3 replies to this topic

[MkoTop]

Good day,

I'm having some trouble with the holding time calculation for a LNG tank.

In the attached xls file you have the calculation, please review it.

I got the max heat flow to the tank  through daily evaporation loses and entalpy difference from loading to discharge. 

The safety valve open at 6 bar. With that info i calculated that the first opening of the

safety valve will be in 28 days (not sure about that calculation).

 

How would I calculate if this setup would hold for 60 days

within the required parameters? For now, I presumed the heat flux through the

insulation at 2W/m2 ?

If I left something out in the xls file please let me know.

 

 

Any help is appreciated, thank you very much.

Attached Files

•  60days.xls   19KB   820 downloads

[kkala]

Since no response has been received for 10 days, following notes are presented subject to criticism. Not all points have been understood. Comments on the spreadsheet of post no 1 are noted on the attached "60daysR1.xls", where more sheets have been added. Results are briefly as below.

1. BOG rate = 0.54% daily is usual for LNG, <http://www.cheresour...-boil-off-rate/>, amounting to about 69 metric ton /day of vaporized LNG.

2. Heat gained by LNG (essentially CH4) during whole cycle (when pressure increases from 1.5 bara to 6.0 bara) had better take both gas and liquid enthalpies into account, see sheets "Case1" and "Case2". It is estimated at about 0.93E9 kJ, not far from calculated value in post no 1. Max liquid fill occurs at end of cycle due to liquid expansion, so "Case1" starts cycle with liquid fill 77.9% to end up with 85%.

3. Heat input rate is needed to specify time of whole cycle T.  With the used value (in post no 1) of 423 kW, time for whole cycle is estimated at about 26 days, lower than 60 days.

Someone with experience could "see" and give further explanations, but I have to limit the investigations to arithmetic operations. More notes are written on the attachment.

 

 

Attached Files

•  60daysR1.xls   64KB   792 downloads

[narendrasony]

Dear Kostas,

There is no need of using Goal Seek function for mass balance. Mass balance equation will directly give the result.

 

Final liquid volume =   [ (dL - dV) at start X  Initial liquid volume  + (dV at start  - dV at end) X Total tank volume ]  /  (dL - dV) at end

 

Where dL and dV   are density  of liquid and vapor phase.

 

One intresting obsevation at >= 89% starting level (though not practical to operate at this level), mass of liquid phase increases with heat transfer and tank pressure i.e. vapor is getting converted to liquid phase even though heat is transferred to the tank. Why such anomaly at those levels?

 

Regards

Narendra  

Edited by narendrasony, 12 February 2013 - 07:10 AM.

[kkala]

1.You are right Narendra, formula by you is straight forward, also expressing the mass balance. Using same symbols with additional subscripts o and 1 for start and end of cycle: VoL*doL+VoV*doV = V1L*d1L+V1V*d1V. From this V1L*(d1L-d1v) = VoL*(doL-dov)+Vt*(dov-d1v), where liquid+vapor volume = Vt . It's a fact that PC makes me think less!

2. If D=initial liquid mass - final liquid mass

α. Indeed for initial filling  88%, 89%, 91%, 94%, D= +1.36, -1.44, -7.06, -15.5 metric ton correspondingly. I feel that  expanded liquid "pushes" the limited vapor volume (sort of a piston) and condensation occurs, despite the increase in temperature.

β. If desity of final liquid = density of initial liquid = 415 kg/m3 (no liquid expansion) corresponding D = +29.9, +27.3, +22.3, +14.9 metric ton, indicating no condensation. D is getting smaller and smaller due to decreasing vapor space.