Scholarly article on topic 'The Influences of Key Factors on the Consequences Following the Natural Gas Leakage from Pipeline'

The Influences of Key Factors on the Consequences Following the Natural Gas Leakage from Pipeline Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Hongya Zhu, Zhanli Mao, Qingsong Wang, Jinhua Sun

Abstract The effects of the environmental dispersion (i.e. atmospheric stability, wind speed, temperature, humidity and ground roughness) and source release factors (i.e. pipeline diameter, length, pressure and release opening area) on the suffocation distance, flammable vapor cloud distance, overpressure distance and thermal radiation distance after the natural gas released from pipeline were evaluated and analyzed. The results show that all the environmental dispersion factors except humidity have an effect on the flammable vapor cloud distance. The more stable atmospheric condition, lower wind speed and smaller ground roughness lead to the longer flammable vapor cloud distance. The atmosphere temperature has a very limited influence on the flammable vapor cloud distance. The higher ambient temperature and larger humidity result in the longer downwind thermal radiation distance, while the atmospheric stability, wind speed and ground roughness nearly does not. All the four source release factors significantly influence the flammable vapor cloud distance and thermal radiation distance, which is due to the different release amount, release rate and initial momentum.

Academic research paper on topic "The Influences of Key Factors on the Consequences Following the Natural Gas Leakage from Pipeline"

Available online at www.sciencedirect.com

ScienceDirect Procedia

Engineering

Procedia Engineering 62 (2013) 592 - 601

www.elsevier.com/locate/procedia

The 9th Asia-Oceania Symposium on Fire Science and Technology

The influences of key factors on the consequences following the natural

gas leakage from pipeline

Hongya Zhua, Zhanli Maoa,b, Qingsong Wanga*, Jinhua Suna

aState Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China bDepartment of Fire Protection Engineering, Chinese People's Armed Police Force Academy, Langfang 065000, China

Abstract

The effects of the environmental dispersion (i.e. atmospheric stability, wind speed, temperature, humidity and ground roughness) and source release factors (i.e. pipeline diameter, length, pressure and release opening area) on the suffocation distance, flammable vapor cloud distance, overpressure distance and thermal radiation distance after the natural gas released from pipeline were evaluated and analyzed. The results show that all the environmental dispersion factors except humidity have an effect on the flammable vapor cloud distance. The more stable atmospheric condition, lower wind speed and smaller ground roughness lead to the longer flammable vapor cloud distance. The atmosphere temperature has a very limited influence on the flammable vapor cloud distance. The higher ambient temperature and larger humidity result in the longer downwind thermal radiation distance, while the atmospheric stability, wind speed and ground roughness nearly does not. All the four source release factors significantly influence the flammable vapor cloud distance and thermal radiation distance, which is due to the different release amount, release rate and initial momentum. © 2013 InternationalAssociation for FireSafetyScience. Published byElsevierLtd.AllRights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology

Keywords: Natural gas; Pipeline release; Flammable vapor cloud distance; Thermal radiation distance

Nomenclature

C concentration downwind at location (x, y, z) (kg/m3) Ca ambient velocity of sound (m/s)

Q release rate (kg/s) *P positive phase duration (s)

V wind speed (m/s) R fuel-air charge radius (m)

X downwind distance (m) R scaled distance (-)

Y crosswind distance (m) f combustion efficiency factor

z vertical distance (m) Xp transmissivity (-)

E energy of explosion or combustion (J) Xi distance from the source (m)

V0 volume of a flammable gas (m3) q total thermal radiation flux (kW/m2)

Hc heat of combustion (J/m3) q thermal radiation flux of point i (kW/m2)

P0 side-on absolute blast overpressure (Pa) D downwind impact distance (m)

P scaled blast overpressure (-) T temperature (°C)

Pa ambient pressure (Pa) H height of source release (m)

Greek symbols

Gy horizontal dispersion coefficient (m) vertical dispersion coefficient (m)

* Corresponding Author. Tel.: +86 551 6360 6455; fax: +86 551 6360 1669. E-mail address: pinew@ustc.edu.cn.

ELSEVIER

1877-7058 © 2013 International Association for Fire Safety Science. Published by Elsevier Ltd. All Rights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology doi:10.1016/j.proeng.2013.08.104

1. Introduction

Natural gas as a clean, efficient and convenient fuel has been used extensively in the residential, commercial and industrial sectors. The International Energy Agency predicted that the global average growth rate per annum of the amount of natural gas consumption will be 2.1%. The amount of natural gas consumption will reach 4060 billion m3 in the 2020 while 4790 billion m3 in the 2030 [1]. In China, with the energy strategy changing, the West-East natural gas transmission project and other branches have covered the most of the large and medium-sized cities, and a dense national natural gas pipeline network is being formed. Due to the corrosion, defect of pipeline itself, human errors or destruction from earthquake, the release of natural gas from pipeline sometimes is inevitable. Once the release occurs, it will induce serious consequences to human and environment as the characteristics of flammability, easy explosivity and dispersion of natural gas. Therefore, it is necessary to evaluate the hazardous distance of natural gas released from pipeline.

The studies on the natural gas release and dispersion have been widely conducted, and a series of theories and models were proposed. For the estimation of gas release rate and quantity, a small hole release model and a full-bore rupture release model of pipeline were presented based on the one-dimensional compressible flow equations [2-5]. Then, the concept of a large hole model of steady and unsteady pipeline release was first established and discussed [6]. Some approximate calculation formulas of release rate in specific conditions were also set up, and these models can promptly and rationally give the results compared with the complex theoretical equations [7-9]. On the other hand, for the research on dispersion process in the atmosphere, types of dispersion models which include Gaussian puff/plume model, Britter and McQuaid model, Sutton model, Box and similar model (e.g. HEGADAS, DEGADIS, Cox and Carpenter, Eidsvik, Fay, DENZ, Van Ulden), Shallow model (e.g. SLAB, TWODEE) and computational fluid dynamical (CFD) model, etc., have been developed. Furthermore, the risk analysis or consequence calculation based on the release and dispersion models were studied [10, 11].

However, CFD simulation is a very time consuming work, sometimes spending tens of hours or even more according to the complexity of models. And thus the complete computational simulations are carried out only in a few cases, for example, the worst credible scenarios, the most probable scenarios and so on for some shortcuts. It's useful to find a way that applying a small amount of calculation in typical scenarios to estimate the hazardous distance and trends close to the actual one in the other scenarios. In order to adopt this approach, the effects of the variability of the parameters on the results of the consequences should be required. At present, most of the studies addressing this aspect are conducted at the same time with model and no consistent thorough investigation [12, 13].

In this work, ALOHA [14], a widely known and used simulation software, developed jointly by NOAA and the Environmental Protection Agency (EPA), can be applied to model many release scenarios: toxic gas clouds, BLEVEs (Boiling Liquid Expanding Vapor Explosions), jet fires, vapor cloud explosions, pool fires and evaluate different types of hazard (depending on the release scenario): toxicity, flammability, thermal radiation, and overpressure. It was adopted to investigate the hazardous distance, including suffocation distance, flammable vapor cloud distance, overpressure distance and thermal radiation distance, corresponding to the potential accident consequence of natural gas when it escapes from the pipeline. The effects of the variation of the key parameters in each consequence were analyzed.

2. Simulation procedure

2.1. Models and key hazardous levels

The release of natural gas from pipeline is a complex process which includes five steps: (i) isothermal flow within a pipeline; (ii) isentropic expansion at the leaking point; (iii) jet release from the leaking point of a pipeline; (iv) formation of a flammable vapor cloud; (v) dispersion in the atmosphere. The potential major hazards include downwind suffocation effects, vapor cloud flash fire, overpressure from vapor cloud explosion and the thermal radiation from the combustion. To predict the corresponding hazard distance, the following models, including atmosphere dispersion model, explosion model and jet fire model, were employed.

The Gaussian plume model [15] is the most widely used dispersion model, which is suitable for neutral gas. As the molecular weight of natural gas is 16.04 g/mol, which is much lighter than that of air (the average molecular weight of air is about 29 g/mol). And then, this dispersion model was employed to estimate the natural gas concentration after leakage. The concentration of natural gas released from a continuous elevated point source can be given by Eq. (1)

The ground-level concentration is more useful as people live on the ground. Thus, for the concentration at ground-level,

i.e., z = 0, Eq. (1) can be simplified to

c(x, y,0, H ) =

—[expC-iexpC--^ )]

According to the calculated concentration distribution, the suffocation distance and flammable vapor cloud distance can be obtained by comparing it with the temporary emergency exposure limit (TEEL) and lower explosive limit (LEL) levels of natural gas as listed in Table 1. In an actual vapor cloud, there will be areas where the concentration is higher than the average and areas where the concentration is lower than the average. This is called concentration patchiness. Because of concentration patchiness, even the average concentration is below the LEL, it cannot ensure that the whole zone is safe. In this paper, it uses 60% of the LEL as the red threat level and 10% of the LEL as another common threat level.

Table 1. Key hazardous levels of pipeline release of natural gas

Classification Accidental consequence

Level-3

Level-2

Level-1

No burning Suffocation (suffocation distance)

Flash fire (flammable vapor cloud distance)

Blast (overpressure distance) Burning Jet fire (thermal radiation distance)

TEEL-3: 200000 ppm 60% LEL (26400 ppm)

destruction of buildings (55.16 kPa)

potentially lethal within 60 s (10.0 kW/m2)

TEEL-2: 5000 ppm

serious injury likely (24.13 kPa)

2nd degree burns with 60 s (5.0 kW/m2)

TEEL-1: 3000 ppm 10% LEL(4400 ppm)

shatters glass(6.89 kPa)

pain within (2.0 kW/m2)

Explosion is another potential accidental consequence of natural gas vapor cloud when it is ignited by spark, flame or detonation. Overpressure, also called a blast wave, refers to the sudden onset of a pressure wave after an explosion which is a major hazard associated with any explosion. The pressure wave radiates outward like a giant burst of air, crashing into anything in its path, which can damage buildings or even knock them flat and cause injuries or deaths to people. The overpressure effect distance can be calculated by TNO explosion model [16] associated with an overpressure level of

concern, which is a threshold level of pressure from a blast wave.

The total vapor cloud energy: E = V0 Hc (3)

Scaled overpressure: P0 = (P0 -Pa)/Pa (4)

Scaled positive phase duration: tp = tpca(Pa /E)1/3 (5)

Scaled distance: R = R(Pa /E)1/3 (6)

A jet fire will occur when the natural gas is rapidly released from pipeline and immediately catches on fire [17]. Thermal radiation is the primary hazard associated with a jet fire. The length of a jet fire is defined from jet tip to the location of LEL while heat is regarded as being sent out by a number of equivalent radiation sources at the central axis. The heat flux of every point at the central axis is:

E = fQHc / n (7)

The thermal radiation flux of a point at a distance xi from the source can be calculated by:

q = XPE /(4nxf) (8)

Then the total thermal flux equals the sum of radiation from every point.

q = Z qi

2.2. Key influential factors and values design

To study the effect of the variability of key factors on the accident consequences, a serious of release scenarios were designed and calculated, and the conditions are close to a real accidental event as possible. In this paper, the parameters are divided into environmental dispersion parameters and source release parameters. The former includes atmospheric stability, wind speed, temperature, humidity and ground roughness, and the latter includes pipeline diameter, pipeline length, pipeline pressure and release opening area.

As the atmospheric stability and wind speed are not isolated, they are valued according to corresponding relation between Pasquill-Gifford atmospheric stability-category classification [18] and wind speed as well as daytime and nighttime are considered, respectively. Likewise, different temperature and humidity distances have been used for daytime and night-time conditions, the values being derived from the meteorological records of Hefei in China. The ground roughness is a raw measure of the obstacles surrounding the release point which also can influence the dispersion of the vapor cloud. The value of ground roughness can represent different ground types. In the present analysis, a value of 3 cm represents open country while 10 cm represents crop area, 30 cm represents village, suburb or scattered trees, and 100 cm represents urban or forest.

For the source release parameters, the variability is designed based on the conditions of natural gas pipeline used in China. Based on the data and trend in the literature [19], the natural gas pipeline can be divided into big diameter pipeline (<p > 500 mm), medium diameter pipeline (350 mm < (p < 500 mm) and small diameter pipeline (<p < 350 mm) by diameter. Also, it can be divided into high-pressure pipeline (P > 9.8 MPa), medium-pressure pipeline (1.568 MPa < P < 9.8 MPa) and low-pressure pipeline by design pressure of pipeline. The variability of pipeline diameter and pipeline pressure parameters are assumed as big diameter pipeline (occupied 59%) and medium-pressure pipeline (occupied 66%) [20]. The values of release area are consistent with pipeline release location and damage size [21].

All the values of parameters are reported in Table 2. The parametric analysis was carried out by changing one parameter in its variability distance, and keeping all the others in constant. The constant value of each parameter which is average or typical is marked in bold.

Table 2. Variability of environmental dispersion parameters and source release parameters

Period Environmental dispersion parameters of the

Source release parameters

day Atmospheric Wind speed Temperature Humidity Roughness Pipeline Pipeline Pipeline Release area

stability (m/s) (°C) (%) (cm) diameter (cm) length (km) pressure (MPa) (%A)

A B C D

1.5;2;2.5 1.5;3;4.5 2;4;6 5;6;7

10;20;30;40 30;40;50;6

Night E F

1.5;3;4.5 -10;0;10;20 50;60;70;8 1.5;2;2.5

3;10;30;100 30;40;50;60 1;3;5;7 3;5;7;9

20;50;80;100

3. Results and discussion

The potential hazards from natural gas after leaking are downwind suffocation, vapor cloud flash fire and overpressure (blast force) from vapor cloud explosion. It also could result thermal radiation hazard if it is ignited after escaping from pipeline. Thus, all the hazard zones respectively according to each potential accident consequence were calculated. It was found that the effects of the variability of the factors on the suffocation distance, flammable vapor cloud distance and overpressure distance are similar, just with different absolute values of each hazard distance. To avoid repetition, only the flammable vapor cloud distance and thermal radiation distance are presented here and the effects of the parameters were also analyzed. At the end of this section, the comparison among all the four hazard distances was conducted.

3.1. Flammable vapor cloud distance of natural gas vapor cloud

• Effects of environmental dispersion parameters

According to the design schemes in Table 2, the simulation results of the flammable vapor cloud distance of the natural gas corresponding to 60% LEL are plotted in Fig. 1, and the effect of the five environmental dispersion parameters (i.e. atmospheric stability, wind speed, temperature, humidity and roughness) considered were studied. The results show that the parameter of humidity almost has no effect on the flammable vapor cloud distance. And the effects of the other environmental dispersion parameters are shown in Fig. 1.

It can be seen that the higher wind speed leads to the shorter flammable vapor cloud distances for any atmosphere stabilities. This is because the wind push the natural gas vapor cloud to disperse forward the downwind direction and the concentration of the cloud is diluted continuously, then shortening the flammable vapor cloud distance. Therefore, the wind speed has a positive effect on the flammable vapor cloud distance. For the atmospheric stability influence, the flammable vapor cloud distances decrease with the reduction of the stability (from class F to A). Under the stable class F condition, it has the longest distance while it has the shortest distance under the unstable class A. This behavior is due to the high stability hampers a fast dispersion of the cloud. Meanwhile, there is no significant difference for class D in daytime and night-time.

Ground roughness (cm)

Fig. 1. The downwind flammable vapor cloud distance corresponding to 60% LEL, as a function of (a) wind speed, (b) atmosphere temperature and (c) ground roughness.

For the effect of atmosphere temperature, Fig. 1(b) shows that the downwind flammable vapor cloud distance increasing very slightly with the temperature rising up under the stability ranks from A to E and the temperature is almost not effect under the stability of F. The increasing temperature promotes the turbulence of atmosphere and then the flammable vapor cloud distance is larger. However, the effect of the variability of the atmosphere temperature on the flammable vapor cloud distance is limited which can be seen from the absolute value of the slightly increasing distance, and even no effect when the stability is much greater F. The absolute values of the distance in stability class B are nearly approach to that of class A, only with slightly bigger. This indicates that the temperature is more important than the atmosphere stability when it is unstable (i.e. class A and class B). Again no big significant difference between class D in daytime and night-time, but slightly longer distances were found during night-time at the same temperature condition.

The ground roughness influences the flammable vapor cloud distance too, as shown in Fig. 1(c). The larger values of ground roughness, the shorter flammable vapor cloud distances are, but the relation between them is not linear. The effect of average ground roughness is not significant when the value is below 10 cm, or above 30 cm. When the ground roughness is between 10 cm and 30 cm, it decreases sharply with the increasing of the ground roughness. On the whole, rough ground hampers the dispersion of the natural gas vapor cloud, but the effect is very limited when the roughness is smaller than 10

cm, sometimes even accelerate the turbulence and then increase the flammable vapor cloud distance. When the roughness distance is from 10 cm to 30 cm, it seriously hampers the dispersion and the flammable vapor cloud distance decreases significantly. And then for the rough ground above 30 cm, it no longer affects the flammable vapor cloud distance. • Effects of source release parameters

The effects of four source release parameters including pipeline diameter, length, pressure and release opening area are shown in Fig. 2. It can be found that all the four source release parameters significantly affect the flammable vapor cloud distance and the roles of which are more important than the atmosphere stability parameter when it is unstable (i.e. class A and class B).

E 1600

e 1400

(O 1200

el 1000

a m 800

al 600

ni 400

n % 200

. —■— A-day —i—i—i—i—i—i—i—

. —•— B-day

, —C-day

—w— D-day

" —4— D-night

' —E-night

♦ F-night

;........i ;

35 40 45 50 55 Pipeline diameter (cm)

—m— A-day

—•— B-day

• —▼— D-day

- —«— D-night

. —E-night

—♦— F-night / >--^ -

2 3 4 5 6 7 Pipeline length (km)

Fig. 2. The downwind flammable vapor cloud distance corresponding to 60% LEL, as a function of (a) pipeline diameter, (b) pipeline length, (c) pipeline pressure and (d) release opening area.

Figure 2(a) shows that the parameter of the pipeline diameter almost shows a linear relationship with the downwind flammable vapor cloud distance. Under any atmosphere stabilities, the bigger the pipeline diameter, the larger the distance is which is due to the bigger release amount and release rate. Class F shows the largest distance while class A has the shortest distance. A slightly higher distance for class D during night-time than that of daytime can be found. This is the competition result of the smaller wind speed (increasing the distance) with the lower temperature (decreasing the distance) during nighttime, which agrees with the trend studied in previous.

It can be seen from Fig. 2(b), the pipeline length influences the flammable vapor cloud distance greatly. On the whole, the distance increases nonlinearly with the increase of pipeline length, mainly due to the varying of the release amount. The increasing rate is becoming smaller and smaller with the pipeline length (i.e. slope) increasing gradually. Because the natural gas is accumulated along the way when it releases from the pipeline and disperses as a plume forward the downwind direction, which causes the distance not proportional to the release amount (presented pipeline length here).

Figure 2(c) shows that the flammable vapor cloud distance increases linearly with the increasing pipeline pressure. This coincides with the real accident and is easy to understand. First, the pipeline with higher pressure contains more amount of natural gas, thus the total release amount is larger. Second, the release rate increases with pressure. Third, the initial momentum of release is also bigger. All of the three reasons lead to the longer flammable vapor cloud distance when the pipeline pressure is higher.

The release opening size effects on the flammable vapor cloud distance as well. The downwind flammable vapor cloud distance increase greatly at first and then gradually decreases at a smaller rate with release opening size increasing. When

the release opening area is equal to the pipeline area (a full-bore rupture release), the release distance is larger than that of release opening area equal to 20%.

3.2. Thermal radiation distance of natural gas jet fire

• Effects of environmental dispersion parameters

The thermal radiation is the main hazard of a jet fire when the natural gas release from pipeline and catches on fire immediately. The distances of which corresponding to 2nd degree burns with 60 s (5.0 kW/m2) under the five environmental dispersion parameters are shown in Fig. 3. The results show that the effects of the wind speed and ground roughness on the thermal radiation distance are very small and nearly can be ignored. As the jet speed is usually much bigger than the wind speed and the rough ground does not hinder the thermal radiation.

-15 -10 -5 0 5 10 15 20 25 30 35 40 45 Atmosphere temperature (°C)

o 195-

E 190-

40 50 60 70 Humidity (%)

Fig. 3.The downwind thermal radiation distance corresponding to 2nd degree burns with 60 s (5.0 kW/m2), as a function of (a) atmosphere temperature and (b) humidity.

The influence of the temperature on the downwind thermal radiation distance covered by the jet fire is plotted in Fig. 3(a). It can be observed that a quite linear trend is presented between them. However, it's different from the effect of the temperature on the flammable, even completely opposite. The downwind thermal radiation distance decreases with the increasing of temperature, which is because that the thermal radiation flux is larger with the higher temperature and thus makes the distance shorter according to Eq. (8). Furthermore, the atmosphere stability almost doesn't affect the thermal radiation distance at the same temperature. There is no great difference from stability class A to F, within the disparity of several meters.

The humidity effect is shown in Fig. 3(b), and the thermal radiation distance is shorter when the humidity is larger. However, the effect of humidity on the distance is not very great, which can be seen from the absolute value of the difference among different humidity. At the same moment of the day, the average temperature is the same, and the thermal radiation distance in the condition of neutral atmosphere stability is larger than the other atmosphere conditions, whatever it is stable or unstable. • Effects of source release parameters

The effects of the four source release parameters on the thermal radiation distance associated with the jet fire are great. Likewise, the parameter of the atmosphere stability doesn't affect the thermal radiation distance.

The thermal radiation distances of four sources release parameters are shown in Fig. 4. It can be seen from Fig. 4(a), linear relationship between the downwind thermal radiation distance and the pipeline diameter was observed. The thermal radiation distance increases with the increasing pipeline diameter. Because the pipeline contains more natural gas with the bigger pipeline diameter, thus the total heat flux is larger and expands the impact distance of the thermal radiation.

Figure 4(b) shows that the downwind thermal radiation distance nonlinear increases with increasing pipeline length. The slope is gradually falling down, i.e. the growth rate of the downwind thermal radiation distance for the same interval pipeline length is decreasing. Compared to the downwind flammable vapor cloud distance, the downwind thermal radiation distance is much smaller either in absolute or increased value. Another difference in the effect of the atmosphere stability is that the downwind thermal radiation distance is not affected by the atmosphere stability as the downwind flammable vapor cloud distance does.

The downwind thermal radiation distance in Fig. 4(c) is linearly proportional to the pipeline pressure with all the other parameters keep in same. The higher pipeline pressure gives rise to the longer downwind thermal radiation distance because of the larger jet release rate.

It can be observed from Fig. 4(d) that the effect of the release opening area on the downwind thermal radiation distance is rather significant, but the trend is not single. The downwind thermal radiation distance increases at the beginning and up to the maximum at 50% opening area, and then slightly decreases again.

—■— A-day

—•— B-day

—A— C-day

—T— D-day

—4— D-night

—►— E-night

♦ F-night

35 40 45 50 55 Pipeline diameter (cm)

® 180

"rä 170

—■— A-day

—•— B-day

—a— C-day

-w— D-day

—D-night

► E-night

♦ F-night

Pipeline length (km)

co 150 m

Fig. 4. The downwind thermal radiation distance corresponding to 2nd degree burns with 60 s (5.0 kW/m2), as a function of (a) pipeline diameter, (b) pipeline length, (c) pipeline pressure and (d) release opening area.

thermal radiation distance

flammable distance

i suffocation distance

200 400 600 800 1000

Downwind hazardous distance

Fig. 5. comparison within the downwind suffocation distance, flammable vapor cloud distance, overpressure distance and thermal radiation distance in the same condition.

3.3. Comparison within the different accident consequence distance

When the natural gas escapes from the pipeline, the potential accident consequences including suffocation, flash fire, blast and jet fire maybe occur under some certain conditions. Every hazard distance corresponding to the potential accident consequence should be evaluated, and by comparison within the calculation results of four accident consequence distances, the maximum hazard distance which is an important index for the emergency decision can be known.

The comparisons are given in Fig. 5. It is worth noting that, only the downwind hazardous distances were considered, all the environmental dispersion and source release factors are the same. The atmosphere stability is class D, the average wind

speed is 6 m/s, the ambient temperature is 20 °C, the humidity is 50%, the ground roughness is 100 cm, pipeline diameter is 50 cm, pipeline length is 5 km, pipeline pressure is 5 MPa and release opening diameter equals to pipeline diameter.

It can be observed that, (i) generally the flammable vapor cloud distance corresponding to the 10% LEL is the maximum hazardous distance; (ii) all the thermal radiation distances corresponding to three different levels of concern are shorter than any other hazardous distances, but the thermal radiation not only affects the downwind area but also the upwind area. and the upwind thermal radiation distance can be considered as approximately equal to the downwind thermal radiation distance; (iii) the overpressure distance (417 m) and the flammable vapor cloud distance (530 m) respectively corresponding to the most serious level of concern both are larger than that of the suffocation distance (135 m), which indicates that blast and flash fire are more dangerous than the suffocation accident.

4. Conclusions

In this study, the potential hazardous distances including suffocation distance, flammable vapor cloud distance, overpressure distance and thermal radiation distance of natural gas released from the pipeline were calculated. Two types of parameters, respectively environmental dispersion parameters (i.e. atmospheric stability, wind speed, temperature, humidity and roughness) and source release parameters (i.e. pipeline diameter, pipeline length, pipeline pressure and release area) were considered in the evaluations. And the effects of the factors on the hazard distance were analyzed. The mainly results can be summarized as follows:

• The effects of the factors on the suffocation distance, flammable vapor cloud distance and overpressure distance are similar, and the difference lies in the absolute values of each hazardous distance.

• The flammable vapor cloud distance is almost not affected by the humidity. The stable atmospheric condition and a lower wind speed lead to the longer flammable vapor cloud distance. The atmosphere temperature has a very limited influence on the flammable vapor cloud distance. The effect of the average ground roughness is not significant when it is below 10 cm or above 30 cm, but it shows a great influence when the roughness is between 10 cm and 30 cm. All the source release factors have a great effect on the flammable vapor cloud distance. The pipeline diameter and the pipeline pressure both almost show a positive linear correlation with the downwind flammable vapor cloud distance. The increasing pipeline length gives rise to the longer impact distance with the gradually smaller increasing rate. And the release opening size shows a specific influence on the flammable vapor cloud distance that increases greatly at first and then gradually decreases at a smaller rate.

• The effects of the environmental dispersion parameters on the thermal radiation distance are different in the flammable vapor cloud distances. The atmospheric stability, wind speed and ground roughness nearly does not affect the thermal radiation distance. The higher ambient temperature and larger humidity give rise to the longer downwind thermal radiation distance. All the four source release parameters significantly influence the thermal radiation distance. The pipeline diameter and pressure are linear to the thermal radiation distance while the pipeline length and release area are nonlinear to that.

• By comparing the four accident consequence distances, it was found that the maximum hazardous distance is flammable vapor cloud distance.

Acknowledgements

This work is supported by the Key Technologies R&D Program of China during the 12th Five-year Plan Period (Grant no. 2011BAK07B01 and 2012BAK13B01), National Natural Science Foundation of China (No.91024025) and Program for New Century Excellent Talents in University (no. NCET-12-0514).

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