Numerical and Experimental Study on Rotating Stall in Industrial Centrifugal Compressor

Centrifugal compressors employed in the oil and gas industry are operated at high gas pressure conditions and are used in a wide operation range. Accurate prediction of the rotating stall and the destabilizing aerodynamic force is one of the key technologies for these compressors. The aim of this study is to establish a method of accurately predicting the inception of rotating stall and its effect on shaft vibration. To achieve this, numerical investigations are carried out by unsteady flow and rotordynamic simulations. To validate the accuracy, an experiment is carried out at relatively high gas pressure conditions. In the first part of the study, the accuracy of compressor performance prediction is studied by steady computational fluid dynamics (CFD) simulation. It is found that by taking the wall roughness effect into account, the predicted performance shows good agreement with the experimental result. In the second part of the study, the accuracy of predicting the rotating stall is studied. In the experiment, two types of rotating stalls are measured. One is a multiple-cell stall induced in the vaneless diffuser and the other is a one-cell stall induced in the impeller. It is found that the simulation can predict the inception of the rotating stall with relatively high accuracy as the predicted results show good agreement with the experimental results in terms of cell count, rotation speed, pressure fluctuation level, and the effect on shaft vibration. Through this study, the effectiveness of simulation is validated for the stall and vibration prediction.

Development of Pump-Drive Turbine Module with Hydrostatic Bearing for Supercritical CO2 Power Cycle Application

The turbomachinery used in the sCO2 power cycle requires a high stable rotor-bearing system because they are usually designed to operate in extremely high-pressure and temperature conditions. In this paper, we present a pump-drive turbine module applying hydrostatic bearing using liquid CO2 as the lubricant for a 250 kW supercritical CO2 power cycle. This design is quite favorable because stable operation is possible due to the high stiffness and damping of the hydrostatic bearing, and the oil purity system is not necessary when using liquid CO2 as the lubricant. The pump-drive turbine module was designed to operate at 21,000 rpm with the rated power of 143 kW. The high-pressure liquid CO2 was supplied to the bearing, and the orifice restrictor was used for the flow control device. We selected the orifice diameter providing the maximum bearing stiffness and also conducted a rotordynamic performance prediction based on the designed pump-drive turbine module. The predicted Campbell diagram indicates that a wide range of operation is possible because there is no critical speed below the rated speed. In addition, an operation test was conducted for the manufactured pump-drive turbine module in the supercritical CO2 cycle test loop. During the operation, the pressurized CO2 of the 70 bar was supplied to the bearing for the lubrication and the shaft vibration was monitored. The successful operation was possible up to the rated speed and the test results showed that shaft vibration is controlled at the level of 2 μm for the entire speed range.

Numerical and Experimental Study on Rotating Stall in Industrial Centrifugal Compressor

Centrifugal compressors employed in the oil and gas industry are operated at high gas pressure conditions and are used in a wide operation range. Accurate prediction of the rotating stall and the destabilizing aerodynamic force is one of the key technologies for these compressors, because rotating stall can sometimes cause severe problems with subsynchronous shaft vibration and limit its operation range. Thus, the aim of this study is to establish a method of accurately predicting the inception of rotating stall and its effect on shaft vibration. To achieve this, numerical investigations are carried out by unsteady Reynolds-averaged Navier-Stokes (RANS) simulation with a full annulus model of the compressor stage. Also, to assess the accuracy of the simulation qualitatively and quantitatively, a high-pressure compressor test rig that contains a shrouded impeller and a vaneless diffuser is built. To investigate the effect of the rotating stall on the shaft vibration, an experiment is carried out at relatively high gas pressure with the inlet pressure level exceeding 30 barA. In the first part of the study, the accuracy of compressor performance prediction is studied by steady computational fluid dynamics (CFD) simulation. It is found that by taking the wall roughness effect into account, the predicted performance shows good agreement with the experimental result. Thus, a subsequent study of the rotating stall is also carried out by considering its effect. In the second part of the study, the accuracy of predicting the rotating stall is studied. In the experiment, two types of rotating stall are measured. One is a multiple-cell stall induced in the vaneless diffuser, for which the speed of rotation is relatively low and the other is a one-cell stall induced in the impeller region. The properties of the multiple-cell stall agree with the previous experimental and numerical studies, and the rotating stall has the limited effect on shaft vibration. Conversely, the one-cell stall shows severe subsynchronous vibration. In this study, both types of stall prediction are examined by CFD simulation. It is found that the simulation can predict the inception of the rotating stall with relatively high accuracy as the predicted results show good agreement with the experimental results in terms of cell count, rotation speed, pressure fluctuation level and the effect on shaft vibration. Through this study, the effectiveness of unsteady CFD simulation is confirmed for these types of stall and vibration prediction.

The Experimental Rotordynamic Stability Evaluation Method Using Magnetic Excitation System for an Integrally Geared Compressor

In Turbomachinery the rotordynamic design is highly important to keep lower shaft vibration in the high speed rotating machinery for personal safety & machine protection. To resolve same, the authors have developed a direct rotor excitation technique using the magnetic exciters designed in-house. The development of compact magnetic exciters enables to be incorporated into the casing and can be an effective method of exciting rotor vibration modes. Four magnetic exciters are installed facing the back side of an integrally geared compressor impeller in the circumferential direction. Test procedure implemented a rotational excitation method to excite forward mode and backward mode independently based on phase controlled magnet currents. In this case rotor has totally ten shaft vibration sensors which detect the rotor vibration modes excited by magnetic excitation technique. The directional frequency response functions (dFRFs) are separate results of forward mode component and backward mode component included in FRF. It achieves more accurate modal analysis to obtain natural frequency and log decrement. The dFRFs with rotational excitation denotes enough amplitude response and thus the magnetic excitation method proves to be more effective. This study successfully carry’s out the measurement of natural frequencies and log decrements of operating rotor. The comparison test results of tilting pad journal bearing (TPJB) with and without the integral squeeze film damper (ISFD), that show the ISFD reduces natural frequency and considerably increases log decrements. These test results demonstrated the effectiveness of the developed rotor excitation method for integrally geared compressor pinion rotors.

COMPARISON OF GEOMETRICAL CHARACTERISTICS AGAINST ROTATING SHAFT VIBRATION

Geometrical dimensioning and tolerance (GD&T) are an important element of the industry that uses high-speed rotation. Poor geometrical tolerance (GT) to components will cause the rotor to become unbalanced. Unbalanced rotor and shaft misalignment are the two major sources of vibration in the rotating system. This paper compares geometrical characteristics (GCs) to investigate the effects of vibrations generated by different GCs. Only four GC shafts were compared, straightness, parallelism, cylindricity, and concentricity, referring to the GD&T standard as ASME Y14.5-2009. These four GCs were selected owing to their direct involvement in the rotating system. Specimens are constructed with parameters of the same dimensions, length, and GT values only differ from GCs. Specimens were measured using a digital gage to find the GT value near 3000 micron at 3 mm. The magnitude of the shaft vibration during rotation was recorded using a VA-12 vibration analyzer with different rotational speeds: 510, 770, and 900 rpm. From the vibration data, the GCs’ effect on the rotation shaft will be determined. GCs are found to have significant effects on the rotation of the shaft that should be considered in the design, installation, and maintenance of rotating shafts. The impact and degree of damage to critical parts of the system can serve as a benchmark for further studies for the optimization of tolerance values and for the maintenance of component performance.