5 replies to this topic

[William.K]

Hi, I am a junior process engineer of LNG power plant or off shore.

And nowadays I am calculating some PSVs on the liquid line.

But I don't know the relief temperature values exactly.

I already saw API and ASME, but there is nothing to satisfy my curiosity.

 

I am going to show you my questions as below;

 

Fluid: LNG

Set Pressure: 10 barg

Set Temperature: -163'C

Relieving Pressure: 11 barg (10% of Set Pressure, normal case)

and Relieving Temperature: ???

 

I saw a lot of comment of this problem and they say T1=Tn*(P1/Pn) but I don't understand this formula and another opinion is

relieving temperature will be determined by the relieving pressure cause of the saturated temperature of that pressure.

So in case of above condition, when LNG relief to the air at 11 barg, at that time the saturate temperature will be abt.-80'C.

(Commonly 1atm LNG will be evaporate at -161.8'C, so at 11 barg, evaporation temperature will be -80'C)

then the relieving temperature is -80'C?

 

I have some calculation sheets of PSV and the fluid is air. At that time the relieving temperature is 0'C. Why?

I am going to black hole when I try to figure it out. Please Help ME.

 

 

[Art Montemayor]

William:

 

LNG is a LIQUID fluid (Liquefied Natural Gas)

 

A “PSV” is a Pressure Safety Valve.  Therefore, the set point is based on PRESSURE – not temperature as you request!  The only reason to know the relieving temperature is to mechanically design the valve --- not to set its capacity.

It seems you need to do some work on your thermodynamics and phase equilibria.  If you are relieving LNG at its saturated state, all you need to do is refer to its saturated conditions at the relieving pressure and look up the resulting evaporation temperature when you adiabatically expand the liquid through the PSV orifice from whatever set pressure you have down to atmospheric pressure.  The process isn’t truly 100% free adiabatic, isentropic expansion, but close enough.

You get the expansion answer with a simulator or with a thermodynamic data base or Natural Gas Mollier Diagram.

 

You should be clear and concise in your basic data.  You are expanding a liquid – not a gas; and it is saturated at storage conditions.  (why you would relieve a liquid is beyond me.  Normally, the GASEOUS phase is relieved, not the liquid.)

[William.K]

Thank you Art Montemayor.

And I had a mistake. the liquid relieves to the tank again not in the air and I thought that's right in case of liquid, I don't need to relieving temperature.

relieving temperature is usually important to calculate the orifice area due to the fluid volume, but liquid won't be change the volume a lot, right?

 

And also while I post this questionnaire, I understand the T1=Tn*(P1/Pn). the thing is the unit conversion. Thank you again.

[fallah]

 

And also while I post this questionnaire, I understand the T1=Tn*(P1/Pn). the thing is the unit conversion. Thank you again.

 

William,

 

It's not a unit conversion...it's an estimation method for relieving temperature in gas expansion (due to fire) case...

 

As Art well mentioned you need to do some work on your thermodynamics and phase equilibria...

[William.K]

Thank you fallah,

 

and unit conversion which I said is....

 

I didn't calculate the right relieving temperature because I misunderstood about the unit of temperature and pressure. That's what I were saying.

 

Anyway thank you for your advice.

[fallah]

 

and unit conversion which I said is....

 

I didn't calculate the right relieving temperature because I misunderstood about the unit of temperature and pressure. That's what I were saying.

 

 

William,

 

Ok, thanks for your clarification, anyway that formula isn't applicable for liquid relief at all...