the most promising, either increasing the concentration of carbide forming elements, or increasing the concentration of nickel in the coatings.