Giuseppe Vannini

Identification and Prediction of Force Coefficients in a Five-Pad and Four-Pad Tilting Pad Bearing for Load-on-Pad and Load-Between-Pad Configurations

This paper presents the identification of the rotordynamic force coefficients for direct lubrication five-pad and four-pad tilting pad bearings. The bearing is 110 mm in diameter with a L/D of 0.4 pad axial length (44 mm). The experiments include load-on-pad and load-between-pad configurations, with 0.5 and 0.6 pivot offsets, for rotor speeds ranging from 7500 rpm to 15,000 rpm. The bearing force coefficients are identified from multiple frequency excitations (20–300 Hz) exerted on the bearing housing by a pair of hydraulic shakers and are presented as a function of the excitation frequency and rotor speed for a 300 kPa unit load. The experimental results also include temperatures at the trailing edge of three pads. The direct force coefficients, identified from curve-fits of the complex dynamic stiffness, are frequency independent if considering an added mass term much smaller than the test device modal mass. The force coefficients from the four-pad bearing load-between-pad configuration show similar coefficients in the loaded and orthogonal directions. On the other hand, as expected, the five-pad bearing load-on-pad shows larger coefficients (∼25%) in the loaded direction. The maximum pad temperature recorded for the 0.5 pivot offset configurations is up to 20°C higher than those associated to the 0.6 offset configuration. Results from a predictive code are within 50% of the experimental results for the direct stiffness coefficients and within 30% for the direct damping coefficients.

Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (FPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6 mm (6.716 in) diameter seals for rotor speeds up to 15 kprm, inlet air pressure of 6.9 bar (100 psi), ambient back pressure, and with inlet gas preswirl. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250 Hz. Three different test methods were employed to give confidence in the rotordynamic coefficients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefficients for all seal configurations while paying special attention to the crossover frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswirl, where the straight-through FPDS possessed the lowest cross over frequency of 40 Hz at 15 krpm. The testing also revealed that the preswirl parameter had significantly more influence on effective damping levels and crossover frequencies when compared to rotor speed.

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

Instability Of A High Pressure Compressor Equipped With Honeycomb Seals.

A two casing (three-stage) high-pressure injection compressor train driven by a gas turbine via a gearbox was full load and full speed performance tested to demonstrate satisfactory aerothermal and rotordynamic performance. During the PTC 10 (1997) Type I testing, rotor instability was observed on the back-to-back (first and second stage) compressor casing. The third stage final compressor casing exhibited anomalous rotor centerline positions in the bearings and rotating stall. This paper discusses the technical analysis, resolution, and retesting to resolve the rotor instability, rotor equilibrium/synchronous response, and rotating stall issues experienced during the testing. It also shows the need for additional joint industry research to better understand and analyze the use of honeycomb seals in hydrocarbon-gas centrifugal compressors. THE KIZOMBA A PROJECT AND COMPRESSOR TRAIN The Kizomba A Project high pressure (HP) compressor trains’ objectives are to compress associated (hydrocarbon) gas for fuel gas source and reinject as well as compress lift gas for crude oil production. The Kizomba A project is designed to produce 250,000 barrels of oil per day. The associated gas and lift gas will require the two HP compressor trains to handle 144.4 MMscm per day per train. The HP compressor trains will be installed and operate on a floating production storage and off-loading (FPSO) vessel located approximately 100 km (62 miles) offshore Angola. The Kizomba A facilities are operated by Esso Exploration Angola (Block 15) Limited under a production sharing agreement with Sonangol (Angola’s national petroleum company). Coventurers in the production sharing agreement are BP Exploration (Angola) Limited, Agip Angola Exploration B.V., and Statoil ASA. The HP compressor trains are composed of a gas turbine as the driver, a gearbox, and two centrifugal compressor casings with three process stages. The first compressor casing is a back-to-back two-stage compressor (HP first and second stage) with four impellers for each stage. The HP third stage final compressor casing is a straight-through compressor with five impellers. The train layout is shown in Figure 1. The main compressor design features are summarized in Table 1. The compressors are equipped with the traditional five tilting pads radial bearing with load-on-pads built by Nuovo Pignone. The main bearing geometrical features are summarized in Table 2. The compressor internal seal original design is shown in Table 3. Use Figure 2 and 3 for seal locations on the cross sectional drawing. The impeller eye seals were tilted teeth labyrinth design to reduce gas leakage, thus providing improved compressor efficiency for both compressor casings. Each tooth of the eye seals is on a smaller diameter than the tooth upstream of it (“stepped”). The balance piston seal on the second stage end for the back-toback compressor (2BCL 458/A) used teeth-on-rotor type, 39 INSTABILITY OF A HIGH PRESSURE COMPRESSOR EQUIPPED WITH HONEYCOMB SEALS by Massimo Camatti Giuseppe Vannini Centrifugal Compressor Design Engineer GE Oil & Gas Nuovo Pignone Florence, Italy John W. Fulton Distinguished Engineering Associate ExxonMobil Research Engineering Fairfax, Virginia and Fred Hopenwasser Division Staff Engineer ExxonMobil Development Company

A Systematic Approach to Estimate the Impact of the Aerodynamic Force Induced by Rotating Stall in a Vaneless Diffuser on the Rotordynamic Behavior of Centrifugal Compressors

One of the main challenges of the present industrial research on centrifugal compressors is the need of extending the left margin of the operating range of the machines. As a result, interest is being paid in accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine to further operate, in case their amplitude is too high. These vibrations are often contained thanks to the stiffness of the oil journals.Centrifugal compressors design is, however, going towards alternative journal solutions having lower stiffness levels (e.g. Active Magnetic Bearings or Squeeze Film Dampers), which hence will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in presence of dynamic external forces like those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions.Within this scenario, experimental tests were carried out on an industrial impeller operating at high peripheral Mach number. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings.The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement.Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made.© 2013 ASME

Development Of A High Pressure Rotordynamic Test Rig For Centrifugal Compressors Internal Seals Characterization

The current centrifugal compressor design for the Oil & Gas market is more and more challenging since the cost requirements and the presence of many competitors is pushing towards casing size reduction and rotational speed increase. The first requirement basically leads to increase the number of wheels per rotor and the second to cross more critical speeds requiring the proper degree of damping. The two consequences together lead also to increase the rotor flexibility ratio (defined as the ratio between the Maximum Continuous Speed and the first critical speed as per the Fulton diagram and API617 7 th ed. [1-2]) and finally the rotordynamic stability is very much challenged. The centrifugal compressors rotordynamic stability is then strictly related to the internal seals’ dynamic behaviour and for this reason the authors’ Company decided several years ago to develop internally a High Pressure Seal Test Rig to measure seals’ stiffness and damping. The rig is now in operation. This paper aims to describe the main test rig capabilities, the applied identification procedures and the preliminary test results on a long labyrinth seal (smooth rotor straight toothed stator). Due to the pressure level (500bar design pressure), the test rig plant appears like a high-pressure industrial plant equipped with the testing cell (a 1:1 scale high pressure compressor) and all the relevant auxiliaries: a 400 kW electric motor (driven by a VFD), a speed increaser gear box, a high pressure reservoir (6

Experimental Results and Computational Fluid Dynamics Simulations of Labyrinth and Pocket Damper Seals for Wet Gas Compression

The most recent development in centrifugal compressor technology is toward wet gas operating conditions. This means the centrifugal compressor has to manage a liquid phase which is varying between 0% and 3% liquid volume fraction (LVF) according to the most widely agreed definition. The centrifugal compressor operation is challenged by the liquid presence with respect to all the main aspects (e.g., thermodynamics, material selection, thrust load) and especially from a rotordynamic viewpoint. The main test results of a centrifugal compressor tested in a special wet gas loop (Bertoneri et al., 2014, “Development of Test Stand for Measuring Aerodynamic, Erosion, and Rotordynamic Performance of a Centrifugal Compressor Under Wet Gas Conditions,” ASME Paper No. GT2014-25349) show that wet gas compression (without an upstream separation) is a viable technology. In wet gas conditions, the rotordynamic behavior could be impacted by the liquid presence both from a critical speed viewpoint and stability-wise. Moreover, the major rotordynamic results from the previously mentioned test campaign (Vannini et al., 2014, “Centrifugal Compressor Rotordynamics in Wet Gas Conditions,” 43rd Turbomachinery Symposium, Houston) show that both vibrations when crossing the rotor first critical speed and stability (tested through a magnetic exciter) are not critically affected by the liquid phase. Additionally, it was found that the liquid may affect the vibration behavior by partially flooding the internal annular seals and causing a sort of forced excitation phenomenon. In order to better understand the wet gas test outcomes, the authors performed an extensive computational fluid dynamics (CFD) analysis simulating all the different types of balance piston annular seals used (namely, a tooth on stator (TOS) labyrinth seal and a pocket damper seal (PDS)). They were simulated in both steady-state and transient conditions and finally compared in terms of liquid management capability. CFD simulation after a proper tuning (especially in terms of LVF level) showed interesting results which are mostly consistent with the experimental outcome. The results also provide a physical explanation of the behavior of both seals, which was observed during testing.

Dynamic Characterization of Tilting Pad Journal Bearings From Component and System Level Testing

The dynamic response of a direct lube, 5-pad, rocker-back pivot tilting pad bearing is characterized in a controlled motion (component level) test rig, and in a spin bunker (full system level) using a dummy rotor mounted on two identical bearings. In the component level test, the force coefficients (stiffness, damping, mass) are identified from pseudorandom excitations using a 2-DOF model. N-DOF system including the pad motions has been shown to yield frequency dependent coefficients that warrant the use of asynchronous coefficients for stability analysis in centrifugal compressors. However, experimental results showed that the real part of the dynamic stiffness is well represented as a constant stiffness and mass coefficients while the imaginary part yields a constant damping coefficient (i.e. frequency independent).In the system level test, a dedicated dummy rotor (representative of a high speed centrifugal compressor rotor) is excited by a magnetic shaker throughout a frequency range covering the rotor modes of interest while spinning at constant speed. From the rotor harmonic response the damping of each mode is extracted using a curve-fitting method based on a 1-DOF model for a given set of speeds.The dummy rotor test provides reference values for system logarithmic decrement and further validates the component level test results. The logarithmic decrement prediction using identified bearing force coefficients are in good agreement with the experimental results. In addition, using for prediction identified coefficients in a classical K-C-M or synchronous K-C form yields similar results (within 15%). This indicates that for the given bearing geometry (clearance, offset and size) and operating conditions, synchronously reduced force coefficients are adequate for stability analysis. Comparison of the identified force coefficients with results from commercially available code yielded reasonable agreement on direct coefficients while some discrepancies are highlighted on the cross-coupled coefficients.Copyright

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.

Rotordynamic Evaluation of Full Scale Rotor on Tilting Pad Bearings With Integral Squeeze Film Dampers

In the last 10 years, major centrifugal compressor manufacturers have been investing in developing technologies to improve their products. Following the increasing demand in terms of performance, efficiency and compactness, the current trend in the compressor industry is to increase the “power density”. One big challenge of this “power density” approach is the increase of the rotational speed which may be related to rotordynamic concerns (e.g. crossing of higher rotor modes, stability). Commonly used in the aircraft gas turbines [16], the squeeze film dampers represent an efficient solution to deal with high vibrations and to ensure stable operation for supercritical rotors. In the Oil & Gas Centrifugal Compressors world, SFDs are not so often utilized by the manufacturers but sometimes chosen by the end users as a retrofit solution when high level of synchronous/sub-synchronous vibrations are experienced in the field. The experimental activities described in this paper represent the authors’ Company effort to validate the behavior of a special, integrated SFD type in order to add this component in the available technology portfolio of a centrifugal compressor using it since the design phase. To accomplish this target, the SFD testing was performed originally at the component level and finally at a system level on a “dummy rotor”, specifically designed to mimic the rotordynamic behavior of a real rotor (e.g. running across both the two first rigid modes and the first bending mode). The main objectives of the testing activity were: to check the benefit of using SFDs in order to increase the rotor system damping, to check the SFD overall operational performances, and finally to validate the rotordynamic predictability of this new rotor system. The system level testing program was performed in a high speed balancing bunker where the rotor was equipped with a magnetic exciter able to deliver sub-synchronous excitation. The main test results which will be described in details in the paper are anticipated here. SFDs showed a significant increase in the damping of rigid modes compared to a baseline configuration (rotor running on traditional journal bearings); the SFDs behavior was fully assessed both from rotordynamic viewpoint (rotor and damper housing vibrations) and from operational viewpoint (oil temperature and pressures directly measured in the damper land); finally the rotor modal damping identification techniques are applied to this highly damped rotor system in order to compare the experiment with the relevant predictions. As a conclusion the testing activity provided the authors’ Company with confidence in the use of this integrated SFD technology and enabled a new option for centrifugal compressor design.Copyright