Unignited High-Pressure Methane Jet Impinging a Pipe Rack: Practical Tools for Risk Assessment

https://doi.org/10.1016/j.jlp.2020.104378Get rights and content

Highlights

  • A high-pressure methane jet outflowing from a loss of containment is simulated.

  • The influence of a pipe rack on the LFL extension is investigated.

  • A simple procedure that allow to estimate the hazardous distance is proposed.

Abstract

Although the diffusion of its storage and transport under liquefied conditions, nowadays it is common to have methane in gaseous form in several industrial applications. This leads to safety implications to be considered: hazards are linked to both the high-pressure at which the gas is kept and to its flammability. Scenarios where flammable jets impact an obstacle are of paramount importance because of their possible occurrence. Following a numerical approach, literature shows up that their assessment can be reliably performed by means of only Computational Fluid Dynamics tools. However, despite the improvements of computing power, Computational Fluid Dynamics costs still limit its use in daily risk analysts’ activities. Therefore, considering an accidental jet-obstacle scenario of industrial interest, the present work investigates how a pipe rack can influence the development of a high-pressure methane jet. Based on a Computational Fluid Dynamics analysis, main achievements of this work are a simple criterion able to identify the situations where the pipe rack does not influence the high-pressure methane jet behavior, therefore allowing to identify the scenarios where simpler models can be used (i.e., analytical correlations known for the free jet situation), and, if present, a simple analytical relationship that roughly predicts the influence of the pipe rack without the need of performing complex Computational Fluid Dynamics simulations.

Introduction

Although the diffusion of Natural Gas (NG, mainly constituted by methane) storage and transport under liquefied conditions, nowadays it is still common to have high pressure facilities using methane (or NG) in gaseous form in several industrial applications (Deng et al., 2018; Khraisheh et al., 2020).

This leads to several safety implications, related to both the High-Pressure (HP), at which the gas is stored, and its flammability. In particular, in case of late ignition, the release scenario can evolve into a Flash Fire (FF), whose hazardous distance is usually quantified as the Lower Flammability Limit (LFL) distance of the unignited cloud. Therefore, to predict how severe may be the consequences of a FF, the extension of the unignited flammable cloud needs to be estimated (Souza et al., 2019b). Commonly, in the risk analysis framework, for the case of a leakage of a flammable substance such evaluation traduces in the prediction of the Maximum axially-oriented Extent (ME) of the cloud (Tchouvelev et al., 2007; Houf et al., 2010; Pontiggia et al., 2014; Colombini et al., 2020a).

Broadly speaking, two different situations of HP gaseous release can be identified: the free jet (intended as a release occurring in an unconfined environment (Dey et al., 2017) and the impinging jet (intended as a release interacting with structures or facilities in the surroundings (i.e., obstacles) (Schefer et al., 2009).

For the latter, which can be expected to be the most probable accidental scenario in an industrial environment (Xu et al., 2011), it has been shown that, in some cases, an increase of the hazardous area (i.e., the ME of the jet cloud) can occur (Kotchourko et al., 2014; Hall et al., 2017).

Despite this, in the past a large amount of the research in the process safety framework has been focused on the free jet scenario (e.g., Lockwood and Moneib (1980), Chen and Rodi (1980), Birch et al. (1984), Schefer and Dibble (1986); Birch et al. (1987), Becker et al. (1988), Pitts (1991), TNO (1997), Witlox and Holt (1999) and Shell (2004)), while, for what concerns impinging jets, only recently some works have been performed with the aim of understanding how an obstacle can influence the jet behavior. In particular, Kim et al. (2013) investigated experimentally the self-ignition near an obstacle of HP hydrogen jets. Pontiggia et al. (2014) compared the performances of two different modeling approaches (namely, integral and Computational Fluid Dynamics (CFD) models) in predicting ME values for both impinged and non-impinged HP methane jets. Tolias and Venetsanos (2015), with the aim of giving best practices guidelines for hydrogen impinging jet simulations, investigated how to model an accidental impinging hydrogen jet through CFD models. Bénard et al. (2016) investigated the effect of a near surface on the ME of high-pressure horizontal and vertical jets of both hydrogen and methane using CFD models. Gerbec et al. (2017) performed a CFD analysis of the release, and the subsequent environmental dispersion, of a vertical impinging propane jet out of an over-filled car tanker. Hall et al. (2017) investigated, both experimentally and numerically through CFD models, the ME of HP hydrogen jets impinging near surfaces. Uggenti et al. (2017) discussed the state-of-the-art of CFD models used for offshore installations risk assessments where impinging jets are usually involved: the aim was to compare the CFD benchmark case with the industrial standard. Hu et al. (2018) presented an improved version of the two-layer partitioning model based on the Abel-Noble equation of state (Jhonston, 2005), which is able to predict more accurately the gas concentrations of HP under expanded hydrogen jets. The proposed model was applied to the flow of a horizontal HP hydrogen jet impacting a vertical obstacle. Colombini and Busini (2019a and 2019b) investigated, using a CFD model, the accidental scenarios of an unignited HP methane jet impacting a horizontal and a vertical cylindrical tank to quantify the influence of some geometric parameters on the ME of the impinging jet. Colombini et al. (2020a and 2020b) investigated the effect of the ground (considered as a lateral impinging obstacle) on unignited methane high-pressure jets.

We can note that almost all the aforementioned works used CFD-based models. As discussed by Batt et al. (2016), Souza et al. (2019a), and Tolias et al. (2019), this is because only CFD-based numerical approaches are able to account properly for complex geometry. Therefore, although the high computational costs and the user knowledge demanded (Zuliani et al., 2016), CFD-based models are the most suitable numerical tools to model HP jet-obstacle scenarios and therefore they were used extensively in this work to provide useful insights in the HP methane jets-obstacle interaction.

In particular, going beyond the study of the simple interaction between a HP jet and a single well-shaped obstacle, this work investigated a credible, whilst probably rare, scenario, focusing on how a typical industrial structure (i.e., a pipe rack) can influence the development of a HP methane jet in terms of ME of the flammable cloud. Through an extensive CFD-based analysis performed with Ansys Fluent 19.1 (Ansys Fluent User Guide, 2017), the main geometrical parameters on which the impinging jet behavior depends were identified allowing for developing a simple analytical relationship that roughly predicts the influence of the pipe rack without the need of performing complex CFD simulations. Moreover, a simple criterion able to identify the situations where the pipe rack does not influence the HP methane jet behavior was developed, therefore allowing to identify the scenarios where simpler models (e.g., analytical correlations able to estimate the ME of a free jet) can be used. Both these outcomes can be quite valuable for practitioners daily involved in industrial safety assessments.

Section snippets

Materials and methods

The commercial platform Ansys Workbench v. 19.1 (Ansys Workbench User Guide, 2017) was used to model, through a CFD approach, an unignited HP methane jet impinging a pipe rack.

The computational domain was created with Ansys DesignModeler, the grid was built using Ansys Meshing and the computations were performed with the numerical solver Ansys Fluent (Ansys Workbench User Guide, 2017; Ansys DesignModeler User Guide, 2017; Ansys Meshing User Guide, 2017; Ansys Fluent User Guide, 2017).

Moreover,

Scenario description

The scenario analyzed in this work mimes an industrial situation involving a horizontally oriented high-pressure release of methane impinging a pipe rack as sketched in Fig. 1, where also the relevant geometric dimensions are labelled.

The methane source was modelled as a nozzle while the pipe rack was represented by a rectangular structure housing several pipes far from the ground. The HP jet nozzle was located at 4.85 m above the ground (which corresponds to the mid-height of the pipe case).

Results

At first, the geometrical characteristics of the pipe rack (namely: dp, ns, and nPS) were changed within realistic ranges, as shown in Table 3, while the values of all the other parameters were kept unchanged: p = 65 bar, T = 278 K, d = 0.0254 m, D = 7.68 m (i.e., half of ME of a free jet at methane LFL), α = 0° (that is, the pipe rack is perpendicular to the jet axis) and c = 5.3% (methane LFL).

As an example, Fig. 6a–d shows the isosurfaces of some of the runs listed in Table 3. Qualitatively,

Discussion

Based on the results achieved and detailed in previous Section, the main findings of this work can be summarized in the following procedure, which allows estimating the ME of a methane flammable jet cloud impinging a pipe rack without any demanding CFD-based computation (in terms of both time and analyst skill):

  • 1

    From both the obstacle and source characteristics, estimate VBR, ABR, and VFP values (as detailed in Section 3):VBR=nPS·nS·π·dP24+2·(nS+1)·s·hH·WABR=h·(nS+1)+dP·nSHVFP=dFJ(D)Hto

Conclusions

The impinging high-pressure jet release of methane can be a relevant scenario in several industrial facilities. Among the others, the credible, whilst probably rare, scenario involving the impingement of a pipe rack was deeply investigated through a CFD-based model, showing that the presence of a rack either enhance (for VBR∙ABR∙VFP lower than about 0.3, see Fig. 10) or does not influence (for VBR∙ABR∙VFP larger than about 0.3, see Fig. 10) the ME of the flammable jet with respect to the free

Credit author statement

Cristian Colombini Methodology; Software; Writing - Original Draft; Visualization. Giuliana Maugeri Methodology; Software. Gianluca Zanon Methodology; Software. Renato Rota Supervision; Formal analysis; Writing - Review & Editing. Valentina Busini Conceptualization; Methodology; Data Curation; Writing - Review & Editing.

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.

Acknowledgements.

References (48)

  • R. Schefer et al.

    Experimental evaluation of barrier walls for risk reduction of unintended hydrogen releases

    Int. J. Hydrogen Energy

    (2009)
  • A.V. Tchouvelev et al.

    Effectiveness of small barriers as means to reduce clearance distances

    Int. J. Hydrogen Energy

    (2007)
  • I.C. Tolias et al.

    Best practice guidelines in numerical simulations and CFD benchmarking for hydrogen safety applications

    Int. J. Hydrogen Energy

    (2019)
  • B.P. Xu et al.

    The effect of an obstacle plate on the spontaneous ignition in pressurized hydrogen release: a numerical study

    Int. J. Hydrogen Energy

    (2011)
  • Release 19.0

    (2017)
  • Release 19.0

    (2017)
  • Release 19.0

    (2017)
  • Release 19.0

    (2017)
  • R. Batt et al.

    Modelling of stably-stratified atmospheric boundary layers with commercial CFD software for use in risk assessment

    Chem. Eng. Trans.

    (2016)
  • H.A. Becker et al.

    Turbulent mixing in the impingement zone of dual opposed free jets and of the normal wallimpinging jet

    Chem. Eng. Commun.

    (1988)
  • P. Bénard et al.

    High pressure hydrogen jets in the presence of a surface

    Int. Conf. Hydrog. Saf.

    (2007)
  • A.D. Birch et al.

    The structure and concentration decay of high pressure jets of natural gas

    Combust. Sci. Technol.

    (1984)
  • A.D. Birch et al.

    Velocity decay of high pressure jets

    Combust. Sci. Technol.

    (1987)
  • C.J. Chen et al.
    (1980)
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