Operation Of Centrifugal Compressor Research Articles

Numerical Investigation of Kelvin–Helmholtz Instability in a Centrifugal Compressor Operating Near Stall

The present work details the occurrence of the Kelvin–Helmholtz instability in a centrifugal compressor operating near stall. The analysis is based on unsteady three-dimensional simulations performed on a calculation domain covering the full annulus for the impeller and the vaned diffuser. A detailed investigation of the flow structure is presented, together with its evolution consequent to the mass flow reduction. It is demonstrated that this reduction leads to an enlargement of the low-momentum flow region initially induced by the combination of the secondary and leakage flows. When the compressor operates near stall, the shear layer at the interface between the main flow and this low-momentum flow becomes unstable and induces a periodic vortex shedding. The frequency of such an unsteady phenomenon is not correlated with the blade-passing frequency. Its signature is thus easily isolated from the deterministic rotor/stator interaction. Its detection requires full-annulus simulations with an accurate resolution in time and space, which explains why it has never been previously observed in centrifugal compressors.

Investigation of Efficiency Deterioration Causes in Process Centrifugal Compressor Operation

The high operating efficiency of centrifugal compressor is a basic requirement to maintain the gas productivity and machine availability. However, there are several factors influencing the stage efficiency in the operating environment leading to a deterioration in the compressor performance in addition to its impact on the mechanical integrity of the internal components. These variables can be classified into three main groups which are suction parameters variation, flow profile distortion, and compressor component damage. The determination of the root cause will help for a proper maintenance plan and to reduce the machine downtime. Hence, this paper will introduce a new approach to identify the inefficient compressor operation causes based on the available operation data. The investigated case is a three-stage gas transport centrifugal compressor driven by 2.9 MW two-shaft gas turbine. The outcomes from the conducted optimisation are compared with the measured discharge parameters and the findings from the internal inspection observation to emphasize the outcomes from the derived approach.

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.

Study on Impeller Fracture Model Based on Vibration Characteristics and Fractal Theory

During the operation of centrifugal compressor, failure easily occurs in the presence of complicated external forces. The failure process characterizes with strong nonlinearity, and hence it is difficult to be described by conventional methods. In this paper, firstly, the cracks in different positions are described using crack fractal theory. The basic failure modes of the impeller are summarized. Secondly, a three-dimensional finite element model of the impeller is constructed. Then the von Mises stress under the centrifugal force is calculated, and the corresponding impeller failure process is simulated by “element life and death technology” in ANSYS. Finally, the impeller failure mechanism is analyzed. It can be found that the static stress is not the main cause of the impeller failure, and the dynamic characteristics of the impeller are not perfect because of the pitch vibration modes which appeared in the investigated frequency range. Meanwhile, the natural frequency of the impeller also cannot avoid the frequency of the excitation force.

Impact of Piping Impedance and Acoustic Characteristics on Centrifugal Compressor Surge and Operating Range

The performance of a centrifugal compressor is usually defined by its head versus flow map, limited by the surge and stall regions. This map is critical to assess the operating range of a compressor for both steady state and transient system scenarios. However, the compressor map does not provide a complete picture on how the compressor will respond to rapid transient inputs and how its surge behavior is affected by these events. Specifically, the response of the compressor to rapid transient events such as single or multiple (periodic) pressure pulses, is also a function of the compressor's upstream and downstream piping system's acoustic response and impedance characteristics. This unique response phenomenon was first described in the 1970 s and came to be known as the “compressor dynamic response (CDR) theory.” CDR theory explains how pulsations are amplified or reduced by a compression system's acoustic response characteristic superimposed on the compressor head-flow map. Although the CDR theory explained the impact of the nearby piping system on the compressor surge and pulsation amplification, it provided only limited usefulness as a quantitative analysis tool, mainly due to the lack of computational numerical tools available at the time. To fully analyze pulsating flows in complex centrifugal compressor suction and discharge header piping systems, the principles of the CDR theory should be implemented in a dynamic flow model to quantify the magnitude of the amplifications of pressure pulses near the surge region. When designing centrifugal compressor stations within a transmission piping system, it is critically important to have a full understanding of the impact of the station's piping system on compressor dynamic behavior. For example, if a compressor system's piping impedance amplifies the suction side pulsations, the compressor's operating range will be severely limited and will produce unacceptable discharge piping vibrations. Whereas it is usually desirable to limit the downstream volume between the compressor discharge and the check valve to reduce the potential for transient surge events, a small discharge volume results in high piping impedance. This will amplify pressure pulsations passing through the compressor. The small downstream volume provides limited ability for any transient peak (such as a pressure pulse) passing through the compressor to be absorbed quickly, and an amplified discharge pressure spike will be the result. Also, if any periodic pressure excitation from upstream vortex shedding or any other continuously varying flow disturbance couples with a pipe resonance length, the result can be high fluctuations of the compressor operating point on its speed line, effectively resulting in a reduced operating range and higher than expected surge margin (surge line moves to the right). Both acoustic resonance and system impedance are functions of pipe friction, pipe and header interface connections, valve/elbow locations, pipe diameter, and valve coefficients, i.e., the entire piping system connected to the compressor. Thus, a careful acoustic and impedance design review of a compressor station design should be performed to avoid impacting the operating range of the machine. This paper describes the methodology of such a design review using modern pulsation analysis software. Examples and parametric studies are presented that demonstrate the impact of system impedance and piping acoustics on the dynamic operating response of the compressor in a typical compressor station. Some recommendations to reduce the risk of pulsation amplification and unsteady operation are also provided.

Evaluation of Gas Turbine Outboard Bleed Air on Overall Engine Efficiency and CO2e Emission in Natural Gas Compressor Stations

Gas turbine (GT) engines employed in natural gas compressor stations operate in different modes depending on the power, turbine inlet temperature, and shaft speeds. These modes apply different sequencing of bleed valve opening on the air compressor side of the engine. Improper selection of the GT and the driven centrifugal gas compressor operating conditions can lead to larger bleed losses due to wider bleed valve openings. The bleed loss inevitably manifests itself in the form of higher overall heat rate of the GT and greater engine emission. It is, therefore, imperative to determine and understand the engine and process conditions that drive the GT to operate in these different modes. The ultimate objective is to operate the engine away from the inefficient modes by adjusting the driven gas compressor parameters as well as the overall station operating conditions (i.e., load sharing, control set points, etc.). This paper describes a methodology to couple the operating conditions of the gas compressor to the modes of GT bleed valve opening (and the subsequent air bleed rates) leading to identification of the operating parameters for optimal performance (i.e., best overall efficiency and minimum CO2e emission). A predictive tool is developed to quantify the overall efficiency loss as a result of the different bleed opening modes and map out the condition on the gas compressor characteristics. One year's worth of operating data taken from two different compressor stations on TransCanada Pipelines' Alberta system were used to demonstrate the methodology. The first station employs a GE-LM1600 gas turbine driving a Cooper Rolls-RFBB-30 centrifugal compressor. The second station employs a GE-LM-2500+ gas turbine driving NP PCL-800/N compressor. The analysis conclusively indicates that there are operating regions on the gas compressor maps where losses due to bleed valves are reduced and, hence, CO2 emissions are lowered, which presents an opportunity for operation optimization.

Dynamic Model of Variable Inducer Shroud Bleed for Centrifugal Compressor Surge Suppression

The requirements in widening the range of centrifugal compressor operation, combined with competent efficiency cannot be achieved using conventional methods. The need for low energy consumption, rapidly reacting actuation systems is inevitable also in the field of surge control. The goal of this paper is to develop the model of the Variable Inducer Shroud Bleed, focusing on the dynamic behavior of the system, which can be an alternative for other active surge suppression. In order to accomplish this requirement, the model has been realized in MATLAB Simulink environment, where a linear quadratic regulator (LQR) for optimal control also has been designed. The simulation results have been compared to measurement data from the compressor test bench at the Department Lab.

Operation of Centrifugal Compressor as Annular Array of Blading

We demonstrate that the polytropic efficiency of modern centrifugal compressors can be raised to the level reached by axial-flow compressors. We consider systems of circulation flows around the impeller, the flows past the blading of axial and centrifugal compressors. And we make recommendations for the compressor blade design.

Positive feedback stabilization of centrifugal compressor surge

Stable operation of axial and centrifugal compressors is limited towards low mass flows due to the occurrence of surge. The stable operating region can be enlarged by active control. In this study, we use a control valve which is fully closed in the desired operating point and only opens to stabilize the system around this point. As a result, only nonnegative control values are allowed, which complicates the controller design considerably. A novel positive feedback controller is proposed which is based on the pole placement technique. This controller has been successfully applied to a laboratory-scale gas turbine installation. Initial experiments show that the surge point mass flow can be reduced by at least 7%. Using this efficient control strategy, stable operation in the desired operating point is maintained with small average control valve mass flow.

Effect of corrosive contaminants on lubricating properties of turbine oil

In the operation of centrifugal and piston compressors on natural gas transmission lines, it is found that air, moisture, and hydrogen sulfide get into the lubricating oil. These contaminants, especially hydrogen sulfide, affect the lubricating properties of the oil to a great degree; however, this problem, which is directly related to the operating reliability of the moving parts of compressors, has not been studied adequately. Oxygen dissolved in the oil forms an iron oxide film on rubbing metal surfaces, protecting them from wear. When no oxygen is present, the wear becomes much more severe, all the way up to grabbing of the rubbing surfaces and pitting. Oil contaminants that form surface films with a different composition and structure, for example oxysulfide films, have received less attention. Using a procedure that we had developed, in which the oil can be saturated with hydrogen sulfide or other gases, we investigated the effects on wear rate and coefficient of friction from the presence of corrosive contaminants that find their way into turbine oil in the process of natural gas transmission.

Methods for increasing the fail-safe operation and service life of centrifugal compressors

Methods for increasing the fail-safe operation and service life of centrifugal compressors

The Time Domain of Centrifugal Compressor and Pump Stability and Surge

New evidence about the time domain operation of centrifugal compressors and pumps in compliant systems is presented. Data from Toyama [1,2] plus unpublished data from another compressor indicate that the flow rate oscillates continuously, at large amplitudes, when a compressor is operating in its supposedly stable regime. A tentative flow model predicts similar oscillations. The model assumes that the compressor operation at all times is described by its quasi-steady characteristic; no hysteresis or complex aerodynamic phenomena have been invoked.

Results of investigation of centrifugal compressor operation

Results of investigation of centrifugal compressor operation

Rotating Stall in a Radial Vaneless Diffuser

This paper is concerned with the theoretical and experimental aspects of unsteady flow in a radial vaneless diffuser. Specifically, the appearance of self-excited oscillations of large amplitude is investigated in great detail. The waves appear when the angle between the tangential and total velocity becomes very small. The predicted wave speed and perturbed velocity distribution versus radius agree quite well with the experimentally obtained values, however, an undeterminable constant appears in the equation which describes the disturbed velocity distribution. A stability criterion indicates that such pulsating flow can occur if and only if there is a region of radially inward flow. The hypothesis is proposed that the waves are a result of the interaction of the three-dimensional boundary layer near the side walls and the free stream. Unsteady flows of this type can be linked to unstable centrifugal pump, blower, or compressor operation.