Legionella
is an opportunistic waterborne pathogen associated with Legionnaires' disease and Pontiac fever. Despite improved public awareness, the incidence of
Legionella
associated infections has ...been increasing. Aerosols generated from engineered potable water systems are a demonstrated cause of both nosocomial and community-acquired legionellosis. The ecology of
Legionella
in these systems is complex with multiple factors impacting their colonization and persistence. Flow dynamics has been identified as an important factor and stagnation in cooling towers is an accepted risk for increased
Legionella
growth; however, less is known about the impact of flow dynamic on
Legionella
in potable water systems. This is especially complex due to the inherent intermittent and variable usage observed within outlets of a potable water system. This systematic literature review examines the role of fluid dynamics and stagnation on the colonization and growth of
Legionella
in potable water systems. Twenty two of 24 identified studies show a positive association between stagnation zones and increased colonization of
Legionella
. These zones included dead legs, dead ends, storage tanks, and obstructed water flow (such as intermittent usage or flow restriction). Prolonged stagnation in building plumbing systems also deteriorates the quality of thermally or chemically treated potable water. This stimulates the colonization of
Legionella
established biofilms. Such biofilms are intrinsically resistant to disinfection procedures and accelerate the rate of decay of chemical disinfectants. Sub-lethal doses of disinfectants and the presence of protozoan hosts in stationary water promote generation of viable but non-culturable
Legionella
cells. This results in false negatives in surveillance methods that use culture methodology. In conclusion, elimination of temporal and permanent stagnation points can improve the quality of potable water, efficacy of disinfectants, and reduce the risk of legionellosis. Current guidelines and water safety plans recognize the risks associated with permanent stagnation point (dead ends and dead legs); however, there is a need for greater emphasis on controlling temporal stagnation arising from intermittent usage.
Identifying dead-legs and related corrosion issues continues to be a challenge in the process industry. Pipeline corrosion has been a factor in several recent incidents involving releases and fires. ...A review of incident reports and citations over the past ten years indicates that Process Hazard Analysis (PHA) revalidations have been noted for not addressing the hazards of a process including corrosion mechanisms and dead-legs. In order for the hazards to be addressed, they must first be accurately identified in a PHA and documented along with any recommended actions for preventive maintenance. This paper describes a methodology for identifying and addressing dead-legs and related corrosion issues in a PHA that can be used to update corporate PHA procedures to be more robust in preventing corrosion related incidents.
•Identifying dead-legs and related corrosion issues challenges the process industry.•Review of corrosion related incidents presented.•A structured methodology for PHA to identify corrosion hazards is presented.•A structured methodology for PHA to identify dead-leg hazards is presented.•Table of corrosion concerns for PHAs included.
Trailing oil is the tail section of contamination in oil pipelines. It is generated in batch transportation, for which one fluid, such as diesel oil follows another fluid, such as gasoline, and it ...has an effect on the quality of oil. This paper describes our analysis of the formation mechanism of trailing oil in pipelines and our study of the influence of dead-legs on the formation of trailing oil. We found that the oil replacement rate in a dead-leg is exponentially related to the flow speed, and the length of the dead-leg is exponentially related to the replacement time of the oil. To reduce the amount of mixed oil, the main flow speed should be kept at about 1.6 m/s, and the length of the dead-leg should be less than five times the diameter of the main pipe. In our work, the Reynolds time-averaged method is used to simulate turbulence. To obtain contamination-related experimental data, computational fluid dynamics (CFD) software is used to simulate different flow rates and bypass lengths. MATLAB software was used to perform multi-nonlinear regression for the oil substitution time, the length of the bypass, and the flow speed. We determined an equation for calculating the length of the trailing oil contamination produced by the dead-leg. A modified equation for calculating the length of the contamination was obtained by combining the existing equation for calculating the length of the contamination with new factors based on our work. The amounts of contamination predicted by the new equation is closer to the actual contamination amounts than predicted values from other methods suggested by previous scholars.
•The purpose of this work is to create a novel experimental database to investigate dead leg flows in pressurized water reactors (PWR).•The interaction between the cold, stagnant branch line water ...and the high-flow hot main line generates a secondary flow known as the “dead leg vortex.”•The dead leg vortex can interact with a stratified layer in the branch, leading to temperature variations on the solid surface.•Such a flow behavior is not well understood, and current screening methods do not adequately account for thermohydraulic forcing.•The aim is to develop more accurate tools by validating advanced prediction methods, with the methodology involving experiments to reproduce the phenomena and validate computational fluid dynamics (CFD) simulations.•A new pressurized experimental setup of a T-junction has been designed and constructed at Vattenfall R&D labs, with temperature, flow rates, and pressures controlled and stable.•Measurements include temperature fluctuations inside the solid pipe wall material in the isolated branch line and temperature measurements on the outer pipe surface using forward-looking infrared (FLIR) cameras.
The current screening criteria for thermal fatigue used in isolated branch lines at nuclear power plants are based on simplified approaches, which, on one hand, tend to be overly conservative, and on the other hand, appear to miss important physics. This is demonstrated by the fact that a number of affected pipes were missed by the current screening methods.
The purpose of the present work is to produce novel and accurate experimental data, which can be used for a benchmark of Computational Fluid Dynamics (CFD) in order to advance the prediction capabilities of thermal mixing and fatigue in dead leg T-junctions and improve the accuracy of the screening criteria. The ultimate value creation is increased safety and availability at nuclear power plants. This is of great interest and utmost importance to the nuclear reactor safety community.
A new pressurized experimental setup of a T-junction has been designed and constructed for the purpose. The experimental setup, with a temperature difference of 120 °C, more closely resembles plant conditions than in earlier experiments. The boundary conditions in terms of the flowrates and temperatures are controlled and stable within their measurement uncertainty – a prerequisite for CFD-grade experiments.
The novelty of the present work is measurements of the temperature fluctuations both in the near wall fluid mixing region and throughout the solid pipe wall material.
It has been shown that the interaction between the swirl penetration of hot water and the stratified layer in the cold dead leg has an intermittent character and that random clusters of bursts of hot water penetrate into the stratification and the stagnant cold water below. The average time scale of this process is of the order of hundreds of seconds.
The stratification between the hot and the cold water in the dead leg has also shown to have a low frequency oscillation of the order of 0.1 Hz, which wiggles in the circumferential direction. After hot water has penetrated downward and excited the instability, the temperature falls back to the initial one in a manner similar to a damped oscillation.
The vertical temperature profiles have shown to be self-similar, for moderate temperatures, when scaled with the temperature difference between the hot and the cold water and with the penetration length.
The penetration length increases with the velocity in the hot leg confirming earlier reported correlations.
A small (less than 0.1 % of the hot main flow) cold leakage flow into the T-junction is sufficient to suppress the swirl penetration and substantially reduces the penetration length. However, the temperature fluctuations have a larger amplitude and range across the whole mixing zone. The fluid temperature fluctuations are shifted towards a more high frequency content, however, the low frequency nature of the swirl penetration is still present.
The obtained novel dataset will be provided for a blind CFD benchmark exercise as part of an OECD/NEA project in 2023 and 2024.
•Compressible governing equations and improved compressible phase change model were used to calculate the thermal hydraulic phenomena in enclosed cavity.•Coolant natural convection result in ...significant thermal stratification in vertical direction.•Vapor accumulates in upper part of pipe and the corrosion in the region is more serious.•Increasing the initial pressure is an effective means to alleviate “dead-leg” phenomenon.
The “dead-leg” in nuclear power plants (NPP) are pipes, which are connected with the primary loop and isolated by two check valves setting at both ends. It is a typical enclosed cavity. As the temperature and pressure in primary side is higher, the coolant in tube is heated by the valve and becoming two phase state, resulting in the corrosion of valves. In present work, the CFD method was employed to study the single phase and two phase thermal hydraulic phenomena in enclosed cavity. The buoyancy was calculated by solving compressible governing equations, and the mixture multiphase model and improved compressible phase change model were used to calculate the boiling process. The models are validated against with the experiment data and the simulation results show great agreement. The coolant natural convection occurs and the significant thermal stratification exists in the vertical direction. The void fraction increases at first and tends to a stable value finally. The vapor accumulates in the upper part of the pipe and the corrosion in the region is more serious. The trend of pressure increasing is consistent with void fraction and self-pressurization phenomenon is caused by the compression of vapor. Increasing the initial pressure could reduce boiling process greatly and it is an effective means to alleviate “dead-leg” phenomenon. This work is meaningful for the deep understanding of thermal hydraulic phenomena in enclosed cavity and provides valuable guidance for engineering.
This paper discusses the root causes and operational mitigations of corrosion anomalies reported for the wet gas system of a floating production storage and offloading (FPSO) vessel. This system is ...classified in general into the flowing and dead leg sections and operated below 62 °C, where a protective FeCO3 film will not form. Corrosion which is temperature dependent is general. The root cause of the anomalies in the flowing wet gas system was found to be due to CaN and solid deposition in the LP separator, increasing levels of corrosive condensate, which drips down the pipe walls, resulting in wall losses of less than 50%. Evacuation of deposits in the LP separator and subsequent control of its formation rate would mitigate anomalies attributed to increasing levels of corrosive condensates. The corrosion pattern at elbows, modified by turbulent flow of emergent fluid streams through the complex-geometry elbows, is mitigated by velocity control. The root cause of the inspection anomalies at dead leg locations was due to the lifting of the PSV and/or infrequent flow through a bypass valve and directly to flare. Infrequent transportation of condensed, corrosive water thereby resulted in dead leg corrosion. Proposed mitigation entails replacement of defective PSVs and modifying flow frequencies. This paper summarizes all the identified root causes and proposed mitigations and provides a greater understanding of the internal degradation mechanisms operating in wet gas systems in general.
Introduction
The suitability of vapor-phase hydrogen peroxide for the decontamination of different-sized narrow cavities and complex geometrical structures were investigated in this paper.
Methods
A ...cavity test block was used, and cavities made from different materials were tested with variable entrance heights and cavity depths. At the end of each cavity, biological indicators were exposed as a microbiological challenge for vapor-phase hydrogen peroxide penetration. Within this study, the test block with the biological indicators was subjected to different decontamination cycles in a production isolator. Inoculation level, cycle length, hydrogen peroxide, and water concentration were varied.
Results
The ratio of cavity entrance height to depth was found to be critical for decontamination success by biological indicators exposed inside the cavities. The higher the ratio, the more spores could be inactivated. Inactivation is also effected by exposure time and hydrogen peroxide concentration.
Conclusion
The results indicate that the entrance height of the cavities should not be smaller than 6 mm and the cavity depth should not exceed 30 mm. If smaller cavities cannot be avoided, high peroxide concentration (800 ppm) and prolonged cycle times were shown to significantly enhance the penetration into dead-ended cavities under diffusive conditions.
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