LNG Level에 따른 BOG 발생량과 탱크 안전성에 관한

모사 연구

신명욱 . 윤인섭

서울대학교 공과대학 응용화학부

A Simulation Study on LNG Tank and Safety

Myoung Wook Shin and En Sup Yoon

Department of Chemical Engineering, Seoul National University,

Shinlim-Dong, Kwanak-Ku, Seoul 151-742, Korea

1. Introduction

Prestressed concrete : To pull and wind up concrete tank by steel cable with a strong tensile strength, that is, to increase the tensile strength of the concrete greatly.

In P/T LNG receiving tank, if the concrete reaches an appropriate strength after the sheath pipe is installed, prestressing cables are inserted and the form of post tension to add tension strength is applied.

Working ranges are as follows. Bottom slab horizontal is 12T15 cable, wall horizontal 12T15 cable, outer ring horizontal 12T15 cable,wall vertical 12T15 cable, and U-tube 4T15 cable,respectively.

Prestressing schedules are as follows. Minimum of tensile strength is is 200 to 270kg/□, The number of tensile cables is 379, and the sustained weight per  cable for 12T15 and 4T15 is 21.37 and 21.20 ton, respectivly.

Neo-Prene-Pad : To weaken a strong seismic forces acting on structures. Its location is between tank raft and basic column, and its size is 750mm × 750mm × 97mm. Its materials are composed of steel plates regulated by KSD 3503 and elastic rubber which is a poly-chloro putene compound. Specifically, tensile strength, elongation at break, and compression are T≥14MPa, E≥350%, and C≤25%, respectively.

When Neo-Prene-Pad is used, frequency and acceleration are 0.75Hz and 0.2g, respectively. When Neo-Prene-Pad is not used, frequency and acceleration are 4Hz and 0.5g, respectively.

Advantages of using Neo-Prene-Pad are to absorb the vibration of structures, almost to alleviate an increasing amplitude of average velocity proportional to structure height, and to deviate the intrinsic frequency of structures corresponding to the most accelerated velocity in a response spectra.

Membrane : Material components are based on ANSI 304 I. Its size is 1300mm × 3550mm having thickness of 1.2mm per sheet. In physical properties yield strength, tensile strength, and elongation are 22 to 32kg/□, more than 55kg/□, and more than 40%, respectively.

In forms, large corrugation is applied to vertical walls and radial bottoms, small corrugation to rings of walls and circular bottoms, and jogging to overlapping between sheets.

Steel cupola : Material components are based on ASTM-A.283-GR.C.   Its thickness and total weight are 6mm and 798.5ton, respectively. Its structures are composed of steel plate, structure, pipe, fitting, ceiling, concrete ring, and so on.

2. Modeling and simulation for BOG

2-1. Normal operation before loading

Total influx heat(Kcal/H) :       Total amount of BOG(Ton/H) :

2-2. Loading operation

Test condition :

- Pressure of test tank : 145 ± 5 g/□.g

- Flow : Not to discharge and inflow LNG

The total average amount of BOG for LNG terminal :

(Unit : T/H)

Tank Condition	T/K - 706	T/K - 707
Normal	29.83	28.93
Loading	40.23	39.93
Difference	10.4	11.00
Average diff.	10.70

The effect on the amount of BOG with LNG level increasing :           - T/K - 706                                  - T/K - 707

An empirical equation is derived as follows :

Y = 3.86628 logX + 29.0924 (T/K-706)

Y = 3.86196 logX + 29.1028 (T/K-707)

3. Conclusion

The simulation results of BOG generated according to LNG level show that the amount of BOG is almost linealy increased with the increase of LNG level, based on theoretical heat balance equation. Also, the amount of BOG to be generated, based on actual experimmental data for loading, is slowly increased in a form similar to an exponential one. Accordingly, tank receives additional forces which are equivalent to this amount; not static, but dynamic. In actual loading conditions, LNG is mostly mixed through a complex bulk convection mass and heat transfer giving birth to turbulence and vortex flow, and then a lot of BOG is generated. However, fortunately, operation pressure of tank is 50 to 170 g/□.g; tank is sustained by BOG compressors which are the functions of an internal tank pressure. Therefore, BOG generated in loading never influences tank safety of Pyeong Taek LNG Receiving Terminal.

Considerig the velocity of external air, it is showed that BOG is increased very slowly until 5 m/s, almost constant more than it.

4. Future

This paper tried to illustrate correlation between LNG tank and BOG for a loading operation. BOG based on experimental data is obtained in a bulk concept, but how the loading operation influences dynamic flows of LNG is not defined.

In the future, the dynamic flows of LNG need to be analyzed in a minute concept through modeling and simulating internal flows of the loading operation.

5. Appreciation

This study was supported by POSECE.ERC'fund and KOGAS.

6. References

Smith, K.A and Virk, P.S (1982) : " LNG storage : Safety analysis ", Annual report, MIT LNG research center.

Hashemi, H.T and Wesson, H.R (1971) : " Cut LNG storage costs ",    Hydrocarbon processing (august).

Bennett, C.O and Myers, J.E (1985) : " Momentum, heat, and mass transfer ", McGraw- Hill company.

KOGAS (1989 to 1995) : " Constructing process and paper for Pyeong-Taek LNG receiving terminal ".

KOGAS (1996) : " Operating manual for Pyeong-Taek LNG receiving terminal ".