5 replies to this topic

[prav]

Hi All,
I want to calculate flow through a restriction orifice. The diameter is 0.6 mm. The liquid propylene is entering at 24 bar and the gaseous propylene is exiting at 6 bar. There is a phase change here because of the difference in pressures. How do I calculate the flow through the orifice ?

Thanks in advance

[Zauberberg]

The way I understood it, you have single phase (liquid) flow through the orifice, and two-phase flow downstream of the orifice. You need to calculate back-pressure on the orifice by knowing the final (receiver) pressure and calculated 2-phase pressure drop through the line downstream of the orifice, and then see if calculated pressure differential across the orifice is sufficient for passing that much flow.

This is an iterative procedure, as you have to match total flow through the orifice at given pressure differential (a variable) across the orifice. DP across the orifice is a variable since it depends on pressure drop in the piping downstream, and this again depends on the flow through the orifice at the calculated pressure differential. Setting up an Excel sheet and then using Goal Seek or trial and error procedure should bring you to the point where these two calculations result in identical values.

[prav]

You need to calculate back-pressure on the orifice by knowing the final (receiver) pressure and calculated 2-phase pressure drop through the line downstream of the orifice, and then see if calculated pressure differential across the orifice is sufficient for passing that much flow.

Sorry I did not understand your solution. The DP across the orifice is constant and which is equal to 18 bar. The phase change occurs at the RO. What is the "2-phase pressure drop" and how it is calculated ?

The situation is that the upstream and downstream pressure across orifice are fixed. The propylene flashes at the orifice and changes phase which is now mixture of liquid and vapor. As we don't have any calculations of orifice sizing for two phase, I don't know which formula to use.

Thanks for the response

[Erwin APRIANDI]

Hi All,
I want to calculate flow through a restriction orifice. The diameter is 0.6 mm. The liquid propylene is entering at 24 bar and the gaseous propylene is exiting at 6 bar. There is a phase change here because of the difference in pressures. How do I calculate the flow through the orifice ?

Thanks in advance

Hi prav, please see the link

http://en.wikipedia....i/Orifice_plate

[Zauberberg]

The DP across the orifice is constant and which is equal to 18 bar.
The situation is that the upstream and downstream pressure across orifice are fixed.

DP across the orifice is not known, as you don't know what is the pressure drop from downstream of the orifice to the final receiving pressure. And this pressure drop will depend on how much flow you have through the orifice, and this flow depends on what is the pressure downstream of the orifice. So you have to iterate the figures till you get the solution.

Example:

1. Set the flow;
2. Calculate pressure drop for the same flow in piping downstream of the orifice, for known receiver pressure;
3. Add this DP to the receiver pressure and calculate pressure downstream of the orifice;
4. Now, calculate if you can have the flow you anticipated in the point No.1, with given differential pressure across the orifice;
5. If the calculated flow is less than what you assumed, go to step No.1 and change the flow;
6. Iterate till you reach the solution.

Pressure downstream of the orifice can be assumed as constant only if you have short piping between the orifice and the final (receiver) pressure, because pressure drop in that case can be considered as negligible. In that case, the calculation is straightforward - just see how much flow of liquid you can get through fixed orifice at fixed differential pressure. However, if piping downstream of the orifice is with significant length, or if there are other fittings with pressure drop highly dependent on the flow, you need to use this iterative approach.

[kkala]

Flow through orifice with phase change requires special method of calculation per DIERS, applicable for PRVs (and why not for any other orifice). At least in my working environment this DIERS method is mentioned, yet not practically applied in general.
For a quicker solution, you can see "Flashing flow and condensing flow" in Perry, 7th edition (1997, Section 6, Multiphase flow). Under stated pressures, flow looks chocked and mass flow flux through the orifice can be calculated. The flashing flow is assumed to produce vapor in equilibrium to the pressure.
Above is resulting in volumetric flow rates a bit higher than DIERS (per opinions at work).
It is also assumed that resulting two phase flow is spray. It is expected so in the orifice, due to its small sectional area.
Reason of need to estimate flow through the orifice would have been useful.