JT effects to calculate lowest temperature
JT effects to calculate lowest temperature
(OP)
Dear Sirs,
I wonder if the JT effect that occurs in the orifice plate to depressurize natural gas from a pressure of 70 bar a is enough to calculate the gas lowest temperature at downstream orifice flow.
My concern come from the fact that the JT effect does not consider the kinematics energy required for increase the velocity till the choked velocity; the phenomena produce a not complete isenthalpic expansion, then, I conclude, that the temperature will be lower than calculated considering just JT effect.
Can anybody help me please?
I wonder if the JT effect that occurs in the orifice plate to depressurize natural gas from a pressure of 70 bar a is enough to calculate the gas lowest temperature at downstream orifice flow.
My concern come from the fact that the JT effect does not consider the kinematics energy required for increase the velocity till the choked velocity; the phenomena produce a not complete isenthalpic expansion, then, I conclude, that the temperature will be lower than calculated considering just JT effect.
Can anybody help me please?





RE: JT effects to calculate lowest temperature
Maybe this is a bit over the top?
If you are concerned and want a conservative measure why dont you just calculate the adiabatic temperature drop and use that for the inlet value to the orifice? Since you dont have choked flow upstream your orifice i would believe your pressuredrop is not that big?
Best regards
Morten
RE: JT effects to calculate lowest temperature
Visiting thread391-78680 you may find some help.
RE: JT effects to calculate lowest temperature
RE: JT effects to calculate lowest temperature
At the throat, the KE change is substantial.
If flow and downstream pressure are known, a reasonable approx is to use adiabatic flow to determine the temperature.
The analysis will be more complicated if heat transfer is to be accounted for as Morten A suggests.
RE: JT effects to calculate lowest temperature
Although rafcu refers to discharge through an orifice, the quotation I'm bringing herebelow from a short section of a Chem Eng article dated October 25, 1976 by Les Driskell titled Coping with high-pressure letdown, may be of help:
RE: JT effects to calculate lowest temperature
Thank you for your kind help;
25362, I found the theoretical answers in your quote, please, could you send me the complete article?
Now, my concern is how calculate lowest temperature in the vena-contracta
Regards
RE: JT effects to calculate lowest temperature
Rafcu, estimate the pressure at the vena contracta and move isentropically from the high pressure, easily done for a single component.
Provide a fax number and I'll try to send the article to you (6 pages). Disregard notes and marks.
RE: JT effects to calculate lowest temperature
Generally, it can be shown that the change in elevation has negligible effect. However,if change in upstream and downstream velocities is significant the isenthalpic assumption is not valid.
RE: JT effects to calculate lowest temperature
Ao^2=A^2 + (gamma-1)/2 *U^2 (1)
where A=acoustic speed
U=local velocity
gamma=ration of specific heats, Cp/Cv
subscript o, stagnation conditions
at choked conditions (vena contracta) A=U, Mach=1
(1)yields (A/Ao)^2 = T/To= 2/(gamm+1)
where T is absolute temperature.
For gamma =1.4 T/To =1/1.2
If upstream stag temp is about 80F,
then T approx =450R, -10F
RE: JT effects to calculate lowest temperature
May be some help can be obtained from
www.grc.nasa.gov/WWW/K-12/airplane/isentrop.html
and links therein
RE: JT effects to calculate lowest temperature
25362,
My fax number: +33 2 40 91 06 19 attentions to: Process Engineer
Exactly my problem is that my customer wants estimate the lowest temperature downstream and upstream orifice flow, for depressurizing a natural gas line at 70 bar a till 7 bar a at the worst temperature condition: The ESD start at -3°C,
Using Hysys simulation, depressurization utility, I obtain -60°C upstream (isentropic depressurization efficiency 90%) during the peak flow.
The problem is that Hysys does not consider the kinetic energy to arrive at sound velocity in orifice flow.
Some people recommended me use OLGA, but this software is not adapted to piping, is better for pipelines
The CS pipe was already damaged during start up. I will suggest use LTCS 3 meters upstream the OF, but the design temperature for this material is -49°C, I hope that the material allowances allows lower temperatures for few minutes…
RE: JT effects to calculate lowest temperature
To rafcu, I've sent the pages. Hope you received them.