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Time Taken To Increase Pressure From 0 Psig To 100 Psig

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#1 nivedita12


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Posted 18 June 2019 - 01:47 PM



I have a 4 inch pipe  of 150 ft in length which is assumed to be 90 % full of LNG.

If the initial pressure is 0 Psig How much time does it take to reach 100 Psig.


I have calculated till this part


I have assumed a heat in leak rate 138 Btu/hr   for the pipe to calculate a vapor flow rate of 0.63 lb/hr based on 220 Btu/lb of mass heat of vaporization  since LNG vaporizes slowly. 


I assumed that since LNG is 90% full then rest of it is vapor.


I calculated the initial mass of vapor form of LNG to be  0.2 lb.


How can I calculate the time increment to pressure increments ?



Thank you 

Edited by nivedita12, 18 June 2019 - 01:49 PM.

#2 Art Montemayor

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Posted 18 June 2019 - 02:23 PM



You must be specific and concise when you define an engineering problem.  Please furnish a sketch and basic data.


For example, is your LNG flowing steadily through the 4" pipe?   Or is your 150 ft length of pipe blocked (or sealed shut) at both ends and being subjected to a heat leak through its wall?   If so, than state it and give our members the conditions of this static condition - such as the pipe material, its insulation (if any), its surroundings (is it buried or exposed to the atmosphere?), and the external conditions around it.


As you should already know, all these conditions will play a part in any heat transfer that is expected to occur between the exterior of the pipe and its cryogenic conditions inside the pipe.  Why don't you supply this essential information to our members in a normal, concise, and detailed engineering manner?   This is an engineering problem to discuss, not a cocktail party chit-chat.   Please furnish the essential basic data and our members will surely lend a hand in helping you to resolve the issue you may have.

#3 nivedita12


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Posted 19 June 2019 - 08:22 AM

Dear Art

The pipe is above ground used to carry liquid from a Barge to Iso-Containers.The ship comes once in 5 days filled with LNG. 

Once the loading is done . Ship leaves.


This pipe is assumed to be blocked in from both ends and assumed to be full of LNG.

Pipe is 4 inch sch 40 VJP pipe. Since the LNG vaporizes slowly and little amounts of it  may escape due to heating of the pipe due to hot  ambient conditions .Assuming the ambient condition to be 75 deg F. 


Cryogenic liquid is at -260 deg F. 


Attached is the sketch .Hope this information is enough. Please let me know 


Thank you 

Attached Files

#4 PhilippM


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Posted 19 June 2019 - 02:25 PM

Why does it matter how long it takes until a certain pressure is reached? I'm assuming you are checking the overpressure protection of the pipe? The main thing you need to worry about is whether or not a relief valve for overpressure protection is required (I would say so, as the line is completely filled with liquid) and how to design the relief system.

I suggest you take a look at API 520, API 521 as well as this thread: https://www.cheresou...c-pipe-systems/

#5 Bobby Strain

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Posted 19 June 2019 - 04:07 PM

Most on't know what VJP is. It doesn't really matter as long as you have the heat leak information from the supplier. It's not goo practice to use acronyms that may not be understood by your audience.


You seem to have no experience with such systems. Unless you are positive about a vapor space you may need to relieve liquid. And you must be certain that the relieving point is such that any vapor can freely collect here. Since you didn't provide an isometric sketch we have no idea what the installation looks like. If the pipe is such that it is liquid full, you must relieve liquid due to liquid thermal expansion. At an increasing temperature. Is this system already operating?



Edited by Bobby Strain, 19 June 2019 - 04:13 PM.

#6 nivedita12


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Posted 08 July 2019 - 10:23 AM

Hi Bobby


This is a new system. I believe it is close to the airport hence we are trying to send the excess vapor  to an iso-container instead of the flare .

All the PSVs at this point on the liquid LNG piping are connected to an Iso-container .

The thing is Barges of LNG  will  come and go and they send the  LNG through barge pumps to the Iso-containers through this piping. 


Hence what happens is if the barge leaves and any liquid is left in the piping it will vaporize eventually and then the vapors are collected in an Iso-container may be should be half -filled so that the vapors can condense. 


Does that answer your question?


Thank you  

#7 Bobby Strain

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Posted 08 July 2019 - 12:49 PM

Eventually vapor must be disposed somewhere along the transfer chain.



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