8 replies to this topic

[agu1]

Hi all,

 

I arrived this forum by looking for information about Flarenet isentropic flash option at relief valves, and after reading some topics i have to ask something that i dont understand

 

Outlet valve temperature is not changing in my case by varying the isentropic flash option and its efficiency, and i think that by setting this option on, and efficiency>0, outlet temperature should be lower than considering isenthalpic expansion (this is what flarenet considers when setting isentropic flash option off, isnt it?

 

Do i have to choose any specific VLE of enthalpy method? I dont know where the problem is...

 

Thank you !!!!

[Bobby Strain]

Well, first, the operation is adiabatic, not isentropic. The outlet stream will have the same enthalpy as the relief valve inlet. You are confused because the capacity of a nozzle is based on isentropic behavior to the choke point (stagnation pressure). But taken from inlet to outlet, the flow is adiabatic. Since you didn't share the stream composition, we can't help in selecting the proper EOS.

 

Bobby

Edited by Bobby Strain, 12 March 2013 - 03:45 PM.

[agu1]

hat happen in a PRV is an isentropic flash from inlet pressure to sonic velocity and pressure at the nozzle throat, then an isenthalpic flash from there to the valve outlet, and I understand that “isentropic flash” & “isentropic efficiency” options in flarenet are for modeling this “mixture”, so this was my doubt about.

 

The stream is 1950kg/h of pure C3, 22,5barg&38ºC, with 0,3barg backpressure. Outlet temperature obtained is -36,17ºC, no matter if isentropic flash option is marked or not. I’m using Peng-Robinson (VLE) and Lee-Kesler (enthalpy).

 

I’ve tried with different vapor streams (LPG, water, ammonia), and I always get the same results. Flarenet is always considering an adiabatic flash through the valve and my question was if this could be changed in flarenet.

Cheers

[Gatts]

Hey Agu,

 

In order to make the Isentropic Flash applicable to your simulation, do the following:

 

At the top of the screen, go to Calculations -> Options -> Methods

 

For your Overall VLE Method, Change it from Compressible Gas to something like Peng Robinson.

 

For your Overall Enthalpy Method, change it from Ideal Gas to something like Peng Robinson.

 

This should fix your problem.  Now, enable the Isentropic Flash and trying fooling around with the isentropic flash efficiencies and this should vary your outlet temperature.  You may find that these temperatures change drastically or not much at all, depending on the case.

 

Keep in mind that this will change the basis for your simulation.  Also, Calculations -> Options is the place in which you can set all of your "default" settings.

Cheers

[Bobby Strain]

Agu,

   You didn't read my response very carefully. From inlet to outlet state the flow is isenthalpic. Not just from the choke point.

Bobby

[agu1]

@Gatts

 

I have tried with every single VLE method-enthalpy method combination and outlet temperature is not varying

 

Could it be any kind of bug or something ?

 

@Bobby Strain

 

So, why flarenet would include an "isentropic efficiency" option?

Edited by agu1, 22 March 2013 - 03:10 AM.

[Bobby Strain]

You should ask them this question. And let us know their answer.

 

Bobby

[flarenuf]

Hello Agu

 

A PSV is really isentropic to the valve throat and then isentahlpic from thereon.  see
http://webwormcpt.bl...tropic-and.html

 

most folk model it as isenthalpic and have done for many a year, its only with the advent of computers and more than 1 decimal place that you notice the difference if you model it as 2 parts.
I have run your C3 stream in FNT and get the following
using peng R for all methods


isentropic flash = No                  T out = -37.02 C

isentropic flash = yes   with efficiency       0.00     0.25      0.5      0.75     1.00
                                                  T out = -37.02  -37.22  -37.43  -37.67  -37.87  C

so varying the efficiency does work

You will find you see more effect from actiavting teh KE option in the model as this will give you a much more accurate model

flarenuf

 

[agu1]

Hi guys,

 

It was a version problem, as after installing Cummulative Patch 3 for v7.3 it works well.

 

Thanks to all