LNG vapor pressure

[helpyou]

HI
on an LNG (liquified natural gas) pipe, I understand that there is LNG vapor pressure with the Liquid. Is the LNG vapor pressure equal to the LNG pressure (Liquid pressure)? Reason is, there are pressure gauges mounted on top of pipe and reading pressure. I assume they are reading vapor pressure. Can someone clarify this point from experience please?

Thank you

 

[Latexman]

Is the LNG vapor pressure equal to the LNG pressure (Liquid pressure)?

If there is a pump or compressor upstream, or the LNG is inerted, probably not. If the pressure gauge is below the liquid/vapor interface, there is a pressure component associated with the depth of liquid. Not so much vapor as it has a low density.

I assume they are reading vapor pressure.

It could be a bad assumption. Maybe not. See above.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Is the LNG vapor pressure equal to the LNG pressure (Liquid pressure)?
Depends on the temperature of the LNG.

The pressure your are seeing is the pressure in the pipe.
Now if the temperature of the LNG rises and there is no where else for the pressure to relieve itself then this could be the vapour pressure or it could the pressure from the pump upstream.

This is the vapour pressure curve for LNG. So e.g. if the temperature of the LNG is -150, but the pressure is say 10 bar then this is just the liquid pressure created by a pump or change in elevation. If the pressure is 2.5 bar then this is the vapour pressure of the LNG.

This is the phase envelope. Note that about -80 and 45 bar getting warmer but higher pressure you enter the supercritical fluid zone where strange things happen.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

The minimum pressure in an LNG pipe is the vapor pressure. Since pipes containing LNG are generally intended to convey only liquid the pressure will be higher than this to prevent vapor formation and cavitation. This higher pressure is created by liquid head, or a pump.

 

[chicopee]

In a pipe where you have liquid and vapor mix, LNG is a saturated and the quality of LNG would be between 0% and 100% being the saturation region of LNG. So yes the pressure in saturated LNG vapor and liquid will have the same pressure as long as there is no external pressure exerted in the saturated LNG liquid. The best way as mentioned above is to take the temperature of the external surface of the pipe and that temperature should very close to the saturated LNG within the piping. Once you have a temperature reading get either a Mollier diagram or a Temperature, Pressure table and determine if the pressure of that saturated liquid is similar to your pressure gage. Beware that the pressure gage should be under a no flow of LNG within the piping otherwise if flow occurs then the pressure gage may not record accurately a static pressure value. If the pressure gage has a higher value then the pressure temperature chart of of that of the Mollier diagram then there is an external force acting on the LNG. If the pressure gage reads a lower value the there is either a good chance continuous flow is taking place or the liquid is draining from the piping.