United states criteria for repair and replacement of BWR piping and steam generator tubing

This paper discusses the criteria currently used by the US Nuclear Regulatory Commission in dealing with repairs and replacement of BWR piping and steam generator tubing and provides the bases for these criteria. As a result of extensive cracking in BWR pipes caused by intergranular stress corrosion, a program of augmented inspection and leak testing has been instituted. Emphasis is placed on returning such piping to original design conformance either by replacement with non-susceptible material or by a combination of stress improvement and environmental control. As a result of a number of degradation mechanisms in steam generator tubes, inspection and criteria for plugging or sleeving have been instituted along with broad recommendations aimed at an integrated program for resolving steam generator tube problems.     

Mechanical methods of improving resistance to stress corrosion cracking in BWR piping systems

Pipelocks and the mechanical stress improvement process (MSIP) have been applied in BWR plants. Pipelocks restore the integrity of the weldments with identified cracks. MSIP removes residual tensile stresses from weldments, thus preventing initiation of cracks or retarding growth of pre-existing flaws in piping systems. The first 12-in pipelock installed at Commonwealth Edison's Quad Cities plant was inspected after operating for 18 months. Pipelocks for 10-in, 12-in and 28-in reactor safe-ends were fabricated for Carolina Power & Light's Brunswick plant. MSIP was applied at Commonwealth Edison's Dresden and LaSalle BWR plants. Extensive qualification has been completed for MSIP under US Nuclear Regulatory Commission and Electric Power Research Institute sponsorship. Sectioning of the pipe wall by Argonne National Laboratories provided stress distribution before and after MSIP for the 12-in and 28-in pipes. Measured ‘as welded’ tensile stresses before MSIP were within the range 30–50 000 psi. Compressive stresses after MSIP at the inside surface of the weldment reached more than 30 000 psi in both hoop and axial directions. The axial compressive stresses extended to the middle plane of the wall. Hoop stresses remained compressive through the wall. The stresses were uniformly distributed around the circumference of the pipe. J.A. Jones Applied Research Company completed an evaluation of MSIP applied to a precracked weldment between 28-in pipe and elbow. The pipe was squeezed to about 1·7% in the presence of cracks 25%, 50% and 90% through-wall. High compressive stresses were measured after MSIP. The cracks did not extend and could be identified after completion of the process by the usual UT technique. The use of mechanical methods becomes especially adequate for reactor safe-ends including bi- or tri-metallic joints. The use of overlay technique or induction heat stress improvement is more difficult due to high thermally induced strains at the strong discontinuity interface between materials of different thermal expansion. Basic concepts and practical application of mechanical methods to inhibit stress corrosion attack are described.     

A new monitoring system for piping systems in fossil fired power plants

A CEC-funded project has been performed to tackle the problem of producing an advanced Life Monitoring System (LMS) which would calculate the creep and fatigue damage experienced by high temperature pipework components. Four areas were identified where existing Life Monitoring System technology could be improved:

1. 1. the inclusion of creep relaxation

2. 2. the inclusion of external loads on components

3. 3. a more accurate method of calculating thermal stresses due to temperature transients

4. 4. the inclusion of high cycle fatigue terms.

The creep relaxation problem was solved using stress reduction factors in an analytical in-elastic stress calculation. The stress reduction factors were produced for a number of common geometries and materials by means of non-linear finite element analysis. External loads were catered for by producing influence coefficients from in-elastic analysis of the particular piping system and using them to calculate bending moments at critical positions on the pipework from load and displacement measurements made at the convenient points at the pipework. The thermal stress problem was solved by producing a completely new solution based on Green's Function and Fast Fourier transforms. This allowed the thermal stress in a complex component to be calculated from simple non-intrusive thermocouple measurements made on the outside of the component. The high-cycle fatigue problem was dealt with precalculating the fatigue damage associated with standard transients and adding this damage to cumulative total when a transient occurred.

The site testing provided good practical experience and showed up problems which would not otherwise have been detected.     

Energy- and exergy-based comparison of coal-fired and nuclear steam power plants

The results are reported of energy- and exergy-based comparisons of coal-fired and nuclear electrical generating stations. A version of a process-simulation computer code, previously enhanced by the author for exergy analysis, is used. Overall energy and exergy efficiencies, respectively, are 37% and 36% for the coal-fired process, and 30% and 30% for the nuclear process. The losses in both plants exhibit many common characteristics. Energy losses associated with emissions (mainly with spent cooling water) account for all of the energy losses, while emission-related exergy losses account for approximately 10% of the exergy losses. The remaining exergy losses are associated with internal consumptions, mainly in components which generate heat by combustion or nuclear reactions, and in components which transfer heat across large temperature differences. It is anticipated that the results will prove useful to those involved in the improvement of existing and design of future electrical generating stations.     

Transient thermal modelling of heat recovery steam generators in combined cycle power plants

Heat recovery steam generator (HRSG) is a major component of a combined cycle power plant (CCPP). This equipment is particularly subject to severe thermal stress especially during cold start‐up period. Hence, it is important to predict the operational parameters of HRSGs such as temperature of steam, water, hot gas and tube metal of heating elements as well as pressure change in drums during transient and steady‐state operation. These parameters may be used for estimating thermal and mechanical stresses which are important in HRSG design and operation. In this paper, the results of a developed thermal model for predicting the working conditions of HRSG elements during transient and steady‐state operations are reported. The model is capable of analysing arbitrary number of pressure levels and any number of elements such as superheater, evaporator, economizer, deaerator, desuperheater, reheater, as well as duct burners. To assess the correct performance of the developed model two kinds of data verification were performed. In the first kind of data verification, the program output was compared with the measured data collected from a cold start‐up of an HRSG at Tehran CCPP. The variations of gas, water/steam and metal temperatures at various sections of HRSG, and pressure in drums were among the studied parameters. Mean differences of about 3.8% for temperature and about 9.2% for pressure were observed in this data comparison. In the second kind of data verification, the steady‐state numerical output of the model was checked with the output of the well‐known commercial software. An average difference of about 1.5% was found between the two latter groups of data. Copyright © 2007 John Wiley & Sons, Ltd.     

Actuator line/Navier–Stokes computations for the MEXICO rotor: comparison with detailed measurements

In the European collaborative MEXICO (Model Experiments in Controlled Conditions) project, a series of experiments was carried out on a 4.5 m diameter wind turbine rotor to validate numerical diagnostics tools. Here, some of the measured data are compared with computations of the combined actuator line/Navier–Stokes (AL/NS) model developed at the Technical University of Denmark. The AL/NS model was combined with a large eddy simulation technique and used to compute the flow past the MEXICO rotor in free air and in the DNW German‐Dutch wind tunnel for three commonly defined test cases at wind speeds of 10, 15 and 24 m s ^?1. Two sets of airfoil data were used. Comparisons of blade loadings showed that the AL/NS technique with the modified airfoil data is in better agreement with the measurements than with the original 2D airfoil data. Comparisons of detailed near‐wake velocities showed good agreement with the measurements. Computations including the influence of the geometry of the wind tunnel showed that tunnel effects are not significant and the effect of the geometry of the wind tunnel only results in a speedup of 3% at a thrust coefficient of C_T = 1.Copyright © 2011 John Wiley & Sons, Ltd.     

电厂主蒸汽流量测量与计算方法分析比较

介绍了目前获取发电机组主蒸汽流量的几种常用方法,对它们进行了分析比较,指出影响主蒸汽流量测量精度的几个主要因素,并提出了提高发电机组主蒸汽流量测量精度的相应对策。     

Thermodynamic performance comparison of SOFC-MGT-CCHP systems coupled with two different solar methane steam reforming processes

Multi-energy complementary distributed energy system integrated with renewable energy is at the forefront of energy sustainable development and is an important way to achieve energy conservation and emission reduction. A comparative analysis of solid oxide fuel cell (SOFC)-micro gas turbine (MGT)-combined cooling, heating and power (CCHP) systems coupled with two solar methane steam reforming processes is presented in terms of energy, exergy, environmental and economic performances in this paper. The first is to couple with the traditional solar methane steam reforming process. Then the produced hydrogen-rich syngas is directly sent into the SOFC anode to produce electricity. The second is to couple with the medium-temperature solar methane membrane separation and reforming process. The produced pure hydrogen enters the SOFC anode to generate electricity, and the remaining small amount of fuel gas enters the afterburner to increase the exhaust gas enthalpy. Both systems transfer the low-grade solar energy to high-grade hydrogen, and then orderly release energy in the systems. The research results show that the solar thermochemical efficiency, energy efficiency and exergy efficiency of the second system reach 52.20%, 77.97% and 57.29%, respectively, 19.05%, 7.51% and 3.63% higher than those of the first system, respectively. Exergy analysis results indicate that both the solar heat collection process and the SOFC electrochemical process have larger exergy destruction. The levelized cost of products of the first system is about 0.0735$/h that is lower than that of the second system. And these two new systems have less environmental impact, with specific CO₂ emissions of 236.98 g/kWh and 249.89 g/kWh, respectively.     

Gases in steam from Cerro Prieto geothermal wells with a discussion of steam/gas ratio measurements

As part of a joint USGS-CFE geochemical study of Cerro Prieto, steam samples were collected for gas analyses in April, 1977. Analyses of the major gas components of the steam were made by wet chemistry (for H₂O,CO₂,H₂S and NH₃) and by gas chromatography (He,H₂,Ar,O₂,N₂ and hydrocarbons).The hydrocarbon gases in Cerro Prieto steam closely resemble hydrocarbons in steam from Larderello, Italy and The Geysers, California which, although they are vapor-dominated rather than hot-water geothermal systems, also have sedimentary aquifer rocks. These sedimentary geothermal hydrocarbons are characterized by the presence of branched C_4–6 compounds and a lack of unsaturated compounds other than benzene. Relatively large amounts of benzene may be characteristic of high-temperature geothermal systems. All hydrocarbons in these gases other than methane most probably originate from the thermal metamorphosis of organic matter contained in the sediments.     

In-service corrosion potential measurements in a series of feedwater deaerator systems in small peat-fired steam raising units

This paper gives an assessment of a series of studies aimed at establishing the corrosion potentials prevalent in working deaerator systems of small 20–40 MW peat fired steam raising units. Two Ag/AgCl reference electrodes were inserted into each deaerator and their tips positioned at different feedwater storage vessel weld run locations. As well as corrosion potential, water chemistry parameters, unit load and feedwater temperature details were recorded.

It was shown that the reference electrodes: (i) sensibly recorded the corrosion potential in deaerator feedwater storage vessel weldments, (ii) the level of corrosion potential was similar at different weld locations, (iii) the agreement between both readings in any vessel was excellent, and (iv) exhibited a range in durability with the 0·1 m reference electrode being better than the 3·0 m.

An initial potential hysteresis, where corrosion potential (CP) values remained at higher levels than those predicted from potential-dissolved oxygen trends, was consistently observed in all three deaerators. Also, various sudden changes in corrosion potential were recorded and it was shown that these coincided with either normal working unit load fluctuations or off-load events. A significant influence of surface condition was evidenced, in that a bright clean weld surface recorded a CP value which was consistently over 150 mV more positive than that shown by an oxidised weld surface.

Finally, it was concluded that the present deaerator system corrosion potential data compared well with other reported potential-dissolved oxygen trends and exhibited reasonable agreement with modern day empirical and deterministic models which predicted crack growth behaviour in stainless and low alloy steels in hot aqueous environments.     

Comments on probabilities of leaks and breaks of safety-related piping in PWR plants

Leaks or failures with a safety significance in Cl 1 or Cl 2 piping of nuclear power plants (NPP) in Germany are very rare events. This excellent record in operating experience is matched by many NPPs in other countries.

The advances achieved in the understanding of fracture behaviour, in the methods of non-destructive testing and surveillance, together with operating experiences, can be used in the re-evaluation of piping systems that have been designed and manufactured to the standards given at the time of construction.

Comments and examples are presented for determining the probability of leaks and breaks in the whole range of Cl 1 and Cl 2 piping systems.     

Mathematical modelling of heat transfer in dedusting plants and comparison to off-gas measurements at electric arc furnaces

A mathematical simulation tool is presented in order to model enthalpy flow rates of off-gas and heat transfer of cooling systems at dedusting plants in electric steel making sites. The flexibility of the simulation tool is based on a user-defined series of modular units that describe elementary units of industrial dedusting systems, e.g. water-cooled hot gas duct, air injector, drop-out box, mixing chamber, post-combustion chamber, filter, etc. Results of simulation were checked with measurements at industrial electric steel making plants in order to validate the models for turbulence, heat transfer and chemical reaction kinetics. Comparison between computed and measured gas temperature and composition yield excellent agreement. The simulation tool is used to calculate off-gas temperature and volume flow rate, where off-gas measurements are very difficult to apply due to high gas temperatures and high dust load. Heat transfer from the off-gas to the cooling system was calculated in detail for a pressurised hot water EAF cooling system in order to investigate the impact of the cooling system and the dedusting plant operation on the energy sinks of the electric arc furnace. It is shown that optimum efficiency of post-combustion of EAF off-gas in the water-cooled hot gas duct requires continuous off-gas analysis. Common operation parameters of EAF dedusting systems do not consider the non-steady-state of the EAF off-gas emission efficiently.     

Local measurements in the flow of a steam injector and visualisation

A steam injector is a thermocompression device where the steam drags directly a subcooled liquid and delivers a liquid with an outlet pressure higher than both upstream fluid pressures. The functioning principle of injectors involves sophisticated thermohydraulic processes whose present knowledge is essentially empirical. This work presents an experimental approach of steam injector in order to finely understand and quantify the physical laws driving the flow in the mixing chamber. Local measurements (2D) are carried out on a special device with a rectangular section, where the flow was visualised. These measurements are the void fraction, the static pressure and the static temperature. They permit the calculation of all the variables and a well understanding of the physical phenomena involved in the flow. Especially, we show, in some well-defined zones, the existence of important non-equilibrium kinetic, thermal and thermodynamic phenomena.     

Transient phenomena in a PEMFC during the start-up of gas feeding observed with a 97-fold segmented cell

To measure local phenomena in a PEMFC during a transitional state induced by changing of the feeding gas, a segmented cell was fabricated and the local current and local potential distribution were measured under open-circuit conditions. The anode or cathode was divided into 97 segments of 1.5 mm each. A change in the anode gas from nitrogen or oxygen to hydrogen induced momentary internal currents among the segments. The potential distribution in the electrolyte was observed simultaneously using three quasi-reference electrodes located locally. The results supported the reverse-current decay mechanism, which is known to be a mechanism of cathode degradation. Furthermore, internal currents were observed when the cathode gas was changed from nitrogen to oxygen. While the cathode was not subjected to a harmful potential, a large potential distribution was induced in the anode.     

High temperature jacketed piping: Review of the suited thermal models and comparison with experimental results

A comparison is presented between a theoretical model and the experimental data of the thermal performance of a jacketed pipe which belongs to an experimental facility aimed at testing the critical components of the externally fired combined cycle (EFCC) technology.The pipe consists of two concentric tubes, the inner with hot air (nominal conditions above 1000 °C) and the outer with cold water, whose function is to make the inner tube wall temperature to be tolerated by a traditional steel (e.g. an AISI series stainless steel).A proper model is identified to calculate the fluid temperatures at the pipe exit, by considering a spatial discretization of the system, such that on each resulting section a differential equation is iteratively solved which gets as boundary conditions the output values arising from the preceding section.The model predictions are compared to the data coming from an experimental facility, resulting in a good agreement: about 87% of the air side (and 92% of the water side) experimental data is falling within a ±8% deviation band from the expected values. Such difference is widely acceptable, being probably due to the uncertainties connected with to the use of closed-form correlations for calculating the convective heat transfer coefficients.     

On a simplified system for steam production in nuclear power plants with pressurized water reactors

The paper develops the idea of a new system of steam production (Simplified System for Steam Production, SSSP) in nuclear power plants (NPPs) with PWR reactors, which is simplified as compared to the system used in the classical NPP of this type where steam is produced by steam generators (SGs). With the SSSP, expanders are used instead of SGs. The particularities of a NPP with a SSSP are analyzed in comparison to the classical NPP equipped with PWR and BWR reactors, which are used almost exclusively at present to produce electric energy at the industrial level.     

A comparison of luminous efficacy models with illuminance and irradiance measurements

Several luminous efficacy models have been tested against simultaneous illuminance and irradiance measurements in Helsinki, Finland (60°11′N, 24°50′E). When compared with the measured values, the Perez luminous efficacy model had the lowest relative root mean square difference (RMSD) of 6.7%. All tested models had an RMSD within 2% of the constant model which used the measured yearly average of 110 lm/W as the constant. For all models, the error was smaller for high solar altitudes but greater for the low altitudes. The monthly average luminous efficacy was fairly constant between April and October but it was clearly lower during the winter months when the sun is very low.     

Transient modeling of combined conduction and radiation in wood fibre insulation and comparison with experimental data

This is an innovative study of wood wool used for building insulation and the reported results could be important for improving the cooling and the heating efficiency of buildings. Wood fibre boards are non-grey, absorbing and scattering media. The radiative properties of the wood wool (albedo, optical thickness and phase function coefficients) were identified using an inverse method based on infrared experimental measurements of reflection and transmission. The radiative part was then found negligible with respect to the phonic part in steady state for the material tested. Transient one-dimensional coupled radiative and conductive heat transfer was solved in a wood wool material. The transient numerical results were validated by comparing them with fluxmeter measurements when temperatures were fixed at the boundaries. The temperatures and the heat capacity had strong influence on the transient numerical results. Finally, our computations showed that a purely conductive model gives the same fluxes at the boundary as the coupled radiative-conductive model.