On the non-monotonic wind influence on flammable gas cloud from CFD simulations for hazardous area classification

https://doi.org/10.1016/j.jlp.2020.104278Get rights and content

Highlights

  • CFD simulations predict compressible gas emission and dispersion.

  • Wind velocity has an important effect on hazardous distance of flammables.

  • The behavior of the gas cloud extent and volume depends on the gas concentration.

  • Wind speed and direction affect the zone type of hazardous area classification.

Abstract

Hazardous areas are defined as a result of a variety of variables as storage temperature, pressure, leak orifice size, physical properties of flammable substance, and wind characteristics. The potential formation of an explosive atmosphere must be accurately assessed to ensure process safety. Therefore, computational fluid dynamics (CFD) arises as an important tool for accurate predictions as recommended by the international standard IEC 60079-10-1 (2015). This study aims to analyze the influence of wind velocity magnitude and direction on the hazardous area classification. The authors evaluated the extent and volume for methane, propane, and hydrogen leakages from a CFD model. For each flammable gas, the wind velocity magnitude and direction were regularly varied. The outcomes show that the behavior of the plume size as the wind varies mainly depends on the gas concentration. Counter-flow wind directions lead to zero relative velocity closer to the release point, which concentrates the gas, and wind in the release direction promotes a higher dilution of the gas cloud increasing the hazardous extent while decreases the volume. As a consequence, the wind also influences the zone type, which was accurately predicted from CFD simulations and significant differences were found when compared to the standard analyses. These differences are, to some extent, related to the consideration of wind velocity effects on the gas jet release.

Introduction

Hazardous area classification is a methodology that analyzes the potential risk in a flammable leak and defines the extent to which safety precautions must be taken. This technique is assessed by the international standard IEC 60079-10-1 (2015) that suggests several guidelines to reduce the risk of explosion. A hazardous area is described as an area where an explosive atmosphere is or is likely to be present, and to evaluate the risk some parameters must be considered. The primary concern is the grade of release that reports the frequency of emission. The physical properties of the substance provide information about flammability level, while the process variables and orifice size determine the discharge rate. In addition, the characteristics of the location affect the dispersion performance, and the nature of emission leads to a one-phase or two-phase flow release.

Regarding gas emission, empirical and numerical models validated against experiments are presented in the literature (Alves et al., 2019). For instance, computational fluid dynamics (CFD) tools are widely used to predict gas dispersion in different scenarios, such as accidental releases (Silgado-Correa et al., 2020; Li et al., 2020a; Li et al., 2020b; Li et al., 2019) and fugitive emissions (Souza et al., 2018, 2019). Accidental release scenarios are relevant studies in the field of safety management but differ from continuous (grade of release) fugitive emissions that occur during normal operation from small openings such as in pipe fittings. Hazardous area classification encompasses fugitive emissions analysis and is mainly considered at the project level. This methodology analyses the extent and volume of flammable gas clouds as a result of the interaction between storage temperature and pressure, orifice diameter, and molecular weight. The concentration of the flammable gas after the release is also directly affected by the local ventilation effectiveness, which depends on wind velocity, distribution of the ventilation, obstacles and its geometry, and leakage location (Ivings et al., 2010; Webber et al., 2011).

Moreover, the wind influence in the dispersion of flammable materials changes according to the release kinetic energy. When momentum forces predominate, the jet is characterized by a well-defined shape with entrainment of large quantities of air, which is favored by a co-flow airspeed. If buoyancy forces predominate, the leak generates a plume. In both scenarios, atmospheric turbulence represents a significant factor after the momentum or buoyancy decay and it is important for further dispersion (Lees, 2005). Therefore, the numerical analysis of wind effects contributes meaningfully to area classification studies as it introduces this parameter for the calculations of the hazardous extent.

IEC 60079-10-1 (2015) states that for areas where there is natural ventilation, the wind velocity must consist of a value that is exceeded 95% of the time. Further information about wind velocity magnitude and direction at a particular location can be obtained from a wind rose, and for safety reasons the calculations must consider the worst-case scenario. The ventilation velocity and jet release rate indicate the degree of dilution of the environment, which is combined with the availability of ventilation and grade of release to classify the area according to the risk level (non-hazardous, Zone 0, Zone 1, Zone 2). However, the hazardous classification following IEC 60079-10-1 (2015) does not take into consideration the effect of wind direction. In that way, the international standard recommends computational modelling as an alternative approach to assess ventilation and to estimate the hazardous zone extent that results from the interaction of different variables.

The present study aims to evaluate the influence of wind velocity magnitude and direction on gas cloud extent and zone type classification as a result of numerical experiments using computational fluid dynamics. The scope of the analysis covers high momentum free jet emission in open and unobstructed areas for continuous releases. Further implications on hazardous area classification are outlined to enhance the comprehension of wind effects. This work was organized with a background of the release scenario and the problem statement in section 1. Section 2 addresses the hazardous area classification methodology. In section 3, an overview of the mathematical modelling referring to the gas release phenomenon and the CFD model for the jet dispersion is presented. Section 4 includes information regarding numerical simulation, introducing details of the geometry and mesh definition, boundary conditions, and the CFD turbulence model. Simulations of the gas leakage and analyses of the wind influence considering both CFD results and the international standard IEC 60079-10-1 (2015) are addressed in section 5. In section 6, final considerations are presented and important contributions are highlighted.

Section snippets

Hazardous area classification overview

The main objective of the hazardous area classification technique is to reduce the probability of accidental ignition in explosive atmospheres. It does not take into consideration catastrophic failures; hence, it analyzes leakages that occur within the design parameters of a process plant. Good practices are outlined by the international standard IEC 60079-10-1 (2015), which classifies hazardous areas as Zone 0, Zone 1, and Zone 2 for a given degree of dilution and grade of release. Zone 0

Mathematical model

The jet release was modeled as a function of the storage conditions considering an isentropic expansion. Moreover, the CFD model for jet dispersion forming a flammable gas cloud is based on the conservation laws of mass, species, momentum and energy as follows.

Numerical modelling

A set of cases of high momentum gas dispersion in an open environment without obstructions was modeled using ANSYS CFX 16.1 software. The wind velocity magnitude and direction (uw) were varied to analyze the influence of those variables in the plume extent.

Results

An important outcome of this study is the concentration profile of flammable gas along the release axis for different wind velocity magnitudes and directions. Fig. 5 illustrates the behavior of methane molar fraction along the central distance considering the wind in the jet direction. It shows that for all values of molar fraction, a higher wind velocity magnitude implies a greater hazardous extent. This increasing pattern is also observed in Fig. 6 as the wind velocity in the jet direction

Final considerations

This work studied the effect of wind velocity and direction on the hazardous extent and volume considering three different gas leakage conditions for hydrogen, methane, and propane. It contributed to enhance the comprehension of different wind scenarios using computational fluid dynamics simulations. The major findings from this work can be summarized as follows:

  • The molar fraction profiles for wind velocity in the opposite direction of the release demonstrate a particular behavior. It decays

CRediT authorship contribution statement

Paloma L. Barros: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing - original draft. Aurélio M. Luiz: Conceptualization, Investigation, Funding acquisition, Methodology, Resources, Supervision. Claudemi A. Nascimento: Conceptualization, Data curation, Investigation, Methodology, Software, Visualization. Antônio T.P. Neto: Conceptualization, Data curation, Investigation, Methodology, Software, Visualization. José J.N.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are grateful to CAPES and PETROBRAS for financial and technical support.

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