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Vapor Balance How It Works

vapor balance lpg tank

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#1 dhindsa_mexican


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Posted 03 August 2018 - 01:18 AM

Hello All, 

I am trying to understand the working of a vapor balance system when filling LPG from mounded bullets to trucks. I have two scenarios and associated questions:


1. When I pump liquid LPG from bullet to a truck using pumps, my understanding is that initially pressure will rise in the truck until vapor pressure is sufficient to overcome friction losses etc. and reach back to tank. For e.g. lets say liquid LPG arrives at truck at 140 psig, and pressure inside truck is 90 psig (vapor pressure @ ambient), then pressure in the truck will rise and when it overcomes friction losses to go back to bullet, it will stay the same. Is this what will normally happen ?


2. Now I have a case where the truck is connected to a vapor header. The vapor header is simultaneously being used for vapor return and another operation wherein the compressor is putting vapor into that header. Hence the vapor header is at high pressure. Now because truck vapor space is connected to this header...what happens inside the truck? for e.g. vapor header is at say 200 psig, truck vapor space is initially at 90 psig (vap pr. at ambient)...I assume for vapor to go back to header, it has to be above 200 psig? would that be possible...pump makes the liquid lpg reach the truck at 140 psig...what will happen in such a case?? 



#2 Pilesar


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Posted 03 August 2018 - 02:39 AM

The vapor spaces are connected before the liquid is transferred. So at the beginning of the liquid transfer, the trailer pressure is the same as the storage pressure or vapor header. The pump discharge pressure must be higher than the trailer pressure for liquid to flow to the trailer.

#3 Art Montemayor

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Posted 03 August 2018 - 12:17 PM

Your understanding of what is happening when you transfer a liquid from storage tanks to portable trailers using a vapor balance system is flawed.


There is negligible or non-perceptible pressure drop through the vapor balance line - if it is properly designed and installed.  What is happening is no different if you were pumping water from a storage tank into a portable container - with both the storage tank and the portable container open to the atmosphere.  The two water vessels would be “vapor balanced” through the common existence of the atmosphere on both tanks.  However, if you have a closed vapor space on the portable container, your transfer pump would probably begin to transfer liquid at first, but would soon develop difficulty because the liquid it pumps into the portable vessel starts to compress the vapor space and develops an increasing discharge pressure that overcomes the ability of the transfer pump or it makes it more difficult to overcome the amount of work required to effect the total transfer.  That is why, in your system, both tanks are equalized in a common vapor space pressure.  It allows for liquid transfer without increasing the vapor space pressure in the receptacle tank.


In your second case, you must have the header directly connected to and have total accessibility to the same vapor spaces existing between the source tank and the receptacle tank.  If you have other pressure sources affecting the vapor header (something that is out of the ordinary and highly unusual) you will not have total vapor equalization and probably erratic pumping efficiency.  Why would anyone tolerate such a system????  

#4 dhindsa_mexican


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Posted 08 August 2018 - 07:53 PM

thank you art and pilesar. I think I understand it better based on your response.

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