Contents Issue 9 (2000)

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D. A. Palmer, P. Bénézeth, D. J. Wesolowski, S. A. Wood, and C. Xiao

The Solubility of Metal Oxides and Hydroxides at High Temperatures Results and Implications of Recent ORNL Measurements

The results of high temperature solubility studies at ORNL are presented in which mainly direct pH measurements were made of aqueous solutions in contact with the crystalline solid phases: Al(OH)3, AlOOH, Fe3O4, Mg(OH)2, Nd(OH)3, and ZnO. Examples are highlighted of specific phenomena such as: the kinetics of gibbsite and boehmite dissolution and precipitation; the appearance of metastable equilibria in the dissolution of Fe3O4; the extremely rapid precipitation of crystalline brucite, Mg(OH)2; and anomalies in the apparent solubility profiles of AlO(OH) and ZnO. General trends associated with the effects of temperature and ionic strength are mentioned. Some of the potentiometric investigations were augmented by conventional batch {AlO(OH) and ZnO}, and flow-through column {ZnO} experiments. In the additional case of ZnCr2O4, the extremely low solubility of this spinel permitted application of only the latter technique and these results are discussed in terms of the measured chromium levels that resulted from incongruent dissolution.

PowerPlant Chemistry 2000, 2(9)

Andrzej Anderko and Malgorzata M. Lencka

Modeling Transport Properties of Electrolyte Solutions in Wide Concentration and Temperature Ranges

Comprehensive models have been developed for calculating electrical conductivity, viscosity, and self-diffusion coefficients in multicomponent electrolyte solutions. In the infinite-dilution limit, the temperature dependence of limiting ionic conductivities and diffusivities is predicted on the basis of the concept of structure-breaking and structure-making ions. The same concept is used to calculate the temperature dependence of the viscosity Bvis coefficients. At finite concentrations, the transport property models combine contributions of long-range (Coulombic) and short-range interactions. In the case of electrical conductivity and diffusivity, the long-range effects are calculated from the dielectric continuum-based mean spherical approximation (MSA) theory for the unrestricted primitive model. In the case of viscosity, the long-range contribution is obtained from the Onsager-Fuoss theory with Debye-Huckel distribution functions. The short-range interactions are represented using property-specific models. In particular, a hard-sphere model is used for the non-electrostatic contributions to diffusivity. The effect of complexation and hydrolysis of ions is taken into account by combining the transport property models with thermodynamic speciation calculations. The models reproduce the transport properties of aqueous systems ranging from dilute to concentrated solutions (up to ca. 30 M) with an accuracy that is appropriate for modeling industrially important systems. In particular, the properties of multicomponent systems can be predicted using data for single-solute systems.

PowerPlant Chemistry 2000, 2(9)

Robert Svoboda, Maurice Bodmer, and Harald Sandmann

Impact of Organic Impurities on Steam Turbine Operation

Organic impurities in steam can lower local pH in the steam turbine, and may possibly cause additional ion-specific corrosion effects. Some practical cases of organic impurities in the steam are illustrated. Limited quantities of carbon dioxide in the steam did not prove to be detrimental as long as the alkalization of the water was sufficient. Low local pH can be compensated within limits by conditioning of feedwater or steam. The possibility of specific corrosion effects for the individual organic impurities, especially for acetate deserves further research.

PowerPlant Chemistry 2000, 2(9)

Joseph Lyons and Jason E. Bane

Characterization of Organic Matter and a Field Study of Its Role in Turbine Corrosion

Field studies were carried out in five power stations involving three different sources of raw water, namely: surface, municipal and groundwater, where the organic matter content was measured using the total organic carbon (TOC) parameter. The constituent compounds that contribute to the TOC, such as polysaccharides, humics, hydrolysates, low molecular weight organics and low molecular weight neutrals, were then identified. The effectiveness of resin and flocculation as removal processes for the different organic compounds was studied. Two different analytical techniques were used to measure the TOC for comparison purposes with somewhat different results and these are discussed.

The failure in the LP section of a 45MW single cylinder steam turbine, which resulted in the replacement of four rows of blades due to corrosion at the blade root, was studied. The steam quality, in this particular turbine and in an identical 45MW turbine with similar operating hours but with no evidence of corrosion, was analysed in detail. Organic matter that enters the boiler undergoes thermal decomposition to low molecular weight organic acids such as: acetic, formic, propionic, oxalic and carbonic acid and these acids were measured during the course of this work, together with conductivity after strong cation resin (CAC) and chloride and sulphate ions. The reason for the blade failure was due to the use of the wrong material in the phase transition zone of the turbine.

PowerPlant Chemistry 2000, 2(9)

Geoff J. Bignold

Sources of Corrosive Conditions in Low Pressure Steam Turbines

The alloys used in steam turbines generally rely on a passive oxide layer for protection against corrosion. This will normally contain oxides of iron, chromium and may have nickel and small proportions of other metals. All of the engineering materials are thermodynamically unstable in steam and thus the integrity of the passive film is vital to their long working lifetime. Ingress of corrosive materials which disrupt the passive film can lead to pitting corrosion, which may be the precursor of stress corrosion cracking or of corrosion fatigue. The assembly of a steam turbine inevitably involves the creation of crevices (e.g. at blade roots, shroud fixings, etc.) and these can represent areas of particular vulnerability. Sudden failures deriving from these mechanisms can be hazardous, and repair costs may be very high. It is therefore important to ensure that all sources of input of corrosive materials are effectively controlled.

Many of the sources of impurity ingress to steam turbines are naturally controlled by the constraints of the rest of the plant. Boiler water concentrations are controlled within target ranges that should help to ensure that the concentration of solutes directly dissolved in main steam are as low as practicable. Nonetheless, as steam expands through the turbine, its ability to hold ionic solutes in solution diminishes very dramatically, and there is always a risk of deposition from this source. Boiler design should prevent the occurrence of gross carry-over of droplets. Any units that have a high risk of gross carry-over are likely to have correspondingly high levels of deposition in their turbines.

Other sources may require specific consideration if they are not to compromise the operation of the unit. The practice of steam temperature control by spraying feed-water in via attemperators can lead to contamination of the steam if the feed-water itself becomes contaminated (i.e., periods of condenser leakage). Leakage of reheaters can result in furnace gasses (containing carbon dioxide and oxides of sulphur and nitrogen) being drawn into the turbine during the shut-down and start-up procedures, when sub-atmospheric pressures extent back into the reheater. The use of condensate to cool the large final stage blades via hood sprays during start-up can lead to the exposure of those blades to any contaminants that have accumulated in the condenser during outages.

Guidance on steam purity requirements for steam turbines must take account of all of these sources of potentially costly corrosion damage. The alloys used in steam turbines generally rely on a passive oxide layer for protection against corrosion. This will normally contain oxides of iron, chromium and may have nickel and small proportions of other metals. All of the engineering materials are thermodynamically unstable in steam and thus the integrity of the passive film is vital to their long working lifetime. Ingress of corrosive materials which disrupt the passive film can lead to pitting corrosion, which may be the precursor of stress corrosion cracking or of corrosion fatigue. The assembly of a steam turbine inevitably involves the creation of crevices (e.g. at blade roots, shroud fixings, etc.) and these can represent areas of particular vulnerability. Sudden failures deriving from these mechanisms can be hazardous, and repair costs may be very high. It is therefore important to ensure that all sources of input of corrosive materials are effectively controlled. Many of the sources of impurity ingress to steam turbines are naturally controlled by the constraints of the rest of the plant. Boiler water concentrations are controlled within target ranges that should help to ensure that the concentration of solutes directly dissolved in main steam are as low as practicable. Nonetheless, as steam expands through the turbine, its ability to hold ionic solutes in solution diminishes very dramatically, and there is always a risk of deposition from this source. Boiler design should prevent the occurrence of gross carry-over of droplets. Any units that have a high risk of gross carry-over are likely to have correspondingly high levels of deposition in their turbines.

Other sources may require specific consideration if they are not to compromise the operation of the unit. The practice of steam temperature control by spraying feed-water in via attemperators can lead to contamination of the steam if the feed-water itself becomes contaminated (i.e., periods of condenser leakage). Leakage of reheaters can result in furnace gasses (containing carbon dioxide and oxides of sulphur and nitrogen) being drawn into the turbine during the shut-down and start-up procedures, when sub-atmospheric pressures extent back into the reheater. The use of condensate to cool the large final stage blades via hood sprays during start-up can lead to the exposure of those blades to any contaminants that have accumulated in the condenser during outages.

Guidance on steam purity requirements for steam turbines must take account of all of these sources of potentially costly corrosion damage.

PowerPlant Chemistry 2000, 2(9)

Miroslav Št'astný and Miroslav Šejna

Analysis of Heterohomogeneous Condensation of the Steam Flowing in a Turbine Cascade by Numerical Two-Population Model

An advanced computational model and code, COCHEM FLOW, of the wet-steam flow through turbine cascades with homogeneous and parallel heterogeneous condensation is described.

This computational model is based on governing equations of the flow with homogeneous condensation supplemented by another equation for the Euler equation system and by the equation of liquid phase transport to heterogeneous droplets. This model enables separate observation of the radii and other parameters of homogeneous and heterogeneous droplets in the flow field. The associated code for numerical solution of the whole system of equations is also described.

The two population numerical model of heterohomogeneous condensation is used for the calculation of the steam flow with condensation through the 2D nozzle cascade of the first LP wet stage of a condensing steam turbine. The parallel heterogeneous condensation is evaluated on the assumption that, at the steam saturation line, droplets are present that originate by nucleation on chemical impurities in Salt Solution Zone close above the steam saturation line.

The calculated results are described and the effects of heterogeneous and/or homogeneous condensations are discussed.

PowerPlant Chemistry 2000, 2(9)

Dirk J. Hanekom

Cooling Water Chemistry Limitations and the Method of Evaluating the Best Value for Money Options

Power generation in a water scarce country necessitates high cycles of concentration in open evaporative cooling water systems. Operation with high cycles of concentration is not only necessary to minimise raw water intake but also to prevent pollution of the natural water resources.

Eskom presently operates seven large power stations, each generating in excess of 2,000 MW, with open evaporative cooling systems at cycles of concentrations varying between 20 and 50 without the addition of corrosion inhibitors. Materials of construction include concrete, epoxy coated mild steel, muntz metal, admiralty brass, stainless steel, and titanium. One station employs induced current cathodic protection for the protection of the cooling water ducts.

Achieving optimal cycles of concentration is a function of the materials of construction of the power plant, the different hardness species of the makeup water and the processes selected for the control of alkalinity, microbial activity, and organic reduction. Lime softening or alkalinity control is vital in achieving high cycles of concentration. However, the composition of the makeup water dictates the feasibility of lime treatment.

The paper discusses these aspects and the employment of techniques such as the application of so-called "anion-free" flocculants and crystal modifiers in some detail. In addition, the implications of concentrated organic species in the cooling water circuits will be addressed.

PowerPlant Chemistry 2000, 2(9)