On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

2015 ◽  
Vol 137 (10) ◽  
Author(s):  
Adam Beevers ◽  
Said Havakechian ◽  
Benjamin Megerle
Keyword(s):  
Steam Turbine ◽  
Temperature Increase ◽  
Low Pressure ◽  
Volume Flow ◽  
Flow Conditions ◽  
Steam Extraction ◽  
Last Stage ◽  
Low Volume ◽  
Critical Regions ◽  
Ventilation Conditions

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade (LSB). Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the LSB increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of LSBs continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effects from geometry, mesh density, turbulence model, and time dependency. The CFD results were verified against steam turbine data from a scaled test facility. The proposed solutions include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

2014 ◽  
Author(s):  
Adam Beevers ◽  
Said Havakechian ◽  
Benjamin Megerle
Keyword(s):  
Steam Turbine ◽  
Temperature Increase ◽  
Low Pressure ◽  
Volume Flow ◽  
Flow Conditions ◽  
Steam Extraction ◽  
Last Stage ◽  
Low Volume ◽  
Critical Regions ◽  
Ventilation Conditions

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade. Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the last stage moving blade increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of last stage moving blades continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effect of geometry, mesh density, turbulence model and time dependency. The CFD results were verified against steam turbine data from a test facility. The proposed complimentary solutions to limit the temperature increase include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

Effect of Steam Injection on Last-Stage Blade Temperature of Low Pressure Steam Turbine at Low Volume Flow Condition

2017 ◽  
Author(s):  
Can Ma ◽  
Jun Wu ◽  
Yuansheng Lin
Keyword(s):  
Steam Turbine ◽  
Nuclear Power ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Steam Injection ◽  
Low Pressure ◽  
Volume Flow ◽  
Pressure Steam ◽  
Last Stage ◽  
Low Volume

In the nuclear power plant, the last stage of the low pressure steam turbine is characterized by long blades. These long blades operate under severe working conditions with wet steam flow and strong mechanical stress. At the start up and shut down operating condition where the volume flow is extremely low, the last stage blades operate in ventilation conditions where there is significant reverse flow in the exhaust and the last stage. In such a condition, the reverse flow would cause significant increase in the blade temperature. In addition, the rotating reverse flow would increase the vibration of the rotor blade. Such temperature increase and enhanced vibration can cause blade damage and force the machine to be shut down. In previous work, the steam injection in the last stage has been proposed as a promising method to decrease the reverse flow at low volume flow conditions, which reduces the stall cell size in the last stage blade. This work investigates the effect of the steam injection process on the blade temperature distribution by conducting three-dimensional flow simulations. Various steam injection configurations are compared in this work and the major consideration to be noted in the design process is discussed.

Detached Eddy Simulation of Rotating Instabilities in a Low-Pressure Model Steam Turbine Operating Under Low Volume Flow Conditions

2021 ◽  
Author(s):  
Ilgit Ercan ◽  
Damian M. Vogt
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Volume Flow ◽  
Steam Turbines ◽  
Detached Eddy Simulation ◽  
Flow Conditions ◽  
Flow Physics ◽  
Part Load ◽  
Eddy Simulation ◽  
Low Volume

Abstract Rotating instability (RI) in steam turbines is a phenomenon occurring during operation at very low volume flow conditions. Whereas RI is well-known in compressors, it is rather uncommon in turbines, where it is limited to the last stages of low-pressure steam turbines. The phenomenon has been studied numerically by means of viscous 3D CFD simulations employing mainly URANS equations. Given the possible difficulties to accurately predict heavily separated flows using such methods, this paper deals with the question whether the more sophisticated Improved Delayed Detached Eddy Simulation (iDDES) model is applicable in an industrial environment and whether it is capable of capturing the complex unsteady flow physics in a more realistic manner. For this purpose, the commercial CFD solver STAR-CCM+ is employed. A three-stage low-pressure model steam turbine featuring a non-axisymmetric inlet and an axial-radial diffuser is used as a test object. In order to capture the asymmetry, the model spans the full annulus and comprises the inlet section, all three stages, the diffuser as well as the exhaust hood. URANS and iDDES simulations have been performed at various low-volume flow part-load operating points and compared to test data. Unsteady pressure fluctuations at the casing as well as time-resolved probe traverse data have been used to validate the simulations. It is found that both models capture the overall flow physics well and that the iDDES model is superior at the most extreme part-load operating condition. In addition to the model accuracy and applicability of the CFD tool used, the paper discusses the challenges encountered during simulation setup as well as during initialization.

Numerical and Experimental Investigation of the Aerodynamic Excitation of a Model Low-Pressure Steam Turbine Stage Operating Under Low Volume Flow

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
Benjamin Megerle ◽  
Timothy Stephen Rice ◽  
Ivan McBean ◽  
Peter Ott
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Volume Flow ◽  
Operating Conditions ◽  
Single Stage ◽  
Tip Clearance ◽  
Working Fluid ◽  
Steam Turbines ◽  
Excitation Mechanism ◽  
Low Volume

The diversification of power generation methods within existing power networks has increased the requirement for operational flexibility of plants employing steam turbines. This has led to the situation where steam turbines may operate at very low volume flow conditions for extended periods of time. Under operating conditions where the volume flow through the last stage moving blades (LSMBs) of a low-pressure (LP) steam turbine falls below a certain limit, energy is returned to the working fluid rather than being extracted. This so-called “ventilation” phenomenon produces nonsynchronous aerodynamic excitation, which has the potential to lead to high dynamic blade loading. The aerodynamic excitation is often the result of a rotating phenomenon, with similarities to a rotating stall, which is well known in compressors. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. The analysis revealed that the rotating excitation mechanism observed in operating steam turbines is reproduced in the model turbine. A 3D computational fluid dynamics (CFD) method has been applied to simulate the unsteady flow in the air model turbine. The numerical model consists of the single stage modeled as a full annulus, along with the axial-radial diffuser. An unsteady CFD analysis has been performed with sufficient rotor revolutions to obtain globally periodic flow. The simulation reproduces the main characteristics of the phenomenon observed in the tests. The detailed insight into the dynamic flow field reveals information on the nature of the excitation mechanism. The calculations further indicate that the LSMB tip clearance flow has little or no effect on the characteristics of the mechanism for the case studied.

CFD Modelling of Steam Turbine Last Stage Blades at Low Load Using Multiple Mixing Plane Approach

2020 ◽  
Author(s):  
Antonio Mambro ◽  
Francesco Congiu ◽  
Enzo Galloni
Keyword(s):  
Steam Turbine ◽  
Flow Behavior ◽  
Volume Flow ◽  
Operating Conditions ◽  
Continuous Increase ◽  
Multiple Mixing ◽  
Wide Range ◽  
Low Load ◽  
Last Stage ◽  
Low Volume

Abstract The continuous increase of variable renewable energy and fuel cost requires steam turbine power plants to operate with high flexibility. This situation leads to steam turbines running at very low volume flow (LVF) for an extended time. Ventilation power and temperature predictions have a significant impact on the thermo-economic optimization of the power plant and lifetime assessment of the ventilating stages. In the last decade with increasing capabilities of CFD and computational resources, significant steps have been made in assessing complex flow behavior. Full size or scaled experimental testing of different last stage blades for a wide range of low load operating conditions is expensive, therefore CFD provides new opportunities in low load assessment. However, prediction of the flow structure of the ventilating stages still represents a challenge for the current CFD tools in terms of calculation time and reliability of the results. There are many different approaches in assessing this phenomenon, which require different computer resources and may not be necessary for most industrial applications. This paper presents the validation of the multiple mixing plane approach (MMP) presented by [9] for low-pressure steam turbine running at low load. Through a comparison with measurements results and more sophisticated methods, it is shown that this approach is able to sufficiently accurately predict the flow field and hence the ventilation power and temperature at low volume flow.

Effects of Upstream Sprayed Water on Steam Flow Conditions and Blade Vibrations of a Low Pressure Steam Turbine

2016 ◽  
Author(s):  
Naoki Shibukawa ◽  
Yoshihiro Ishikawa ◽  
Yoshifumi Iwasaki ◽  
Kota Chiba
Keyword(s):  
Steam Turbine ◽  
Low Pressure ◽  
Steam Flow ◽  
Flow Conditions ◽  
Large Size ◽  
Low Load ◽  
Vibration Stress ◽  
Experimental Works ◽  
Last Stage ◽  
Two Stages

A shutdown operation of a large size steam turbine could possibly cause flashing phenomena of the pooled drain water in low-pressure heaters. The boiled steam is sometimes in the same amount as the main flow in the case where shutdown is executed during low load conditions, and returns to the steam flow path through the extraction lines. A series of experimental works with a subscale model turbine facility has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressures under the flashing back conditions. It was observed that the blades of the two stages before the last stage (L-2) and a stage before the last stage (L-1) endured their peak vibration stresses immediately after the flash-back flow reached the turbine. In the meantime, the vibration stresses of the last stage (L-0) blades were reduced. In this paper, the behavior of the water droplets and their vaporization in the steam path were mainly investigated. A series of experiment was conducted in which several amounts of controlled sprayed water were continuously supplied into the turbine. The transient steam condition and blade’s vibration stresses were measured at the same time. The results showed the possibility that sprayed water upstream can change the mass flow rate and temperature downstream to avoid the unstable steam flow and overheating of the long blades during low load operation.

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