Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow

Non-synchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D CFD has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multi stage environment. The numerical results have been validated with measurement data from a multi stage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.

Evaluation of GT 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 GE-LM1600 gas turbine driving a Cooper Rolls-RFBB-30 centrifugal compressor. The second station employs 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.

Speed Dependency of Coupled Rotor-Blade Torsional Natural Frequencies

The natural frequencies of blades depend on the rotational speed of the rotor train as the stiffness changes with centrifugal loading. In the case of low pressure turbines with shrunk-on-disc design the coupled rotor-blade torsional natural frequencies can also show this property. For proper analysis of the speed dependency, a complete rotor-blade model which takes the elasticity of the blades into account is required. In this paper the torsional natural frequencies calculated with a complete rotor-blade model are compared with those calculated with a model in which blade elasticity is not included. The analysis clearly demonstrates that calculations without blade elasticity lead to different natural frequencies. By modeling the complete rotor and taking blade elasticity into account, it is demonstrated that the torsional natural frequencies of a complete rotor-blade model can also become speed dependent. As a consequence, a distinction between the natural frequencies at nominal speed and natural frequency at critical speeds becomes necessary. In the following, measured torsional natural frequencies at different rotating speeds of an individual low pressure rotor are presented. A comparison of the measured speed dependency of the torsional natural frequency with calculation results thereby taking the blade elasticity into account is conducted. The analysis shows that the measured speed dependency can be predicted with a high level of accuracy and can become important for modes which are dominated by the blades of the last stages. As a consequence of this analysis, a clear distinction between natural frequency at nominal and at critical speed has to be made for certain rotor and blade designs. It is shown that the use of the Campbell diagram is highly beneficial for designing rotor trains with large blades with regard to their torsional vibration behavior.

Influence of Partial Admission on the Downstream Stages of a Reaction Turbine: An Analytical Approach Verified by Measurements

Field measurements on an industrial steam turbine with a rated power output of 5.8 MW, consisting of an impulse type control wheel and a reaction part, showed a significant gap of efficiency from the design calulations. It was suspected, that this gap results from underestimation of the loss created by non-uniform inflow conditions to the reaction part due to partial admission. The experimental results and data of experiments done in the 1990s are therefore recalculated to find possible explanations. It turns out, that probably the data considered for verifcation is not complete. When taking the complete data into account, and using an averaging method, the verification calculations show, that the models used for design and recalculation of industrial steam turbines are accurate enough for industrial purposes, but a calculation model for efficiency loss due to partial admission has to be added. In this work non-uniformity between the flow passages was not observed for the test turbine. Non-uniformity of the flow in radial direction was observed for the test turbine, but was not taken into consideration here, as the whole rotor was treated integrally. Flow seperations as unsteady effects were not considered, as a steady-state investigation was conducted. The calculation models are verified by comparison with field measurement data from industrial steam turbines, by comparison with the results of a 9 MW steam driven test turbine and by recalculated results from literature. Not all verification calculations are presented in detail here.

A Novel Method of Coupling the Steam Turbine Exhaust Hood and the Last Stage Blades Using the Non-Linear Harmonic Method

The exhaust hood of a steam turbine is a vital area of turbomachinery research its performance strongly influences the power output of the last stage blades. It is well known that accurate CFD simulations are only achieved when the last stage blades are coupled to the exhaust hood to capture the strong interaction. This however presents challenges as the calculation size grows rapidly when the full annulus is calculated. The size of the simulation means researchers are constantly searching of methods to reduce the computational effort without compromising solution accuracy. This work uses a novel approach, by coupling the last stage blades and exhaust hood by the Non-Linear Harmonic Method, a technique widely used to reduce calculation size in high pressure turbine blades and axial compressors. This has been benchmarked against the widely adopted Mixing Plane method. The test case used is the Generic Geometry, a representative exhaust hood and last stage blade geometry that is free from confidentiality and IP restrictions and for which first calculations were presented at last year’s conference [1]. The results show that the non-uniform exhaust hood inlet flow can be captured using the non-liner harmonic method, an effect not previously achievable with single passage coupled calculations such as the mixing plane approach. This offers a significant computational saving, estimated to be a quarter of the computation time compared with alternative methods of capturing the asymmetry with full annulus frozen rotor calculations.

Using CFD to Enhance the Preliminary Design of High-Pressure Steam Turbines

This paper focuses on the use of the CFD for improving a steam turbine preliminary design tool. Three-dimensional RANS analyses were carried out in order to independently investigate the effects of profile, secondary flow and tip clearance losses, on the efficiency of two high-pressure steam turbine stages. The parametric study included geometrical features such as stagger angle, aspect ratio and radius ratio, and was conducted for a wide range of flow coefficients to cover the whole operating envelope. The results are reported in terms of stage performance curves, enthalpy loss coefficients and span-wise distribution of the blade-to-blade exit angles. A detailed discussion of these results is provided in order to highlight the different aerodynamic behavior of the two geometries. Once the analysis was concluded, the tuning of a preliminary steam turbine design tool was carried out, based on a correlative approach. Due to the lack of a large set of experimental data, the information obtained from the post-processing of the CFD computations were applied to update the current correlations, in order to improve the accuracy of the efficiency evaluation for both stages. Finally, the predictions of the tuned preliminary design tool were compared with the results of the CFD computations, in terms of stage efficiency, in a broad range of flow coefficients and in different real machine layouts.

The Effect of Turbulence and Real Gas Models on the Two Phase Spontaneously Condensing Flows in Nozzle

The aim of the paper is to analyse the effect of turbulence and real gas models on the process of spontaneous condensation in converging diverging (CD) nozzle by using commercial Computational Fluid Dynamics (CFD) code. The calculations were based on the 2-D compressible Navier-Stokes (NS) equations coupled with two-equation turbulence model, and the non-equilibrium spontaneous condensing steam flow was solved on the basis of the classical nucleation theory. The results were validated to the available experimental data.

Safety Issues With Reciprocating Compressor Design, Operating Practices and Maintenance

Reciprocating compressors are extensively used on high pressure and critical applications in all industries. These equipment needs through review of design of various components for a failsafe design. API-618 specifies various details about construction of various components, but due to OEM design by compressor vendor, proper review is not done or due to cost economic of equipment these are not given importance. There are many examples when accident took place when these issues were ignored or correct assembly procedure for assembly of components is not followed and there were mistakes in design of piping system. In this paper, some of such cases are discussed and improvements are suggested for preventing any safety incidents. The issues described are designs of unloaders and venting system, unloader and vent piping, cylinder tap off connections, end clearance pocket unloaders, distance piece and crank case venting system, purge system for motors. One of the risks with valve unloaders is the contamination of the instrument air with flammable gas specially on high pressure application. The instrument air may get contaminated with hydrocarbon if unloader leaking from stem and leak off gas tubing which is connected to flare may get interchanged with the instrument air connection. The same mistake if not noticed & compressor started, it may lead to the incident where compressor can show showed a strange vibration pattern, and instrument air header may get contaminated with hydrocarbon gases. Another issue is crank case pressurisation due to improper venting system which can lead to pressurisation of crank case, failure of oil seals leading to flywheel end bearing failures. Further checking for finding reasons of crank case pressurisation, it is observed that excessive rod packing leaks results from incorrect leveling of compressor, distance and cylinders. Motor purge system is one of the essential requirements for starting large reciprocating compressors; mistake in design/fabrication of vent system may create unsafe condition leading to explosion in motor. All issues have been discussed in paper and various solutions and fail safe design have been suggested to improve safety and reliability. The studies also indicates that proper start up procedure and follow up of correct checklists during overhauling and start up can eliminate/ identify such mistakes.

An Upwind Eulerian-Eulerian Model for Non-Equilibrium Condensation in Steam Turbines

The present paper proposes an Eulerian-Eulerian two-phase model for non-equilibrium condensing flow in steam turbines. This model is especially suitable for upwind finite volume scheme. An approximate Roe type flux using real water/vapor property is constructed to calculate the upwind wet-steam flux. This flux fully couples the wetness fraction with other conservative variables in the Jacobian Matrix whose eigen-vector and eigen-value are analitically derived. A novel treatment of real wet-steam property is developed by constructing a 3-DOFs TTSE table according to IAPWS97 formulas. The table is actually a cubic and uses the mixture’s density, the mixture’s internal energy and wetness as independent variables. Besides homogeneous condensation, heterogeneous condensing is also integrated into the model, which facilitates simulating the effect of salt impurities. The above methods are validated through two nozzle and one turbine cascade calculations and finally applied to a model LP steam turbine stage. Results show that the current model is very robust and is able to correctly capture the non-equilibrium condensation phenomena.

The Role of Rotor Welding Design in Meeting Future Market Requirements

The demand for current and future steam turbine components is driven by higher efficiency but also by higher plant cycling needs and optimized cost balance. An increase in efficiency increases the demand for higher life steam temperatures of up to 620/630°C for today’s units and of even up to 720°C for future steam power plants. The gap between required material properties in the hot and cold running parts of a steam turbine rotor is widened by the increased live steam temperatures and the increased demand for flexibility and adaptability to current and expected future energy market conditions. Besides further material development, welding is one measure to realize such contradictory rotor characteristics. Whereas 720°C is more a future related task, solutions for 560°C / 620°C apply already welded rotors. The paper discusses from a perspective of a steam turbine manufacturer the technical features to enable flexible high efficient rotor components with a focus on advanced welding technologies suitable for different large steam turbine components and what further steps for new welding technologies are under way.