Orifice sizing - Two Phase Flow

[Emiliano1]

Hi to everybody in the forum,

I'm looking for some bibliografy/ design experience in two phase flow orifice design.

As I know the normal sizing method considered to specify a two phase flow orifice is to sum up the areas required by liquid flow with vapour flow and get the total area. This bring to a conservative approach.

But I have some doubts on this theme:

1) should be considered the inlet or the outlet vapour/liquid mass ratio for sizing? I guess it depends on where the vaporization takes place, if in the inner part of the orifice or exactly at the outlet.

2) the formula for sizing the orifice are very weel knowed for single phase flow (gas or Liquid) is there any formula used for mixed phase? or reliable approach?

3) I'm suppose to produce the depressurization curve (Pr vs Time) generated by the orifice, considered the area installed, to produce some documentation that will be the base for an on site test of the depressurization system of the plant. I've some experience on this point with single phase flow, has somebody some experience with two phase depressurization curve defition and on site test?

Many thanks in advance
Ciao
Emiliano

 

[Ashereng]

Emiliano, adding the areas of the individual phase may not always result in a "conservative" answer.

A better way to calculate the orifice requirement is DIERS method.

For control valves, I had thought that Cv was additive in regards to 2-phase flow. I was wrong. In the Fisher control valve handbook, there is a section on 2-phase flow correction factor.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[Latexman]

1 and 3) Don't be surprised when you look at actual performance numbers and see flows greater than those calculated. I've seen this several times. The problem? Equilibrium is not reached instantaneously. It's called "frozen equilibrium". Google it. During the short period the flow is in the orifice, it may not reach equilirium. It probably doesn't flash in the orifice. It remains more liquid and flashes somewhere downstream.

Good luck,
Latexman