1

Hazard Range in Case of Natural Gas Jet Fire

K. M. Marghany# and M. S. Abdel-Kader

Marine Engineering Technology Department,

College of Maritime Transport and Technology

Arab Academy for Science, Technology and Maritime Transport

P.O. Box: 1029, Alexandria, Egypt

# marghanykhaled@yahoo.com

Abstract- The gas industry involves production, processing and

conveying Natural Gas (NG), in addition to a number of

hydrocarbon gases in relatively large quantities and at high

pressures. An accidental release of such pressurized gases can

lead, if ignited, to a serious jet fire hazard to personnel and to

equipment and properties. Hence it is important for operators to

have good understanding of the fire hazards posed by their

installation and to be able to demonstrate that their safety

procedures can manage the risks effectively.

The present endeavor is primarily concerned with this

particular issue and uses a specialized computer software

developed by Trinity Consultants, USA, which is called

"DEGADIS+". This software is used to study the consequences

in case of different scenarios of jet fire. In each case the

distance to safe area, which corresponds to a heat radiation level

of 5 kW/m², is sought.

The results obtained are thoroughly analyzed and

compared to similar results of other investigators and pertinent

analyses are furnished. Results are illustrated in a number of

forms in order to make them easier to comprehend and use.

Tremendous amount of computer outputs are available, but

because of space limitation are not included in this paper. The

distance to safe area, within the context used herein, is found to

change in direct proportion with pipeline pressure and hole

diameter and to change inversely with pipeline temperature.

Index Terms- DEGADIS+, Hazard, Jet fire, Natural gas, Risk

assessment

I. INTRODUCTION

Natural Gas (NG) is one of the most important sources of

clean, environmentally friendly and cheap energy. Recently,

the NG market has been rapidly growing in demand,

production and consumption. Many studies have been

conducted to assess the risks that may arise during the

individual stages of this industry, and to avoid the adverse

effect in capital recovery and project safety. A continuous

attention to the safety of NG transmission and distribution is

needed, as NG is always transported in pressurized pipelines.

Once the pipeline is ruptured, accidently or because of

corrosion, the high pressure NG rapidly releases to the

surrounding and likely causes fire or explosion if ignition

source is found near the pipeline zone [ 1].

The release of NG from a high pressure pipeline to

atmosphere is, in general, a complex process which includes:

(a) gas flow within the pipeline, (b) gas isentropic expansion

near a hole on the pipeline, (c) gas jet release from the hole,

and (d) gas dispersion in the atmosphere. Following these

steps, or even earlier, jet fire may take place, if the dispersed

NG is ignited.

Computational models developed to study this process

fall into three categories: (a) empirical, e.g. Joy and Ahn [2]

and Lou et al. [3], (b) integral, e.g. DEGADIS+ [4] and SLAB

[5] and (c) Computational Fluid Dynamics (CFD), e.g. Chan

[6], Luketa et al. [7], Garcia et al. [8], Sklavounos and Rigas

[9], and Cormier et al. [10].

Empirical models have the lowest accuracy, but are less

expensive and easy to use. Integral models, though less accurate than

CFD models, have the advantage of lower computational cost. As

such, the present study is primarily concerned with assessing the

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hazard range associated with NG jet fire using DEGADIS+. The

model is used to calculate the distance to safe area, which

corresponds to a heat flux of 5 kW/m². The effect of different source

release parameters of NG jet, namely: temperature, pressure and hole

diameter, on the distance to safe area is investigated.

II. PREVIOUS WORK

A number of important studies, [11] to [16] have been

conducted, which address the different aspects of safety

related to NG and LNG fires. The present paper, however,

will be confined to assessing the hazard range of NG jet fires

only. For example, Lehr and Beatty modeled jet fire as a

circular cylinder that radiates upward and uniformly over the

cylinder’s surface. Flame tilt due to wind was not considered.

Incident thermal radiation to an object was determined by

calculating the product of the average emissive power at the

flame surface, an atmospheric transmission factor, and a

geometric view factor. Average emissive power was

calculated by an empirical correlation. The transmission factor

was calculated by a relation based on thermal radiation from a

nuclear bomb explosion. Burn regression rate was determined

experimentally. The rates were found to vary from 0.4 to 1

mm/s.

The radiant flux to an object was estimated by taking a

fraction of the heat release rate, averaged over the fire’s

duration, and dividing by the square of the distance to an

object [13]. Lowesmith et al. [14] studied jet fire associated

with NG principally related to gas transmission pipelines, as

large and full scale experiments were undertaken at test site to

study fire characteristics.

The flame was modeled as an elliptical cylinder ( tilted

flame). The base of the flame was assumed to increase due to

flame drag and was approximated by an empirical correlation.

Flame angle and flame length were calculated by using

empirical formulae. The flame is divided into two zones: a

clear zone with no smoke, and a zone in which a fraction of

the flame is obscured by smoke. The length of the clear zone

is determined by an empirical correlation [13].

Dong et al. [1] investigated the hazard range of the NG jet

released from a high pressure pipeline using a one-

dimensional integral model combined with a release model.

Source release parameters, as well as environmental and time

parameters were considered, and the former, including the

pipeline length, the operation pressure of the pipeline and the

release hole diameter, were found to dominate the gas release

rate through a hole and, consequently, the length and width of

gas jet release. The present endeavor, therefore, will be

concerned with source release parameters only.

III. MODLEING

Fire modeling is a technique for calculating the effect of

different types and scenarios of fire. Modeling can be used to

estimate fire parameters at thousands of sites for the cost of

measuring data at a single site with nearly the same accuracy.

Also modeling can be used where measurements are not

available or difficult to take [17]. Previous studies have

indicated that the merits of the modeling results obtained

depend on the potential capabilities and the salient features of

the model used. Furthermore, model results should be

compared with other models' results, or preferably to

experimental measurements, in order to judge the validity of

such results. The DEGADIS+ model [4,18] is one of the most

widely used and internationally recognized models in this

field. For this reason and for availability reasons as well, this

particular model will be used in the current study.

The DEGADIS+ model was developed to model heavier-

than-air gaseous release, although it has been used to model

lighter-than –air gases, such as NG, as well. In the current

study, the model is used to evaluate the hazard range

associated with NG jet fires in terms of the distance at which

the heat flux is 5 kW/m², i.e. the distance to safe area. This

particular value was chosen since the main concern is to

determine the distance to safe area for personnel as

recommend by the USA National Fire Protection Association

(NFPA). For equipment and properties, the distance to safe

area is definitely greater and corresponds to a heat flux of 20

kW/m² [19].

A. Program of Study

Table 1 shows the different scenarios of jet fire considered

in this study.

Table 1 Program of study

No. Parameter Values considered

1 Pipeline temperature (ºC) -25, -50, -75, -100 and -125

2 Pipeline pressure ( bar) 1.8, 2.0, 2.5, 3, 3.5 and 4

3 Hole diameter (mm) 3.2, 9.5, 15.9 and 25.4

3

B. Model input data

The input data include meteorological, chemical, as well

as source and release data.

1) Meteorological data

In this study the following meteorological conditions are

considered:

i) Ambient temperature: 25℃

ii) Ambient pressure: 1 atmosphere

iii) Relative humidity: 50%

iv) Wind speed: 1.5 m/s

v) Stability Class: very stable

2) Chemical data

The chemical data include:

i) Type: NG Light (methane)

ii) Molecular weight: 16.04 g/mole

iii) Lower Flammable Level (LFL): 5%

iv) Upper Flammable Level (UFL): 15%

3) Source and Release data

The source and release data include:

i) Source of leak: pipeline

ii) Pipeline temperature, pressure and hole diameter:

model is run for different values of each of these

parameters, as detailed in Table 1.

C. Model Output Data

Model output results are obtained in a variety of forms,

namely:

1) Summary report, 2) Chart and 3) Map

1)Summary--report: Table 2 shows a typical summary report

on a jet fire for a pipeline temperature of -100℃, pressure

of 4.05 bar and hole diameter of 3.2 mm. As may be seen

from this figure the report is divided into 5 distinct parts:

i) Physical data, which include molecular weight,

boiling point and vapour density,

ii) Pipeline data, which include pipe diameter,

temperature, pressure, and hole diameter,

iii) Ambient conditions, which include air

temperature, ambient pressure and wind speed,

iv) Results, which include flame length, average

flame diameter and maximum emissive power,

and finally

v) Output, which includes different levels of

radiation and related distances from the center of

jet.

2) Chart

Figure 1 shows the relation between radiation level

(or heat flux) and distance from the centre of jet for jet

fire in a natural gas pipeline with a pressure of 2.0 bar

and a hole diameter of 25.4 mm. Note that three levels of

heat flux are shown on the chart; these are: 5 kW/m²,

10kW/m², and 15 kW/m², corresponding to different

types of objects exposed to fire radiation. This particular

point will be handled in more detail later in the analysis.

Note also that the height of interest at which all results

are obtained is 0.5 m, because the effect of heat flux on

human beings at this particular height is the main

objective of the present study.

3) Map

Figure 2 shows the relation between heat flux and distance

from center of jet for a jet fire of natural gas in a pipeline with

a pressure of 4.0 bar , a temperature of –75oC and a hole

diameter of 25.4 mm. The map shows that the heat flux is 15

kW/m² at a distance of 6.76 m from the centre of jet , 10.0

kW/m² at 8.69 m and 5 kW/m² at 12.76 m.

IV. RESULTS AND DISCUSION

For the different scenarios of NG jet fire the model is run

to calculate the distance at which the heat flux is 5 kW/m², i.e.

the distance to safe area. This particular value is adopted by

many international fire protection associations, such as the

American NFPA. The reason why this particular value of heat

flux (5 kW/m²) was chosen is because higher values would

cause severe casualties to human beings, as may be seen from

Fig. 3. For instance, a heat flux of 5 kW/m² is likely to cause

second degree burns to human beings exposed to this flux for

about 45 seconds, whereas the same harm may be caused

when a shorter exposure time of about 18 seconds is reached

for a heat flux of 10 kW/m². As such, fire fighters should be

able to operate conveniently with reasonable safety at a heat

flux level of 5 kW/m². It is to be noted that higher values of

heat flux can be considered when the effects of different fire

scenarios on equipment and properties are studied. Moreover,

it is worth- mentioning at this point that the value of 5 kW/m²

has recently been reduced to 4.7 kW/m² for higher safety

[20].

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Table 2 A typical summary report on a jet fire for a pipeline temperature of – 100 °C, a pipeline pressure of 4.05 bar and a

hole diameter of 3.2 mm.

Jet fire model

Gas outflow due to pipeline leak fuel

Name: NG light (Methane) physical state : vapor phase only

Constant properties

Molecular weight : 16.04 g/mol Boiling point : -161.0 °C Critical temperature : 190.55 K

Critical pressure : 46.0 bar Ratio of specific heats of vapor : 1.11

Calculated Properties

Vapor compressibility factor : 0.956 Vapor density : 4.73 kg/cu m

Pipeline Data

Pipeline temperature : -100.0 °C Pipeline pressure (absolute) : 4.05 bar

Pipeline diameter : 500.0 mm Hole diameter : 3.2 mm Choked flow : Yes

Discharge coefficient : 0.63 Substance release rate : 4.36E-03 kg/s

Local Ambient Conditions

Air temperature : 25.0 °C Ambient pressure : 1.01 bar Wind Speed : 1.5 m/s

Results

Maximum flame extent : 0.99 m Visible flame length : 0.8 m

Flame lift-off : 0.19 m Average flame diameter : 0.27 m

Maximum emissive power: 380.0 kW/m² Height for radiation calculation : 0.5 m

---------------------------------------------

Distance from Radiation

jet centerline

(m) (kW/m²)

-----------------------------------------------

1.00 25.10

1.30 15.0

1.60 10.0

2.00 6.31

2.23 5.0

3.00 2.75

4.00 1.52

5.00 0.95

6.00 0.65

7.00 0.47

10.00 0.23

25.00 0.03

50.00 0.01

--------------------------------------------------

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Table 3 is a summary of results of jet fire scenarios with

different pipeline temperatures, pressures and hole diameters,

where the distance to safe area ( distance to 5 kW/m² radiation

flux) in each case, as evaluated by the model, is given. From

this table it is noted that:

i) Increasing gas temperature inside the pipeline reduces

the distance to safe area. This may be attributed to the

fact that higher temperature of gas increases dispersion

of jet. Consequently, the heat radiation of fire is reduced.

ii) When increasing the gas pressure inside the pipeline

the distance to safe area increases. This may be

explained as follows; increase of pressure inside the

pipeline increases gas density and outlet velocity.

Consequently, jet volume and resulting heat radiation

increase.

iii) Increasing the hole diameter increases the distance to safe

area, primarily because of the proportional increase in the

release rate. Dong et al. [1], however, have concluded that

this trend holds true for hole diameters below 200 mm

only; for higher values of hole diameter, they argued, the

effect is negligible.

It is to be noted that the current study is essentially a

parametric study, where a single parameter is changed at a

time. A more involved study should consider the

consequences of changing any of these parameters on the rest

of the parameters considered. For instance, Dong et al. [1]

have shown that both pressure and temperature of the NG

released though the hole decrease with increasing hole

diameter. The current study, however, has not considered the

effect of changing pipeline temperature, pressure or hole

diameter on the parameters of the NG released through the

hole.

It should also be mentioned here that a source height of 0.5

m was chosen in the current study, primarily because the

study is concerned with the effects on human beings.

However, Dong et al. [1] have considered a number of such

heights, in the range from zero to 20 m, since their study

involved both personnel and equipment; they found that at the

height of 2 m the hazard range reaches its largest value.

The results listed in Table 3 are also depicted in Figures

4 through 6, where distance to safe area, corresponding to 5.0

kW/m² heat radiation level, is plotted as function of different

NG pipeline temperatures, pressures and hole diameters. On

the same figures, the results are also drawn and

corresponding correlation coefficients listed. It is clear from

these figures that the distance to safe area reduces when

increasing NG temperature inside the pipeline and increases

when increasing NG pressure inside the pipeline and hole

diameter.

If the jet is confined and within the flammable range then

the flame can accelerate and result in an explosion. The

magnitude of the explosion damage depends on the amount of

NG above the lower flammable range [21].

For the heat flux of 4.7 kW/m2, the distance to safe area has

also been evaluated, corresponding to different values of

pipeline temperature, pressure and hole diameter, as listed in

Table 3 and shown in Figs. 4-6. It is clear from these results

that higher values of distance to safe area are generally

obtained. Moreover, the relative difference, in percent,

between the distances to safe area corresponding to both heat

flux values was also calculated, cf. Table 3, and found to

range from 3.1% to 3.85%. As such, the value of 4.7 kW/m2

represents a more conservative choice when assessing the risk

associated with jet fires.

V. CONCLUDING REMARKS

In the present study the integral model DEGADIS+ was

used to evaluate the hazard range associated with ignition of a

NG jet released horizontally from a high-pressure pipeline in

terms of distance to safe area corresponding to a heat flux of 5

kW/m2. The effects of NG temperature and pressure, as well

as hole diameter were investigated. The results are, in general,

in good agreement with results of other investigators and

show that the hazard range, within the context used herein,

increases with operating pressure and hole diameter, and

decreases with temperature.

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Table 3 Jet fire model results

I) Effect of temperature ( Pipeline pressure = 4 bar and hole diameter = 25.4 mm )

Pipeline temperature ( oC ) -125 -75 -50 -25

Distance to safe area (m) 5 kW/m2# 14.2 12.76 12.2 11.7

4.7 kW/m2 14.67 - 12.63 12.15

Percentage relative difference, ∆D* (%) 3.31 - 3.52 3.85

II) Effect of pressure ( Pipeline temperature = - 100 oC and hole diameter = 25.4 mm )

Pipeline pressure ( bar ) 1.8 2.0 2.5 2.8 3.0 3.55 4

Distance to safe area (m) 5 kW/m2 10.6 10.8 11.5 11.9 12.2 12.8 13.4

4.7 kW/m2 10.97 - 11.92 - 12.59 - 13.85

Percentage relative difference, ∆D* (%) 3.5 - 3.65 - 3.2 - 3.36

III) Effect of hole diameter ( Pipeline pressure = 4 and Pipeline temperature = - 100 oC )

Hole diameter (mm) 3.2 9.5 15.9 22.0 25.4

Distance to safe area (m) 5 kW/m2 2.23 5.81 9.0 11.97 13.41

4.7 kW/m2 2.31 - 9.28 - 13.85

Percentage relative difference, ∆D* (%) 3.6 - 3.11 - 3.28

# Heat flux level,

* ∆D (%) = {(D4.7 – D5.0)/ D5.0} * 100, where D5.0 and D4.7 are the distances to safe area corresponding to heat

flux levels of 5 and 4.7 kW/m2, respectively.

It is to be noted that the present study has considered small

hole diameters ranging from 3.2 to 25.4 mm, which are

relatively small values. In real life, hole diameters as large as

the pipeline diameter, as it is the case when the pipeline

ruptures, may be encountered. Previous studies have indicated

that some combinations of hole diameter and operating

pressure can produce unstable flame, e.g. Lowesmith et al.

[14]. This particular point is worthy of further investigation.

The present study has taken into consideration three

source release parameters only, namely: NG temperature and

pressure inside the pipeline as well as hole diameter. An

augmented study of the same problem should include other

source release parameters, such as pipeline length and

diameter, environmental parameters, such as ambient wind

speed, atmospheric stability and height of release source, as

well as time parameters, such as NG concentration averaged

time and valve closing time for unsteady gas release. Another

important point related to the above point of dependence of

results on time is the time elapsed from the moment of NG jet

release. In the current study, all calculations and results

obtained were based on a 10- minute elapsed time. Dong et al.

[1] have indicated that the NG pressure and release rate

remain almost constant for a valve closing time of less than 10

minutes, consistent with the current chosen value.

Development of events and dependence of consequences of jet

release on higher elapsed time values, with possible unsteady

flow, are yet to be considered.

References

[1] D. Dong, L. Yue, Y. Yong, and J. Yong, "Evaluation of

hazard range for the natural gas jet released from a high-

pressure pipeline: a computational parametric study", J.

Loss Prevention, Accepted for publication, 2010

[2] D. Joy, and B. Ahn, "A simple model for the release rate

of hazardous gas from a hole on high-pressure pipelines ",

J. Hazard. Mater, 2003 A97, pp 31-46.

7

[3] J. Lou, M. Zheng, X. Zhao, C. Huo, and L.Yang,

"Simplified expression for estimating release rate of

hazardous gas from a hole on high-pressure pipeline", J.

Loss Prevention, 2006 Vol. 19, pp 362-366.

[4] T. Spicer, and J. Havens, "User's guide for the DEGADIS

2.1: Dense gas dispersion model", Environmental

Protection Agency, 1989 EPA-450/4-89-019.

[5] D. Ermak, "SLAB: an atmospheric dispersion model for

denser-than-air releases", UCRL-MA-105607, LLNL,

Livermore, 1990.

[6] S. Chan, "A three-dimensional model for simulating

atmospheric dispersion of heavy gases over complex

terrain", UCRL-JC-127475, Lawrence Livermore National

Laboratory, 1997.

[7] H. Luketa, R. Koopman, and D. Ermak, "On the

application of computational fluid dynamics codes for

liquefied natural gas dispersion", J. Hazard. Mater, 2007,

Vol. 140, pp 504-517.

[8] J. Garcia, E. Migoya, J. Lana, and A. Crespo, "Study of

the dispersion of natural gas issuing from compressor

stations through silencers with upper cover", J. Hazard.

Mater, 2008, Vol. 152, pp 1060-1072.

[9] S. Sklavounos, and F. Rigas, "Estimation of safety

distances in the vicinity of fuel gas pipelines", J. Loss

Prevention, 2006,Vol. 19, pp 24-31.

[10] B. Cormier, R. Qi, G. Yun, Y. Zhang, and S. Mannan,

"Application of computational fluid dynamics for LNG

dispersion modeling: a study of key parameters", J. Loss

Prevention, 2009, Vol. 22, pp 332-352.

[11] A. Loppz, L. Gritzo, and M. Sherman, "Risk assessment

compatible fire models", Sandia National Laboratories,

California, USA, 1998.

[12] G. Chamberlain, "Management of large LNG hazards",

23rd World Gas Conference, Amsterdam, Netherlands,

2006.

[13] Lehr, Beaty, Fay, and Quest "Risk assessment application

for the marine and offshore oil and gas industries",

American Bureau of Shipping (ABS) , NY, USA, 2000.

[14] B. Lowesmith, G. Hankinson, M. Acton, and G.

Chamberlain, "An overview of the nature of hydrocarbon

jet fire hazards in the oil and gas industry and a simplified

approach to assessing the hazards". Trans. IChemE, Part

B, Process Safety and Enviromental Protection, 2007

85(B3), pp 207-220.

[15] J. Flemming"LNG storage and transit in Boston",

Technical Report, Boston Fire Department, Boston,

USA, , 2007.

[16] K. Marghany, "Risk assessment in case of LNG dispersion

and fire associated with LNG spillage", M. Sc. Thesis,

AASTMT, Alex., Egypt, 2009.

[17] Turner, B. and Richard, S. "Practical guide to atmosphere

dispersion modeling", 2nd Ed., Trinity Consultant, USA,

2004.

[18] J. Havens, and T. Spicer, "New models to predict

consequences of LNG releases" University of Arkansas

Chemical Hazards Research Center, USA, 2007.

[19] NFPA, "Life Safety Code", National Fire Protection

Association, Massachusetts, USA, 2000.

[20] A. Shehata, EGAS, Edko, Egypt, Private communication,

2009.

[21] A. Furness, and M. Muckett,. "Introduction to fire safety

management" Elsevier Ltd. London, UK, 2007.

8

Fig. 1 Dependence of heat flux on distance from jet centre.

Fig. 2 A typical map showing variation of distance with radiation in case of NG jet fire for a pipeline temperature of -75°C,

a pressure of 4.0 bar and a hole diameter of 25.4 mm.

Hea

t fl

ux (

kW

/m²)

Distance from jet centre (m)

Radiation (kW/m²)

■15 ■ 10

■ 5

9

Fig. 3 Dependence of heat flux effect on exposure time [15]

Fig. 4 Dependence of distance to safe area on NG pipe temperature.

( Pipeline pressure = 4 bar and Hole diameter = 25.4 mm )

11

11.5

12

12.5

13

13.5

14

14.5

15

-140 -120 -100 -80 -60 -40 -20 0

Dis

tan

ce t

o s

afe

are

a, D

(m

)

Pipeline temperature, T ( ̊ C )

5 kW/m2

4.7 kW/m2

Exposure time (s)

Hea

t fl

ux (

kW

\m² )

Exposure time (S)

10

Fig. 5 Dependence of distance to safe area on NG pipe pressure.

( Pipeline temperature = -100°C and Hole diameter = 25.4 mm )

Fig. 6 Dependence of distance to safe area on hole diameter.

( Pipeline pressure = 4 bar and Pipeline temperature = -100°C )

10

10.5

11

11.5

12

12.5

13

13.5

14

14.5

1.5 2 2.5 3 3.5 4 4.5

Dis

tan

ce t

o s

afe

are

a, D

(m

)

Pipe pressure, p (bar)

5 Kw/m2

4.7 Kw/m2

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30

Dis

tan

ce t

o s

afe

are

a, D

(m

)

Hole diameter, d (mm)

5 Kw/m2

4.7 Kw/m2