High Pressure Steam Turbine Research Articles

Comparative evaluation of solarized triple combined cycle for different ORC fluids

Triple combined cycles have the synergetic operation of three power cycles for utilizing the heat which is otherwise lost for improving the overall cycle performance. The integration of Brayton cycle with the Rankine cycle run on steam and organic Rankine cycle (ORC) run on different working fluids yields a triple combined cycle. This paper presents a comparative evaluation of the triple combined cycle powered by a heliostat based central solar receiver system for different working fluids in the low-temperature cycle. The influence of pressure ratio, steam bleeding, and working fluid in ORC has been studied upon the performance of the triple combined cycle under consideration. The organic Rankine cycle is run on different working fluids namely R141b, R245fa, and R236ea. The study concludes that the combined cycle has the maximum thermal efficiency and overall specific work output as 33.86% and 338.62 kJ/kg respectively at 24 cycle pressure ratio, 20% steam bleeding, steam generation of 50 bar & 20 bar at 600 K in high pressure steam turbine & low pressure steam turbines, and R236ea as ORC working fluid. It is seen that the R236ea working fluid offers best combined cycle performance in comparison to R141b and R245fa working fluid.

Upgrade of the thermal power plant flexibility by the steam accumulator

Thermal power plants are forced to operate under frequent load changes in energy systems with increased share of solar and wind power plants. These load changes increase thermal power plant aging and reduce their efficiency. Therefore, solutions for energy storage are sought in order to accumulate energy in periods of lower power demand and produce excess power in periods of increased demands. In the present paper the steam accumulator as the thermal energy storage device is applied in a 650 MWe coal-fired thermal power plant to increase its flexibility under the demand for the variable load changes. The steam accumulator is charged during periods of lower power demand, as during the night hours, and discharged during peak load demands. The steam accumulator with volume of 600 m3 is charged with superheated steam (pressure 4.5 MPa and temperature 335 °C) extracted from the exit of high pressure steam turbine. The steam discharged from the steam accumulator replaces the turbine steam extraction for two low pressure heaters: a 59.5 MWt and a 54.6 MWt heater operating at 0.56 MPa and 0.23 MPa steam pressure. This replacement results in an increase of the steam flow rate through the turbine and the turbine power gain of 27.3 MWe during 27 min and 11.5 MWe for additional 13 min. The proposed solution of the steam accumulator installation in the power plant is a new design. The upgrade of power plant flexibility by steam accumulator is demonstrated.

Leakage effects due to bristle deflection and wear in hybrid brush seal of high-pressure steam turbine

This study investigated the effect of clearance between a rotor and hybrid brush seal for leakage flow in terms of the axial deflection and wear. The leakage flow was experimentally measured according to pressure difference, and the Darcian porous medium-based CFD model was used to predict leakage flow according to clearance and rotational speed. The bristle's axial deflection was estimated according to the external force, the brush seal compliance was derived based on the cantilever beam theory, and the bristle wear and oxidation under superheated steam and high-speed operation environments were studied. The accuracy of the CFD model that considered the clearance change was improved by 11% compared to that of the previous CFD model.

Optimal control of flexible natural gas combined cycles with stress monitoring: Linear vs nonlinear model predictive control

In future energy markets, traditional thermal power plants are expected to cycle more to adapt their operation to the intermittent power generation of renewable energy sources. Gas turbine load ramps and stresses in thick-walled equipment of the steam cycle are arguably the main limitations in the flexible operation of natural gas combined cycles. This work proposes a control strategy based on model predictive control with stress monitoring to overcome both limitations and enhance the flexible operation of thermal power plants. The linear and nonlinear formulation of the problem included in the model predictive control strategy are described, and two different modelling approaches for the stresses in the high pressure drum and steam turbine rotor are presented. The results demonstrate that the proposed control strategy is capable of computing optimal control sequences without exceeding the maximum allowable stress in critical components and the ramp rates of the gas turbine. The comparison between the linear and nonlinear formulations shows the superior performance of linear model predictive control and suggests that the nonlinear formulation should only be used when the stress models can not be expressed as a linear system of equations.

ASPECTS OF STABILIZATION OF THE HEAT CONDITION OF HIGH PRESSURE CYLINDRICAL CASES OF STEAM TURBINES IN THE STARTING AND VARIABLE MODES

An application of the method of creating inverse temperature non-uniformity for steam turbine high pressure cylinder inner split housings by controlling their heat state in the starting and variable modes is studied. Criteria are defined, namely: temperature differences between steps; constant difference of total tangential stresses from the pressure difference on the wall of the cylinder body; tangential stresses of the temperature with the linear law of temperature change along the radius of the cylinder, regulation of which, according to their respective programs, allows to stabilize the heat state of the cylinder body when starting a turbine from various thermal conditions and exploiting it in variable modes. Technical solutions are proposed aimed at increasing the technical and economical characteristics of the operation of steam turbines and power unit.

Optimal dynamic operation of natural gas combined cycles accounting for stresses in thick-walled components

Flexible natural gas combined cycles will play a fundamental role in future electric markets. Stresses in thick-walled components and gas turbine load ramps are arguably the main limiting factors during transient operation. Classical control strategies as PID are not suitable to incorporate technical constraints such as stress limits. This work presents a control methodology based on model predictive control where the stress in the walls of the high pressure drum and the first high pressure steam turbine rotor are computed simultaneously with the optimal control sequence. Thus, the maximum allowable stress in this equipment can be set as a constraint and the control actions imposed in the power plant ensure that these limits are not exceeded. Two cases simulating flexible operation under realistic conditions and tight constraints on the stress limits are included. Results show that with the proposed control methodology the natural gas combined cycle can respond to load step changes of 165 MW in 300 s, and can operate close to the material maximum stress limit without exceeding it. The robustness and flexibility of this methodology allows its application to different operation conditions such as start-ups and shut-downs.

Investigation on the unsteady pressure fluctuation characteristic in the blade tip seal of steam turbine based on spectrum

The paper presents the unsteady numerical simulation results of tip leakage flow in high pressure steam turbines, and also presents the influence analysis of leakage vortexes on pressure fluctuation characteristics of rotor cascade under different blade tip seal clearances. The numerical method for calculating is based on the large eddy simulation turbulence model and the pressure fluctuation of rotor cascade which induced by the unstable leakage vortexes is obtained by frequency spectrum analysis. The results show that the vortex frequencies in tip seal cavity contain both the wheel rotating frequency and the high frequency caused by the tip leakage flow breaking into small scale vortexes. The unsteady characteristics of tip leakage flow also induce steam exciting force which changes with the time.

Test of flow characteristics and parameter optimization of steam turbine DEH

The characteristics of steam turbine high pressure cylinder intake valve directly affect the performance of AGC and primary frequency modulation, In DEH system, the nonlinear characteristics of the high valve flow need to be corrected to ensure a smooth and linear relationship between the integrated valve level instruction and the steam flow of the turbine. When the nonlinear correction of the high valve flow is not correct, there may be nonlinear intervals such as “dead zone”, “jump”, and “inflection point” between the valve comprehensive opening instruction and the steam turbine flow, The valve integrated opening command mutates the gain of steam turbine flow, causing the valve to be insensitive or oscillating when the valve is in a certain opening range under the coordinated control system of the furnace and machine.

Analysis of Causes of Valve Rod Fracture of Steam Turbine High Pressure Main Steam Valve

A 300MW sub-critical coal-fired thermal power plant steam turbine generator set # 1 high pressure steam valve stem is fractured. The turbine unit has been running 46000 hours before the crack happened. The chemical component was examined which was in the standard range. they observed the metallographic structure of the broken stem to find the nitriding treatment of fine grain martensite structure at surface area and lath martensite at central section, healthy structure condition. they also found the existing of circumferential oxidized cracks with white corrosion production. According to that, they believe it was caused by residual stress when grinding. Finally, SEM was used to observe the fracture to further confirm our ideas.

Multi-criteria performance optimization and analysis of a gas–steam combined power system

This paper reports a comprehensive investigation of performances for a gas–steam combined power system (GSCPS) by considering exergy and thermo-ecology criteria. The most known and important parameters such as power density, power output, exergy destruction, exergetic efficiency, ecological coefficient performance (ECOP) and effective ecological power density (EFECPOD) are investigated. The influences of turbine design parameters such as turbine speed, mass flow rate of the working fluid, pressure ratio, equivalence ratio, intake air temperatures and pressures, residual gas fraction, fluid temperatures in heat exchanger, the pressure of high-pressure steam turbine, the pressure of medium-pressure steam turbine, the pressure of low-pressure steam turbine, the pressure of open feedwater heater, the pressure of condenser on the performance specifications have been evaluated. The results indicated that the component properties have remarkable impacts on the performance parameters of the GSCPS.

Leakage and Cavity Pressures in an Interlocking Labyrinth Gas Seal: Measurements Versus Predictions

Gas labyrinth seals (LS) restrict secondary flows (leakage) in turbomachinery and their impact on the efficiency and rotordynamic stability of high-pressure compressors and steam turbines can hardly be overstated. Among seal types, the interlocking labyrinth seal (ILS), having teeth on both the rotor and the stator, is able to reduce leakage up to 30% compared to other LSs with either all teeth on the rotor (TOR) or all teeth on the stator. This paper introduces a revamped facility to test gas seals for their rotordynamic performance and presents measurements of the leakage and cavity pressures in a five teeth ILS. The seal with overall length/diameter L/D = 0.3 and small tip clearance Cr/D = 0.00133 is supplied with air at T = 298 K and increasing inlet pressure Pin = 0.3–1.3 MPa, while the exit pressure/inlet pressure ratio PR = Pout/Pin is set to range from 0.3 to 0.8. The rotor speed varies from null to 10 krpm (79 m/s max. surface speed). During the tests, instrumentation records the seal mass flow (m˙) and static pressure in each cavity. In parallel, a bulk-flow model (BFM) and a computational fluid dynamics (CFD) analysis predict the flow field and deliver the same performance characteristics, namely leakage and cavity pressures. Both measurements and predictions agree closely (within 5%) and demonstrate that the seal mass flow rate is independent of rotor speed. A modified flow factor Φ¯=m˙T/(PinD1−PR2) characterizes best the seal mass flow with a unique magnitude for all pressure conditions, Pin and PR.

Axial-Swept Influence on Inner Flow Performance of HP Steam Turbine Based on CFD

Swept blade technology was used to redesign the supercritical steam turbine sets that could improve the inner-efficiency of turbine sets and decrease the consumption of coal the noxious gas such as NOx. The eighth high-pressure stages, including static and rotor cascades, were selected as tested prototypes that were blew in the low-velocity wind tunnel. We used the five-hole ball head needle to measure the aerodynamic parameters distribution along the width and span direction of the high-pressure stage cascades. With the inkblot display technology, the limit flow spectrums were displaced in the blade surface and the endwalls. These tested data could be used to check the simulation results of CFD software. To improve the efficiency of the steam turbine high-pressure (HP) stage, we selected the supercritical steam turbine HP stage cascade blade as the prototype to research into its inner flow performance of the axial-swept blade by the CFD software. Two different redesigned blades, with ±20°, swept angle, and 30% swept height, named axial fore-swept and axial aft-swept, were built up. The stage passages flow field of the prototype blade, and the two redesigned swept blades were simulated using CFD software with stage interface planes between the stages. The CFD simulation results indicated that the leading edge of swept blades influenced the inlet flow field; the pressure in aft-swept blade stage in both endwalls was higher than in the middle and was beneficial to improve the passage flow properties of HP stage. But for the fore-swept HP stage, its pressure distribution was lower in both endwalls than in the middle and not beneficial to passage flow.

Technological and economical analysis of flare recovery methods, and comparison of different steam and power generation systems

One of the most important environmental issues in the oil, gas, and petrochemical industries is the appropriate disposal of waste hydrocarbon gases in industrial units. The most commonly employed method for the safe disposal of these gases is burning them in flare stakes followed by releasing them in the environment. To date, various methods and procedures, preponderantly focusing on the modification of the units which generates the gases directed to flare stacks, have been introduced to reduce and/or recover these gases. These methods face with a number of issues, such as the lack of economic justification, high systemic risks, and operational limitations. In this research, four methods for flare gas recovery have been introduced, namely high-pressure steam generation, steam turbine, electricity and heat generation, and combined cycle, and they have been simulated using Aspen HYSYS software. According to the results, producing electric power from flaring gases in the last three methods was 7.323 e+5, 4.350 e+5, and 1.442e+006 kW, respectively, and it caused less pollution and saved energy. In order to assess the economic justification of the proposed methods, the economic assessment was conducted for each method, and economic return was calculated. The rate of investment return for the four processes of the high-pressure steam generation, steam turbine, electricity and heat cogeneration, and combined cycle was 18.66, 19.76, 25.79, and 31.97, respectively, reflecting the high economic return of the combined cycle method.

COMPARISON OF ENERGY FLOW STREAM AND ISENTROPIC METHOD FOR STEAM TURBINE ENERGY ANALYSIS

In this paper, a comparison of two different methods for a steam turbine energy analysis is presented. A high-pressure steam turbine from a supercritical thermal power plant (HPT) was analysed at three different turbine loads using the energy flow stream (EFS) method and isentropic (IS) method. The EFS method is based on steam turbine input and output energy flow streams and on the real steam turbine produced power. The method is highly dependable on the steam mass flow rate lost through the turbine gland seals. The IS method is based on a comparison of turbine steam expansion processes. Observed energy analysis methods cannot be directly compared because they are based on different sources of steam turbine energy losses, so, an overall steam turbine energy analysis is presented. Unlike most steam turbines from the literature, the analysed HPT did not have the highest overall energy efficiency at a full load due to exceeding the water/steam critical pressure at the turbine inlet during such operation.

Thermoecology-based performance simulation of a Gas-Mercury-Steam power generation system (GMSPGS)

In this study, thermo-ecology based performance investigations for a combined system consists of Brayton gas cycle, Rankine mercury cycle and Rankine steam cycle, which is called Gas/Mercury/Steam power generation system (GMSPGS), are carried out depending on the most used performance characteristics such as power generation, density of power generation, destructed exergy, second law efficiency (exergetic performance efficiency) with respect to destructed exergy and power generation, ECOP and EFECPOD. The impacts of system design and operating parameters for gas cycle part, mercury cycle part and steam cycle part on the performance characteristics are parametrically examined in detailed. The used parameters are inlet temperature and pressure of the air, mass flow rate of the air, pressure ratio, equivalence ratio, turbine speed, residual gas fraction (RGF), mercury temperature of the gas-mercury heat exchanger (GM-HEX), low-pressure mercury turbine (LPMT), medium-pressure mercury turbine (MPMT), high-pressure mercury turbine (HPMT), steam temperature at the mercury-steam heat exchanger (MS-HEX), the pressures of low-pressure steam turbine (LPST), medium-pressure steam turbine (MPST), high-pressure steam turbine (HPST), open feed water heater (OFWH) and condenser (COND). The results demonstrated that the performance specifications are substantially affected by the component properties. Beside the theoretical model examined in the paper, a data obtained from artificial neural networks (ANNs) is also presented to model the system. It is shown that a limited number of parameters is enough for good approximations.

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part I: Design Concept and Typical Performance of a Swirl Breaker

In high and intermediate pressure (HIP) steam turbines with shrouded blades, it is well known that shroud leakage losses contribute significantly to overall losses. Shroud leakage flow with a large tangential velocity creates a significant aerodynamic loss due to mixing with the mainstream flow. In order to reduce this mixing loss, two distinct ideas for rotor shroud exit cavity geometries were investigated using computational fluid dynamics (CFD) analyses and experimental tests. One idea was an axial fin placed from the shroud downstream casing to reduce the axial cavity gap, and the other was a swirl breaker placed in the rotor shroud exit cavity to reduce the tangential velocity of the leakage flow. In addition to the conventional cavity geometry, three types of shroud exit cavity geometries were designed, manufactured, and tested using a 1.5-stage air model turbine with medium aspect ratio blading. Test results showed that the axial fin and the swirl breaker raised turbine stage efficiency by 0.2% and 0.7%, respectively. The proposed swirl breaker was judged to be an effective way to achieve highly efficient steam turbines because it not only reduces the mixing losses but also improves the incidence angle distribution onto the downstream blade row. This study is presented in two papers. The basic design concept and typical performance of the proposed swirl breaker are presented in this part I, and the effect of axial distance between a swirl breaker and rotor shroud on efficiency improvement is discussed in part II [8].

Improvement of Steam Turbine Stage Efficiency by Controlling Rotor Shroud Leakage Flows—Part II: Effect of Axial Distance Between a Swirl Breaker and a Rotor Shroud on Efficiency Improvement

The basic principle of a distinct idea to reduce an aerodynamic mixing loss induced by the difference in tangential velocity between mainstream flow and rotor shroud leakage flow is presented in “Part I: Design Concept and Typical Performance of a Swirl Breaker.” When the swirl breaker is installed in the circulating region of leakage flow at the rotor shroud exit cavity, the axial distance between the swirl breaker and the rotor shroud is a crucial factor to trap the leakage flow into the swirl breaker cavity. In Part II, five cases of geometry with different axial distances between the swirl breaker and the rotor shroud, which covered a range for the stage axial distance of actual high and intermediate pressure (HIP) steam turbines, were investigated using a single-rotor computational fluid dynamics (CFD) analysis and verification tests in a 1.5-stage air model turbine. By decreasing the axial distance between the swirl breaker and the rotor shroud, the tangential velocity and the mixing region in the tip side which is influenced by the rotor shroud leakage flow were decreased and the stage efficiency was increased. The case of the shortest axial distance between the swirl breaker and the rotor shroud increased turbine stage efficiency by 0.7% compared to the conventional cavity geometry. In addition, the measured maximum pressure fluctuation in the swirl breaker cavity was only 0.7% of the entire flow pressure. Consequently, both performance characteristics and structural reliability of swirl breaker were verified for application to real steam turbines.

Critical assessment of oxy-fuel integrated coal gasification combined cycles

Critical assessment was performed for a type of oxy-fuel integrated coal gasification combined cycles (IGCC) that was comprised of proven components. A type of two-stage entrained-flow gasifier consisting of combustor and reductor sections was simulated by a one-dimensional model that has been proven through application to gasifiers of industrial scales. It was successfully reproduced on Aspen Plus® and integrated together with the other components into a commercial-scale IGCC system. The oxidizing agents were not only O2 and CO2 (carrier gas for conveying the coal) but also additional CO2 or CO2/H2O, that was required to suppress hydrocarbons formation and maintain the combustor temperature allowing molten ash to have sufficiently low viscosity. The net thermal efficiency was predicted as a function of steam/coal mass ratio (S/C) within a range of 0–0.4. Increasing S/C up to 0.2 increased the cold gas efficiency slightly but resulted in decrease in the net thermal efficiency of the system. This was mainly due to the extraction of steam from the high pressure steam turbine exhaust, causing loss of power output. Resulting in lower cold gas efficiency due to higher oxygen ratio, higher moisture content of the pulverized coal gave higher net thermal efficiency due to less steam consumption for coal drying. The net efficiency was optimized at approximately 39% on a higher-heating-value basis without steam feeding, which was higher by 6–7 points than that for conventional IGCC with oxygen-blown gasification combined with CO2 recovery.

Numerical Analysis on the Impact of Interstage Flow Addition in a High-Pressure Steam Turbine

This study analyzes the fluid dynamic characteristics of an ultrasupercritical (USC) high-pressure turbine with additional steam supplied through an overload valve between the second and third stages. The mixing between the main and admission flows causes complex flow phenomena such as swirl and changes of velocity vectors of the main flow. This causes a pressure drop between the second-stage outlet and third-stage inlet, which could potentially affect the performance of the turbine. First, a single-passage computational analysis, which is usually preferred in predicting the performance of multistage turbomachines, was performed using a simple model of an admission flow path and a single passage (SP) for the second and third stages of the turbine. However, the actual flow in the overload valve is supplied through the admission flow path, which has the shape of a casing that circumferentially surrounds the turbine, after flowing in two directions perpendicular to the turbine axis. This necessitates full-passage computational analyses of the two stages and the flow paths of the admission flow. To achieve this, we implemented a full three-dimensional (3D) geometric model of the admission flow path and conducted a full-passage computational analysis for all the flow paths, including those of the second and third stages of the turbine. The focus of analysis was on the pressure drop due to the admission flow. The results of the single and full-passage analyses were compared, and the effects of two different methods were analyzed.

Total Site Analysis (TSA) and Exergy Analysis for Shaft Work and Associated Steam and Electricity Savings in Low Temperature Processes in Industrial Clusters

Low temperature process cooling is an energy demanding part in many chemical production processes. Cooling systems operating at very low temperatures consume a large amount of high quality energy such as electricity or high pressure steam, used to drive refrigeration compressor units. Hence decreasing refrigeration load can make a major improvement on the process energy balance. In industrial process clusters with several processes operating at low temperatures, it is important to investigate opportunities for exchange of low-temperature energy between processes. This paper presents an investigation for a chemical cluster located in Stenungsund on the West Coast of Sweden. One chemical plant within the cluster operates two compression refrigeration systems at its steam cracker plant. One system is a propylene-based system with three temperature levels between 9 °C and -40 °C, driven by high pressure steam turbine drivers with a capacity of ca. 22 MW. The other is an ethylene refrigeration system with three temperature levels between -62 °C and -100 °C, electrically driven with a capacity of ca. 4.5 MW. A previous Total Site Analysis (TSA) study of the cluster focused on integration opportunities within the cluster above ambient temperature, thereby decreasing the overall hot utility and cooling water and air demand. Utility savings below ambient temperature were not investigated in detail. This paper demonstrates how Heat Integration (HI) tools such as TSA and exergy analysis can be applied to target for shaft work and hot utility savings for processes and utility systems operating below ambient temperature. In total a savings potential corresponding to 15 % of the total shaft work consumption of the refrigeration systems was identified. In addition ca. 6.3 MW of utility steam which is currently used for sub-ambient process heating can be saved in addition to shaft work savings.