Compressible Flow Basic Question

[shahyar]

Is there any way that I can calculate the properties of flow(in a pipe) after choke?
Let's assume I have a long horizontal pipe and the flow could be Adiabatic or Isothermal. Because of huge flowrate I have a choke somewhere at the middle of the pipe(and I can calculate that). But what would be the pressure, temperature, and density at the end of pipe which is atmospheric pressure?
The answer to this question can also help me to visualize the phenomenon of double choke which is not rare in discharge side of PSV's.
Thanks

 

[gaswell]

something is not clear to me,
if you model as isothermal tout=tin and pout (as you write) = patm
for adiabatic Hout = Hin,
you may consider (or not) the contribute of kinetic energy,
see the other posts discussing this topic in Safety Relief Valve forum,
or am I misunderstanding the question ?

 

[Matt17]

I guess you have not the support of any software, right?

If you're speaking about a vent line, gas velocity inside should be sufficiently high to approximate the thermal behaviour to adiabatic ... in this way you can estimate the outlet temperature by means of an isenthalpic. However, be aware of the gas velocity: if it's very high (e.g. 0.3 Mach or higher) the cooling effect could be higher than pure isenthalpic ... in this case you can have an estimation with the Fanno Flow.
In both cases, above suggestions only work for a first estimation, but it could be enough for a first conceptual design.

Good luck.

Matt17
Process Engineer

 

[Latexman]

A "long" pipe usually follows the isothermal model more closely than the adiabatic model.

Good luck,
Latexman

 

[Matt17]

Latexman rises the right question: what are we speaking about?

Matt17
Process Engineer

 

[sshep]

I have never heard of a "double choke" and am curious about the inferance that a flow could be simultaniously limited at two points, especially in a staight pipe for an isothermal model. The limiting flow as calculated by Crane TP410 always occurs at the outlet, and the calculation method for high dP should be done by segmenting the pipe. If your flow hits the limit in the middle of the pipe, then it usually means you are guessing too high of a flow.

Obviously my understanding is conventional. I am very anxious to hear more.

best wishes,
sshep

 

[shahyar]

Thanks gentlemen!
I found a portion of my answer in the below link:

http://www.eng-tips.com/viewthread.cfm?qid=100632

As zdas04 (Mechanical)says:
"One point that is missing is that sonic flow is not sustainable in straight pipe. Period. If you enter a pipe at sonic velocity (downstream of a choke for example), friction effects will begin to reduce the velocity immediately and the duration of M=1.0 is meters, not kilometers or even tens of meters."
So, it means after a choke, within a certain distance(how can we calculate this distance?)the flow drops to subsonic again, and again opportunity for another choke if situations is suitable.

 

[zdas04]

So I participated even though I didn't have time to open eng-tips.com today. At least you picked a quote that is not embarrassing.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[TD2K]

I can't see two choke points in a constant diameter line for the reasons given above by sshep. If you had a size change, I could see two choke points. One at the end of the small line and another at the end of the large line if you had the right conditions though I haven't tried to come up with a case.