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Heat Transfer Engineering

ISSN: 0145-7632 (Print) 1521-0537 (Online) Journal homepage: http://www.tandfonline.com/loi/uhte20

Numerical Simulation on the Dispersion of NaturalGas Releases from a Buried Pipeline

Yajun Deng, Hao Hou, Lichao Fang, Qing Yuan, Bo Yu & Yongtu Liang

To cite this article: Yajun Deng, Hao Hou, Lichao Fang, Qing Yuan, Bo Yu & Yongtu Liang (2017):Numerical Simulation on the Dispersion of Natural Gas Releases from a Buried Pipeline, HeatTransfer Engineering, DOI: 10.1080/01457632.2017.1325681

To link to this article: http://dx.doi.org/10.1080/01457632.2017.1325681

Accepted author version posted online: 22May 2017.Published online: 22 May 2017.

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HEAT TRANSFER ENGINEERING, VOL. , NO. , –https://doi.org/./..

Numerical Simulation on the Dispersion of Natural Gas Releases from aBuried Pipeline

Yajun Denga, Hao Houa,b, Lichao Fanga, Qing Yuana, Bo Yuc, and Yongtu Lianga

aNational Engineering Laboratory for Pipeline Safety, Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University ofPetroleum, Beijing, China; bChina Petroleum Pipeline Bureau International, Langfang, China; cSchool of Mechanical Engineering, Beijing Instituteof Petrochemical Technology, Beijing, China

ABSTRACTPipeline is amajorway for natural gas transportation. Accidental gas release fromapipelinemight leadto great economic losses and casualties. Therefore, it is important to investigate the dispersion charac-teristics of natural gas release from pipelines. Most previous studies on accidental natural gas releasefrom pipelines mainly focused on bare pipelines and adopted simplified 2D models. This paper firstestablisheddispersionmodels of natural gas release fromburiedpipelineswith high and lowpressure,respectively. The numerical methods were validated by Trial 26 of the Thorney Island field experimentand the dispersionmodel was found to be reasonable and reliable. Then, comparative study between2D and 3D dispersion model of released natural gas was carried out, which proved that 3D modelhad superiority upon the 2Dmodel. Then the 3Dmodel was employed to simulate dispersion processof released natural gas by changing pipeline pressure, orifice diameter, wind speed, and soil prop-erties. Finally, the effect of leakage conditions on consequence distance was analyzed. The numericalresults could provide technical supports for the design of emergency disposal equipments and urgentpipeline repairs.

Introduction

Natural gas is a clean and efficient fossil energy source. Itis widely used in the fields of power generation, indus-trial combustion, urban civil use of gas, and commer-cial areas [1–3]. Pipeline as the main way for natural gastransportation plays a crucial role inmaintaining nationalenergy supply. However, because of perforation caused bycorrosion and third-party damage, accidental gas releasefrom pipelines occurs. The main component of naturalgas is methane, which is flammable and explosive. Whenmeeting fire, if the concentration of the mixed gas (nat-ural gas and air) approaches explosion limit, the mixedgas will ignite and explode, which will lead to great eco-nomic losses and casualties. Considering cost savings,long-distance gas transmission pipelines are in trend withbigger diameter and higher pressure, which increases therisk of pipeline release. Therefore, it is necessary to gaina better understanding of gas dispersion characteristics toprovide technical guides for the design of emergency dis-posal equipments and urgent pipeline repairs.

At present, experimental study on natural gas pipelinerelease is expensive and dangerous. Besides, it is difficult

CONTACT Professor Bo Yu yubobox@vip..com School of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Qingyuan, DaxingDistrict, Beijing, , China.Color versions of one or more of the figures in this paper can be found online at www.tandfonline.com/uhte.

to obtain sufficient and accurate data from experiment.With the rapid development of computing technique,numerical simulation approaches have been widelyemployed to study the dispersion of released gas inthe atmosphere [4–10]. Numerical study on accidentalnatural gas release from pipelines went through threestages. In the early stage, gas in the whole computationaldomain was treated as incompressible. Then, gas in thewhole computational domain was treated as compress-ible. Nowadays, the gas in the near field and far field weretreated as compressible and incompressible, respectively.Qin [4] used the Fluent software to simulate the 2D nat-ural gas release from elevated pipelines and analyzed theeffect of some factors on dispersion process, such as windspeed, pressure, barrier and terrain. Sang [5] establishedthe 3D release model of high pressure pipelines and usedFluent software to simulate the dispersion process withrealizable k-epsilon model. Then he studied some factorswhich have effects on the dispersion characteristics, suchas orifice diameter, wind speed, initial release velocity andshape of release orifice. In the aforementioned studies,both Qin and Sang treated the natural gas in the whole

© Taylor & Francis Group, LLC

2 Y. DENG ET AL.

dispersion region as incompressible. However, becauseof the large pressure ratio between the pipeline andatmosphere, supersonic under-expanded free jet occursat the release orifice. The pressure and physical propertiesof natural gas vary greatly in the near field. Therefore,considering natural gas in whole region as incompressiblebrings large errors for simulation and results could notmatch with actual physical process.

Based on fluidmechanic theory and porousmedia the-ory, Cheng [6] established the mathematical and phys-ical model of dispersion process of released natural gasfrom buried pipelines. The dispersion processes with dif-ferent operation pressure, orifice diameters, release posi-tions and atmosphere temperatures were simulated byFluent software. In his study, Cheng treated natural gas inwhole region as compressible, then he simulated the pro-cess by the density-based solver. As stated above, the pres-sure and physical properties of natural gas near the releasehole change greatly. In order to capture details of jet flow,extremely refined mesh and small time step are needed,which demands a great deal of computational resource asthe dispersion region is generally very large.

Novembre et al. [7] adopted numerical simulationcombined with semi-empirical equation to study the dis-persion of released natural gas in the far field. First, theycalculated the flow state in the section where pressure ofjet flow reached atmospheric pressure. Then they treatedsome parameters such as velocity, pressure and temper-ature at this section as input parameters and considerednatural gas in the far field as incompressible for simula-tion. The simulation results were very close to the resultsof treating the natural gas in whole region as compress-ible and the errors are accepted in engineering. Liu et al.[8, 9] divided the gas release into jet flow and disper-sion process, then they established jet model and disper-sion model, respectively. In jet model, flow parametersat the section where the pressure of jet flow approachedatmospheric pressure were calculated. Then flow param-eters were taken as inlet boundary condition for disper-sionmodel. Natural gas was treated as incompressible andcompressible in jet model and dispersion model, respec-tively. Thismethodhadmodest amount of calculation andhigh precision compared with experimental results.

In field engineering, natural gas pipelines are mostlyburied in soil. Compared with exposed pipeline in air,natural gas releases from buried pipelines have their owncharacteristics. For gas release from a buried pipeline withhigh pressure, the high pressuremakes the natural gas dis-charge into the surroundings at a velocity of hundreds ofmeters per second. The high-velocity jet can remove soiland produce a crater. So the released gas expands directlytowards the air which is similar to the bare pipeline. Forgas release from a buried pipeline with low pressure, the

released gas velocity is low which is unable to removesoil. Thus gas firstly disperses through soil pores untilit reaches the ground, and then it disperses into air. Atpresent, study on dispersion characteristics of releasednatural gas from buried pipelines is limited and lacks sys-tematic research under different leakage conditions.

This paper first established dispersion models ofreleased natural gas from buried pipelines with highand low pressure, respectively. Then, Fluent softwarewas employed to simulate the 3D dispersion processfrom buried pipelines under different conditions suchas pipeline pressure, orifice diameter and wind speed.Finally, the consequence distances under different leak-age conditions were analyzed. Research results can pro-vide great guidance for emergency disposal equipmentsdesign and urgent pipeline repairs.

Physical models

According to the characteristics of buried pipelines withhigh and low pressure, physical models for dispersion ofreleased natural gas from buried pipeline with high andlow pressure are given below, respectively. To simplify thephysical problems, some assumptions are stated below.

(1) Since methane makes up nearly 90% in commer-cial natural gas pipeline generally, this paper usedphysical properties of methane as natural gas.

(2) There is only one release hole in pipeline and thediameter of hole is given.

(3) For buried pipelines with high pressure, pipelinepressure is high enough and the diameter of releasehole is large enough for natural gas to remove soillayer.

(4) For buried pipelines with low pressure, releasednatural gas could not remove soil layer.

Dispersionmodel for released natural gas fromburied pipelines with high pressure

After natural gas released from high pressure pipeline,gas would disperse through release hole into soil layer. Ifreleasing hole is large enough, natural gas would removesoil and disperse directly in the atmosphere. In view ofsafety analysis, the focus of engineering is on the disper-sion process in the far field rather than the jet process inthe near field. Therefore, this research adopts the com-binedmethod of semi-empirical equations and numericalsimulations to investigate the dispersion process in the farfield. The pseudo source defined in Birch et al. [11, 12] isused to replace the actual release orifice. Pseudo sourceis considered as the cross-section when jet reaches theatmospheric pressure. Parameters of pseudo source are

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Figure . Schematic map of dispersion model for released naturalgas from buried pipelines with high pressure.

calculated by Birch model and treated as the inlet bound-ary conditions for far field in which the natural gas istreated as incompressible. Figure 1 depicts the schematicmap of dispersion model for released natural gas fromburied pipelines with high pressure.

Dispersionmodel for released natural gas fromburied pipelines with low pressure

When the pressure of natural gas in pipeline is low,released gas velocity cannot be able to break through soillayer. The released gas firstly disperses from pores of soilto ground floor and then reaches atmosphere. This papertreats soil as an isotropic porous media. Figure 2 showsthe schematic map of dispersion model for released natu-ral gas from buried pipelines with low pressure.

Numerical methods

Governing equations

The dispersion process of released natural gas fromhigh pressure pipelines satisfies the continuity equation,momentum equation, energy equation, and species trans-port equation.

Continuity equation:

∂ρ

∂t+ ∇ · (ρu) = 0 (1)

Figure . Schematic map of dispersion model for released naturalgas from buried pipelines with low pressure.

Momentum equation:

∂

∂t(ρu) + ∇ · (ρuu) = −∇p+ ∇ · τe f f + ρg (2)

Energy equation:

∂

∂t(ρE) + ∇ · (

u(ρE + p

))

= ∇ ·⎛⎝ke f f∇T −

∑j

h jJ j + τe f f · u⎞⎠ + Sh (3)

Species transport equation:

∂

∂t(ρYi) + ∇ · (ρuYi) = −∇Ji + Si (4)

For dispersion of released natural gas from low pres-sure pipelines, porous media model should be supple-mented and a source term should be added to the generalmomentum equation.

Si = −(

μ

αui +C

12ρ |u| ui

)(5)

where Si is the source of momentum equation in i direc-tion, |u| is the magnitude of velocity, α stands for perme-ability andC is the inertia resistance factor.

The calculation methods for inertia resistance factorand viscous resistance factor are shown as follows.

Viscous resistance factor:1α

= 150(1 − ε1)2

D2Pε1

3 (6)

Inertia resistance factor:

C = 3.5DP

(1 − ε1)

ε13(7)

4 Y. DENG ET AL.

Turbulencemodel

Natural gas dispersion can be treated as turbulent flowresulting from the combined effects of multi-componentgases. The realizable k − ε model is chosen as the turbu-lent model including full buoyancy effects in this study.Transport equations of k and ε are shown as follows,respectively.

∂

∂t(ρk) + ∇ · (ρku) = ∇ ·

[(μ + μt

σk

)∇k

]+Gk + Gb − ρε + Sk (8)

∂

∂t(ρε) + ∇ · (ρεu) = ∇ ·

[(μ + μt

σε

)∇ε

]

+ ρC1Sε − ρC2ε2

k + √vε

+C1εε

kC3εGb + Sε (9)

whereC1 = max[0.43, η

η+5 ], η = Skε, S = √

2Si jSi j.In these equations,Gk represents the generation of tur-

bulence kinetic energy caused by mean velocity gradi-ents. Gb is the generation of turbulence kinetic energybrought by buoyancy. C1, C2, C1ε and C3ε are constants.σk and σε are the turbulent Prandtl numbers for kand ε, respectively. Sk and Sε are user-defined sourceterms.

Computational domain andmesh

The computational domain for gas dispersion fromburied pipelines with high pressure is a cube with edge of100 m. The pipeline diameter is 1016 mm and the burieddepth of pipeline is 1.5 m. Because of the symmetry of themodel, only half of the dispersion area is considered inthis study. Figure 3(a) shows the computational domain. Itis divided into multi-block structured hexahedral mesheswith themesh generator integrated computer engineeringand manufacturing code for computational fluid dynam-ics (ICEM-CFD). Figure 3(b) depicts the computationalmesh.

The computational domain for gas dispersion fromburied pipelines with low pressure includes the airdomain and soil domain. The range of air domain isthe same to that of pipeline with high pressure. Thelength, width and height of soil area are set as 100 m,100 m, and 4.016 m, respectively. The pipeline diameterand the buried depth of pipeline are the same to that ofhigh pressure pipeline. Also, the ICEM-CFD software isemployed to generate multi-block structured hexahedralmeshes.

Figure . Computational domain andmesh for gasdispersion fromburied pipelines with high pressure.

Boundary conditions andmain parameters

The boundary conditions for dispersion of released natu-ral gas from buried pipelines with high pressure are deter-mined as follows. Pseudo source is defined as velocityinlet. The pseudo source parameters can be calculatedthrough the following equations [12].

The pseudo diameter:

dps = d

√CD

(p1pa

)(2

γ + 1

)1/(γ−1)V2

V3(10)

Velocity at the pseudo source:

V3 = V2

{CD +

[1 − pa

p1

(2

γ + 1

)−γ /(γ−1)] /

γCD

}

(11)

Distance between the pseudo source and orifice:

xs = 10xm (12)

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Table . Main parameters.

Name Value

Air density (kg/m) .Ratio of specific heat of air .Air viscosity (Pa·s) .× −

Natural gas density(kg/m) .Ratio of specific heat of natural gas .Natural gas viscosity(Pa·s) .× −

Soil density(kg/m) Soil thermal conductivity (W/ (m �K)) .Soil heat capacity (J/(kg �K)) Soil porosity .Mean soil diameter(mm) .

where xm is the distance between Mach disc and orificethat can be calculated through:

xm = 0.645d√

p1pa

(13)

Top side, front side and right side of the computationaldomain are regarded as pressure outlet boundaries. Backside is specified by symmetry plane. Left side is consideredas pressure outlet boundary without wind. And left side isdefined as velocity inlet boundary with wind.

Wind speed is one of the most significant parametersin dispersion model, as it determines how quickly the gaswill be diluted by the flowing air. In order to account forthe variation of wind speed with height near the groundlevel, a power law is used to describe the vertical windprofile [9]:

v = v1 ×(hh1

)β

(14)

where v is the wind speed at height h over the ground;v1 is the reference wind speed at reference height h1 overthe ground; β is the wind shear exponent, which dependsupon the atmospheric stability class and the groundsurface roughness, and it is set as 0.12 in this study.

Boundary conditions for dispersion of released nat-ural gas from buried pipelines with low pressure in airarea are similar to those of pipelines with high pressure.In this paper, only boundary conditions of soil area arepresented. The boundary condition of release orifice isdefined as pressure inlet boundary. The soil is treated as aporousmedia. The front side, back side, left side and rightside of soil area are considered as pressure outlet bound-aries and the down side is wall. The soil porosity, iner-tia resistance factor and viscous resistance factor are set,respectively.

Main parameters used in the simulation are presentedin Table 1. The information of simulation cases for highpressure pipelines is listed in Table 2, and variablesare pipeline pressure, orifice diameter and wind speed.The Table 3 illustrates the simulation cases for lowpressure pipelines.

Table . Simulation cases for high pressure pipelines.

Case Pipeline pressure (MPa) Orifice diameter(mm) Wind speed(m/s)

Simulationmethod

Fluent software is used to carry out the simulation. Pres-sure and velocity are coupled calculated using SIMPLEalgorithm. The advection term of momentum equation,energy equation, species transport equation, turbu-lent kinetic energy equation and turbulent dissipationrate equation are discretized using the second upwindscheme. The central difference scheme is employedfor diffusion term of the equations stated above. Sym-metry plane is chosen as the observation plane. Thesimulation is allowed to proceed until the methane vol-ume concentration at observation plane reaches steadystate.

Results and discussion

Validation ofmodel

Trial 26 of Thorney Island field experiment [13] wassimulated to validate the numerical methods adoptedin this research. Thorney Island tests were designed toinvestigate the dispersion process of dense gas and werewidely used to validate dispersion model [9, 14, 15].In Trial 26, the gas source was cylindrical tent of 14 mdiameter, 13 m height and the total volume was 2000 m3.The composition of gas was 68.4% N2 and 31.6% Freon12 (relative density was 2.0). The wind speed at 10 mheight was 1.9 m/s and wind shear exponent was 0.07. A9 m × 9 m × 9 m obstacle was placed at 50 m downwindaway from the center of cylindrical gas tent. After thestart of the trial, the sides and top of cylindrical tent wasquickly removed, and the heavy gas was exposed to theatmosphere. Then the dense gas dispersed in atmospheredue to gravity. Although released gas was not natural gas,the experiment can still be used to validate the dispersion

Table . Simulation cases for low pressure pipelines.

Pipeline Orifice WindCase pressure (MPa) diameter(mm) porosity speed(m/s)

. . . . .

6 Y. DENG ET AL.

Figure . Comparison of the predicted gas concentrations and theexperimental data at two monitoring points.

model of released natural gas. According to Liu et al. [9],the computational domain with 300 m × 260 m × 80 mwas used in this study to simulate Trial 26 of ThorneyIsland. In the experiment, there were two concentrationsensors installed on the obstacles: on the windward faceat 6.4 m height and on leeward face at 0.4 m height.Figure 4 compares the predicted gas concentrations andthe experimental data at two monitoring points. Forcomparison, the results predicted by Liu et al. [9] andHsieh et al. [15] are also shown. As can be seen fromthe figure, the predicted maximum concentrations areclose to the experimental data with reasonable deviationsat both monitoring points. The time variations in theconcentrations at both monitoring points are also wellreflected by the present simulation. This shows that the3D dispersion model established in this study is reliableand can be used to simulate the dispersion of releasednatural gas from pipelines.

Choice ofmodel dimensions

The dispersion process of released natural gas is actually around hole dispersion. If the 2Dmodel is used to simulate

the problem, the dispersion process is considered as aslit dispersion. Some researchers have improved the 2Dmodel by specifying turbulent intensity and hydraulicdiameter as turbulence parameters. This method limitslongitudinal length of slit by setting the equivalent diam-eter of inlet and thus ensures that simulation result of 2Dmodel is orifice leakage instead of slit leakage. However,by means of consulting literature, we found that thenumerical results between 2D model and 3D model havebig differences with the same boundary conditions andparameters [4, 5]. Therefore, this article simulated thecase 1 for gas dispersion from a high pressure pipelineusing the 2D model and 3D model, respectively, withsame boundary conditions and parameters. Figure 5shows the contours of methane volume concentrationwith 3Dmodel and 2Dmodel under the same conditions.It can be found that the calculated result of 2D model isasymmetric when there is no wind. And the calculatedmethane dispersion area is significantly larger than thatof 3D model. Therefore, the 3D model has many advan-tages over 2D model and is employed to perform thesubsequent calculation.

Dispersion of released natural gas from buriedpipelines with high pressure

This part investigates the effects of pipeline pressure, ori-fice diameter and wind speed on gas dispersion processof released gas from buried pipelines with high pressure.To study the effect of pipeline pressure on the disper-sion process, simulations were conducted by changingthe pipeline pressure while leaving other parametersunchanged. Figure 6 shows the contours of methanevolume concentration at symmetry plane for differentpipeline pressure. Because of the introduction of pseudosource, pipelines with different pipeline pressure mighthave different pseudo diameters, even the real releaseorifices are the same. Pseudo diameters under 10 MPaand 5 MPa are 0.28 m and 0.199 m, respectively. Thepipelines with different diameters might have differentrelease rates so as to form different dispersion processes.As shown in Figure 6, jet forms when natural gas releasesat high velocity. While the concentration of methane nearorifice is high and the dispersion area is very small. Thedensity difference between natural gas and air contributesto the quick dispersion. With the development of disper-sion process, gas dispersion area becomes larger whilethe concentration decreases rapidly. This might occurthat the air resists the flow of natural gas. Besides, it canbe easily seen from the Figure 6 that pipeline pressurehas a significant effect on the height of jet. Jet height is70.2 m for 10 MPa while 36.5 m for 5MPa. The higherthe pipeline pressure, the bigger release rate of naturalgas and the higher the jet height.

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Figure . Comparison of methane volume concentrations with D model and D model under the same conditions.

The orifice diameter plays a crucial role in the dis-persion of released natural gas. Therefore, we changedthe orifice diameter to figure out its effect on dispersionprocess. The contours of methane volume concentrationat the symmetry plane for different orifice diameters areshown in Figure 7. It can be seen that methane volumeconcentrations for different orifice diameters vary greatly.The jet height increases with the increment of orificediameter. When the orifice diameter is 20 mm, jet heightis 14.5 m. When orifice diameter is 50 mm, jet height is70.2 m. When the orifice diameter reaches 100mm, jetheight exceeds the height of the computational domain.With the increase of orifice diameter, the height andwidthof the jet core increase gradually and the increase in heightdirection is larger than that in radial direction. The mainreason is that the pipeline with larger orifice might havelarger release rate.

The long distance natural gas pipeline with high pres-sure is generally built in the wild. Hence, the dispersionof released natural gas in atmosphere is strongly affectedby wind. We simulated three cases with wind speed of0 m/s, 10 m/s, and 20 m/s, respectively. The methane

volume concentration distributions at symmetry planeare shown in Figure 8. It can be seen that the methanevolume concentration distributions for different windspeeds vary a lot. Firstly, the jet develops towards down-wind direction with the increase of wind speed. Thelarger the wind speed, the bigger the angle between jetand vertical direction. This might occur because thatwind could enhance the heat and mass transfer betweenthe natural gas and the air. Advection transportation ofnatural gas clouds is intensified by the wind, whichmakesthe natural gas clouds transport towards the downwinddirection. The larger the wind speed, the bigger thetransportation effects. Besides, since the release sourcevelocity is very high, wind speed has little effect on initialjet. The further the jet is from the source, the greaterthe effects of wind speed are. Furthermore, the jet heightdecreases with increment of wind speed. For example,when the velocity magnitudes are 0 m/s, 10 m/s and20 m/s, the corresponding maximum jet heights are70.2 m, 20.8 m, and 17.9 m, respectively. The reason forsuch difference is the advection transportation of thewind.

Figure . Methane volume concentrations for dispersion of released gas from high pressure pipelines with different pipeline pressure.

8 Y. DENG ET AL.

Figure . Methane volume concentrations for dispersion of released gas from high pressure pipelines with different orifice diameters.

Figure . Methane volume concentrations for dispersion of released gas from high pressure pipelines with different wind speeds.

Dispersion of released natural gas from buriedpipelines with low pressure

This section investigates the effect of orifice diameterand soil porosity on the gas dispersion characteristic forreleased gas from a buried pipeline with low pressure.Figure 9 shows the dispersion process of released gas insoil when the orifice diameter is 100 mm. As can be seenfrom Figure 9, natural gas firstly releases from the dam-aged pipeline and then disperses into the soil. Becauseof the influence of soil resistance, natural gas lost bigkinetic energy and cannot form free jet like bare natu-ral gas pipeline. Hence, dispersion rate of natural gas inthe soil is slow. Only a little amount of gas releases at 0.1sand increases gradually with time. However, during theinitial stage of perforation leakage, because of the highpressure and release rate, the influence of the soil resis-tance on the decaying of concentration and velocity ofnatural gas dispersion is limited. When running time is530 s, the released natural gas breaks through the poresand reaches the ground surface, then it expands rapidlynear the ground.

Figure 10 shows the distributions of methane volumeconcentration of released gas from low pressure buried

Figure . The dispersion process of released gas from a low pres-sure pipeline in soil when the orifice diameter is mm.

pipeline with the orifice diameters of 20 mm, 50 mm, and100 mm, respectively. It can be seen that the orifice diam-eter has an important influence on the concentration andspeed of dispersion.When the orifice diameter is 100mm,the release rate is higher than that when the orifice diam-eter is 50 mm, and the dispersion concentration reachessteady state in a shorter time. In addition, when the ori-fice diameter is 20 mm, the released natural gas cannotreach to the ground surface. This is mainly because ori-fice diameter is too small for natural gas to overcome thesoil resistance.

Because the difference of soil property could lead to thegreat varied ability of released natural gas to get throughthe soil, the dispersion processes with the soil porositiesof 0.1, 0.43, and 0.7 were simulated. Then the effect of soilporosity on the dispersion processwas analyzed. Figure 11shows the methane volume concentration distributionsof released gas from buried pipelines with low pressurewhen the soil porosities are 0.1, 0.43, and 0.7, respectively.It can be seen that the soil porosity also has an impor-tant influence on the dispersion concentration. When thesoil porosity is bigger, gas dispersion area is bigger andthe required time to reach steady state is shorter. Further-more, when the porosity is 0.1, the gas cannot spread tothe ground surface. Thismight occur because the pressureof the gas is not high enough for natural gas to overcomethe resistance of soil when soil is very compact.

Consequence distance

Once accidental natural gas releases occurs, urgentpipeline repairs should be conducted. The treatment of

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Figure . Methane volume concentrations for dispersion of released gas from low pressure pipelines with different orifice diameters.

Figure . Methane volume concentrations for dispersion of released gas from low pressure pipelines with different soil porosities.

released natural gas in atmosphere should be cared firstbefore urgent pipeline repairs. At present, drainage deviceis widely used to transfer released gas to safe areas, whichcould shorten the accident emergency response time andreduce risk of repair construction effectively. Figure 12shows the field test diagram of drainage devices of buriednatural gas pipeline release. In the design of drainagedevices, designing the size of drainage cover is a majorissue. Consequence distance analysis of released naturalgas plays an important role in designing the proper size ofdrainage cover.

Figure . The field test diagramof drainage devices of buried nat-ural gas pipeline release.

The explosion limit of natural gas in air is 5% ∼ 15%,area of 5% concentration curve is considered as conse-quence region. And the diameter of biggest cross-sectionparallel to the ground for consequence region is consid-ered as consequence distance. Figure 13 shows the con-sequence distances of released gas from buried pipelineswith high pressure under different leakage conditions.Figure 13(a) shows consequence distances for differentpressure. As can be seen from the diagram, pipeline pres-sure has an important influence on consequence distance.The consequence distance multiplies with the incrementof pipeline pressure.When the pipeline pressure is 5MPa,the consequence distance is 3.2 m. When the pipelinepressure approaches to 10 MPa, the consequence dis-tance is 4.4 m. The consequence distances for differentorifice diameters are shown in Figure 13(b). When theorifice diameters are 20 mm, 50 mm, and 100 mm, thecorresponding consequence distances are 70.2 m, 20.8 m,and 17.9 m, respectively. The larger the orifice diameter,the longer the consequence distance. Figure 13(c) showsconsequence distances for different wind speeds. Theconsequence distances are 4.4 m, 6.5 m, and 5.9 m whenwind speeds are 0 m/s, 10 m/s, and 20 m/s, respectively.High wind speed stimulates the dispersion of gas byenhancing the transportation of natural gas in air. Thepipeline with higher wind may have longer consequencedistance. When the wind speed reaches a certain limit,the consequence distance decreases.

10 Y. DENG ET AL.

Figure . Consequence distances for high pressure pipelinesunder different leakage conditions.

Figure 14 shows the consequence distances of releasedgas from buried pipelines with low pressure under differ-ent conditions. The consequence distances for differentorifice diameters are shown in Figure 14(a). The conse-quence distances are 0 m, 0.7 m and 3 m when the orificediameters are 20 mm, 50 mm and 100 mm, respectively.The larger the orifice diameter, the bigger the conse-quence distance. The consequence distances for differentsoil porosities are shown in Figure 14(b). When the soilporosities are 0.1, 0.43, and 0.7, the corresponding conse-quence distances are 0 m, 0.7 m, and 2.9 m, respectively.

Figure . Consequence distances for low pressure pipelinesunder different leakage conditions.

The consequence distance increases with the incrementof soil porosity. When orifice diameter is very small orsoil is very compact, the influence of the leakage could beignored.

Conclusions

This paper established the dispersion model of releasednatural gas from buried pipelines with high and low pres-sure, respectively. The Fluent software was used to simu-late the gas dispersion under different leakage conditions.The main conclusions are shown as follows:

(1) For gas release from buried pipelines with highpressure, the higher the pipeline pressure, thelarger the release rate and the higher the jet height.With the increase of orifice diameter, the heightand width of the jet core increase gradually andthe increase in height direction is larger than thatin radial direction. With the increment of windspeed, the jet moves towards downwind direction,and the angle between jet and ground decreases.

(2) For gas release from buried pipelines with lowpressure, when the orifice diameter is bigger, thegas dispersion velocity is faster and the requiredtime to reach steady state is shorter. Soil porosity

HEAT TRANSFER ENGINEERING 11

has a great influence on the dispersion process ofnatural gas in soil. Natural gas is more difficult tospread in soil with low porosity than that in soilwith high porosity. In particular, when the orificediameter is very small or the soil is very compact,natural gas could not spread to the ground surface.

(3) The pipeline pressure, orifice diameter, wind speedand soil porosity have great influence on the con-sequence distance, and the consequence distanceanalysis can provide the guide for the design of theproper size of drainage cover.

In this research, the dispersion model of released nat-ural gas was established under certain assumptions andhad some differences from the actual situation. In futurestudy, we plan to combine numerical simulation with alarge number of field data and experiments to improve theapplication of the model.

Nomenclature

C inertial resistance factor, 1/mC1 model constantC1ε model constantC2 model constantC3ε model constantCD discharge coefficientd orifice diameter, m

dps pseudo diameter, mDp mean particle diameter, mE total energy of fluid element, Jg acceleration of gravity, m/s2

Gb generation of turbulence kinetic energy due to buoyancy,kg/(m·s3)

Gk generation of turbulence kinetic energy due to the meanvelocity gradients, kg/(m·s3)

h height, mh1 reference height, mhj enthalpy of species j, J/kgJj diffusion flux of species j, mol/(m2·s)k turbulent kinetic energy, m2/s2

keff effective conductivity, W/(m·K)p pressure, Pap1 pipeline pressure, Pap2 pressure at orifice, Pap3 pressure at the pseudo source, Papa ambient pressure, PaSh volume heat source, kg·J/(m3·s)Si the source term for the ith (x, y, or z) momentum

equationt time, sT temperature, KT1 temperature in pipeline, KT2 temperature at orifice, KT3 temperature at the pseudo source, KT0 ambient temperature, Ku gas velocity, m/sv wind speed at the height h, m/s

v1 wind speed at the height h1, m/sV2 gas velocity at the orifice, m/sV3 gas velocity at the pseudo source, m/sx coordinate variable

xm location of Mach disc, mxs distance from the pseudo source to the orifice, mYi local mass fraction of each species

Greek symbols

α permeabilityβ surface roughnessε dissipation rate of turbulent kinetic energy, m2/s3

ε1 soil porosityγ specific heat ratioμ dynamic gas viscosity, N·s/m2

μt turbulent viscosity, N·s/m2

ρ density, kg/m3

ρ1 gas density in pipeline, kg/m3

ρ2 gas density at orifice, kg/m3

ρ3 gas density at the pseudo source, kg/m3

τ eff stress tensor, Pa

Acknowledgements

The study is supported by the National Science Foundation ofChina (Nos. 51325603, 51636006).

Notes on contributors

Yajun Deng is a Ph.D. studentof Oil & Gas Storage and Trans-portation Engineering at theChina University of Petroleum(Beijing). He obtained his bach-elor’s degree from YangtzeUniversity, China in 2013. Hisresearch interests include theaxial flow cyclone’s fundamentalresearch and industrial appli-cation, numerical simulation ofnatural gas pipeline release anddispersion process.

Hao Hou is an engineer in ChinaPetroleum Pipeline BureauInternational. He received hismaster degree of Oil & GasStorage and TransportationEngineering from China Uni-versity of Petroleum (Beijing)in 2016. His research interestis long-distance transportationtechnology of waxy crude oil andnatural gas.

12 Y. DENG ET AL.

Lichao Fang is a master studentin Oil & Gas Storage and Trans-portation Engineering at theUniversity of Petroleum (Bei-jing), China. She received herB.S. degree in Oil & Gas Storageand Transportation Engineeringfrom theUniversity of Petroleum(Beijing), China. She is currentlyworking on numerical simu-lation for heat transfer, waterflow and mechanical structure offrozen soil around pipeline.

Qing Yuan is a Ph.D. student ofOil & Gas Storage and Trans-portation Engineering at theChina University of Petroleum(Beijing). He obtained his bach-elor’s degree from SouthwestPetroleum University, Chinain 2014. His research interestis long-distance transporta-tion technology of waxy crudeoil.

Bo Yu is a professor of Mechan-ical Engineering and Oil & GasStorage and TransportationEngineering at Beijing Instituteof Petrochemical Technology,China. He received his Ph.D.in 1999 from Xi’an JiaotongUniversity, China. He workedat Kyushu University fromJune 1999 to March 2001 as apostdoctoral fellow and at theNational Institute of AdvancedIndustrial Science and Tech-

nology, Japan, from April 2001 to March 2005 as a specialresearch associate. He is a senior member of the ChineseSociety of Theoretical and Applied Mechanics and a memberof the Society of Petroleum Engineers. He is an academiccommittee member of several international symposiums andthe Key Laboratory of Railway Vehicles & Thermal Techniqueof Ministry of Education, China. Since 1999, he has publishedmore than 100 papers in archival journals and conferenceproceedings and obtained four national and provincial naturalscience and technology awards. His current research interestsinclude long distance transportation technology of waxy crudeoil, turbulent drag-reducing flow, and computational fluiddynamics.

Yongtu Liang is a profes-sor of Oil & Gas Storageand Transportation Engi-neering at China Universityof Petroleum (Beijing). Hereceived his Ph.D. in 2009 fromChina University of Petroleum(Beijing). His current researchinterests include long-distancetransportation technology ofwaxy crude oil, multiphase flowin pipes, and ocean engineering.

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