Andrew H. Lerche

Dynamic Stress Prediction in Centrifugal Compressor Blades Using Fluid Structure Interaction

A computational model is developed that predicts stresses in the blades of a centrifugal compressor. The blade vibrations are caused by the wakes coming off stationary inlet guide vanes upstream of the impeller, which create a periodic excitation on the impeller blades. When this excitation frequency matches the resonant frequency of the impeller blades, resonant vibration is experienced. This vibration leads to high cycle fatigue, which is a leading cause of blade failure in turbomachinery. Although much research has been performed on axial flow turbomachinery, little has been published for radial machines such as centrifugal compressors and radial inflow turbines. A time domain coupled fluid-structure computational model is developed. The model couples the codes unidirectionally, where pressures are transferred to the structural code during the transient solution, and the fluid mesh remains unaffected by the structural displacements. A Fourier analysis is performed of the resulting strains to predict both amplitude and frequency content. This modeling method was first applied to a compressor in a single stage centrifugal compressor test rig. The analysis results were then validated by experimental blade strain measurements from a rotating test. The model correlated very well with the experimental results. In this work, a model is developed for a liquefied natural gas (LNG) centrifugal compressor that experienced repeated blade failures. The model determined stress levels in the blades, which helped to predict the likely cause of failure. The method was also used to investigate design changes to improve the robustness of the impeller design.Copyright

Development Of Advanced Centrifugal Compressors And Pumps For Carbon Capture And Sequestration Applications

In order to reduce the amount of carbon dioxide (CO2) greenhouse gases released into the atmosphere, significant work has been made in sequestration of CO2 from power plants and other major producers of greenhouse gas emissions. The compression of the captured CO2 stream requires significant power, which impacts plant availability, capital expenditures, and operational cost. Preliminary analysis has estimated that the CO2 compression process alone reduces the plant efficiency by 8-12 percent for a typical power plant. The goal of the present research is to reduce this penalty through development of novel compression and pumping processes. The research supports the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) objectives of reducing the energy requirements for carbon capture and sequestration in electrical power production. However, the technology presented here is applicable to other gases including hydrocarbons as well as smaller scale carbon capture projects including CO2 separation from natural gas. The primary objective of this study is to boost the pressure of CO2 from near atmospheric to pipeline pressures with the minimal amount of energy required. Previous thermodynamic analysis identified optimum processes for pressure rise in both liquid and gaseous states. Isothermal compression is well known to reduce the power requirements by minimizing the temperature of the gas entering downstream stages. Intercooling is typically accomplished using external gas coolers and integrally geared compressors. Integrally geared compressors do not offer the same robustness and reliability as in-line centrifugal compressors. The current research develops an internally cooled compressor diaphragm to remove heat internal to the compressor. Results documenting the design process will be presented including 3-dimensional (3D) conjugate heat transfer computational fluid dynamics (CFD) studies. Experimental demonstration of the design was performed using a centrifugal compressor closed loop test facility at the authors’ company. A range of operating conditions was tested to evaluate the effect on heat transfer. At elevated pressures, CO2 assumes a liquid state at moderate temperatures. This liquefaction can be achieved through commercially available refrigeration schemes. However, liquid CO2 turbopumps of the size and pressure needed for a typical power plant were not readily available. This paper describes the test stand design and construction as well as the qualification testing of a 150 bar cryogenic turbopump. A range of suction pressures were tested and net positive suction head (NPSH) studies were performed.

Rotordynamic Comparison of Built-Up versus Solid Rotor Construction

Most manufacturers of multi-stage centrifugal compressors for the oil and gas industry utilize a solid shaft rotor construction. The impellers use a shrink fit onto the shaft with spacers in between the impellers. With the introduction of the guidelines in the 7th edition of API 617, built-up rotors for centrifugal compressors using a tie-bolt are recognized by API. This study compares the rotordynamic performance of the identical compressor using both a tie-bolt design and a more conventional solid rotor for a two-stage pipeline application. A full API 617 lateral analysis is performed on the two designs, assuming identical impeller flow path, stage spacing, and hub diameter. The critical speed and unbalance response are computed, and a full Level 2 stability analysis is performed for each case. The results show the tie-bolt construction to be slightly lighter and stiffer, resulting in a higher critical speed and improved rotordynamic stability.Copyright

Experimental Validation of Empirical Methods for Dynamic Stress Prediction in Turbomachinery Blades

Turbomachinery blade fatigue life estimation requires reliable knowledge of actual static and dynamic stresses occurring within the blades. A common method for predicting dynamic stresses is to construct a finite element model of the blade and simulate the dynamic response to aerodynamic loads. Although this method is powerful and very useful, modeling errors (geometry, boundary conditions, stress concentrations, damping, etc.) may result in inaccurate stress predictions. Furthermore, unavoidable variability in manufacturing results in blade mistuning, which significantly affects stress amplification at resonance. This paper presents two empirical methods for predicting dynamic stresses in turbomachinery blades that include the actual effects of structural damping and mistuning. Both methods use strain gauge measurements from a blade modal test to obtain load to strain transfer functions, which are applied to predict the blade strain or stress response to a simulated load. The advantages and disadvantages of each method are discussed. The predictions of each method are compared with dynamic blade strain data acquired during a rotating test of a centrifugal compressor impeller.Copyright

Rotordynamic Force Prediction of an Unshrouded Radial Inflow Turbine Using Computational Fluid Dynamics

Cross-coupled forces due to bladed components, bearings and seals can contribute to destabilizing a rotor system and are an important input to the rotordynamic design of turbomachinery. Alford (1965) developed a simple formula for describing the cross-coupled mechanism of an unshrouded axial turbine stage. The high flow radial inflow turbine studied here can exhibit similar characteristics due to its long stage length. In this work, a transient computational solution is developed to predict cross-coupling stiffness of an unshrouded turbo-expander. The three-dimensional computational fluid dynamics (CFD) model includes the flow path from the inlet guide vanes (IGV’s) to the exit of the radial inflow turbine. A 360 degree model of the flow path is used to simulate the turbine centered at its axis of rotation while the shroud is displaced a small distance from the axis of rotation. This offset simulates the uneven blade tip clearance that is present in a whirling rotor. Unsteady effects are included using a time-transient simulation while time-averaged forces acting on the turbine are used to calculate the cross-coupling aerodynamic coefficients. The rotordynamic coefficients calculated using this method are compared to both the Alford equation and formulations used for shrouded centrifugal compressor impellers.Copyright

Revisiting the SAFE Diagram for Analysis of Mistuned Bladed Disks

The SAFE diagram is a design tool that is often used for analyzing modes of concern in bladed disks. The diagram implements the concept of phase cancellation in order to show what bladed disk modes can and cannot be excited by upstream flow obstructions. In many cases, phase cancellation theory dictates that even if a modal frequency corresponds with an excitation order, the mode may not be excited if the total work calculated by integrating the excitation profile around the mode shape is equal to zero. This paper reviews the theory behind the SAFE diagram and explores the effects of blade mistuning on SAFE diagram analysis from both theoretical and experiential viewpoints. Modal test data, mistuned finite element analysis, and two case studies all indicate that mistuned modes may not exhibit the pure nodal diameter patterns that are predicted by a symmetric analysis and are required for phase cancellation. These mistuned asymmetric modes may result in high blade stresses and blade failure even if a SAFE diagram analysis indicates that the mode will not be excited.© 2012 ASME

Development of an Internally Cooled Centrifugal Compressor for Carbon Capture and Storage Applications

In order to reduce the amount of carbon dioxide (CO2) released into the atmosphere, significant progress has been made into capturing and storing CO2 from power plants and other major producers of greenhouse gas emissions. The compression of the captured carbon dioxide stream requires significant amounts of power and can impact plant availability, and increase operational costs. Preliminary analysis has estimated that the CO2 compression process reduces plant efficiency by 8% to 12% for a typical power plant. This project supports the U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) objective of reducing energy requirements for carbon capture and storage in electrical power production. The primary objective of this study is to boost the pressure of CO2 to pipeline pressures with the minimal amount of energy required. Previous thermodynamic analysis identified optimum processes for pressure rise in both liquid and gaseous states. Isothermal compression is well known to reduce the power requirements by minimizing the temperature of the gas entering subsequent stages. Intercooling is typically accomplished using external gas coolers and integrally geared compressors. For large scale compression, use of straight through centrifugal compressors, similar to those used in oil and gas applications including LNG production, is preferred due to the robustness of the design. However, intercooling between each stage is not feasible. The current research develops an internally cooled compressor diaphragm that removes heat internal to the compressor. Results documenting the design process are presented including 3D conjugate heat transfer CFD studies. Experimental demonstration of the design is performed on a sub scale centrifugal compressor closed loop test facility for a range of suction pressures.Copyright

Experimental Study of Blade Vibration in Centrifugal Compressors

Blade vibration in turbomachinery is a common problem that can lead to blade failure by high cycle fatigue. Although much research has been performed on axial flow turbomachinery, little has been published for radial flow machines such as centrifugal compressors and radial inflow turbines. This work develops a test rig that measures the resonant vibration of centrifugal compressor blades. The blade vibrations are caused by the wakes coming from the inlet guide vanes. These vibrations are measured using blade mounted strain gauges during a rotating test. The total damping of the blade response from the rotating test is compared to the damping from the modal testing performed on the impeller. The mode shapes of the response and possible effects of mistuning are also discussed. The results show that mistuning can affect the phase cancellation which one would expect to see on a system with perfect cyclic symmetry.Copyright