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How to find Thermal Expansion Relieving Temperature

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NatawanS

Petroleum
Jun 4, 2020
2
Dear All,

My liquid-full pipeline maximum operating pressure and temperature is 160 Barg and 20 degC.
I have PSV installed for thermal relief with set point of 190 Barg. What temperature increase will increase pressure from 160 Barg -> 190 Barg?
1. I use equation 3 from API 521 (section 4.4.12.4.1) to estimate pressure rise due to simultaneous heating of the pipe and blocked-in liquid.
From that equation, at 23 degC, I will get the pressure rise from 160 -> 190 Barg. Only a few degC and pressure rise to PSV set point already!
2. Technical advisor at my company disagreed with the calculation result and suggested me use HYSYS simulation on the thermal relief calculation to find relieving temperature.
From HYSYS safety analysis, with thermal expansion sizing case, the relieving temperature at PSV set point 190 Barg is 360 degC (I use same stream condition at 160 Barg, 20 degC).

My question is, which one is the right method to find temperature increment to increase blocked-in liquid pipeline pressure from 160 Barg -> 190 Barg?
If using equation from API is the right way, why the relieving temperature I got from HYSYS is so high compared to the API equation?

Thank you very much,
NatawanS (Process Engineer)
 
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NatawanS,

There is a quick and fast method presented by eng-tips user 25362 in this thread.

There is also a FAQ by prex here.

I don't think you would need a 340°C temperature rise to increase 30 bar of pressure, unless your piping is made of latex.

Hope those links assist you.

Daniel
Rio de Janeiro - Brazil
 
as suggested the normal procedure is to solve considering fluid and pipe expansion (due to temperature, pressure),
for a full liquid system, you could presume constant values for fluid expansivity and compressibility and solve directly (see the formulation in many Eng-Tips threads),
with some evaporation, or additional accuracy, you can solve a flash operation at specified volume and pressure (pset), I use the method t = VPF() in PRODE PROPERTIES,
a iterative solution is required but you can easily write a macro in Excel, LibreOffice or Python ...
 
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