5 replies to this topic

[Murphy]

Hi.

I have two pressurised LPG tanks; Tank A, which is a spherical tank of capacity 2000 m3 (About 16m diameter) and tank B which is a horizontal tank of capacity 150 m3 (3m dia x 23m length). The bottom of tank A is at the same level as the centre of tank B. My question is:

1. If Tank A contains 10m of LPG and tank B 1m of LPG, and if the pressure in both tanks is 4 Bars, will there be flow of product from tank A to tank B?

2. If I push product into tank A from tank B via a pump, will the level of product in the tank A affect the rate of filling of the tank (e.g. level in tank A=2m while pressure= 3 bars & level in tank A=10m while pressure is still = 3 bars)?

Thank you in advance,

Murphy

[rxnarang]

Murphy, Please see the attached sketch. Does this help?

Regards
Rajiv

Attached Files

•  rajiv.pdf   21.39KB   91 downloads

[Murphy]

Thank you very much for your reply, Rajiv, but I still cannot visualise how this works.

For the first scenario, I understand that this would apply for atmospheric tanks; but in this case, the pressure in both tanks is the same in all directions. What is the mechanism which will make the liquid flow from tank A to B? Does gravity apply even in this case?

For the pump scenario, this would be applicable if the tank A was atmospheric. The point of operation of the pump should not change with change in height since the overall pressure between tank A and pump discharge is the same even if the height is 2m or 10m.

Maybe I am forgetting some basic engineering principles here, but still I am confused. Please help...

Thanks,

Murphy

[rxnarang]

Thank you very much for your reply, Rajiv, but I still cannot visualise how this works.

For the first scenario, I understand that this would apply for atmospheric tanks; but in this case, the pressure in both tanks is the same in all directions. What is the mechanism which will make the liquid flow from tank A to B? Does gravity apply even in this case?

Gravity will do the trick. I assumed that the pressure is same in both the tanks, which is why I show an equalisation line in the vapour space connecting both the tanks. Remember, the pressure in the tanks is the vapour pressure of the contents, which is dependent only on the temperature.

For the pump scenario, this would be applicable if the tank A was atmospheric. The point of operation of the pump should not change with change in height since the overall pressure between tank A and pump discharge is the same even if the height is 2m or 10m.

The pressure difference between the tanks in the vapour space is the same; i.e., vapour pressure of the fluid. The friction losses, or the shape of the system curve will also be the same. The curve will shift vertically, as the differential head requirement is lower with 10 m. Whereever the system curve meets the pump curve, the flow is established.

Looking at it differently now: A centrifugal pump will always generate the same head, so if the suction pressure rises, the discharge pressure will also rise. As the destination pressure is fixed, we should be able to pump more, till the friction losses increase to such an extent, that the discharge pressure of the pump meets the pressure requirement to flow the incremental amount.

Maybe I am forgetting some basic engineering principles here, but still I am confused. Please help...

Thanks,

Murphy

[Art Montemayor]

Murph:

Welcome to our Forums.

Please look at my attached Excel Workbook and read the included comments.
Await your reply, comments, or questions.

 LPG_Tank_Transfer.xls   210KB   94 downloads

[Murphy]

Hi Art,

Thank you very much for your explanation. It's clearer for me now; I really appreciate.

Regards,

Murphy