Zirong Lin,Shuangfeng Wang,Shuxun Fu,Jiepeng Huo

1 Foshan Public Utilities Holding Co.Ltd.,Guangdong,Foshan 528000,China

2 South China University of Technology,Guangdong,Guangzhou 510640,China

3 Foshan gas group Co.Ltd.,Guangdong,Foshan 528000,China

4 Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangdong,Guangzhou 510640,China

Keywords:Natural gas Leakage and diffusion Cofferdam Numerical simulation

ABSTRACT The leakage and diffusion characteristics of natural gas were investigated in the condition of the leakage of liquefied natural gas(LNG)in the storage tank.Fluent was adopted to simulate the process in a series of three-dimension unsteady state calculations.The effects of different heights of the cofferdam(1.0 m,2.0 m and 3.0 m),wind directions,ambient temperature,leakage location,leakage volume on the diffusion process of natural gas were investigated.The diffusion characteristics of the natural gas clouds over cofferdam were found.Under windless condition,when the gas clouds met,the gas clouds rose due to the collision,which made them easier to cross the cofferdam and spread out.The higher the ambient temperature was,the higher the gas concentration around the cofferdam was,and the smaller the gas concentration difference was.When the leakage occurred,the higher cofferdam was more beneficial to delay the outward diffusion of gas clouds.However,when the leakage stopped,the higher cofferdam went against the dissipation of gas clouds.Under windy condition,the time to form stable leakage flow field was faster than that of windless,and the lower cofferdam further reduced this time.Therefore,considering the effect of barrier and dissipation,it was suggested that the rational height of cofferdam should be designed in the range of 1.0 m to 2.0 m.In case of emergency,the leakage of gas should be deduced reasonably by combining the measurement of gas concentration with the rolling of gas clouds.When windless,the leakage area should be entered between the overflows of gas clouds.

1.Introduction

With the increasing demand for natural gas in China,LNG(liquefied natural gas)industry is developing rapidly[1,2].Because of the flammable and explosive characteristics,the security issues in the process of production,transportation,storage and others of LNG are widely concerned about.There have been many casualties caused by the leakage of LNG tanks both at home and abroad [3,4].In order to control the spread of natural gas leakage and prevent the expansion of the scope of damage,relevant provisions have said that cofferdam must be established outside the LNG tank,such as GB 50183-2004“Code for protection design of petroleum and natural gas engineering“requires that LNG facilities should be equipped with cofferdams,and the effective volume in the cofferdam area should be more than the volume of a maximum storage tank in the tank group when measures have been taken to prevent the impact of low temperature or fire.The effective volume in the cofferdam area should be the total volume of the tank in the tank group when the tank is not affected by low temperature and fire.GB 50156-2014“Code for design and construction of filling station”requires that the top surface of the cofferdam should be at least 0.8 m above the ground.GB 50016-2006“Code of design on building fire protection and prevention"requires that a fire barrier should be provided around them,for the aboveground or semi-underground storage tanks or tank groups of class A,B and C liquids.Therefore,the design of LNG storage tank cofferdam area is the key to ensure the safe operation of LNG gasification station.In particular,under the condition of LNG storage tank leakage accident,all kinds of emergency measures need to be implemented in the cofferdam area.Therefore,it is of great significance to explore and study the leakage and diffusion behavior of natural gas in the cofferdam area of LNG storage tank.

Fig.1.3-Dimensional Simulation model(unit:m)schematic diagram.

In addition to the large-scale leakage field test and wind tunnel simulation,numerical simulation is a widely used research method in the study of natural gas leakage diffusion rule.Wu et al.[3]proposed the improved HNE-DS model effectively to simulate the dynamic leakage process of natural gas liquid tanks under critical and subcritical releasing conditions associated with vapor/liquid phase change.Yang et al.[5]used Fluent to simulate the concentration distribution and velocity distribution of gas among buildings at different time after the leakage.Liu et al.[6]conducted numerical simulation and experiment to investigate the urban natural gas leakage and diffusion under different building layouts.Ulrich et al.[7]investigated how realistic environmental conditions affect methane concentration distributions near leaking underground NG distribution pipelines and ultimately to inform protocols for leak detection by walking surveys.Marsegan et al.[8]suggested that the computational fluid dynamics(CFD)model based on the N-S equation had the strongest theoretical basis,and could simulate the influence of complex ground obstacles on the diffusion of LNG gas clouds by solving the complete system of conservation equations,and collecting an archive of evaporation,dispersion and combustion information through a series of experimental and modeling studies.Zhu[9]combined experiments and simulations to obtain the transient process of LNG leakage and diffusion.The simulation results showed that the range of the lower wind direction danger area firstly increased and then decreased,and the maximum distance of immediately dangerous to life or health(IDLH)increased firstly and arrived at the peak of 52 m at 300 s.Nawaz et al.[10]evaluated the possibility of shift between boiling and evaporation regime,through a comprehensive parameter sensitivity analysis,including pool temperature,wind speed,ambient temperature,and pool size.Animah et al.[11]employed various risk analysis approaches to identify the potential hazards,calculate the probability of accidents,and assess the severity of consequences,in order to assess the risk of accidents associated with LNG facilities and carriers.Lervag et al.[12]presented a coupled fluid-dynamical,heattransfer,and thermodynamical model to predict the onset of delayed RPT,and also derived a general method to predict the distance from the spill source to a possible delayed RPT event for a given steady flow pattern.Recent literature studies on natural gas leakage indicated that the focus was on the safe application of natural gas,but there were few studies on the risk analysis and three-dimension model leakage analysis of LNG storage tank cofferdam,which was the most dangerous area.

Fig.2.the size of liquid phase pipes directly connected to storage tanks.

Table 1 Example settings of different working conditions

Fig.4.1/2 LEL contour profile of the natural gas clouds at 10 s,20 s,30 s and 40 s in Case 3.

Therefore,in order to obtain the effect of cofferdam on the diffusion of LNG,the process of gas diffusion in a cofferdam area was simulated in this paper.The effect of different working conditions on gas leakage diffusion in cofferdam area was investigated.The simulation results were able to provide scientific suggestions for the design and layout of the cofferdam of LNG gas station and emergency responses in case of accident.

2.Numerical Methods

2.1.Physical models

In this paper,fluent was used as computational fluid dynamics software.A series of three-dimension unsteady numerical simulations were carried out for gas diffusion of LNG leakage accident in a LNG storage tank,of which the storage capacity was 8×150 m3.The geometry of the cofferdam area in LNG storage tank was shown in Fig.1.The cofferdam area was 44.5 m×34 m,which had eight cylindrical tanks with outer diameter of 5 m,high of 22 m,and longitudinal center spacing of 9.5 m,transverse center spacing of 13 m.The size of the computational domain of this model was 64.5 m×54 m×28 m.The ground,cofferdam and the external surface of the storage tank in the computational domain were of solid-wall grid structure.The space was divided by structured grid and the grid in cofferdam area was locally encrypted.

Fig.5.1/2 LEL contour profile of the natural gas clouds at 10 s,20 s,30 s and 40 s in Case 15.

Fig.6.1/2 LEL contour profile contrast of the natural gas clouds at different leakage location for two leakage points.

Because the leaked natural gas produced by evaporation of LNG was denser than air,which belonged to the heavy gas diffusion process[9],the model needed to consider the impact of gravity.The direction of gravity-Z direction was 9.8 m·s–2.A standard k-ε turbulence model was chosen for the flow model.Because the gas component was mainly methane(volume ratio between 85%and 95%),we used methane as a diffusion gas component in the calculation.Component diffusion model was chosen,and gas state was calculated by the ideal gas law.

2.2.Initial conditions and boundary conditions

Combined with the real case of LNG tank leakage accident,the most likely place for LNG liquid phase leakage was the interface between the inlet pipe and outlet pipe,outer diameter of which was 57 mm at the bottom of the tank,and the interface of instrument,outer diameter of which was 18 mm,as shown in Fig.2.Therefore,this paper assumed that the liquid phase pipe at the bottom of the LNG storage tank ruptured in two ways,forming a leakage source with an aperture of 45 mm and 14 mm,respectively.Eq.(1)was used to calculate the liquid phase leakage mass flow rate(Ql),which was 23.32 kg·s–1and 2.26 kg·s–1,respectively.

In this equation,Cdwas the liquid leakage coefficient of the circular leakage port,taking 0.65;Alwas the section area of the leakage point;ρ was the density of liquid;P and P0respectively as the pressure of the tank and the ambient pressure,0.6 MPa and 0.101325 MPa respectively;g denoted the gravitational acceleration and h was the height difference between the liquid level in the tank and the leakage source.Compared with h,the pressure difference P and P0was the main influencing factor of the leakage rate,so the leakage could be considered to be continuous and stable in the first few minutes,which was the most critical stage of emergency disposal.

The cryogenic LNG liquid had a strong heat exchange with the surrounding air after leakage,when the liquid reached the ground,so it would form a liquid pool and spread to the surrounding,in the conduction and convection heat transfer process.When the liquid would boil and rapidly evaporate,and the evaporation rate and leakage rate was the same,the size of the liquid pool was in a dynamic equilibrium state.The physical process was very complex.As this paper mainly investigated the diffusion process of gas clouds in storage tank area after LNG leakage,so the gasification process of LNG leakage was simplified.In this paper,the diameter of the liquid pool formed by the leakage LNG was supposed to be the diameter of the storage tank (leak1 to leak8 were shown in Fig.1),and the evaporation process of LNG liquid pool was simulated.The temperature of vaporized gas was the boiling temperature of LNG(111.15 K).

The outer surface of the tank was set to an adiabatic wall due to the good heat insulation of LNG tank.Ignoring the thickness of the cofferdam,the cofferdam and the ground were set to the same fixed temperature,which was the same as the ambient temperature.The perimeter and top of the computing field were set as pressure outlets,and the ambient pressure was 101,325 Pa,and the ambient gas component was air,in which O2accounted for 21%mole volume.

2.3.Grid independence check and case setting

Firstly,the calculation results before and after grid encryptions were compared to verify the grid independence.The specific comparison examples were shown in Cases 1 and 2 in Table 1 Since the lower explosive limit (LEL) of methane (CH4) gas was 5%,the gas leakage behavior was represented by 1/2 LEL contour profile of natural gas clouds in comparison,to ensure that the area beyond the contour profile was a safe area.It could be seen from Fig.3 that the gas concentration contour profile and diffusion range presented by Case 2 were basically the same as that of Case 1.When the natural gas clouds overpassed the cofferdam,there would be obvious contour lines.At the same time,both cases showed that during the gasification of LNG,the frozen heavy gas firstly heated up in contact with the ground,and then expanded outward along the ground under the action of gravity until the rising height exceeded the cofferdam,and then continued to warm up in contact with the ground,forming gradually rising and spreading again,which was consistent with the LNG leakage phenomenon mentioned in literatures[13,14].Therefore,this paper considered that when the grid number was not less than 103,840 for LNG leakage cases,the simulation result of gas clouds diffusion behavior was acceptable and stable,and the minimum grid size was 0.05 m.

The effects of different heights of the cofferdam,wind directions,ambient temperature,leakage location,leakage volume on the diffusion process of natural gas were investigated.Example Settings of different working conditions were shown in Table 1.

3.Results and Discussions

3.1.Effect of leakage volume under windless condition

From the comparison of Cases 1 and 3,it could be seen that the leakage volume had a great influence on the diffusion rate of the natural gasclouds.When the leakage belonged to an instrument valve leakage(2.26 kg·s–1)as shown in Fig.4 and the leakage time was at 40 s,the natural gas clouds overturning only occurred in the cofferdam near the leakage point,and could be controlled in the surrounding position close to the cofferdam.When leakage belonged to the root of the valve leakage (23.32 kg·s–1) as shown in Fig.3 (a) and the leakage time was at 10s,the natural gas clouds overturning already occurred in the cofferdam near the leakage point.At 20 s,the overturning condition expanded,and the natural gas clouds diffused in a fan shape.At 30 s,the overturning condition further expanded,and the natural gas clouds basically covered the whole cofferdam.At 40 s,the natural gas clouds overturning occurred in the entire cofferdam.From the above simulation analysis,it could be seen that in the actual state of windless,the leakage volume of the accident could be calculated through the alarm time and concentration detection in the field area,so as to determine the type of leakage and the corresponding emergency disposal plan.

Table 2 Natural gas concentration data at each point of cofferdam top at different leakage times and different ambient temperatures in Cases 1,4 and 5

3.2.Effect of leakage location under windless condition

From the comparison of Cases 1 and 15 as shown in Figs.3(a)and 5,when the leakage point was at leak2,the diffusion area and velocity of the natural gas clouds were smaller than that at leak1,mainly because the leakage point of leak2 was further away from the cofferdam in the x direction.It took longer for cold gas clouds to form a collision roll after contact with the cofferdam.Therefore,it could be known that the farther the leakage point was from the cofferdam,the lower the velocity of leakage diffusion was.

From the comparison of Cases 12,13 and 14 as shown in Fig.6,when the number of leakage points increased to two,it could be seen that the leakage gas clouds diffused in a symmetrical state.The farther the leakage points were from each other,the faster the gas clouds diffused and the larger the covered area.When the two gas clouds met,they collide,and the gas clouds rose,forming a clear contour line.Then,under the action of heat exchange,the gas clouds moved in a wave and accelerated to the two sides.The climb that occurred when gas clouds met made it easier for them to cross the cofferdam and diffusion.

3.3.Effect of ambient temperature under windless conditions

From the comparison of Cases 1,4 and 5 as shown in Table 2 and Fig.7,it could be seen that the higher the ambient temperature was,the higher the concentration of natural gas around the cofferdam was,and the smaller the gas concentration difference was,which meant that the gas was spreading more evenly.The main reason for this phenomenon was that the higher the temperature difference,the faster the molecules spread.

3.4.Effect of cofferdam height under windless conditions

3.4.1.Effect on gas clouds overflow

Fig.8 showed the1/2 LEL contour profile of the natural gas clouds for the first 40 s of the leakage process at three cofferdam heights.From this comparison,it could be seen that when the leakage time was 10 s,the 1 m high cofferdam had been crossed by the gas clouds,and the gas clouds started to spread out of the cofferdam area.However,the 2 m and 3 m high cofferdams could still prevent the gas clouds from spreading.As the leakage time increased,the 2 m high cofferdam was crossed by the gas clouds at 20 s.The 3 m high cofferdam was crossed by the gas clouds at 30 s.When the leakage time was 40 s,the 1 m high cofferdam had completely lost its function,and the dangerous area of explosion expanded rapidly.The 2 m high cofferdam also began to be surmounted by the gas clouds at the far end from the leakage point.For the 3 m high cofferdam,the gas clouds only appeared a little overflow at the edge end,but the overall gas clouds were basically still in the cofferdam.It could be seen that the higher the cofferdam,the more the horizontal diffusion of the gas clouds could be restrained,and the longer the time required for the gas clouds to cross the cofferdam.

Fig.7.Relationship between leakage time and natural gas concentration data at each point of cofferdam top in different ambient temperatures.

Fig.8.1/2 LEL contour profile contrast of the natural gas clouds at different height of the cofferdam in Cases 1,16 and 17:(a)H=1.0 m,(b)H=2.0 m,(c)H=3.0 m.

Table 3 Natural gas concentration data at each point of cofferdam top at different leakage times in Case 1

Table 4 Natural gas concentration data at each point of cofferdam top at different leakage times in Case 16

Table 5 Natural gas concentration data at each point of cofferdam top at different leakage times in Case 17

Fig.9.Relationship between leakage time and natural gas concentration data at each point of cofferdam top in the 1 m high cofferdam(Case 1).

Fig.10.Relationship between leakage time and natural gas concentration data at each point of cofferdam top in the 2 m high cofferdam(Case 16).

3.4.2.Effect on the leakage concentration at the top edge of cofferdam

Fig.11.Relationship between leakage time and natural gas concentration data at each point of cofferdam top in the 3 m high cofferdam(Case 17).

From the comparison of simulation data as shown in Tables 3–5 and Figs.9–11,the concentration of natural gas gradually decreased with the height of cofferdam increasing in the cofferdam,but the concentration fluctuation would increase.The average concentration of natural gas at the top edge of cofferdam should be combined with the height of cofferdam and the degree of gas cloud overflow.Case 1,in which a large number of gas clouds overflowed,and Case 17,in which the high cofferdam height prevented the gas clouds overflow,their average concentration of natural gas at the top edge of the cofferdam was lower than that in Case 16,in which a small amount of gas clouds overflowed.The concentration at points (1,17,H) and (22,1,H) remained at a low level,while the concentration at points(22,34,H),(44,34,H),(44,17,H)and(44,1,H)rose rapidly and remained at a high level.The causes of this phenomenon might be that leakage of natural gas at low temperature under windless condition,in points(22,34,H),(44,34,H),(44,12 H)and(44,1,H)spread direction,could contact with the larger ground and cofferdam,through the heat exchange,gas cloud temperature and inflation accelerating,as shown in Fig.12.This indicated that under windless condition,there was a higher risk of explosion at the cofferdam farthest from the leakage source.Therefore,when emergency rescue was needed to enter the danger zone after the occurrence of leakage,it was preferred to enter from the direction of point(1,17,H)and(22,1,H),that is between the two gas cloud overflows.

3.4.3.Effect on gas diffusion

In Cases 1,16 and 17,when the leakage time was 200 s,the leakage point was set to close to simulate the control of the leakage situation in the LNG storage tank,so as to study the effect of cofferdam on the dissipation of natural gas.As shown in Table 6,the higher the cofferdam was,the slower the natural gas dissipated and the higher the concentration of natural gas remained in the cofferdam.This was mainly because when the cofferdam height was small,the horizontal diffusion resistance was small,so that the gas clouds could dissipate in the horizontal and vertical directions at the same time.While when the cofferdam height was large,the horizontal diffusion resistance was large,so that the gas clouds mainly dissipated through the vertical direction.As shown in Fig.13,the gas clouds in Case 1(H=1.0 m)completely overflowed the cofferdam in the 50 s for maximum dissipation,while the gas clouds in Case 17(H=3.0 m)needed to gradually heat exchange with the air to dissipate across the cofferdam,so that the concentration decreased slowly.In fact,according to the records in Fig.13,it could be observed that when the gas clouds overflowed the cofferdam to dissipate,there would be obvious bubble phenomenon.The bubble rose in the vertical direction and then spread around.Case 1(H=1.0 m)presented smaller bubbles;while Case 17(H=3.0 m)and Case 16(H=2.0 m)presented larger bubbles.

Although increasing the cofferdam height could effectively hinder the natural gas spreading over the cofferdam and reduce the explosion risk in the space dimension,it also increased the time of the gas clouds dissipation and the risk of explosion in the cofferdam area,especially in the vicinity of the tank.Therefore,the design of the cofferdam height in the tank area needed to be considered comprehensively,and the hazards caused by the leakage accident should be reasonably evaluated.

3.5.Effect of cofferdam height under windy conditions

From the comparative analysis of Figs.14 and 15,it could be seen that the cofferdam height also had a great influence on the diffusion characteristics of natural gas clouds under windy conditions.When the cofferdam height was 1.0 m,it had little resistance to wind,so the wind field played a decisive role on flow field in the cofferdam.Natural gas spread rapidly downstream of the wind after gas leakage.As shown in Figs.14(a)and 15(a),when the wind speed was 5 m·s–1,whether the wind direction was x or y,the time to reach stable diffusion was basically 30 s.Under the action of the wind field,the concentration of natural gas upstream of the leakage source was close to zero,and even there was a large safe area in the cofferdam,which was conducive to the emergency personnel to enter the cofferdam to plug the leak.But low cofferdams allowed large concentrations of gas clouds to spread beyond the cofferdam areas downstream of the wind field,increasing the risk of explosions downstream.

Fig.12.At different cofferdam heights,the vector cloud profile contrast of the temperature distribution in the plane y=7 and x=8 when the leakage time was 280 s.

Table 6 Maximum concentration of natural gas in cofferdam after leakage control

Fig.13.1/2 LEL contour profile contrast of the natural gas clouds after the leakage has stopped in the 50 s,100 s,150 s and 200 s.

When the cofferdam height was 2.0 m,it had a significant effect on the wind field,resulting in significant vortex flow in cofferdams upstream of the wind field.The vortex flow made the gas that was about to escape blow back into the cofferdam,resulting in a higher concentration of gas at the vortex.As shown in Figs.14(b)and 15(b),the safe area presented in the 1 m high cofferdam basically disappeared.In addition,the time needed for leakage diffusion to achieve stable were getting longer.When the wind speed in the x direction is 5 m·s–1,it increased to 40 s;when the wind speed in the y direction is 5 m·s–1,it increased to 70 s.This may be due to obstacles in the direction of the wind.In the x direction,the gas clouds had less contact with the cofferdam and the storage tank,so the heat exchange effect was smaller,and the diffusion flow became stable faster.

When the cofferdam height was 3.0 m,the effect of the wind field on the gas clouds diffusion in the cofferdam was similar to that was 2.0 m,except that the gas clouds diffusion disturbance of natural gas was larger.The time needed for the leakage diffusion to reach stability was further increased.When the wind speed in the x direction is 5 m·s–1,it increased to 80 s;when the wind speed in the y direction is 5 m·s–1,it increased to 110 s.

4.Conclusions

A series of three-dimension unsteady state numerical simulations were carried out for gas diffusion of LNG leakage accident in a LNG storage tank,of which the storage capacity was 8 ×150 m3.The effects of different heights of the cofferdam (1.0 m,2.0 m and 3.0 m),wind directions,ambient temperature,leakage location,leakage volume on the diffusion process of natural gas were investigated.Conclusions of the study could be summarized as follows:

(1) Under windless condition,the larger the leakage volume was,the faster the gas clouds could cross the cofferdam;the further away the leakage was from the cofferdam,the slower the gas clouds diffused.When the number of leakage points increased to two,it was found that if the two leakage points were further apart from each other,the gas clouds would spread faster and cover a larger area.When the gas clouds met,the gas clouds rose due to the collision,which made them easier to cross the cofferdam and spread out.

(2) Under windless condition,the higher the ambient temperature was,the higher the gas concentration around the cofferdam was,and the smaller the gas concentration difference was.

Fig.14.1/2 LEL contour profile contrast of the natural gas clouds at 5 m·s–1 in the x direction.

(3) Under windless condition,when leakage occurred,the higher cofferdam was beneficial to delay the outward diffusion of gas clouds.Moreover,the gas concentration distribution at the top of the cofferdam showed such a trend that the farther the cofferdam was from the leakage point,the greater the concentration was,and the faster the concentration rose.On the contrary,when the leakage stopped,the higher cofferdam went against the dissipation of gas clouds.Therefore,there should be a reasonable cofferdam height for the LNG storage tank.

(4) Under windy condition,the time to form stable leakage flow field was faster than that of windless;and the lower cofferdam further reduced this time.Due to low flow resistance for the low cofferdam,the safe zone even appeared in the cofferdam when the height of the cofferdam was 1 m,but no such safe zone appeared when the height of the cofferdam was 2 m.The wind direction affected the diffusion behavior of the gas clouds by influencing the obstacles it encountered.

Based on the results of the above simulation analysis,the following suggestions were proposed for the emergency rescue measures and design scheme of LNG storage tank.

(1) Considering the effects of barrier and dissipation,it was suggested that the design height of cofferdam should be controlled within 1.0 m to 2.0 m.

(2) During emergency rescue,the leakage volume of the accident could be calculated through alarm time and concentration detection in the field,so as to determine the type of leakage and corresponding emergency disposal plan.

(3) In case of strong wind condition,emergency rescue should try to enter the leakage area from the direction of the opposite wind;In the case of weak wind condition,the preferred entry point should be located at the place between the overflows of gas clouds,such as(1,17,H)and(22,1,H)point directions under leak1 leakage conditions,rather than absolutely farthest from the leakage.

Fig.15.1/2 LEL contour profile contrast of the natural gas clouds at 5 m·s-1 in the y direction.

Acknowledgements

The present study was supported by the Funding for post-doctoral research in Foshan City.