lng vapor dispersion from atmospheric relief valve

LNG vapor dispersion from atmospheric relief valve

Main author Jung Hyun, Yoon

HYUNDAI ENGINEERING CO., LTD. KOREA

[email protected]

Co-author Jung Chul, Ha

HYUNDAI ENGINEERING CO., LTD. KOREA

[email protected]

Ju Chul, Park HYUNDAI ENGINEERING CO., LTD.

KOREA [email protected]

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© Copyright © 2008 IGRC2008

1. ABSTRACT

LNG vapor (or Boil-Off Gas) control is a major concern in LNG receiving terminal design. The BOG generation cannot be prevented but it can be minimized and recondensed by a BOG treatment system which will reduce the LNG loss to atmosphere or flare. Discharge of the flammable BOG to atmosphere is not recommended for safety reason. BOG is generated during normal operation and instead of venting it can be recovered in the BOG treatment system composed of BOG compressors and a recondenser. During abnormal or emergency operation, BOG can be produced excessively which may exceed the capacity of the BOG treatment system in which case the BOG can be partially vented out to the flare. Atmospheric relief valves are installed on LNG storage tanks and vaporizers for venting the BOG to atmosphere in case of such abnormal operation or emergency. The BOG vented to atmosphere is flammable and it can linger around the LNG terminal which presents potential explosion hazard. Flammable gas dispersion modeling has been used to evaluate the safety distance from the potential source of ignition to the point of BOG release in LNG receiving terminal This paper evaluates the required minimum safety distance from the ignition source and point of relief valve for the Incheon LNG Receiving Terminal in Korea by using PHAST program and manual calculation.

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C O N T E N T S

1. Abstract ................................................................................................. 2

2. Introduction........................................................................................... 4

3. Dispersion Study .................................................................................... 4

3.1. PSV on LNG Storage Tank ...................................................................................4

3.2. PSV on Vaporizer ...............................................................................................4

4. Study Basis ............................................................................................ 5

4.1. Dispersion Scenario ...........................................................................................5

4.2. Dispersion Calculation ........................................................................................5

4.3. Weather Condition .............................................................................................5

4.4. LNG Composition ...............................................................................................6

5. Dispersion Modeling with PHAST............................................................ 6

5.1. Scenario 1: PSV On LNG Storage Tank..................................................................7

5.2. Scenario 2: PSV On Vaporizer..............................................................................9

6. Dispersion Estimation by Manual calculation ....................................... 11

6.1. STEP 1: Calculation of Released Gas Discharge Rate ............................................. 12

6.2. STEP 2: Calculation of Concentration for Dispersed Gas (Gaussian Plume Model) ...... 16

6.3. Comparison Table and Analysis.......................................................................... 20

7. Conclusion............................................................................................ 20

8. Reference............................................................................................. 21

9. List of Tables........................................................................................ 21

10. List of Figures ...................................................................................... 22

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2. INTRODUCTION

Incheon LNG Receiving Terminal has been built to send out 3,330 ton per hour of natural gas in Korea. It has 18 LNG storage tanks, several HP pumps and vaporizers. Nowadays, Phase II-4 expansion is under construction with 2 inground tanks to meet the increased demand. In the LNG terminal, pressure relief valves have been installed to protect the equipment or piping from abnormal pressure build-up. Generally, atmospheric vapor release is not recommended as it may create flammable condition. Therefore most of the relief valve discharges are connected to a flare system. In Incheon LNG Receiving Terminal, a flare with a capacity of 140 ton per hour is in operation. However, atmospheric vapor release is provided as the means in preventing any catastrophic failure of the LNG tank when relief load exceeds the designed flare capacity. Atmospheric PSV on LNG storage tank and on discharge line of vaporizer can release flammable gas to the atmosphere which can cause explosion or fire when it is ignited. Therefore, sufficient safety distance should be secured between the relief valve and source of ignition, or the ignition source should be eliminated from the beginning of the design stage.

3. DISPERSION STUDY

3.1. PSV on LNG Storage Tank

LNG storage tank is equipped with low-pressure intank pumps for storing and sending out the LNG. If the excessive BOG causes the increase the internal pressure of tank to build, it is released to the flare stack or atmosphere through a Pressure Relief Valve. According to the data of LNG storage tank manufacturer, the maximum generation of BOG in the 200,000 m3 LNG tank of capacity is estimated to be about 66 ton/hr. As a safety measure, when the pressure of the tank builds up to 265 mbarg, BOG is released to the flare at a rate of 30 ton/hr. When the pressure exceeds 290 mbarg, the BOG is released to the atmosphere at a rate of 40 ton/hr

3.2. PSV on Vaporizer

LNG is sent out from the storage tank through a low-pressure intank pump, and then it is pressurized by a high pressure pump. The pressurized LNG is then sent to Vaporizer. High pressure atmospheric relief valve, sized for block out case, is installed on the discharge pipe of each vaporizer. Therefore, the capacity of each PSV should be same as that of vaporizer. Two PSVs are installed on each vaporizer; one is operational and the other one, stand-by. The PSV should be located at a safe location considering the dispersion of the relief gas.

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4. STUDY BASIS

4.1. Dispersion Scenario

BOG release to the atmosphere has been studied for the following two cases. 1) PSV on LNG Storage Two pilot type PSVs (1 Operation + 1 Stand-by) are installed on LNG storage tank. When the BOG compressor system fails, the excessive BOG will be released through the PSV to the atmosphere at a rate of 40,794 kg/hr when the internal pressure reaches 290 mbarg. The PSV size is 10 inch for inlet and 14 inch for outlet. 2) PSV on Vaporizer Two pilot type PSVs (1 Operation + 1 Stand-by) are installed on the Vaporizer. Their setting pressure is 125.2 barg and the required relieving rate is 99,000 kg/hr. The PSV size is 4 inch for inlet and 6 inch for outlet.

4.2. Dispersion Calculation

Vapor dispersion is calculated in two ways; 1) Using PHAST 6.53 of DNV 2) Manual calculation. PHAST simulation program is used worldwide for Consequence Analysis which can estimate the consequences of release of toxic or flammable materials. The model includes accidental releases from catastrophic ruptures, leaks, line ruptures, relief valves and rupture disks.

4.3. Weather Condition

Weather condition of this study is based on the project design data of the Incheon LNG Receiving Terminal. According to the design data, average wind velocity of the terminal location is 3.7m/s and prevailing direction is NW or WNW. The air temperature is 35.2 °C maximum, and -19 °C minimum with annual mean temperature of 11.4 °C. The relative humidity is 70%.

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4.4. LNG Composition

The following table is a typical composition of natural gas handled in Incheon LNG Terminal. However in this modeling, to simplify the simulation, it is assumed that vapor to be released is 100% methane.

Table 1. LNG Composition

Component Unit Molecular

Weight Composition

(mol%) MW (kg/kg-mol)

Methane 16.043 88.3371 14.172

Ethane 30.069 9.5239 2.864

Propane 44.096 1.5434 0.680

i-Butane 58.123 0.2517 0.146

n-Butane 58.123 0.3346 0.195

Nitrogen 28.013 0.0093 0.003

Total - 100.0000 18.060

5. DISPERSION MODELING WITH PHAST

The dispersion calculations show estimated areas affected by the LNG vapor dispersion and also the forecasted average vapor concentrations. The simplest calculations require an estimate of the release rate of the gas, atmospheric conditions, surface roughness, temperatures, pressures and release diameters. Weather conditions at the time of the PSV release have a major effect on the extent of dispersion. The primary factors are the wind speed and the atmospheric stability. Atmospheric stability is an estimate of the turbulent mixing; stable atmospheric conditions lead to the least amount of mixing and unstable atmosphere conditions, to the most amount of mixing. Wind speed is a significant impact on the dispersion of the released gas. The LNG vapor will first be diluted and mixed with air and as the wind speed increases, the released gas will move downwind faster, and the dispersed gas will be diluted by even a larger quantity of air. In this modeling, three cases have been studied as below table. Table 2. Applied Air Condition

Case Wind Speed

(m/s) Pasquill-Gifford Stability Class

Remark

A 1.5 F (Very Stable) Severest Case B 5 D (Neutral) C 3.7 D (Neutral)

With regard to the concerned concentration limits, 0.1 LEL, 0.5 LEL, 1.0 LEL have been considered for each case.

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5.1. Scenario 1: PSV On LNG Storage Tank

In addition to the above weather data, design data are also required for modeling such as PSV hole size and relieving height or orientation. In case of inground LNG tank of Incheon terminal, the PSV is installed vertically. The height of discharge point is 19.2 meters from the ground level and hole diameter is 250 mm.

5.1.1. Simulation Result

The calculated relieving rate from the PSV is 11.4 kg/s. 1) Case 1A - Wind velocity 1.5m/s, Stability: F (Stable)

The 1.5/F condition is used for the most conservative night time condition. Lower wind speed and stable atmospheric condition can cause the explosive vapor cloud to travel over the longest distance among all the conditions.

Figure 1 A. Case 1A Modeling Result [Wind velocity 1.5m/s, Stability: F (Stable)]

2) Case 1B - Wind velocity 5m/s, Stability: D (Neutral)

The 5/D condition is used for the neutral daytime condition. Medium wind speed and neutral stability condition can lead to show a typical modeling results in most simulations.

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Figure 1 B. Case 1B Modeling Result [Wind velocity 5m/s, Stability: D (Neutral)]

3) Case 1C - Wind velocity 3.7m/s, Stability: D (Incheon Design Condition)

3.7/D condition is used for the Incheon design condition case. This weather condition shows a typical dispersion of LNG in the Incheon LNG terminal area.

Figure 1 C. Case 1C Modeling Result [Wind velocity 3.7m/s, Stability: D (Incheon Design condition)]

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The range of 0.1 LEL (4,400 ppm) is shown in black on the above graph. Gray color shows 0.5 LEL (22,000 ppm), and white color shows actual 1.0 LEL (44,000 ppm). Among the three cases, case 1A shows the largest flammable cloud. The severest dispersion case is case 1A (1.5 m/s wind speed, F condition) among the three cases. It is due to relatively low wind speed and stable air condition. In this case, 0.1 LEL range can be reached 240 meters horizontally and up to 65 meters vertically. If ignition source exists within the LEL range, there will be a possibility of jet, flash or explosion. Thus, every potential ignition source has to be placed outside of this range or every electrical apparatus in the range has to be explosion proofed. In case of Incheon LNG terminal, the whole process area has been classified as hazardous area. And flare (flare is open ignition source) is located more than 300 meters away from the LNG storage tank. According to the Korean City Gas Law, the safety distance between the LNG storage tank and plant boundary is determined based on the stored volume of LNG storage tank. In case of inground LNG tank which has 200,000 m3 capacity, the minimum distance is specified as 85 meters. In the above dispersion results, dispersion range of 1.0 LEL in horizontal direction is less than 10 meters, which is within the regulatory minimum distance.

5.2. Scenario 2: PSV On Vaporizer

The discharge point of PSV which is installed on outlet line of vaporizer is 19.6 meters from the ground level. The PSV discharge direction is vertical and hole diameter of this PSV is 48mm. This system has higher pressure and temperature compared to PSV on LNG storage tank. Therefore, different results can be expected.

5.2.1. Simulation Result

1) Case 2A: Wind velocity 1.5m/s, Stability: F (Stable) The 1.5/F condition is used for the most conservative night time condition. Lower wind speed and stable atmospheric condition can cause the explosive vapor cloud to travel over the longest distance among all the conditions.

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Figure 2 A. Case 2A Modeling Result [Wind velocity 1.5m/s, Stability: F (Stable)]

2) Case 2B: Wind velocity 5m/s, Stability: D (Unstable) 5/D condition is used for the neutral daytime condition. Medium wind speed and neutral stability condition leads to show a typical modeling result in most simulations.

Figure 2 B. Case 2B Modeling Result [Wind velocity 5m/s, Stability: D (Neutral)]

3) Case 2C: Wind velocity 3.7m/s, Stability: D (Incheon Design Condition)

3.7/D condition is used for the Incheon design condition case. This weather condition shows a typical dispersion of LNG in the Incheon LNG terminal area.

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Figure 2 C. Case 2C Modeling Result [Wind velocity 3.7m/s, Stability: D (Incheon Design condition)]

The range of 0.1 LEL is shown in blue, 0.5 LEL in green and actual 1.0 LEL in yellow on the above graph. The severest dispersion case is case 1A (1.5 m/s wind speed, F condition) among the three cases. It is due to relatively lower wind speed and stable air condition. In this case, 0.1 LEL range can reach to 240 meters. Compared with the result of PSV on the LNG tank, relatively high pressure gas is released to atmosphere. Therefore, released gas can reach up to 115 meters vertically due to high momentum. After that, gas disperses to atmosphere by mixing with air. According to the Korean City Gas Law, equipment which contains flammable fluid should be maintained a safety distance from the battery limit. The safety distance is determined based on the volume of gas contained in the equipment. In case of one vaporizer, safety distance is 28 meters in compliance with the regulation. However this regulation also describes if calculated distance is less than 50 meters, 50 meters should be kept as a minimum. According to modeling result, LEL and 0.5 LEL are ranged within 50 meters.

6. DISPERSION ESTIMATION BY MANUAL CALCULATION

The purpose of this section is to compare previous dispersion modeling with Gaussian Plume theory. There are two steps in estimating the concentration of dispersed gas at the concern point. Step 1: Calculation of Released gas discharge rate Step 2: Calculation of concentration for dispersed gas

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6.1. STEP 1: Calculation of Released Gas Discharge Rate

There are two regimes for flow of gases through an orifice; sonic (or chocked) for higher internal pressures, and subsonic flow for lower pressures. The step-1 calculation determines regime of flow present. The step-2 calculation estimates the release rate of gas, using the equation for the specific flow regime. The following equation defines the pressure at which the flow regimes change from sonic to subsonic:

1)1

2()( −

+= γ

γ

γCRsP

P

…………………………………………………………………………... (4-1) Discharge rate can be obtained by the following two equations: Discharge of gases at sonic velocity through an orifice can be calculated as below:

CRss

a

S

CSD P

PPP

forRT

MWgAPCQ ⎟⎟

⎠

⎞⎜⎜⎝

⎛≤⎟⎟

⎠

⎞⎜⎜⎝

⎛+

=−+

,1

2 )1()1(

γγ

γγ

………………………… (4-2) Discharges of gases at subsonic velocity though an orifice can be calculated as below:

CRss

a

s

a

s

a

S

CSD P

PPPfor

PP

PP

RTMWgAPCQ ⎟⎟

⎠

⎞⎜⎜⎝

⎛>

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛−

=

+

,1

2)1(2

γγ

γ

γγ ……….. (4-3)

where Q : Gas discharge rate (kg/s) CD : Discharge coefficient (-) gc : Gas constant (1kg m/N sec2) MW : Molecular Weight (kg/kmol) A : Cross section area (m2) PS : Storage absolute pressure (Pa) TS : Storage temperature (K) R : Gas constant, 8314 (J/kmol K) γ : Specific heat ratio (CP/CV) Pa : Atmospheric pressure (Pa)

Table 3. Mass Discharge Coefficient CD

Release Assembly CD

Venturi Meter / Nozzle 0.95 ~ 0.99 for Sonic & Subsonic

Orifice

0.61 ~ 0.67 for Subsonic

0.75 for near sonic

0.84 for Sonic

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It is noted that below assumptions and limitations to be considered for use of the above equations.

Ideal gas No friction flow No heat transfer Single component gas

1) Discharge Rate Calculation for PSV on LNG Storage Tank

A. Determine the flow regime, sonic or subsonic.

1)1

2()( −

+= γ

γ

γCRsP

P

371.1=γ (PRO/II Simulation Value)

5332.0)1371.1

2()( 1371.1371.1

=+

= −CR

sPP

atmPa 1=

atmPs 3.1=

FlowSubsonicPP

s

a ⇒>=⎟⎠⎞

⎜⎝⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛5332.078.0

3.11

B. Apply to the subsonic flow – Equation (4-3)

KCTKkgmoleJR

kgmolekgMWNmkgg

PaPmA

TableC

S

C

s

D

116157/8314

/04.16sec/1

371.1130325

049.0

)3(67.0

2

2

=−=

⋅==

⋅⋅⋅=

===

=

o

γ

= 10.3 kg/s

CRss

a

s

a

s

a

S

CSD P

PPPfor

PP

PP

RTMWgAPCQ ⎟⎟

⎠

⎞⎜⎜⎝

⎛>

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎠

⎞⎜⎜⎝

⎛−

=

+

,1

2)1(2

γγ

γ

γγ

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛−⎟

⎠⎞

⎜⎝⎛

−×=

+371.1

)1371.1(371.12

5

3.11

3.11

1371.1371.1

)298)(8314()04.16)(1)(2()103.1)(049.0)(67.0(Q

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2) Discharge Rate Calculation for PSV on Vaporizer A. Determine the flow regime, sonic or subsonic.

1)1

2()( −

+= γ

γ

γCRsP

P

816.1=γ (PRO/II Simulation Value)

307.0)1816.1

2()( 1816.1816.1

=+

= −CR

sPP

atmPa 1=

atmPs 126=

FlowSonicPP

s

a ⇒<=⎟⎠⎞

⎜⎝⎛=⎟⎟

⎠

⎞⎜⎜⎝

⎛307.0008.0

1261

B. Apply to the sonic flow – Equation (4-2)

CRss

a

S

CSD P

PPP

forRT

MWgAPCQ ⎟⎟

⎠

⎞⎜⎜⎝

⎛≤⎟⎟

⎠

⎞⎜⎜⎝

⎛+

=−+

,1

2 )1()1(

γγ

γγ

KCTKkgmoleJR

kgmolekgMWNmkgg

PaPmA

TableC

S

C

s

D

2730/8314

/04.16sec/1

816.11026.1

0018.0

)3(84.0

2

7

2

==

⋅==

⋅⋅⋅=

=×=

=

=

o

γ

= 37.8 kg/s

)1816.1()1816.1(

7

1816.12

)273)(8314()04.16)(1)(816.1()1026.1)(0018.0)(84.0(

−+

⎟⎠⎞

⎜⎝⎛

+×=Q

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6.2. STEP 2: Calculation of Concentration for Dispersed Gas (Gaussian Plume Model)

The plume model describes a continuous release of material. The solution depends on the rate of release, the atmospheric conditions, the height of the release above ground, and the distance from the release. The equations for the average concentration for each case are:

A) mz H6.1<σ

B) mz H6.1≥σ

C) If HE, z, σz < 0.4Hm, the equation can be simplified as:

W

where,

C(x,y,z) : Average concentration (kg/m3) Q : Continuous release rate (kg/s) Hm : Mixing height (m) HE : Effective release point height (m) U : Wind speed at the height of release point (m/s) σy : Dispersion coefficient in the y direction (m) σz : Dispersion coefficient in the z direction (m) x : Wind direction distance (m) y : Cross wind direction distance (m) z : Distance above the ground (m)

]}})2(5.0exp[])2(5.0exp[

4)-...(4 .............. ])2(5.0exp[])2(5.0[{exp

])(5.0exp[])(5.0]{exp[)(5.0exp[2

):,,(

22

22

1

222

z

Em

z

Em

z

Em

z

EmN

i

z

E

z

E

yzyE

zHiHzHiH

zHiHzHiH

zHzHyu

QHzyxC

σσ

σσ

σσσσπσ

++−+

+−−+

−−−+

−+−+

+−+

−−−=

∑=

)54.......(........................................].........)(5.0exp[2

),,,( 2

21 −−=

ymy

E

y

uH

QHzyxCσσπ

)64...(]}........)(5.0exp[])(5.0]{exp[)(5.0exp[2

):,,( 222 −+

−+−

−−=z

E

z

E

yzyE

zHzHyu

QHzyxCσσσσπσ

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D) HE (Effective release height)

udvH

uvH

uvduvH

M

DW

DW

35.1for 0

5.1for 5.12

=Δ

≥=Δ

<⎥⎦⎤

⎢⎣⎡ −=Δ

1) Assumptions and Limitations Generally, Gaussian plume models are applicable in risk analysis for neutral and positively buoyant emission. The models have been validated over a wide range of emission characteristics and downwind distances (0.1 to 10km). However, in the beginning stage of gas dispersion, initial momentum of the gas will dominate the dispersion before turbulent mixing occurs. In case of the LNG dispersion in this study, since the concentration of Lower Explosive Limit which has relatively short distance from release point, Gaussian model is not desirable for this study. Therefore 0.1 LEL dispersion case is assessed to meet this assumptions and limitations for the Gaussian plume models in this study.

2) Dispersion Coefficient (Briggs Rural Coefficient) Dispersion coefficient (σy and σz) shall be selected for dispersion manual calculation. In this study, Briggs Rural coefficient which is recommended by AIchE/CCPS is applied as shown in below table; Table 4. BR (Briggs Rural) Coefficient

Stability σy σz

D 0.08 x (1+0.0001 x)-1/2 0.06 x (1+0.0015 x)-1/2

F 0.04 x (1+0.0001 x)-1/2 0.016 x (1+0.0003 x)-1

3) Calculation To calculate the longest dispersion distance of 0.1 LEL, we can assume that ‘x’ direction is variable, ‘y’ direction is 0 and ‘z’ direction is equal to HE (HE is the Plume center line). Q (discharge rate) is determined at 5.1) and σy, σz are shown in table 4.2. Concentration of 0.1LEL can be calculated from the following equations.

lengtheffect Momentum lengtheffect forceDown

height release Actual where,

)74.(............................................................

=Δ=Δ

=

−Δ+Δ+=

M

DW

S

MDWSE

HH

H

HHHH

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(1) Dispersion Calculation (PSV on LNG Storage Tank)

A) Substitute y=0 and z=HE to Equation (4-6)

B) Determine the σy, σz value from table 4.2 (x=192, D condition,

when C=0.1 LEL) - x value to be calculated by trial and error

C) Determine the HE value from Equation (4-7)

MDWSE HHHH Δ+Δ+=

mH

smu

md

smA

Qv

udvH

E

E

447.3

)124)(25.0)(3(02.19

/7.3

25.0

/1249)(1.7)(0.04

10.3

,302.19

=++=

=

=

===

++=

ρ

D) Find the x value (0.1LEL distance) when C=0.1 LEL

mx

ppm

C

Hu

QHHxCz

E

zyEE

192

4400

]})15.10

)44(2(5.0exp[1{)7.3)(15.10)(21.15(2

3.10)44 ,0 ,192(

]})2(5.0exp[1{2

):,0,(

2

2

=⇒

=

−+=

−+=

π

σσπσ

]})2(5.0exp[1{2

):,0,( 2

z

E

zyEE

Hu

QHHxCσσπσ

−+=

15.10))192(0015.01)(192)(06.0()0015.01(06.0

21.15))192(0001.01)(192)(08.0()0001.01(08.0

21

21

21

21

=+=+=

=+=+=

−−

−−

xx

xx

z

y

σ

σ

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(2) Dispersion Calculation (PSV on Vaporizer)

A) Substitute y=0 and z=HE to Equation (4-6)

B) Determine the σy, σz value from table 4.2 (x=392, D condition,

when C=0.1 LEL) - x value to be calculated by trial and error

C) Determine the HE value from Equation (4-7)

MDWSE HHHH Δ+Δ+=

mH

smu

md

smA

Qv

udvH

E

E

277.3

)6.182)(05.0)(3(06.19

/7.3

05.0

/6.18218)(115)(0.00

37.8

,306.19

=++=

=

=

===

++=

ρ

D) Find the x value (0.1LEL distance) when C=0.1 LEL

mx

ppm

C

Hu

QHHxCz

E

zyEE

392

4400

]})66.18

)27(2(5.0exp[1{)7.3)(66.18)(76.30(2

8.37)27 ,0 ,360(

]})2(5.0exp[1{2

):,0,(

2

2

=⇒

=

−+=

−+=

π

σσπσ

]})2(5.0exp[1{2

):,0,( 2

z

E

zyEE

Hu

QHHxCσσπσ

−+=

66.18))392(0015.01)(392)(06.0()0015.01(06.0

76.30))392(0001.01)(392)(08.0()0001.01(08.0

21

21

21

21

=+=+=

=+=+=

−−

−−

xx

xx

z

y

σ

σ

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6.3. Comparison Table and Analysis

Table 5. Comparison of Discharge Rate

Operating condition, hole size, and mass discharge coefficient (CD) are the major factors that effect the discharge rate. The different result between PHAST and manual calculation may be mass discharge coefficient (CD). Although this value was determined from Table 3, it is not only a reference but also sensitive value according to flow type. Some references recommend that this value be 1.0 for the sonic flow on orifice.

Table 6. Comparison of Dispersion Distance (3.7/D condition)

Dispersion distance obtained by manual calculation is much longer than PHAST simulation. In manual calculation, it is assumed that the plume disperses in a direction perpendicular to effective release height (HE) and then spread along the wind direction. But actual plume disperses in a diagonal direction due to the light density of Methane. As a result, it can produce two different results with the same condition.

7. CONCLUSION

In LNG terminal, generally BOG release to the atmosphere is least recommended for safety reason as natural gas is very flammable. However, in case of abnormal operation or emergency, release of BOG to the atmosphere is necessary to release the pressure inside the equipment and plant facilities, and to prevent catastrophic accident of the LNG terminal. In case of such inevitable release of BOG, vapor dispersion modeling can enable estimation of safety distance required between the point of BOG release and ignition source. Based on such study and estimation, safety requirement should be clearly defined and enforced by low. According to the above assessment, dispersion range of lower explosive limit is ranged within plant boundary. Also, modeling result satisfies the safety distance which is defined by the Korean City Gas Law. There are some deviations between the modeling result and the manual calculation result. The manual calculation result gives more conservative figures than PHAST model result. This means that manual calculation can be used as preliminary determination for dispersion. However, for exact and efficient assessment, consequence analysis should be conducted through

Scenario PHAST 6.53 Manual calculation

PSV on LNG Storage Tank 11.4 kg/s 10.3 kg/s

PSV on Vaporizer 51.9 kg/s 37.8 kg/s

Scenario PHAST 6.53 Manual calculation

PSV on LNG Storage Tank 110 m 192 m

PSV on Vaporizer 120 m 392 m

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using a professional simulation program. This modeling can also be used to examine any occupational risk.

8. REFERENCE

Hyundai Engineering Co., Ltd.,(2005) “Technical Review Report for Incheon LNG Receiving terminal. Korea Gas Cooperation (2004) “Calculation Report for Relief Valve for TK-217, 218” Korea Occupational Safety and Health Agency (2008) “Consequence risk analysis” CCPS (2000) “Guidelines for Chemical Process Quantitative Risk Analysis” API RP 520 (2000) “Sizing, Selection and Installation of Pressure Relieving Devices in Refineries” DNV “PHAST Version 6.5 Training Manual”

9. LIST OF TABLES

Table 1. LNG Composition ............................................................. 6

Table 2. Applied Air Condition ....................................................... 6

Table 3. Mass Discharge Coefficient CD........................................ 12

Table 4. BR (Briggs Rural) Coefficient......................................... 17

Table 5. Comparison of Discharge Rate ....................................... 20

Table 6. Comparison of Dispersion Distance (3.7/D condition)... 20

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10. LIST OF FIGURES

Figure 1 A. Case 1A Modeling Result [Wind velocity 1.5m/s Stability : F (Stable)] ................................................ 7

Figure 1 B. Case 1B Modeling Result [Wind velocity 5m/s, Stability : D (Neutral)].............................................. 8

Figure 1 C. Case 1C Modeling Result [Wind velocity 3.7m/s, Stability : D (Incheon Design condition)] ................. 8

Figure 2 A. Case 2A Modeling Result [Wind velocity 1.5m/s,

Stability : F (Stable)] .............................................. 10

Figure 2 B. Case 2B Modeling Result [Wind velocity 5m/s, Stability : D (Neutral)]............................................ 10

Figure 2 C. Case 2C Modeling Result [Wind velocity 3.7m/s, Stability : D (Incheon Design condition)] ............... 11

lng vapor dispersion from atmospheric relief valve

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