J. Jeffrey Moore

Rotordynamic Stability Measurement During Full-Load, Full-Pressure Testing Of A 6000 Psi Reinjection Centrifugal Compressor.

Full-load, full-pressure rotordynamic stability measurements were conducted on a seven-stage, back-to-back centrifugal compressor. To validate rotordynamic predictions, the rotor was excited while operating at full load and full pressure during factory testing. This was accomplished through means of a magnetic bearing, which was attached to the free end of the rotor. This device injected an asynchronous force into the rotor system to excite the first forward whirling mode. This technique measures the rotor’s logarithmic decrement (log dec), which indicates the level of stability, or damping, in the rotor. The device is designed to be nonintrusive to the original dynamics of the rotor and may be easily installed/removed on the test stand. This paper discusses the techniques used to measure the rotordynamic stability from a fullload, full-pressure test of a 6000 psi reinjection compressor. The results demonstrate the effectiveness of swirl brakes and damper seals in producing a compressor that becomes more stable as discharge pressure increases. This approach to compressor design is in stark contrast to traditional designs in which the stability degrades with increasing pressure, ultimately leading to rotordynamic instability. This technology ensures trouble-free startup and operation of these compressors in the field, minimizing risk for the end-user.

Novel Concepts for the Compression of Large Volumes of Carbon Dioxide

In the effort to reduce the release of CO{sub 2} greenhouse gases to the atmosphere, sequestration of CO{sub 2} from Integrated Gasification Combined Cycle (IGCC) and Oxy-Fuel power plants is being pursued. This approach, however, requires significant compression power to boost the pressure to typical pipeline levels. The penalty can be as high as 8% to 12% on a typical IGCC plant. The goal of this research is to reduce this penalty through novel compression concepts and integration with existing IGCC processes. The primary objective of the study of novel CO{sub 2} compression concepts is to boost the pressure of CO{sub 2} to pipeline pressures with the minimal amount of energy required. Fundamental thermodynamics were studied to explore pressure rise in both liquid and gaseous states. For gaseous compression, the project investigated novel methods to compress CO{sub 2} while removing the heat of compression internal to the compressor. The high-pressure ratio due to the delivery pressure of the CO{sub 2} for enhanced oil recovery results in significant heat of compression. Since less energy is required to boost the pressure of a cooler gas stream, both upstream and interstage cooling is desirable. While isothermal compression has been utilized in some services, it has not been optimized for the IGCC environment. This project determined the optimum compressor configuration and developed technology concepts for internal heat removal. Other compression options using liquefied CO{sub 2} and cryogenic pumping were explored as well. Preliminary analysis indicates up to a 35% reduction in power is possible with the new concepts being considered.

An Improved Catcher Bearing Model and an Explanation of the Forward Whirl/Whip Phenomenon Observed in Active Magnetic Bearing Transient Drop Experiments

Though many approaches have been proposed in the literature to model the reaction forces in a catcher bearing (CB), there are still phenomena observed in experimental tests that cannot be explained by existing models. The following paper presents a novel approach to model a CB system. Some of the elements in the model have been previously introduced in the literature; however, there are other elements in the proposed model that are new, providing an explanation for the forward whirling phenomena that has been observed repeatedly in the literature. The proposed CB model is implemented in a finite element rotordynamic package, and nonlinear time-transient simulations are performed to predict published experimental results of a high speed vertical sub-scale compressor; with no other forces present in the model, the agreement between simulations and experimental data is favorable.The results presented herein show that friction between the journal and axial face of the catcher bearing results in a forward cross-coupled force that pushes the rotor in the direction of rotation. This force is proportional to the coefficient of friction between the axial face of the rotor and catcher bearing and the axial thrust on the rotor. This force results in synchronous whirl when the running speed is below a combined natural frequency of the rotor-stator system, and constant frequency whip when the speed is above a whip frequency.Copyright

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

Aerodynamically Induced Radial Forces in a Centrifugal Gas Compressor: Part 1—Experimental Measurement

Net radial loading arising from asymmetric pressure fields in the volutes of centrifugal pumps during off-design operation is well known and has been studied extensively. In order to achieve a marked improvement in overall efficiency in centrifugal gas compressors, vaneless volute diffusers are matched to specific impellers to yield improved performance over a wide application envelope. As observed in centrifugal pumps, nonuniform pressure distributions that develop during operation above and below the design flow create static radial loads on the rotor. In order to characterize these radial forces, a novel experimental measurement and post-processing technique is employed that yields both the magnitude and direction of the load by measuring the shaft centerline locus in the tilt-pad bearings. The method is applicable to any turbomachinery operating on fluid film radial bearings equipped with proximity probes. The forces are found to be a maximum near surge and increase with higher pressures and speeds. The results are nondimensionalized, allowing the radial loading for different operating conditions to be predicted.

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 Analysis of a Large Industrial Turbocompressor Including Finite Element Substructure Modeling

A rotordynamic analysis of a large turbocompressor that models both the casing and supports along with the rotor-bearing system was performed. A 3D finite element model of the casing captures the intricate details of the casing and support structure. Two approaches are presented, including development of transfer functions of the casing and foundation, as well as a fully coupled rotor-casing-foundation model. The effect of bearing support compliance is captured, as well as the influence of casing modes on the rotor response. The first approach generates frequency response functions (FRFs) from the finite element case model at the bearing support locations. A high-order polynomial in numerator-denominator transfer function format is generated from a curve fit of the FRF. These transfer functions are then incorporated into the rotordynamics model. The second approach is a fully coupled rotor and casing model that is solved together. An unbalance response calculation is performed in both cases to predict the resulting rotor critical speeds and response of the casing modes. The effect of the compressor case and supports caused the second critical speed to drop to a value close to the operating speed and not compliant with the requirements of the American Petroleum Institute (API) specification 617 7th edition. A combination of rotor, journal bearing, casing, and support modifications resulted in a satisfactory and API compliant solution. The results of the fully coupled model validated the transfer function approach.

MANUFACTURING AND TESTING EXPERIENCE WITH DIRECT METAL LASER SINTERING FOR CLOSED CENTRIFUGAL COMPRESSOR IMPELLERS

Direct Metal Laser Sintering (DMLS) is an additive manufacturing process that utilizes a high-powered laser to build up a metal part by selectively melting thin layers of metal powder. This process is attractive for the manufacturing of parts with complex geometry such as closed centrifugal compressor impellers. DMLS allows closed impellers to be made in a single piece and eliminates the shroud joint that results from two-piece manufacturing processes. Using a monolithic impeller can allow higher tip speeds with improved fatigue characteristics compared with two-piece and three-piece designs. Prototype parts can be made more economically than investment casting when considering the tooling costs. Manufacturing costs for DMLS parts are marginally higher than for two-piece machined impellers, but qualification efforts for the braze/weld joint at the cover are circumvented. The DMLS process introduces several factors that must be considered in the impeller design to achieve a successful build with the proper strength and surface finish. This paper describes the authors’ experience with manufacturing and testing multiple closed impeller designs constructed from Inconel 718, 17-4 PH Stainless Steel, and Titanium 6Al-4V. A detailed discussion of design factors and manufacturing experience with a DMLS vendor is included for the various metals. Dimensional, post-test destructive inspection, and material test results are provided showing that the DMLS process can produce an impeller with good dimensional accuracy, surface finish, and material strength. Finally, overspeed test results up to maximum tip speeds of over 1400 ft/s (425 m/s) and aerodynamic performance test results are presented and discussed.

Transient Surge Measurements Of A Centrifugal Compressor Station During Emergency Shutdowns.

For every centrifugal compressor installation, the design of the surge control system is vitally important to prevent damage of the compressor internal components, seals, and bearings. While most surge control systems are capable of preventing surge for steady-state operation, emergency shutdowns (ESDs) are particularly challenging, since the surge control system must respond faster than the deceleration rate of the train. The available experimental data are not of sufficient quality and resolution to properly validate current software packages. This paper outlines an experimental test program using a full-scale compressor tested in a hydrocarbon flow loop under controlled, laboratory conditions. Transient compressor surge data during an ESD were captured over a variety of initial speed, pressure, and flow conditions. Furthermore, the anti-surge valve was modified in subsequent tests to simulate a slower and a smaller valve, providing a more varied test condition. Results of the testing and model comparisons will be presented.

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