Sealing in Turbomachinery

Clearance control is of paramount importance to turbomachinery designers and is required to meet today's aggressive power output, efficiency, and operational life goals. Excessive clearances lead to losses in cycle efficiency, flow instabilities, and hot gas ingestion into disk cavities. Insufficient clearances limit coolant flows and cause interface rubbing, overheating downstream components and damaging interfaces, thus limiting component life. Designers have put renewed attention on clearance control, as it is often the most cost effective method to enhance system performance. Advanced concepts and proper material selection continue to play important roles in maintaining interface clearances to enable the system to meet design goals. This work presents an overview of turbomachinery sealing to control clearances. Areas covered include: characteristics of gas and steam turbine sealing applications and environments, benefits of sealing, types of standard static and dynamics seals, advanced seal designs, as well as life and limitations issues

Influence of centrifugal compressor system components on its general rotordynamic characteristics

Nowadays most countries are depending on Oil and Gas for their energy supply. In such operations, centrifugal compressors are dominating most of the used critical machines hence it is important to give these turbomachines more consideration in terms of their technical performance and reliability. Centrifugal compressors are one of many turbomachines that require technical solutions for Enhanced Oil Recovery (EOR). The oil and gas fields have different production environments which require adequate selection of compressors to handle the variance in gas and oil specifications and this in turn force the equipment manufacturers to revise their currently used design specifications. This research presents different types of compressors and their work principles with an emphasis on centrifugal compressor components The literature review carried in this research describes different cases in turbomachinery rotordynamics where failures were encountered at the commissioning and operation stages. Also the literature shows how these machines are improved technically by improving the compressor components performance such using Pocket Damper seals and tilting type bearings. The aim of this research is to study the factors affecting Rotordynamic behaviour of large natural gas centrifugal compressors. The study will review the influence of various conditions of rotor components such as bearings, seals, impellers, etc on the overall Rotordynamic stability at various process conditions ... [cont.]

Turbomachinery Clearance Control

Controlling interface clearances is the most cost effective method of enhancing turbomachinery performance. Seals control turbomachinery leakages, coolant flows and contribute to overall system rotordynamic stability. In many instances, sealing interfaces and coatings are sacrificial, like lubricants, giving up their integrity for the benefit of the component. They are subjected to abrasion, erosion, oxidation, incursive rubs, foreign object damage (FOD) and deposits as well as extremes in thermal, mechanical, aerodynamic and impact loadings. Tribological pairing of materials control how well and how long these interfaces will be effective in controlling flow. A variety of seal types and materials are required to satisfy turbomachinery sealing demands. These seals must be properly designed to maintain the interface clearances. In some cases, this will mean machining adjacent surfaces, yet in many other applications, coatings are employed for optimum performance. Many seals are coating composites fabricated on superstructures or substrates that are coated with sacrificial materials which can be refurbished either in situ or by removal, stripping, recoating and replacing until substrate life is exceeded. For blade and knife tip sealing an important class of materials known as abradables permit blade or knife rubbing without significant damage or wear to the rotating element while maintaining an effective sealing interface. Most such tip interfaces are passive, yet some, as for the high-pressure turbine (HPT) case or shroud, are actively controlled. This work presents an overview of turbomachinery sealing. Areas covered include: characteristics of gas and steam turbine sealing applications and environments, benefits of sealing, types of standard static and dynamics seals, advanced seal designs, as well as life and limitations issues

Reservoirs Modeling of Gas hydrate deposits in North Slope of Alaska and Gulf of Mexico

In order to address the world\u27s growing energy demand, the necessity to explore more and more unconventional sources of energy arises. Recently there has been increased interest in the potential of natural gas hydrates as an alternate energy resource. Methane hydrates are crystalline solids, very similar to ice, in which non-polar molecules are trapped inside the cages formed by water molecules. Methane hydrates could be potentially a vast source of energy. The production of natural gas from hydrates economically poses a big challenge to today\u27s scientific world. Two sites for greatest potential for gas production from gas hydrates as identified by USGS and NETL/DOE are North Slope (ANS) Alaska and the Gulf of Mexico (GOM). In this work specific locations of hydrate deposits are examined, namely the Prudhoe Bay L Pad (PBU L-Pad) and Walker Ridge 313 (WR313) deposits in the ANS and GOM. Reservoir modeling in this work is primarily based on these two gas hydrate deposits.;The uncertainty of reservoir parameters such as hydrate reaction kinetics, the permeability of hydrate bearing sediment, Porosity and permeability of the shale layer boundary on gas production is studied in this work. Gas production from a horizontal well as opposed to a vertical well is evaluated using a mechanistic well bore model. A preliminary assessment of thermal disturbance due to a hot well bore penetrating hydrate deposits in the PBU L pad site is performed using CMG STARS coupled geotechnical model. The results of this study indicate that the extent of hydrate dissociation around a hot wellbore is limited by the thermal diffusion of heat moving radially away from the casing and cement.;In April and May of 2009, the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) in collaboration with the U.S. Geological Survey (USGS), the U.S. Minerals Management Service, an industry research consortium led by Chevron, and others completed a marine hydrate drilling expedition in the Gulf of Mexico called the Joint Industry Project (Leg II) expedition. A complex heterogeneous 3-D model using well log data seismic data are constructed and simulated using CMG STARS and Petrel. An uncertainty assessment of gas production from the WR313 G well on reservoir parameters is performed using a Latin-hyper cube Monte Carlo sampling. Results of the reservoir simulations indicate very high potential for producing methane from these marine hydrate deposits using depressurization due to in situ temperature and pressure related to the great depth of the deposits. The predicted production rates display high (5-40 MMscf/day) rates making the reservoirs to be attractive locations for further exploration. Special cases were considered to estimate influence of permeable over- and under burden on production

Process analysis and design in micro deep drawing utilizing a flexible die

As a result of the remarkable demands on electronic and other portable compact devices, the need to produce various miniaturized parts, particularly those made from metallic sheet is growing. In other words, in order for manufacturing companies to stay in competition, they are required to develop new and innovative fabricating processes to produce micro components with more complex features and a high standard of quality and functionality. Microforming is a micro fabrication process that can be employed efficiently for mass production with the advantages of greatly minimizing material waste and producing highly accurate product geometry. However, since the clearance between the rigid tools, i.e. punch and die, utilized in microforming techniques is relatively very small, there is a high risk of damaging the tools during the forming operations. Therefore, the use of forming tools made of flexible materials in sheet metal forming processes at micro scale has powerful potential advantages. The main advantages include a reduction in the production cost, eliminating the alignment and mismatch difficulties, and also the creation of parts with different geometrical shapes using the same flexible tool. As the workpiece is in contact with a flexible surface, this process can significantly improve the quality of the obtained products. Despite these clear advantages, micro flexible forming techniques are currently only utilized in very limited industrial applications. One reason for this is that the deformation behaviour and failure mode of sheet metals formed at micro scale are not yet well understood. Additionally, the experience-based knowledge of the micro-forming process parameters is not sufficient, particularly when flexible tools are used. Hence, to advance this technology and to improve the production quality of formed micro parts, more investigation of the key process parameters related to the material deformation are needed. The main contribution of this work is the development of a novel technique for achieving micro deep drawing of stainless steel 304 sheets using a flexible die and where an initial gap (positive or negative) is adopted between the blank holder plate and an adjustment ring utilized in the size-scaled forming systems developed for this purpose. The interesting point here is that this study presents the first attempt of employing flexible material as a forming die tool in the micro deep drawing technology to produce micro metallic cups at different scaling levels. Polyurethane rubber materials are employed in this study for the forming flexible die with various Shore A hardness. Also, the stainless steel 304 sheets utilized for the workpieces have different initial thicknesses. Various parameters that have a significant influence on the sheet formability at micro scale are carefully considered, these include initial gap value, rubber material properties, initial blank thickness, initial blank diameter, friction coefficients at various contact interfaces, diameter and height of the rubber die and process scaling factor. The size effect category of process dimension was also taken into account using similarity theory. Three size-scaled micro deep drawing systems were developed correspondingly to three different scaling factors. In each case, finite element simulations for the intended micro drawing systems are performed with the aim of identifying optimum conditions for the novel forming methodology presented in this thesis. The numerical models are built using the known commercial code Abaqus/Standard. To verify the microforming methodology adopted for the proposal technique as well as to validate the predictions obtained from simulations, an appropriate number of micro deep drawing experiments are conducted. This is achieved using a special experimental set up, designed and manufactured to fulfil the various requirements of the micro-forming process design procedure. The new knowledge provided by this work provides, for the first time, a predictive capability for micro deep drawing using flexible tools that in turn could lead to a commercially viable production scale process

Volume 2 – Conference: Wednesday, March 9

10. Internationales Fluidtechnisches Kolloquium:Group 1 | 2: Novel System Structures Group 3 | 5: Pumps Group 4: Thermal Behaviour Group 6: Industrial Hydraulic

Numerical Investigation of Rotors in Floating Ring Bearings using Co-Simulation

In this work, the nonlinear oscillation behavior of rotors in floating ring bearings is investigated numerically by means of transient run-up simulations. Non plain bearing designs are considered, which are commonly used in turbocharger applications. Furthermore, a mass-conserving cavitation model based on two-phase theory is employed, which is well suited for transient journal motions due to subsynchronous oscillations. Special axial boundary conditions are introduced, which better reflect open-ended bearings. In contrast to classical cavitation approaches, which assume degassing of dissolved air as the main mechanism, this boundary condition leads to sucking-in of air from the surroundings during squeeze motion of the journal. The numerical model is separated into a rotor and a bearing model, which are implemented in commercial software tools. Both subsystems are joined by means of one of two coupling methods, which enables the creation of detailed, easily interchangeable and updateable subsystems. Dynamic-static solver coupling is used for rotors in single film bearings with non mass-conserving cavitation models. An explicit co-simulation approach is employed for rotors in floating ring bearings or bearings with a mass-conserving cavitation model. In the first case, only the rotor subsystem is time-dependent. In the latter case, both subsystems are time-dependent and are solved with their respective solvers. All time-dependent subsystems use stable, implicit BDF solvers. The rotor is modeled using multibody dynamics software. The partial differential equation of the bearing subsystem is discretized with the Finite Element method, which yields high flexibility concerning the gap geometry. The influences of the bearing geometry and different parameters are investigated using a symmetric Jeffcott rotor. Also, an asymmetric, heavy turbocharger is considered. A non mass-conserving penalty cavitation approach yields a smooth pressure profile. The run-up behavior and the stability threshold are almost identical to the often-used half-Sommerfeld (Gümbel) condition. The hydrodynamic pressure in pockets or grooves is negligible due to their large gap size. However, the hydrostatic pressure has to be considered, which can stabilize or destabilize the system, depending on the lubricant supply geometry. The differences between non mass-conserving and the mass-conserving cavitation model are small for purely synchronous oscillations due to small unbalance. The two-phase model yields a lower stability threshold rotor speed, which is further decreased for open-ended bearings. Non mass-conserving models implicitly assume that the bearing gap is completely filled with lubricant at all times. During squeeze motion, the pressure build-up upon load-reversal is instantaneous. The pressure build-up is delayed for the mass-conserving cavitation model, which yields reduced radial damping. Furthermore, the circumferential extend of the pressure profile during whirl motion is smaller due to cavitation for mass-conserving models. The load carrying capacity is decreased, which may often yield higher eccentricities during sub-synchronous oscillations. A two-phase model can also entail a different bifurcation behavior, especially for insufficient lubricant supply

Numerical modelling of bidirectional dry gas face seals

The optimization of the geometrical parameters of the aerodynamic lift features and the analysis of the fluid flow in the seal interface are inter-twined. Any small changes in the geometrical parameters of the aerodynamic lift features significantly affect the performance of a non-contacting gas face seal. For a gas face seal to function with optimum performance requires that the optimum geometrical parameters be identified. This can be achieved through a lengthy trial and error process, often heavily dependent on the designer’s depth of insight, itself dependent on experience, or can be achieved through automated numerical methods. The purpose of this research was to develop a reliable numerical model that can serve as a design tool for simulating the performance of both unidirectional and bidirectional dry gas face seals. This was achieved in three steps. The first approach consisted in developing a 2D numerical model that employed the Reynolds equation for seals operating at very low rotating speeds and low pressure differentials. In the second step a 3D-CFD model was assembled and the practicability of using CFD, in a seal design loop, for seals operating in wide range of operating conditions, was investigated. This model employed a commercial CFD package (ANSYS CFX version 11). For last approach both models were incorporated into an automatic optimization tool that can generate optimal seal geometries with a minimum of human intervention. An extensive set of results from the analysis of dry gas face seals spanning across different operating conditions and geometrical seal face profiles, with the inclusion of convergent radial taper, are presented and discussed in this thesis. The results obtained from the Reynolds equation and 3D CFD models are compared and critically analysed. Results obtained with both models are validated against test data obtained from AESSEAL plc, the sponsor of this research. The 3D CFD model predictions showed a better agreement with the test data on the seal leakage than the Reynolds equation model. The leakage rates and fluid film thickness predictions illustrate how the 3D CFD model can be used for seal design while overcoming some of the shortcomings of the Reynolds equation based models. The major limitation of the 3D CFD model is that it is computationally expensive. An automatic optimization tool which can be used for the design of dry gas face seals has been presented. The improvements achieved from the optimization of a spiral groove face seal utilising the automatic optimization tool are: 4.8% increase of opening force, 13.2% reduction of seal leakage, 20.7% increase of design efficiency parameter, 28.3% increase of axial film stiffness and 15.9% reduction of power consumption. A proposed new design of dry gas face seal capable of bidirectional operation has been presented. This type of seal outperformed the spiral groove face seal, in reverse rotation of the sealing shaft, in terms of opening force and positive axial film stiffness

Fourteenth NASTRAN (R) Users' Colloquium

The proceedings of a colloquium are presented along with technical papers contributed during the conference. Reviewed are general applications of finite element methodology and the specific application of the NASA Structural Analysis System, NASTRAN, to a variety of static and dynamic sturctural problems