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# Isentropic Flash Calculations

## Isentropic Flash Calculations

(OP)
Hi guys,
I'm working on some two-phase flow applications, attempting to use API 520's omega method. The methods call out an isentropic/adiabatic flash calculation to determine the system specific volume. Unfortunately I don't have access to ASPEN or similar software.

I tried searching the forums and found lots of references to process simulators and using Prode Properties (which I have but am not very familiar with), but I can't seem to find any source that shows the equations for this calculation.

Does anyone have a source/reference that shows the steps for doing the flash calculations? I also hesitate to use Prode Properties when I'm unfamiliar with it; is there a good guide to using it anywhere?

### RE: Isentropic Flash Calculations

Check the frequently asked question area. FAQ1203-1293: A Simple Numerical Method for Gas/Vapor Flow in a Safety Valve

This is the simple, vapor only venting derivation for a PSV. Two-phase is more complex, but it's derivation and examples can be found if your Google-fu is strong.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Isentropic Flash Calculations

This link shows lots of flash equations and calculations. Unfortunately, they don't give specific equations or examples on their isentropic flash (#7). It's a worthy article nonetheless.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Isentropic Flash Calculations

If a Mollier diagram for you product is available, you can easily determine the specific volume as long as you have the initial condition such as temp. and pressure just follow the isentopic line to the final condition such as pressure, or temperature, or enthalpy and at that point you can interpolate the specific volume.

### RE: Isentropic Flash Calculations

RVAmeche,
in this forum you can find several references to Prode Properties since the library includes several equilibrium (HEM) and non-equilibrium (HNE) models,
there is an Excel page with VBA code for API HEM (posted by PaoloPemi) but (as far as I know) the procedures included in the library calculate the speed of sound (critical flow) with different models for equilibrium and non-equilibrium cases,
as said by chicopee in some cases (for example, pure fluids) you can utilize a mollier chart or a table of values, while for mixtures you need to solve phase equilibria.

### RE: Isentropic Flash Calculations

RVAmeche,
I prefer PRODE for these calc's but you can use another simulator as well,
however, as library, Prode offers several advantages when working with Excel or similar applications (while a simulator is more suitable for solving complex plants).

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