Limiting Operating Conditions Research Articles

Stability analysis of supercavitating underwater vehicles with adaptive cavitator

Supercavitating vehicles are characterized by substantially reduced hydrodynamic drag with respect to fully wetted underwater vehicles. Drag is localized at the nose of the vehicle, where a cavitator generates a cavity that completely envelops the body ( supercavity). The size of the supercavity and the magnitude of the drag at the cavitator are greatly affected by the vehicle's velocity and by the shape and size of the cavitator. This paper investigates the benefits of an adaptive cavitator, capable of adjusting its size with the speed of forward motion of the vehicle. Objective of the cavitator size variation is to maintain the minimum cavity length and the minimum drag at any given speed. The localized drag and the propulsion required to sustain the vehicle's motion can cause the vehicle to buckle. In addition, propulsion acts as a follower force and may be a source of flutter-type instabilities when the vehicle is accelerating. The insurgence of buckling and flutter is investigated to identify limiting operating conditions. The analysis is performed by using a finite element model developed to predict stability limits. The case of pulsating thrust is also addressed through the application of the method of infinite determinants, also known as Bolotin's method.

Hydrodynamic and tray efficiency behavior in parastillation column

This work presents aspects of the parastillation process, which employs a unique distillation column where the vapor stream is divided into two equal parts and the falling liquid is alternately in contact with both vapor parts on a stage-by-stage basis. A laboratory-scale apparatus was used to study the parastillation column. Experiments were carried out under total and partial reflux conditions using an ethanol-water system. Experiments were conducted to analyze the effects of vapor flow rate and initial ethanol concentration in the reboiler on the hydrodynamic conditions. Limiting operating conditions were defined. Murphree separation efficiencies were calculated and discussed.

Retrofitting the schematic and layout solutions of the condensate-feedwater path and its equipment of turbine units at thermal and nuclear power stations

It is shown that the efficiency of the equipment of a regeneration system already in service can be improved through operating it at sliding pressure. The limiting operating conditions of the heat exchanger-pipeline-pump system are determined. The results of the work allowed the operational reliability of a turbine to be improved under static and dynamic conditions.

Failure probability assessment of wall-thinned nuclear pipes using probabilistic fracture mechanics

The integrity of nuclear piping system has to be maintained during operation. In order to maintain the integrity, reliable assessment procedures including fracture mechanics analysis, etc., are required. Up to now, this has been performed using conventional deterministic approaches even though there are many uncertainties to hinder a rational evaluation. In this respect, probabilistic approaches are considered as an appropriate method for piping system evaluation. The objectives of this paper are to estimate the failure probabilities of wall-thinned pipes in nuclear secondary systems and to propose limited operating conditions under different types of loadings. To do this, a probabilistic assessment program using reliability index and simulation techniques was developed and applied to evaluate failure probabilities of wall-thinned pipes subjected to internal pressure, bending moment and combined loading of them. The sensitivity analysis results as well as prototypal integrity assessment results showed a promising applicability of the probabilistic assessment program, necessity of practical evaluation reflecting combined loading condition and operation considering limited condition.

05/01198 Prediction of knock limited operating conditions of a natural gas engine

05/01198 Prediction of knock limited operating conditions of a natural gas engine

Temporal phenomena in inductively coupled chlorine and argon–chlorine discharges

Reproducible modulations in low-pressure, inductively coupled discharges operating in chlorine and argon–chlorine mixtures have been observed and studied. Changes in the light output, floating potential, negative ion fraction, and charged particle densities were observed. Here we report two types of unstable operational modes in an inductively coupled discharge. On the one hand, when the discharge was matched, to minimize reflected power, instabilities were observed in argon–chlorine plasmas over limited operating conditions of input power and gas pressure. The instability window decreased with increasing chlorine content and was observed for chlorine concentrations between 30% and 60% only. However, when operating at pressures below 5mTorr and the discharge circuit detuned to increase the reflected power, modulations were observed in a pure chlorine discharge. These modulations varied in nature from a series of sharp bursts to a very periodic behavior and can be controlled, by variation of the matching conditions, to produce an apparent pulsed plasma environment.

A quantitative assessment of LCOs for operations using system dynamics

Limiting conditions for operations (LCOs) define the allowed outage times (AOTs) and the actions to be taken if the repair cannot be completed within the AOT. Typically plant shutdown is required. In situations where the risk associated with the action, i.e. the risk of plant shutdown given a failure of the safety system, may be substantial, a strategy is needed to control the plant risk. In this study the changing operation modes are evaluated quantitatively and dynamically using the tool of system dynamics. System dynamics has been developed to analyze the dynamic reliability of a complicated system. System dynamics using the Vensim software have been applied to LCOs assessment for an example system, the auxiliary feed water system of a reference nuclear power plant. Analysis results of both full power operation and shutdown operation have been compared for a measure of core damage frequency. The increase in core damage frequency is used as a measure in this study. A time dependent framework developed in this study has been shown to be very flexible in that it can be applied to assess LCOs quantitatively under any operational context of the Technical Specifications in Final Safety Analysis Report of the reference plant.

Prediction of knock limited operating conditions of a natural gas engine

Computer models of engine processes are valuable tools for predicting and analyzing engine performance and allow exploration of many engine design alternatives in an inexpensive fashion. In the present work, a zero-dimensional, two zone thermodynamic model was used to determine the knock limited operating conditions of a natural gas engine. Experimentally based burning rate models were used for flame initiation and propagation calculations. A knock model was incorporated with the zero-dimensional model. Comparison of the measured and calculated cylinder pressure data indicated that the model is able to match the measured cylinder pressure data with less than 8% error in magnitudes if the computations are started at the experimental spark timing. The knock predictions agreed with the measurements also. With the established knock model, it is possible not only to investigate whether knock is observed with changing operating and design parameters, but also to evaluate their effects on the maximum possible knock intensity.

Computer-Aided Conceptual Design of Batch Distillation Systems

During the past decade, several authors worked in the development of methods to cope with different steps of the design and synthesis of batch distillation systems. Among these methods, the conceptual design is based both on the thermodynamic analysis of residue curve maps and on the behavior of composition profiles in batch columns operating at limiting operating conditions, i.e., infinite number of stages and total reflux. In this work, we apply the conceptual design approach to design the recovery of 2-propanol from a mixture containing water by using cyclohexane as entrainer. All the computations were performed through an integration of these methods in a software tool for conceptual design.

Acoustic Analysis of Gas Turbine Combustors

Combustion instability has become a major issue for gas turbine manufacturers. Stricter emission regulations, particularly on nitrogen oxides, have led to the development of new combustion methods, such as lean premixed prevaporized(LPP)combustion,to replacethetraditionaldiffusion e ame.However,LPPcombustionismuchmore liable to generate strong oscillations, which can damage equipment and limit operating conditions. As a tutorial, methods to investigate combustion instabilities are reviewed. Theemphasis is on gas turbine applications and LPP combustion. The e ow is modeled as a one-dimensional mean with linear perturbations. Calculations are typically done in the frequency domain. The techniques described lead to predictions for the frequencies of oscillations and the susceptibility to instabilities for which linear disturbances grow expotentially in time. Appropriate boundary conditions are discussed, as is the change in the linearized e ow across zones of heat addition and/or area change. Many of the key concepts are e rst introduced by considering one-dimensional perturbations. Later higher-order modes, particularly circumferential waves, are introduced, and modal coupling is discussed. The modeling of a simplie ed combustion system, from compressor outlet to turbine inlet, is described. The approaches are simple and fast enough to be used at the design stage.

Concentration polarisation in heterogeneous electrochemical reactions: a consistent kinetic evaluation and its application to molten carbonate fuel cells

Aim of this work is to focus attention on how the concentration polarization, in all electrochemistry applications, is often formulated on the basis of only thermodynamic statements and how it should be written in a kinetic more consistent expression. Moreover, the application of this study to the analysis of molten carbonate fuel cell (MCFC) behaviour has been discussed with reference to diffusion limited operating conditions.

Modelling of carbon dioxide absorption in aqueous solutions of AMP and MDEA and their blends using Aspenplus

In this paper the results of an experimental investigation on the solubility of CO2 in methyldiethanolamine (MDEA), a tertiary amine, and 2-amino-2-methyl-1-propanol (AMP), a primary sterically hindered amine as well as their mixtures are presented. It is shown that AMP CO2 uptake is higher as compared to that of MDEA. The addition of AMP to MDEA enhances the CO2 loading of the latter. The experimental data for single amines are used to generate the electrolyte NRTL model parameters that is available in the AspenPlus Software. The results have indicated that parameters generated using data obtained at limited operating conditions are able to accurately predict the CO2 solubilities at other conditions. Furthermore, the model is also able to estimate the CO2 loading in AMP–MDEA mixtures.

Radial solids flow structure in a liquid–solids circulating fluidized bed

Radial flow structure was studied using a fiber-optical probe in a 7.6 cm ID and 3.0 m high liquid–solids circulating fluidized bed (LSCFB). Radial solids holdup profiles were obtained at four different heights for glass beads and plastic beads of mean diameters 508 and 526 μm, respectively. The results show that local suspension densities are greater near the wall than in the center of the riser for both types of particles, confirming the existence of the radial non-uniformity. Under the same cross-sectional average solids holdup, the radial profiles of solids holdup are the same for each type of particle system under different operating conditions, but the light particles (plastic beads) show a flatter radial profile than the relatively heavier particles (glass beads). Discussion on the solids acceleration explains why a non-uniform axial profile in the LSCFB only exists for very heavy particles and only under limited operating conditions.

Simplified procedure for design of catalytic combustors with periodic flow reversal

An extended simplified model, applicable to every cycle period, for the simulation of the behaviour of an adiabatic reverse-flow reactor for the treatment of waste gases is presented and compared to the complete model. This model is based on the analogy with the steady countercurrent reactor and consists of simple equations that relate the main input and dimensional parameters of the reactor. Such a model permits the setting up the guidelines for the design of the reactor and to obtain some indications for its control system, allowing the prediction of the limiting operating conditions for autothermal operation.

Equilibrium reactor performance analysis for production of iron carbide

Iron carbide is a new material employed in steelmaking. This material is produced by gas–solid reactions in a temperature region lower than for conventional sponge iron. An equilibrium reactor performance analysis has been conducted to define limiting operating conditions with respect to temperature and gas phase composition for a process conducted at a fixed pressure. The configuration of the system incorporates two inlet streams, one composed of the solids feed (iron oxide) and the other of a CH2–H2 gas mixture. The outlet streams are composed of the equilibrated gas mixture and three solids: iron carbide with infinitesimal amounts of both unconverted iron oxide and solid carbon. The predictions agree with those reported in recent patents.

Revised Technique for the Determination of Mean Incident Light Fluxes on Photobioreactors

AbstractThe cultivation of photosynthetic microorganisms in photobioreactors mainly occurs under limitation by radiant light energy transfer at a rate controlled by the total radiant light energy entering the reactor. Consequently, the mean incident radiant light flux onto the reactor is a key technical quantity in both kinetic and energetic studies of such processes. In this paper, a general revised method for determining the incident radiant light flux is proposed using a chemical actinometer, Reinecke's salt. New easy procedures are given for preparing the reactive salt and measuring the photoreaction rate. Additionally, a mathematical model makes it possible to apply the technique to different reactor geometries and lighting systems with no limiting operating conditions for the incident flux. The method gives accurate results and constitutes a simple, general and reliable method for evaluating the maximum performance of photobioreactors. It can also be a tool for developing monodimensional models for simulation.

Vertical Multiphase Flow Correlations for High Production Rates and Large Tubulars

Summary Numerous correlations exist for predicting pressure drop in vertical multiphase flow. These correlations, however, were all developed and tested under limited operating conditions that do not match the high production rates and large tubulars normally found in the Middle East fields. This paper presents a comprehensive evaluation of existing correlations and modifications of some correlations to determine and recommend the best correlation or correlations for various field conditions. More than 400 field data sets covering tubing sizes from 2⅜ to 7 in., oil rates up to 23,200 B/D, water cuts up to 95%, and gas/oil ratio (GOR) up to 927 scf/STB were used in this study. Considering all data combined, the Beggs and Brill correlation provided the best pressure predictions. However, the Hagedorn and Brown correlation was better for water cuts above 80%, while the Hasan and Kabir model was better for total liquid rates above 20,000 B/D. The Aziz correlation was significantly improved when the Orkiszewski flow-pattern transition criteria were used.

Product Composition Regions of Single-Feed Reactive Distillation Columns: Mixtures Containing Inerts

We present a method to determine the product composition regions of single-feed reactive distillation columns. A new set of transformed compositions is introduced, and the resulting residue curve maps are studied for two reacting mixtures containing inerts. Due to the geometrical properties of the new composition variables, by using the residue curves it is possible to determine the feasibility of a desired separation from the extreme limiting operating conditions. As in conventional distillation, the generalization of the feasibility criterion gives a method to obtain the complete product composition region attainable in a single-feed reactive distillation column for a given quaternary feed. These concepts are applied to azeotrope-forming mixtures. An interesting behavior is observed for the highly nonideal reacting mixture isobutene-methanol-MTBE-butane. The minimum reflux ratio, for certain splits, corresponds to a finite number of trays in both column sections. The conditions for the occurrence of such a case are given

Improving allowed outage time and surveillance test interval requirements: a study of their interactions using probabilistic methods

Technical Specifications (TS) define the limits and conditions for operating nuclear plants safely. We selected the Limiting Conditions for Operations (LCO) and Surveillance Requirements (SR), both within TS, as the main items to be evaluated using probabilistic methods. In particular, we focused on the Allowed Outage Time (AOT) and Surveillance Test Interval (STI) requirements in LCO and SR, respectively. Already, significant operating and design experience has accumulated revealing several problems which require modifications in some TS rules. Developments in Probabilistic Safety Assessment (PSA) allow the evaluation of effects due to such modifications in AOT and STI from a risk point of view. Thus, some changes have already been adopted in some plants. However, the combined effect of several changes in AOT and STI, i.e. through their interactions, is not addressed. This paper presents a methodology which encompasses, along with the definition of AOT and STI interactions, the quantification of interactions in terms of risk using PSA methods, an approach for evaluating simultaneous AOT and STI modifications, and an assessment of strategies for giving flexibility to plant operation through simultaneous changes on AOT and STI using trade-off-based risk criteria.

Chlorate electrosynthesis current efficiency equations based on dimensionless groups

Various mathematical models for the electrosynthesis of chlorate are analysed taking into account that the current efficiency is mainly affected by the distribution of the two competing reactions forming chlorate, i.e. the anodic oxidation and the autoxidation of hypochlorite. It is shown that the current efficiency is determined by four dimensionless groups one of which is commonly negligible. Results are compared with a known current efficiency equation. Two new current efficiency equations, representing the limiting operating conditions of chlorate systems, are proposed for industrial application.