Low Pressure Steam Turbine Research Articles

Energy-Based Water Droplet Impact Erosion Studies of Laser-Treated Austenitic and Martensitic Steels and their Applications

Abstract High-power diode laser (HPDL) surface treatments of high-manganese austenitic and high-chromium martensitic steel round specimens have been carried out, and their resistance to water droplet impact erosion (WDIE) in air was evaluated as per ASTM G73-10, Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus. This is because the water droplets always travel along with a fluid medium, either steam or air, and these are influenced by inertial, Coriolis, and centrifugal forces, which are missing in a vacuum. Because of these forces, the water droplets elongate and may break before hitting a target, resulting in WDIE damages entirely different from those in a vacuum. The WDIE damages on the test specimens were observed on the leading edge towards the suction side, where the boundary layer is attached, and the pressure gradients are high. These are similar to those occurring on the blades of an axial flow compressor of a gas turbine, a low-pressure steam turbine, and a helicopter rotor, because their leading edges are round. The WDIE testing is based on water droplet kinetic energy (KEd) and kinetic energy flux (KEfd). It has been proved experimentally that the product of KEd and KEfd can predict WDIE damages in a material, and this product can be used to compare the WDIE test results of different materials received from different laboratories. It is observed from the test results that the resistance to WDIE of HPDL-treated high-manganese austenitic steel (Hadfield’s steel) has reduced drastically, whereas that of high-chromium martensitic stainless steel has improved manifold. Fine microstructure and increased martensitic contents in high-chromium martensitic stainless steel after HPDL treatment are the main reasons for its improved performance, whereas the coarse microstructure having microcracks in HPDL-treated Hadfield’s steel is responsible for poor performance.

Optimization of a 900 mm Tilting-Pad Journal Bearing in Large Steam Turbines by Advanced Modeling and Validation

Modernization of steam turbine components can extend power plant lifetime, decrease maintenance costs, increase service intervals, and improve operational flexibility. However, this can also lead to challenging demands for existing components such as bearings, e.g., due to increased rotor weights. Therefore, a careful design and evaluation process of bearings is of major importance. This paper describes the applied advanced modeling methods and performed validation for the optimization of a novel 900 mm three-pad tilting pad journal bearing that showed high temperature sensitivity to the fresh oil supply temperature. The bearing was developed to cope with increased rotor weights within the low pressure (LP) steam turbine modernization at two 1000 MW nuclear power plants. With a static load of 2.7 MN at a speed of 1500 rpm, it represents one of the highest loaded applications for tilting pad bearings in turbomachinery worldwide. After identification of the reasons for the sensitivity, advanced modeling methods were applied to optimize the bearing. For this purpose, a more comprehensive bearing model was developed considering the direct lubrication at the leading pad edge and pad deformation. The results of the entire analyses indicated modifications of bearing clearances, pad length, thickness, and pivot position. The optimized bearing was then implemented on both units and proved its excellent operational behavior at increased fresh oil supply temperatures of up to 55 °C. In conclusion, the application of advanced modeling methods proved to be the key success factor in the optimization of this bearing, which represents an optimal solution for turbomachinery.

Unsteady Forces in LP Last Stage 380 MW Steam Turbine Rotating and Non-vibrating Rotor Blades with Exhaust Hood

Cracks were found in the roots of the last stage rotor blades. Tip-timing measurements showed high vibration amplitudes in certain working conditions. The aim of this paper is to analyse unsteady forces acting in such working conditions on the last stage of rotor blades, taking into account unsteady flow in the exhaust hood. Using ANSYS CFX, this paper examines unsteady forces in a low pressure steam turbine last stage with an exhaust hood. A sliding mesh model was used for interaction between the stator-rotor blades and the rotor blade- exhaust hood. The study included a 3D transonic viscous flow. The CFD mesh included all the last stage stator, rotor blades. The rotor blades vibrated but do not rotate. Numerical static pressure and temperature values in selected exhaust hood crosssections were compared with experimental ones. Unsteady rotor blade forces and moments were analysed along the lengths of the blades. This study has shown that interaction with the exhaust hood causes the non-symmetrical pressure and temperature along and behind the rotor blades. The low frequency unsteady harmonic is higher towards the tip of the blade. The greatest low component of unsteady forces was 50 Hz, which is far below the natural frequency and therefore irrelevant to rotor blade vibration. The second harmonic, 100 Hz, is much more dangerous, because is relatively close the natural frequency of the blade.

Aeroelasticity Analysis of Unsteady Rotor Blade Forces and Displacements in LP Last Stage Steam Turbine with Various Pressure Distributions the Stage Exit

Several high vibration amplitude problems have been reported regarding the slender last stage blading of commercial LP steam turbines. This paper analyses rotor blade unsteady forces and displacements in the last stage of a low pressure steam turbine with symmetrical and non-symmetrical (exhaust hood induced) pressure distributions behind the rotor blades. The FSI partially integrated method was used for the 3D ideal gas flow. 3D unsteady non-viscous flow was analysed using the Godunov-Kolgan method. Non-symmetrical pressure distributions in the stage exit were taken from 3D viscous CFX ANSYS calculations, using a CFD mesh of all the stator blades, all the rotor blades (rotating and not vibrating) and the exhaust hood. The non-uniform and uniform pressure distributions cause rotor blades to vibrate in different bending-torsion modes. Introducing blade vibration into the flow model causes the appearance of unsteady force and blade displacement harmonics that are not multiples of the rotation frequency. The unsteady rotor blade forces and blade displacements analysis shows the importance of including an exhaust hood in the CFD mesh.

Use of steam jet booster as an integration strategy to operate a natural gas combined cycle with post-combustion CO2 capture at part-load

This paper aims to evaluate the integration of the steam jet booster in a natural gas combined cycle with CO2 capture at low part-load operation. The steam ejector takes a high pressure motive steam flows in a supersonic nozzle while dragging a low pressure steam which comes from the crossover. Both flows mix into one at fixed pressure of 3.5 bar and sent to the reboiler. The results are compared with two integration alternatives: uncontrolled and controlled steam extraction control. Uncontrolled steam extraction provides better part-load performance than controlled. However, with sliding pressure, at 42.3% gas turbine load the low pressure steam turbine operates at 27% of its capacity compared with 66% when the energy plant operates without capture, this imposes a potential risk to the integrity of the turbine. When the steam ejector is integrated, there is no significant improvement in the efficiency compared with sliding pressure strategy. However, the used capacity of the low pressure steam turbine increases from 27% to 42.8%. Therefore, the use of the steam ejector represents a solution to avoid severe damage to the low pressure steam turbine, thus bringing more flexibility, and ensure that steam extraction will not impose any constraint to the energy plant with CO2 capture at part-load.

Development of a New High-Strength Steel for Low Pressure Steam Turbine End-Stage Blades

Influenced by the growing share of Renewable Energies, higher flexibility and increased efficiency of fossil power plants as well as improved cost efficiency in production of turbine components are evident market trends. Daily cycling in turbine operations leads to advanced requirements for robust design especially of rotating parts. Low pressure (LP) steam turbine end-stage blades with larger exhaust areas are one lever to increase the efficiency of the turbine by reduction of exhaust losses and also to realize cost-efficient single flow exhaust applications. Consequently, blade steels with improved mechanical properties are required. The results of the development of a new high-strength precipitation-hardening (PH) steel for LP end-stage blade application with significantly enhanced material properties are reported. The paper covers the testing strategy applied and information on crucial material parameters like improved low cycle and high cycle fatigue (HCF) behavior while keeping good stress corrosion cracking (SCC) resistance and corrosion fatigue (CF) properties. Furthermore, first manufacturing experiences and validation results from a full-scale component test rig are presented.

Effect of Temperature and Hold Time of Induction Brazing on Microstructure and Shear Strength of Martensitic Stainless Steel Joints.

1Cr12Mo martensitic stainless steel is widely used for intermediate and low-pressure steam turbine blades in fossil-fuel power plants. A nickel-based filler metal (SFA-5.8 BNi-2) was used to braze 1Cr12Mo in an Ar atmosphere. The influence of brazing temperature and hold time on the joints was studied. Microstructure of the joints brazed, element distribution and shear stress were evaluated at different brazing temperatures, ranging from 1050 °C to 1120 °C, with holding times of 10 s, 30 s, 50 s and 90 s. The results show that brazing joints mainly consist of the matrix of the braze alloy, the precipitation, and the diffusion affected zone. The filler metal elements diffusion is more active with increased brazing temperature and prolonged hold time. The shear strength of the brazed joints is greater than 250 MPa when the brazing temperature is 1080 °C and the hold time is 30 s.

Efficiency evaluation of a coal-fired power plant integrated with chilled ammonia process using an absorption refrigerator

Chilled ammonia process (CAP) is an alternative process for inhibiting ammonia escape during the CO2 capture process. In this paper, the integration of a coal-fired power plant with CAP, using an absorption refrigerator (AR) to provide the chilling load, is proposed. The thermal energy consumption of the CAP and the AR, derived from a crossover pipe between the intermediate pressure and low pressure (IP-LP) steam turbine sections and from an appropriate port of a low pressure (LP) turbine, are the basic scenarios considered for this study. A systematic evaluation of a 300-MWe coal-fired power plant is conducted and its overall process is compared with the power plant integrated with CAP using the conventional vapor compression refrigerator (VCR). An analysis of the basic scenarios reveals that the efficiency penalties reduced from 13.23% to 9.82% when the steam extractions were from the IP-LP crossover pipe and LP turbine, of which a 4.26% and 2.96% loss, respectively, was contributed by the AR. Decreasing the chilling temperature, while increasing the regenerating temperature can reduce the efficiency penalty. A comparative investigation of the lower efficiency penalties of various COPa and COPc are recommended. Better performance can be achieved by VCR integration if the COPc is higher than 2.0 and 3.5. The results from and an understanding of the two types of refrigerators can be used as the basis for system design and optimization.

Analytical investigation of simultaneous effects of convergent section heating of Laval nozzle, steam inlet condition, and nozzle geometry on condensation shock

Rapid expansion and supercooling of dry vapor in low-pressure steam turbines trigger nucleation phenomenon. Subsequently, following the occurrence of vapor condensation, a vapor–liquid two-phase flow is established. Entropy generation mainly by condensation shock, blade erosion, and ultimately, destruction of equipment and efficiency reduction are among adverse effects of vapor condensation, which should be either attenuated or controlled. In the present research, which is a continuation to the research performed by original authors, a one-dimensional analytical Eulerian–Lagrangian model is used to apply convergent section heating to different supersonic nozzles under various inlet conditions. The results indicate that the flow response to the heating is well dependent on the intensity of condensation shock or inlet conditions. In order to compensate for the mass flow rate resulted from the convergent section heating, effects of simultaneous reduction of inlet stagnation temperature and convergent section heating were investigated. Finally, it was found that, maintaining constant mass flow rate, simultaneous reduction of inlet stagnation temperature and convergent section heating cannot attenuate the condensation shock significantly. Therefore, the best approach to compensate for the reduction in the mass flow rate due to convergent section heating is to simultaneously increase inlet stagnation pressure.

Characterization on the Microstructure Evolution and Toughness of TIG Weld Metal of 25Cr2Ni2MoV Steel after Post Weld Heat Treatment

The microstructure and toughness of tungsten inert gas (TIG) backing weld parts in low-pressure steam turbine welded rotors contribute significantly to the total toughness of the weld metal. In this study, the microstructure evolution and toughness of TIG weld metal of 25Cr2Ni2MoV steel low-pressure steam turbine welded rotor under different post-weld heat treatment (PWHT) conditions are investigated. The fractography and microstructure of weld metal after PWHT are characterized by optical microscope, SEM, and TEM, respectively. The Charpy impact test is carried out to evaluate the toughness of the weld. The optical microscope and SEM results indicate that the as-welded sample is composed of granular bainite, acicular ferrite and blocky martensite/austenite (M-A) constituent. After PWHT at 580 °C, the blocky M-A decomposes into ferrite and carbides. Both the number and size of precipitated carbides increase with holding time. The impact test results show that the toughness decreases dramatically after PWHT and further decreases with holding time at 580 °C. The precipitated carbides are identified as M23C6 carbides by TEM, which leads to the dramatic decrease in the toughness of TIG weld metal of 25Cr2Ni2MoV steel.

Investigation of local brittle zone in multipass welded joint of NiCrMoV steel with heavy section

Abstract

CO2 capture from syngas generated by a biomass gasification power plant with chemical absorption process

In the context of the climate changes and to improve people's life, solutions for reducing the amount of CO2 emissions into the atmosphere from the energy sector are being sought after. One of the solutions is to produce a greater part of the global energy requirement from renewable energy sources. Nowadays, biomass is one of the most easily available renewable energy sources. The biomass gasification is a technology which presents several important advantages of the bioenergy in terms of producing heat, power and biofuels for useful applications. In this article, the biomass gasification with the CO2 capture technology (BIGCC + CAP) by using an aqueous monoethanolamine solution was studied for converting the treated syngas into electricity in a combined cycle power plant. The biomass gasification can be done using either air or oxygen in order to improve the quality of the syngas produced. However, only the gasification with air was studied in this research due to the large energy consumption in the ASU (air separation unit) unit for oxygen separation. Thus, the CO2 post-combustion capture by the chemical absorption technology was studied to determine its effects on the performances of a gas turbine power plant with an HRSG (Heat Recovery Steam Generator) with a total installed power of 10 MW. In order to increase the performances of the combined cycle, the exhaust gases are supplementary used in an HRSG unit in order to produce steam for the steam turbine, with an installed power of 4 MW. The chemical solvent used for the CO2 capture separation was monoethanolamine with a concentration of 30 wt. %, considering a CO2 capture efficiency of 90%. The thermal heat required for the solvent regeneration is provided by the steam extracted from the low pressure steam turbine. To reduce the thermal energy requirement for the chemical solvent regeneration, an HRSG unit was used to recover the heat from the syngas. Thus, the overall efficiency was reduced from 50.9% to 45.8% in the case without the HRSG unit and to 47.03% in the case with the HRSG unit. By comparing the BIGCC + CAP with and without the HRSG unit, the lowest value of the LCOE was obtained for the BIGCC + CAP + HRSG, and was found to be 77.56 €/MWh.

Proposal and analysis of two novel integrated configurations for hybrid solar-biomass power generation systems: Thermodynamic and economic evaluation

Hybridization of solar and biomass energies, as two types of renewables, represents a resourceful way of complementing each other to overcome their individual drawbacks. In the present work, two novel solar-biomass hybrid power generation systems are proposed, modeled and compared with each other as well as with the standard biomass-based system without solar energy. In both the proposed systems biomass is first gasified in a gasifier and then syngas is used in a gas turbine plant combined with a Rankine cycle. In proposed systems the solar energy is used in the bottoming Rankine cycle, where the first system employs it in an indirect way to heat the feed water before deaerator while the second system adopts the solar energy in direct steam generation mode to generate additional steam for low pressure steam turbine. Thermodynamic and economic models are developed to investigate the systems’ performances from the viewpoints of energy, exergy, economic and environmental. According to the results of economic evaluation, the levelized cost of electricity for standard, the first and second proposed configurations are obtained 79.34, 79.88 and 74.94 $/MWh, respectively. Also, the values of 0.79, 0.77 and 0.62 t/MWh are calculated for CO2 emission for standard, the first and second proposed configurations, respectively.

Experimental Investigation of the Grouped Blade Vibration for Steam Turbine by Noncontact Sensors

All turbine blades have mistuned structures caused by manufacturing variations within the manufacturing tolerance, such as the geometrical deviations and variance of material properties. The mistuning effect has a known tendency to increase the dynamic stress, but it is also known to be difficult to predict the maximum vibration response before the operation. This paper studies the blade vibration of grouped blades in a low-pressure steam turbine. The study objectives are to characterize the vibration behavior of the grouped blade structure and to evaluate the maximum response of all blades in a stage experimentally. An experimental investigation is carried out in a vacuum chamber, and blades are excited by an air jet during start-up and shut-down. The circumferential blade amplitude distribution is measured by noncontact sensors (NCSs) and strain gauges (SGs). The circumferential blade amplitude distribution is found to differ depending on vibration modes and nodal diameters (NDs), but the relative tendency is almost the same for all types of operation at each mode and all NDs. Therefore, the median of all experimental results obtained with the NCSs is used in a comparison with calculation results and results of two theoretical curves obtained using equations from the literature. In comparing the measurement results and the calculation results, the circumferential blade amplitude distribution is not the same with all modes and NDs. However, the maximum amplitude magnification is about 1.5–1.8, and all measurement results are lower than the results for the two theoretical equations. This means the maximum response comparison to the tuned blade gives an evaluation on the safe side by the two theoretical equations.

Corrosion Fatigue of a Low-Pressure Steam Turbine Blade

A low-pressure (LP) steam turbine blade of the steam turbine last stage of a thermal power plant had failed after it had been only a few years in service. The failure comprised of surface damage that formed over the leading edge and a tangential crack that formed between the tie-rod hole and the damaged leading edge. The surface damage occurred on the leading edge was typical of pit-like defects of honeycombed texture. Type and factors that may have caused the failure of the LP steam turbine blade are discussed in this paper. The metallurgical assessment was conducted by preparing a number of specimens from the as-received failed LP steam turbine blade. Various laboratory examinations were performed including visual and macroscopic examination, chemical analysis, metallographic examination, hardness testing, and scanning electron microscopy equipped with energy-dispersive spectroscopy analysis. Results of the metallurgical assessment obtained showed that the failed LP steam turbine blade had been experiencing corrosion fatigue due to the combined effect of cavitation erosion and fatigue. Fatigue crack was initiated from the internal wall of the tie-rod hole where several pit-like defects, typical of cavitation erosion present. Formation of this cavitation erosion may be considered as physical corrosion. Similar cavitation erosion was also formed over the damaged leading edge. The fatigue crack was subsequently propagated in tangential direction toward the nearest damaged leading edge of the vane blade and eventually forming the final fracture thereon.

Full-annulus Simulation of Unsteady Flows through Multi-stage Stator-rotor Blade Rows in a Low-pressure Steam Turbine

Full-annulus Simulation of Unsteady Flows through Multi-stage Stator-rotor Blade Rows in a Low-pressure Steam Turbine

Dynamic Stress Analysis of L-1 Low Pressure Steam Turbine Blade: Mathematical Modelling and Finite Element Method

The present paper deals with dynamic stress analysis of the L-1 low pressure steam turbine blade. The mathematical and finite element modeling employed to capture the effect of stresses on the dynamic response of the turbine blade assembly. From the mathematical modeling, the various governing differential equations of motion for lateral vibration of cantilever beam have been derived. The finite element model of AISI420 L-1 low pressure stage steam turbine blade used in the present work is developed by using three dimensional scanning technique.

Effect of chloride and sulfate ions on crevice corrosion behavior of low-pressure steam turbine materials

Crevice corrosion behavior between homogeneous and dissimilar materials of 13Cr and 3.5NiCrMoV steels was investigated in simulated boiler waters containing chloride ions (Cl−) and sulfate ions (SO42−), using open circuit potential (OCP) measurements in addition to morphology analysis. Pitting occurred inside the crevice not only on 13Cr steel but also on 3.5NiCrMoV steel in the test water containing 100 ppm Cl−, while it was suppressed by the coexistence of 50 ppm SO42− in the water with the 100 ppm Cl−. However, the presence of 50 ppm SO42− in the test waters promoted uniform corrosion on 3.5NiCrMoV steel inside the crevice.

Study of the wet steam flow in the blade tip rotor linear blade cascade

Experimental investigations of non-equilibrium spontaneous condensation in transonic steam flows in a linear blade cascade were carried out. The cascade consists of the rotor blades of the low-pressure (LP) steam turbine last stage. The experimental testing facility is a part of a small-scale steam power plant located at the Silesian University of Technology in Gliwice. The steam parameters at the testing facility inlet correspond to the wet steam conditions in a low-pressure steam turbine. Static pressure measurements on the blade surface as well as Schlieren images were used to assess the flow field in the steam turbine rotor tip blade linear cascade for different flow conditions. The experimental results were used to validate an in-house CFD code.

Condition Monitoring of Low Pressure Steam Turbine Blades

Condition Monitoring of Low Pressure Steam Turbine Blades