Two-phase flow 1

[presso]

To verify if we have a two-phase flow in a steam condensate line, is it correct to assume the pressure drop accross the tubing is equal to the pressure drop of the estimated flowrate of steam plus the pressure drop of the remaining flowrate of water? Can it give a rough estimate? If not is there another way to estimate the effect on the pressure drop?
Thank you!

presso

 

[KernOily]

Presso - Adding the liquid phase dP to the vapor phase dP will not give you the correct answer or even a reasonable answer. There are several ways to go about this. 1) Get a copy of Perry's or Marks and use the two-phase pressure-drop correlations listed therein; 2) Use one of the software packages out there that will do this, e.g. PipePhase or PipeFlo. These assume you know the quality of the stream.

If you suspect you have mostly vapor, and your pipe run is all horizontal, with no or very shallow slope, then you can get close by assuming 100% vapor and using one of the steam dP correlations, also from Perry's or Marks.

Thanks!
Pete

 

[Montemayor]

presso:

I'm presuming you're a Chem. Eng. and that you've taken basic Unit Operations and 2-phase flow, so the following explanation on 2-phase flow should be familiar to you. The state of the art in 2-phase flow is such that a reasonable estimate of pressure drop can be made under certain conditions. The major complication in comparison to single-phase flow is the variety of flow patterns that can be produced in a gas-liquid system. The type of flow pattern encountered depends on the fluid properties, flow rates, and on the geometry of the equipment. There is no such thing as just simply one type of 2-phase flow for which we calculate.

For example: Flow can be horizontal or vertical / sloped.
Horizontal 2-phase flow can be one of the following types:
1) Bubble
2) Plug
3) Stratified
4) Wave
5) Slug
6) Annular
7) Mist or Spray (Dispersed)

Vertica/sloped flow can be one of the following types:
1) Bubble
2) Slug
3) Froth
4) Annular
5) Mist

So, which one are you referring to. It makes a big difference in the results. If you know the flow pattern and all the physical and geometric properties of the system, the best correlation can be expected to predict the pressure drop with an accuracy of about 25%. So, as you can see, you are already talking about "rough" figures before you begin an accurate determination of the category type of flow you have.

Additionally, for steam lines under 2-phase flow, certain special techniques have been developed in the industry. There was an article written by Meador & Shah in the January 1969 issue of Hydrocarbon Processing; vol 48, No. 1; page 143; "Steam lines designed for 2-phase". The method uses graphs and the Dukler-et. al. method to predict the frictional pressure drop after the flow conditions have been determined. The graphs used assume horizontal condensate lines. However, Dukler describes an elebation correction term which can be used where the piping is not horizontal. The gas-liquid volume ratio in the pipe must be predicted before the Dukler method can be used.

Perhaps this article can help you out if you can find it at an engineering library.

Hope this helps.


Art Montemayor
Spring, TX

 

[25362]

Chemical Engineering (McGraw-Hill) has published a series of eight articles over the period April 1970-February 1971, titled Chemical Engineering Aspects of Two-Phase Flow by DeGance and Atherton, both working then at Brown & Root, Inc. The articles were were collected in a reprint form which may serve as a good introduction to the subject.