Rollover in LNG tanks. 3

[ris2]

Can anyone provide details about rollover in LNG storage tanks and how can it be prevented?

 

[TD2K]

Have you done a keyword search on this site? I believe I've seen posts on this subject before.

 

[BenThayer]

Rollover in Liquid Ammonia tanks is probably the same thing. A liquid ammonia tank that is stored at essentially atmosperic pressure is ~ -28°F and has a density of about 42.6 #/ft3. If there is perfect mixing, the entire tank would be at this temperature.

But if you have the tank full (~60 feet), you would also have liquid at the bottom of the tank at ~17 psig. Saturated liquid at 17 psig corresponds to ~2°F and a density of about 41.2 #/ft3.

Because the liquid at the bottom of the tank has a lower density, it tends to rise and the liquid at the top tends to sink.

A rollover is typically refered to as a sudden event where the top and bottom suddenly "rollover" and results in massive vapor generation. A typical method of "control" is to have the liquid entering the tank come in through a stand pipe that is disconnected at the top. This allows vapor in the product line to disengage and the cold liquid then descends to the bottom of the tank. Also, careful control of the refrigeration system can help prevent sudden cooling at the top of the tank.

Sudden changes in atmospheric pressure will affect the gauge pressure on the tank and cause upset conditions in the tanks.

 

[Montemayor]

TD2K is precisely correct. Go to:

thread798-63390

where the subject was dissected a year ago.

Art Montemayor
Spring, TX

 

[Thamed57]

Hi ris2,
I agree with Benthayer explanation for the rollover phenomena.
I work in LNG producing Company called Qatargas and I can give you some refernce books to read from.
1- Liquefied Gas Handling Prenciples On Ships and In Terminals by McGuire and White 2nd edition (SIGTTO) page42 chapter2.16 liquid and vapour densities. If you give me your fax or e-mail I will send this chapter for you.

 

[ris2]

Thamed57-- My email is rsingh@atlanticlng.com and my fax is 868-648-2905.
Thanks.

 

[MJCronin]

ris2,

I guess the better question should be:

"Are there any design features that should be incorporated into the tanks/relief valves/storage facilities that will mitigate rollover...????

Or is this just an operations issue ?

By the way, I do not believe NFPA-58 mentions this in any way...

My opinion only...

MJC

 

[JDavisENE]

I am interested in Montemayor's recommended thread 798-63390 but I get a "page not found" message.

Can anyone help me with a copy? (I've e-mailed the site to request that the thread is restored - but no response.)

Also, I am interested to know if anyone has experience with the ENTIS LNG Pro offered by ENRAF - has it successfully predicted a rollover event?

Has mixing/recirculating then prevented the rollover?

Thanks,
J. Davis
EN Engineering