8 replies to this topic

[friendly0123]

Hi, I am currently designing a pressure build up coil in a LNG vaporiser. I need some guildeline to determine the basis for the flow of LNG required to be vaporised back to the LNG tank for maintaining the pressurise.  Usually what should be the LNG level in the tank before it is being topped up? 20% of the total vessel volume? And the guideline for time required for the gaseous natural gas from the LNG vaporiser to pressurise the LNG tank to the required pressure during the topping up process?

 

Thank you

[Art Montemayor]

Friendly:

 

Something doesn’t make sense about your description of your scope of work.  LNG, by nature and process, is a SATURATED fluid.  It does not exist in the process or in storage as a sub-cooled fluid.  That means the lowest pressure it exhibits in its vapor space within the confinements of a vessel is its own saturated vapor pressure.  Why would you have need for “maintaining the pressurize” (Sic)?  The tendency – because the process is not adiabatic and therefore undergoes heat leaks – is for the storage pressure to increase rather than decrease.  Of course, if there is considerable liquid draw off, there can be a pressure decline.  For that effect, you either recirculate a portion of the pump out or separately vaporize a portion and return to tank.  If the pump out is meant for pipeline gas sales, then a portion of that pump out is regulated back to the tank as a vapor.  LNG carriers have been designed to used ambient vaporizers to create make up vapor space gas as their tanks are pumped out.  Is that what you mean?

 

The designer and fabricator of the LNG tank is the one who determines the maximum LNG level in the tank.  No one else should determine that.

 

Your idea of the “time required for the gaseous natural gas from the LNG vaporiser to pressurise the LNG tank to the required pressure during the topping up process” is all mixed up.  Your tank vapor pressure will INCREASE during the “topping up process”, not decrease.  So, why should you be concerned about a vaporizer to pressurize the tank during this time?  At the cryogenic temperature you are operating at (-260 ^oF), only two things will cause your tank pressure to decrease:  a venting of the tank or a very rapid liquid pump out.  I’ve already explained what is done during the liquid pump out.

[friendly0123]

Hi Sir,

 

Thank you for your clear explanation. Sorry that I got the questions a little mixed up. Yes, I am refering to the vaporizer for vaporising a portion of the LNG back to make up for the vapor space gas as the tanks are being pumped out. In this case, how do we detemine the required amount of LNG flow to vaporise and return back to the tank for the making up of vapor space in order to maintain the pressure in the LNG tank? Is there any standard guidelines like 15% or 20% of the total LNG being drawed out from the tank?

[curious_cat]

Is there any standard guidelines like 15% or 20% of the total LNG being drawed out from the tank?

 

 

20% sounds crazy large. Just by a simplistic gas law analysis how could the volume of a flow of x volumes of a saturated liquid be compensated by 0.2 x of the liquid being turned to vapor. 

 

That would mean liq. LNG expands 5 times as it vaporizes. I'd guess it expands a lot lot lot more, perhaps 400x. (Obviously, you need to do a detailed design not my lazy back of envelope calculation)

 

If you repumped 20% back as vapor you'd probably over-pressurize the tank to the point of a loud bang. A very loud bang really. 

 

 

I'm no expert though. I may be wrong. Art will probably correct my simplistic analysis. 

Edited by curious_cat, 23 September 2013 - 12:18 AM.

[Dipankarc84]

curious_cat

yes you are correct. LNG expands by ~600 times as it vaporizes. Hence, 20% sounds too high.

[Bobby Strain]

600 times?

 

Bobby

[Dipankarc84]

Bobby

Yes vapor NG is ~600 (590) times higher in volume than LNG

[Bobby Strain]

Actually, if both the vapor and liquid are at atmospheric pressure and in equilibrium, then the vapor specific volume is only about 270 times the liquid specific volume. I know you all read the same stuff that I read where the 600 times is presented. You can do the math and determine that the vapor is at standard conditions and the liquid is at its bubble point at atmospheric pressure. So, be careful of trick questions and numbers without definition.

 

Bobby

[curious_cat]

Actually, if both the vapor and liquid are at atmospheric pressure and in equilibrium, then the vapor specific volume is only about 270 times the liquid specific volume. I know you all read the same stuff that I read where the 600 times is presented. You can do the math and determine that the vapor is at standard conditions and the liquid is at its bubble point at atmospheric pressure. So, be careful of trick questions and numbers without definition.

 

It's a question of whether the end state is vapor @ -161 C or a vapor at, say, room temperature. 

 

For a lot of hazard analysis etc. the room temp. vapor condition makes more sense I guess as the ultimate state. That's where the oft quoted 600x number comes from I think. It's also a more relevant comparison to make when pushing the point about how much space you save by transporting natural gas as liquid. 

 

But I agree if the vapor is assumed to stay in equilibrium with Liq. the expansion is only 270x.