Blowdown Pipe Research Articles

Numerical investigation of the depressurization performance in blowdown pipe using method of characteristics

The coolant leaks quickly after the break of the high-pressure pipeline system. The rapid pressure decrease and fluctuation will further affect the integrity of the pressure boundary and the effective heat removal of the reactor core in the early phase of blowdown. Generally, numerical simulation plays a vital role in the safety analysis of nuclear power plants (NPPs) under severe accidents like loss of coolant accident (LOCA) conditions. In this paper, one-dimensional two-phase hydrodynamic equations and essential closure models are established to describe the internal pipe flow firstly. Then, method of characteristics (MOC) realized by a self-programming code is introduced, which is different from the numerical scheme of system analysis code RELAP5 in the simplification and discretization. Finally, combined with the Edwards blowdown experiments, the two numerical solution methods are compared on modeling, sensitivity analysis, and calculation results under various working conditions. The influence of back pressure variation on the solution of thermal–hydraulic parameters has been considered during the blowdown process. The solution result of RELAP5 is highly related to the set discharge coefficients, which are usually given according to experience. However, the solution result of MOC can be enveloped by that of RELAP5 with different discharge coefficients. In addition, the results solved by MOC are closer and more conservative to the experimental values during the early phase of blowdown with low void fraction fluid. It lays a foundation for the further calculation and safety analysis of the complex loop system under LCOA conditions.

Cracking Failure Analysis of Pipe to Flange Weld Joint

Cracking failures occurred frequently in blowdown pipe to flange weld joint, and the root crack appeared on the blowdown pipe side of the weld joint and extend into the base metal of pipe. To explore the reason why cracking failures occurred on the blowdown pipe side of the weld joint, the weld joint was analyzed via macroscopic and metallographic structure observation, and blowdown pipe was tested by physical and chemical property testing, and corrosion scale was analyzed by SEM, EDS, and XRD. The dent and thinner wall thickness of pipe are found in the internal pipe to flange weld joint. Widmanstatten metallographic structure is found in heat affected zone (HAZ), fusion zone (FZ) and weld metal zone (WZ) of weld joint. Stress concentration form in the weld joint at blowdown pipe side during welding due to the dent and different wall thickness. Under the action of gas vortex in the dent of weld joint, corrosive droplets aggravate the localized corrosion of the weld joint. Stress corrosion cracking (SCC) were initiated in FZ and WZ from the surface of weld joint and propagated into the base metal of pipe under the combined actions of stress resulting from weld process and during operation, and corrosion.

Richardson number criteria for direct-contact-condensation-induced thermal stratification using visualization

This study aims to suggest a dimensionless number to determine the formation and disappearance of thermal stratification induced by direct contact condensation in a 1/20 scaled-down suppression pool of the Fukushima Daiichi nuclear power plant. The modified Richardson number, which represents the ratio of buoyancy force to inertia of steam, was employed to explain the thermal stratification in the suppression pool. Steam condensation experiments were performed at both sub-atmospheric and atmospheric pressures. A blow-down pipe with an inner diameter of 12.7 mm was used to inject and condense steam in the suppression pool. The effects of steam mass on the thermal stratification behaviors were examined by varying the steam mass flow rate (mass flux) from 0.50 kg/hr (1.10 kg/m2·s) to 2.50 kg/hr (5.48 kg/m2·s). The steam condensation was visualized using a high-speed camera. In the results, thermal stratification was successfully reproduced in this study, and it was observed that the behaviors of thermal stratification (vertical temperature profile) was affected by the steam mass flow rate and the subcooling temperature. The Richardson number was evaluated by measuring the size and frequency of steam bubbles. The time evolution of the Richardson number was analyzed with vertical temperature profiles of water in the suppression pool. The critical Richardson number, which determines the formation and disappearance of thermal stratification, was suggested to be of the order of 1.

Metallurgical aspects of a blowdown pipe failure in a petrochemical plant

This study investigates the premature failure of AISI 304 stainless steel pipe in a blowdown unit used between boiler and blowdown vessel in a petrochemical plant. The first leak was reported in the elbow after a few weeks of service. The leak was repaired. The repaired unit showed a crack in the pipe in the vicinity of the elbow only after <6 months of service at 150 °C. Failed pipe and elbow were taken out of the system, in order to conduct a failure analysis. Visual inspection, optical and electron microscopy, electron dispersive spectroscopy (EDS) analysis, and hardness measurements were employed to identify failure cause. Results showed that intergranular stress assisted cracking facilitated by grain drop, due to oxidation at grain boundaries, were the main failure mechanisms. Possible mitigation strategies are suggested.

Frequency analysis of chugging condensation in pressure suppression pool system with pattern recognition

Abstract Direct contact condensation (DCC) phenomena in boiling water reactor (BWR) pressure suppression pool systems need to be understood to properly assess the performance of the pool as a heat sink and as a safety critical structure. Condensation oscillations in the form of chugging are challenging to predict by computational fluid dynamics (CFD) methods but safety relevant because of associated high dynamic loads on in-pool structures and the pool itself. Recently, new measurement methods for CFD validation purposes have become available. One of these techniques is visual observation using the high-speed cameras and suitable data processing method. Pattern recognition is a well suited technique for the determination of large oscillating bubble dynamics in a pressure suppression pool. In this work, the formation and collapse of the steam bubbles in chugging condensation mode are evaluated by using the pattern recognition algorithm. The pattern recognition algorithm is based on video material recorded during the direct contact condensation experiment DCC-05 of the PPOOLEX test facility. The formation speed, the shape and size of the steam bubbles and the acceleration of collapsing bubbles are estimated with the algorithm. Fast Fourier transform (FFT) is used for frequency analysis of the pattern recognized data. The frequencies found are compared to the frequency data of the pressure transducers collected during the experiment and to the previous results of the NEPTUNE_CFD simulations of the same experiment. The frequency analysis shows that the chugging frequencies of the steam bubbles range from 1 to 3 Hz, as predicted. Also the natural frequencies of the bubbles are visible around 53 Hz. Another frequency spike was observed close to the 125 Hz. This frequency is close to the mechanical resonance frequencies of the suppression pool and the blowdown pipe. Because of neither the pressure suppression pool nor the blowdown pipe are visible to the pattern recognition, the spike of the higher frequencies is most likely from the interfacial area of the bubble which resonates with the suppression pool system, affecting rapid condensation at a certain point.

Modelling of pool stratification and mixing induced by steam injection through blowdown pipes

Containment overpressure is prevented in a Boiling Water Reactor (BWR) by condensing steam into the pressure suppression pool. Steam condensation is a source of heat and momentum. Competition between these sources results in thermal stratification or mixing of the pool. The interplay between the sources is determined by the condensation regime, steam mass flow rate and pool dimensions. Thermal stratification is a safety issue since it limits the condensing capacity of the pool and leads to higher containment pressures in comparison to a completely mixed pool with the same average temperature. The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models were previously developed for predicting the macroscopic effect of steam injection and direct contact condensation phenomena on the development of stratification and mixing in the pool. The models provide the effective heat and momentum sources, depending on the condensation regimes. In this work we present further development of the EHS/EMS models and their implementation in the GOTHIC code for the analysis of steam injection into containment drywell and venting into the wetwell through the blowdown pipes. Based on the PPOOLEX experiments performed in Lappeenranta University of Technology (LUT), correlations are derived to estimate the steam condensation regime and effective heat and momentum sources as functions of the pool and steam injection conditions. The focus is on the low steam mass flux regimes with complete condensation inside the blowdown pipe or chugging. Validation of the developed methods was carried out against the PPOOLEX MIX-04 and MIX-06 tests, which showed a very good agreement between experimental and simulation data on the pool temperature distribution and containment pressure.

Thermal stratification and mixing in a suppression pool induced by direct steam injection

An experimental and numerical investigation of thermal stratification and mixing in a suppression pool is presented. Steam injected into a drywell flows through a blowdown pipe and then down to the pressure suppression pool where direct contact condensation occurs. The steam venting and condensation is a source of heat and momentum. A complex interplay between the two leads either to thermal stratification or mixing of the pool. The experiments are conducted in a scaled down PPOOLEX facility at Lappeenranta University of Technology (LUT). The corresponding numerical simulations are performed using GOTHIC with the Effective Heat Source (EHS) and Effective Momentum Source (EMS) models. The EHS/EMS models, that have been previously proposed, predict the development of thermal stratification and mixing during a steam injection into a large pool of water. The experiments exhibit the development of thermal stratification in the pool at relatively low mass flow rates and then pool mixing when the mass flow rates are increased but later thermal stratification can re-develop even at the same relatively high mass flow rates, which is due to the increasing pool temperature that shifts the condensation to a different regime. The numerical simulations quantitatively capture this complex transient pool behavior and are in excellent agreement with the transient averaged pool temperature and water level in the pool.

CFD simulation of steam condensation in a subcooled water pool

In present work interfacial dynamics of steam injection to a sub cooled stagnant water pool using a vertical blowdown pipe has been simulated using ANSYS FLUENT 14. Multiphase Volume of Fluid (VOF) model is used to track the interface and standard k-є model was adopted for modeling the turbulence. Effects of pool temperature and steam injection velocity are explored to understand the hydrodynamic and thermal characteristics of the pool. Also the effect of both these parameters on the rate of condensation is investigated. It has been observed that increase in the steam injection velocity will decreases the interfacial temperature which in turn enhances the rate of condensation.

Fabrication and evaluation of an attapulgite membrane as the filter for recycling blowdown water from industrial boilers

Continuous blowdown water (CBW) from industrial boilers is of great quantity and energy, and therefore worth recycling. According to the data from a long period of monitoring, carbonate and sulfate were documented as the main contaminants in CBW. Herein, an attapulgite (ATP) membrane was prepared on a macroporous Al2O3 support through solid state sintering. The prepared membrane (attapulgite membrane (ATM)) was characterized by mercury porosimetry, scanning electron microscope, X-ray diffraction, and permselectivity analysis in dead-end mode. The optimal sintering temperature was 800 °C based on the morphology of the sintered active layer and its adhesive strength with support. The active layer exhibited a pore size distribution concentrated on 12.7nm with a thickness of about 80μm. In addition, crystal structures of ATP were retained in powder form after sintering. ATM obtained a pure water permeability of 1,411.87Lm–2h–1MPa–1 with the effective retention of carbonate and sulfate through electrical-related interactions. Several ATM filters were also developed and mounted on the boiler blowdown pipe, which had a high rejection rate of alkalinity and dissolved solids in CBW. The volume of wastewater emitted from the low-pressure boiler was reduced, since the filtrate could be recycled as part of the make-up water.

Generalization of experimental data on amplitude and frequency of oscillations induced by steam injection into a subcooled pool

Steam venting and condensation into a subcooled pool of water through a blowdown pipe can undergo a phenomenon called chugging, which is an oscillation of the steam–water interface inside the blowdown pipe. The momentum that is generated by the oscillations is directly proportional to the oscillations’ amplitude and frequency, according to the synthetic jet theory. Higher momentum can enhance pool mixing and positively affect the pool's pressure suppression capacity by reducing thermal stratification. In this paper, we present a generalization of available experimental data on the amplitude and frequency of oscillations during chugging. We use experimental data obtained in different facilities at different scales to suggest a scaling approach for non-dimensional amplitude and frequency of the oscillations. We demonstrate that the Froude number Fr (which relates the inertial forces to gravitational forces) can be used as a scaling criterion in this case. The amplitude has maximum at Fr≈2.8. There is also a strong dependence of the amplitude on temperature; the lower the bulk temperature is the higher the scaled amplitude. A known analytical theory can only capture the decreasing trend in amplitude for Fr>2.8 and fails to capture the increasing trend and the temperature dependence. Similarly, there is a minimum of the non-dimensional frequency at Fr≈6. A strong dependence on temperature is also observed for Fr>6; the lower the bulk temperature is the higher the scaled frequency. The known analytical theory is able to capture qualitatively the general trend in frequency but not the dependence on temperature.

Numerical investigation of the process and flow erosion of flushing oil tank with nitrogen

Abstract A computational fluid dynamic (CFD) model coupling with volume of fluid (VOF) method and discrete phase model (DPM) has been used to predict the flow erosion rate in an oil tank and blowdown pipe during the process of gas flushing. An initialization method by injecting particles from a tank wall is developed to construct the initialization distribution of residue particles. The flow field distribution of oil–gas–water flow and the erosion rate on the wall surface during emptying process can be captured under different operating conditions with different fluid parameters. The applicability of the approach is verified by comparing the simulated results with the measurements. The effects of fluid parameters such as gas inlet rate, oil density and viscosity, as well as the operating parameters such as residual oil level and residue particle diameter, are evaluated. In general, the required flushing time and erosion rate are all sensitive to fluid parameter changes and operating condition changes. It is found that the flow erosion is related to the emptying process of oil, which has a sharp rise in the end of the process. High inlet speed, large oil density, small oil viscosity, high oil level and large particle diameter can result in severe erosion.

The roles of incident investigations in learning processes at a Scandinavian refinery

This paper examines how investigations played a part in ongoing learning processes at a Scandinavian refinery. Data regarding the roles of investigations was collected during six months of observations at the refinery and interviews with 70 employees. We focus on two investigated incidents, the rupture of a blowdown pipe from a steam drum and an incident that could have led to the injury of mooring operators during the mooring of an oil tanker. We found that investigations were used as part of efforts to learn from incidents at the refinery, but by far from all the interviewed employees. Investigations were described as having several roles. They generated new knowledge about the causes of incidents and confirmed existing knowledge regarding the dangers of corrosion for plant integrity and the importance of following mooring procedures. However, they also suggested measures that were not considered economically feasible or well suited for the refinery. In addition, investigations contributed to learning regarding issues that were not addressed directly in suggested measures, because findings were also applicable to other safety issues at the refinery. In light of these roles, we suggest that investigation groups should ensure closer cooperation with investigated units when developing measures, and work to ensure that investigation reports are used systematically as a learning tool among employees. This may increase the integration of investigation findings in important safety improvement initiatives and thereby the potential for learning from incidents.

Numerical modelling of low-Reynolds number direct contact condensation in a suppression pool test facility

In the safety pressure suppression pool systems of Boiling Water Reactors (BWRs), the condensation rate has to be maintained high enough in order to fulfill their safety function. A major part of this condensation occurs as direct contact condensation (DCC), which governs different modes varying from vigorous chugging of collapsing bubbles to mild condensation on almost flat steam–water interface. This paper discusses the Computational Fluid Dynamics (CFD) simulations of the latter, low-Reynolds number weak condensation regime. The numerical simulations were performed with two CFD codes, NEPTUNE_CFD and OpenFOAM, in which the DCC phenomenon was modelled by using the Eulerian two-fluid approach of interpenetrating continua without interfacial tracking. The interfacial heat transfer between steam and water was modelled by using the DCC models based on the surface renewal and the surface divergence theories. Flow turbulence was solved by employing the standard k–∊ turbulence model. The CFD results of this study were validated against the test results of the POOLEX facility of Lappeenranta University of Technology. In the reference test STB-31, the condensation phenomena were limited to only occur on a stable steam–water interface by very low steam mass flux applied and thermal insulation of the blowdown pipe. The simulation results demonstrated that the surface divergence model predicted the condensation phenomena quite accurately both qualitatively and quantitatively while the surface renewal model overestimated it strongly.

Validation of Effective Models for Simulation of Thermal Stratification and Mixing Induced by Steam Injection into a Large Pool of Water

The Effective Heat Source (EHS) and Effective Momentum Source (EMS) models have been proposed to predict the development of thermal stratification and mixing during a steam injection into a large pool of water. These effective models are implemented in GOTHIC software and validated against the POOLEX STB-20 and STB-21 tests and the PPOOLEX MIX-01 test. First, the EHS model is validated against STB-20 test which shows the development of thermal stratification. Different numerical schemes and grid resolutions have been tested. A48×114grid with second order scheme is sufficient to capture the vertical temperature distribution in the pool. Next, the EHS and EMS models are validated against STB-21 test. Effective momentum is estimated based on the water level oscillations in the blowdown pipe. An effective momentum selected within the experimental measurement uncertainty can reproduce the mixing details. Finally, the EHS-EMS models are validated against MIX-01 test which has improved space and time resolution of temperature measurements inside the blowdown pipe. Excellent agreement in averaged pool temperature and water level in the pool between the experiment and simulation has been achieved. The development of thermal stratification in the pool is also well captured in the simulation as well as the thermal behavior of the pool during the mixing phase.

Preparation of a nano-scale ceramic membrane and its application in the medium-pressure boiler with phosphate treatment

An attapulgite nano-scale (NS) membrane was prepared on the clay-fly ash ceramic substrate. Based on this NS membrane, a membrane filter was developed for the problem of energy waste, thermal and phosphorus pollution resulting from the high blowdown rate of medium-pressure boilers. Mounting this membrane filter on the boiler blowdown pipe, it could separate both particles and ions, which are larger or smaller than the membrane pore size. The quality of treated water met the requirements of feed water standards in medium-pressure boilers. Recycling treated water to the feed water system could reduce the boiler blowdown rate to less than 1%.

A three-dimensional transient two-phase flow analysis with a density pertubation finite volume method

In this paper, we present the analysis of a full three dimensional transient two-phase flow with strong disequilibria. This analysis uses a finite volume scheme with a recently developed density perturbation method. The application test case is a three dimensional extension of a one-dimensional experiment on blowdown pipe. The results are stable with mesh refinements and show realistic three-dimensional effects.

Analytical Studies of Blowdown Thrust Force and Elastic-Plastic Behavior of Pipe at Pipe Rupture Accident

When high-temperature and high-pressure water is ejected from pipe, a blowdown thrust force will act on the pipe and it will cause pipe movement. The analytical methods of blowdown thrust force and pipe movement are presented in the paper. The equation of momentum conservation is used to calculate the blowdown thrust force, together with the results of the thermal-hydraulic analysis code RELAP4/MOD6. The finite element method is used to calculate the dynamic movement of pipe. The equations for calculating the maximum strain at the outer surface of pipe are derived by assuming the static equilibrium condition. The analytical results are compared with experimental ones to verify the analytical methods presented here.

Fluid-structure analysis for the HDR blowdown and snapback experiments with FLUX

FLUX is a special purpose code to analyse three-dimensional fluid-structure interactions during blowdown of a pressurized water reactor. Such a blowdown has been simulated in the HDR experiments. For the first series of blowdown experiments and for snapback experiments in the same facility the results of precomputations are reported and compared with the experimental results. Refinements are desirable with respect to two-phase-damping of pressure waves in the blowdown pipe and vessel wall flexibility. The general quantitative agreement between measurement and computation is satisfactory.

Investigation of Blowdown Problems with the Computer Code DRIX-2D

A model for two-phase (water-vapor) flow in two-dimensional Cartesian or cylindrical coordinates is described that is implemented in the code DRIX-2D. The model includes slip between the phases and accounts for thermodynamic nonequilibrium. The code was designed as a “best-estimate” model for simulation of loss-of-coolant accidents (LOCAs) in light water reactor safety analysis. In this paper results of DRIX-2D applications are reported that can be used to assess the validity of simplified LOCA models.The main results are that both Cartesian and axisymmetric coordinates in two dimensions show considerable disadvantages as far as the pressure history in the downcomer is concerned. Yet, both models yield acceptable results concerning the gross blowdown behavior. Due to a 90-deg change in flow direction, considerable radial profiles in mass flow rate, velocity, and void fraction establish in the blowdown pipe. Nevertheless, a minor difference in the averaged mass flow rate exists only between a one- and two-dimensionally modeled blowdown pipe.A nonequilibrium state establishes at the pipe inlet in the case of subcooled vessel conditions and is maintained up to the orifice at least for pipe lengths <5m. However, the increase in mass flow rate caused by this nonequilibrium state is generally small enough for typical reactor conditions, so that an equilibrium assumption in the blowdown pipe should be appropriate for LOCA calculations.