Geothermal Turbines Research Articles

Corrosion and wear analysis of HVOLF sprayed WC-10CO-4Cr coating on geothermal turbine blade

Purpose The purpose of this paper is to develop the high pressure high-velocity oxy-liquid fuel sprayed WC-10Co-4Cr coatings on geothermal turbine 9Cr-1Mo steel for protection against wear and corrosion. Design/methodology/approach The microstructural characterization of as-deposited and corroded coating was done and presented using X-ray diffraction and scanning electron microscope/energy dispersive spectroscopy analysis. Findings The developed coating offered 50 per cent enhanced microhardness (1,200 HV) and 100 per cent enhanced wear resistance, in comparison to bare geothermal turbine steel, respectively. The coating has shown enhanced life in the simulated working conditions (fog test and dip test). This may be because of the high microhardness of the developed coating as per the proven tribological theories. Originality/value Coating offered excellent corrosion resistance in the harsher simulated environments to geothermal turbines.

Considerations regarding WC-cermets depositions by HVOF and ESD techniques used for reinforced steel components for geothermal turbines

The cumulative effect of erosion-corrosion process in geothermal turbines results in significant damages, especially for rotors and turbine blades; the degree in which they are affected depends on steam temperature, mechanical impact and the geothermal composition of the substances dissolved in steam. Layer composite materials are a feasible option to improve the erosion-corrosion resistance and the durability of turbine components. There cermet powders containing tungsten carbide are used for High Velocity Oxy-Fuel (HVOF) spaying and for Electro Spark Deposition (ESD) to obtain graduate microstructure and to form local hard structures. Both techniques by WC-cermet depositions were found to be most suitable to improve wear resistance, to repair rotors and blades and to provide possible life extension of geothermal turbines. The paper presents a comparative assessment of tungsten carbide and the technological parameters used for HVOF and ESD cermets layers depositions. Also, the work presents the analysis of the micro chemical composition of the microstructure, the micromechanical characterization and erosion-oxidation resistance of HVOF and ESD coatings using WC-cermets on steel substrates. The experimental procedure involved X-ray diffraction of the samples, micro mechanical tests and SEM investigations to provide specifically information about micro-morphological modifications and grade of adhesion of protective layers.

Geothermal steam condition performance monitoring

Turbine scaling from poor purity steam is a major problem in geothermal power stations. The “true” purity and quality of steam going into geothermal turbines can’t easily be measured in stations equipped with direct-contact condensers. Field testing has also shown that wet steam line samples are generally not representative of steam purity. Measuring sodium flux or chemical tracer injected upstream of the separators in drainpot condensate discharges can be used to determine the steam condition. However, the best measurement method seems to be the measuring of sodium flux.Tracers injected upstream of separators can take a long time to stabilise. The results of tracer studies need to be treated with caution and if they are used for a short interval test, they can give misleadingly optimistic results.Even with reliable representative data, there are very few changes that can be made to improve steam purity or quality without either major capital expenditure or significant load loss. This means that the design of a steam delivery system capable of providing clean dry steam is of vital importance in power station design. Results from existing stations’ performance shows that the design of steam delivery systems is currently being poorly done.

Achieving More Efficient and Reliable Operation of Geothermal Turbines by Using a Secondary Flash Steam Superheating System

Problems encountered in operation of saturated steam geothermal turbine units that stem from the specific features of a geothermal heat carrier are considered. A two-phase state, increased content of salts, and corrosiveness of geothermal working medium have a negative influence on the efficiency and reliability of the turbine’s first and last stages. Owing to high concentrations of impurities in the liquid phase, the first stages suffer from intense generation of deposits. The resulting decrease in the power output is due to both fouling of the flow path and significantly growing roughness of the turbine cascade blades. The flow of wet steam in the geothermal turbine flow path is accompanied by droplet impingement erosion of the last-stage blades and corrosion fatigue of the metal of rotor elements. In addition, the losses due to steam wetness in the flow path cause an essential decrease of the geothermal turbine efficiency. The article gives examples of erosioninduced damage inflicted to the last-stage rotor blades, corrosion fatigue of the metal of integrally-machined shroud elements, and deposits in the nozzle vane cascades of geothermal turbine stages. The article also presents the results from numerical investigations of the effect that the initial steam wetness has on the silicic acid concentration in the wet steam flow liquid phase in a 4.0 MW geothermal turbine’s stages. A method for achieving more efficient and reliable operation of the geothermal turbine low-pressure section by applying a secondary flash steam superheating system with the use of a hydrogen steam generator is proposed. The article presents a process arrangement for preparing secondary flash steam supplied to the geothermal turbine low-pressure section in which the flash steam is evaporated and superheated through the use of a hydrogen steam generator. The technical characteristics of the system for preparing secondary flash steam to be used in the intermediate inlet to the turbine were preliminarily assessed (taking the upgrading of the Mutnovsk geothermal power plant as an example), and it has been shown from this assessment that the wetness degree in the low-pressure section can be decreased down to its final value equal to 2.0%.

Recent Advances of the Basic Concepts in Geothermal Turbines of Low and High Enthalpy

A few numbers of countries in the world are involved in geothermal exploration and geothermal development projects. In order to provide a stable power supply without increasing carbon dioxide on global environment problem, a basic condition need to be fulfilled. This condition is high reliability and high maintainability for the geothermal energetic pumps and turbines. Effective efficiency improvement and geothermal turbines system upgrading are very important and also fundamental economical factor. This means that CO2 emissions into the atmosphere are minimal and a higher reliance on geothermal power generation would work on preventing global warming.The solution to climate changes threat is based now, mainly, on renewable and ecological sources of energy. Geothermal energy has the potential to play a significant role in moving the Europe and other regions of the world toward a cleaner and more sustainable energy system. In order to increase the reliability of geothermal steam turbines, assessing the materials life under geothermal environment condition will be an important step. The corrosion process in the geothermal turbine and pumps depends on temperature, pressure, chemistry, mechanical and vaporous carryover of impurities and water treatment (distribution between the vapors, the surface film and rotor blades material, heat transfer properties etc).The aim of this paper is to present a new coating method for geothermal turbines and pumps components using multi composite technology in order to obtain a protective layer to reduce corrosion damages. The results were very promising and the technique used, plasma jet spraying is a very good method to be used on the geothermal turbines and pumps components.

Experiments on life cycle extensions of geothermal turbines by multi composite technology

These days, many countries are involved in evaluation and development of geothermal projects, and this includes the installation and maintenance of geothermal steam turbine. High reliability and maintainability are required as a basic condition for the geothermal turbines and energetic components in order to provide a stable power supply. For increasing the reliability of geothermal steam turbines the lifespan assessment of materials under geothermal environment conditions is important. A leading cause of reduced availability in geothermal power plant, non-conventional energetic systems’ components is corrosion damages in turbines, especially blades and rotors corrosion. Improved turbine mechanical integrity and increased rotor steam-turbine life cycle demand optimization is achieved thermal spray depositing using multi composite systems. This specific paper objective is design and synthesis of new complex powder mixtures NiCr/NiCoCr with different addition of ZrO2 stabilized with Y2O3 that can be used to obtain protective layers with improved wear, thermal shock and abrasion resistance. These coating will be studied also in “in situ” conditions, in Hellisheiði geothermal system, to investigate the actual effects of geothermal steam on the surface coated with multi-composite structures. The results will be reported in a future paper for comparing laboratory and “in situ” conditions tests results.

Pitting Corrosion Damage for Prediction Useful Life of Geothermal Turbine Blade

In the last years, the power industry in Mexico has suffered the consequences of some mechanisms of damage that reduce the efficiency in power generation. These mechanisms of damage originate problems in main components as rotors, nozzle blades and others due to losses of pressure, high vibrations, erosion and corrosion. By the type of work fluid, mixture of water-steam and brine rich in salts, geothermal turbines are in a high corrosion environment. The main forms of damage caused by corrosion in a geothermal turbine are crevice corrosion and pitting corrosion. The latter is a damage mechanism through which the useful life is affected. The blades of the last stage in low pressure (LP) in the geothermal turbines are exposed to corrosion by environment and high load cycles which cause failures by corrosion fatigue, crack initiation and crack propagations. In this work, a methodology for the analysis of the failure by pitting corrosion and its consequences in its useful life was obtained. From the results, it is possible to conclude that this corrosion may influence a mechanism of pitting corrosion, which precedence to stress corrosion cracking and corrosion fatigue.

Estimation of useful life in turbines blades with cracks in corrosive environment

Geothermal turbines of 110MW were installed in the Federal Electricity Commission in Cerro Prieto Mexico, which operating time exceeds 150,000h. Therefore, the critical components which determine the useful life of the turbine should be evaluated to determine the rehabilitation or replacement of them. The critical components are the blades of the last stage in the steam turbine. It has been observed that different blades of the turbine of 110MW with cracks presented corrosion products, which resulted in a failure for corrosion fatigue mechanisms. In this paper, it was studied the effect of crack propagation produced in a geothermal turbine blade of the last stage, L-0, which is made of stainless steel AISI 410 exposed to corrosion under a sea water solution. The corrosion phenomena including localized corrosion suffered by the cracking sample were studied through the electrochemical noise technique in current and potential and polarization curves. The tests were conducted on pieces of blades subjected to fatigue. The results indicated that the exposure to the corrosion solution modified the width and the length of the cracks. Using a scanning electron microscopy (SEM), the surface of the crack was observed, showing that the corrosion mechanism produced a significant increment of the velocity of crack propagation and therefore, a decrement of the useful life of the material. This research will allow us to understand the corrosion process in addition to estimate the useful life of the blades when they are subjected to load cycles.

Failure of last-stage turbine blades in a geothermal power plant

This paper presents an investigation into causes of failure of geothermal steam turbine blades. Several L-0 blades of geothermal steam turbines of 110 MW capacity suffered failures, causing forced outages of the turbines. To assess the causes of failure, the natural frequencies of the blades installed on the rotor were measured in the laboratory. The measured frequencies were compared with the natural frequencies calculated through a finite-element analysis (FEA) of the blade. The FEA was also used to calculate the vibratory stresses on the blade numerically. Also, the investigation analyzed the operational data and the history of the blade failures on several rotors of different units from the same system. The results of previous repairs were reviewed, and metallurgical investigations were conducted to identify the mechanical and metallurgical modes of failure. The results of the investigation showed that the fracture of two blades was attributed to installation and manufacturing errors and aggravated by general deterioration of the blades. The deterioration was caused by the erosion and corrosion process that resulted from moisture condensation in the steam.

Developments in geothermal energy in Mexico—Part twenty. Technical advantages and disadvantages of geothermal well-head units

The technical advantages and disadvantages resulting from locating a steam turbine unit close to a geothermal well-head are presented. The short steam piping system has implications in the design of a well-head turbine. It is concluded that there are some technical differences between large geothermal turbines and well-head units, which may be decisive in the selection of geothermal plant.

Fluid Dynamic and thermodynamic Performance of Geothermal Turbines Operating by Steam mixed with Noncondensable Gases : 2nd Report, Experimental Study

An understanding of the complex behavior of gas mixed steam in its expansion through a turbine stage is essential to the design of efficient and reliable geothermal turbine units. The 1st Report described thermodynamic characteristics and performance of geothermal turbines and showed state equations of steam with noncondensable gases. This 2nd Report presents some experimental results on the behavior of steam with noncondensable gases, such as the equivalent flow coefficient, fluid dynamic loss characteristics and deviation angle in nozzles or cascades. The tests were carried out in a steam tunnel, and measurements were made using the Schlieren method, a pitot probe, and a wedge probe. Results and conclusions obtained from these tests are reported.

Fluid Dynamic and Thermodynamic Performance of Geothermal Turbines Operating by Steam Mixed with Noncondensable Gases : 1st Report, Theoretical Study

An understanding of the complex behavior of gas mixed steam in its expansion through a turbine stage is essential to the design of efficient and reliable geothermal turbine units. The 1st Report described thermodynamic characteristics and performance of geothermal turbines and showed state equations of steam with noncondensable gases. This 2nd Report presents some experimental results on the behavior of steam with noncondensable gases, such as the equivalent flow coefficient, fluid dynamic loss characteristics and deviation angle in nozzles or cascades. The tests were carried out in a steam tunnel, and measurements were made using the Schlieren method, a pitot probe, and a wedge probe. Results and conclusions obtained from these tests are reported.

Fluid Dynamic and thermodynamic Performance of Geothermal Turbines Operating by Steam Mixed with Noncondensable Gases : 3rd Report, Confirmation by Performance Test of Actual Turbine

As to the thermodynamic performance of steam turbine to be operated by geothermal steam mixed with noncondensable gases, the 1st and 2nd Reports were presented to report on a theoretical calculation method. To confirm the accuracy of the calculation method, we have compared calculated values with actually measured values based on the results of performance tests conducted at a geothermal power generating plant. The result of comparison and examination shows that the result of calculation wherein the effect of gaseous components is taken into consideration well coincide with the results of performance tests within the range of measurement tolerance, and proves the reasonability of the calculation method.

Emission control of gas effluents from geothermal power plants.

Geothermal steam at the world's five largest power plants contains from 0.15 to 30% noncondensable gases including CO(2), H(2)S, H(2), CH(4), N(2), H(3)BO(3), and NH(3). At four of the plants the gases are first separated from the steam and then discharged to the environment; at the fifth, the noncondensables exhaust directly to the atmosphere along with spent steam. Some CO(2) and sulfur emission rates rival those from fossil-fueled plants on a per megawatt-day basis. The ammonia and boron effluents can interfere with animal and plant life. The effects of sulfur (which emerges as H(2)S but may oxidize to SO(2)) on either ambient air quality or longterm human health are largely unknown. Most geothermal turbines are equipped with direct contact condensers which complicate emission control because they provide two or more pathways for the effluents to reach the environment. Use of direct contact condensers could permit efficient emission control if coupled to processes that produce saleable quantities of purified carbon dioxide and elemental sulfur.

A pilot geothermoelectric power station in Pauzhetka, Kamchatka

The Geotes in Pauzhetka is the first pilot-commercial installation of its kind in the USSR. The technological scheme of the station is extremely simple. Its exploitation has been improved through modifications of the regulating and hot supply systems brought into the planned design. A two-year long operation of the plant has allowed an analysis of the progressive changes in its technical-economical showing. Starting from the data available on the bi-phase flow notion, a descriptive method of steam-to-water wells hydrodynamical parameters has been elaborated. Rated results are being correlated with experimental ones. Related characteristics of wells make it possible to identify their construction and the optimum steam-pressure value in front of the turbines. As was experimentally shown, under certain conditions the head pressure of a steam-water mixture may be utilized in special hydrodynamical turbines. The field of uses for such turbines is discussed. Experimental data, showing a possible use of water jets and mechanical vacuum pumps for the air to be sucked off geothermal turbines condensers, have been analyzed and compared. Possibilities of a step increase of plant capacity, up to 100,000 kW, are discussed.