3 replies to this topic

[Luigi]

Hi,I need to size a restriction orifice to be installed in a depressurization line. I expect two-phase flow upstream the restriction (liquid/gas) and some additional flashing downstream.
The initial upstream pressure is 1100 psi and the downstream pressure is about 5 psi.

For this particular conditions (two-phase, big pressure drop, flashing), I cannot find a method for sizing the orifice. Could somebody help me with formulas or methodologies to size this orifice plate?

Thank you. Your suggestions will be appreciated.

[Luigi]

Come on, nobody has nothing to say?

[Art Montemayor]

Luigi:

I think I responded to this identical query in Eng-Tips where I wrote -

"You can't find a method or equation for 2-phase choked flow because, in my opinion, there isn't any. At least I haven't found or heard of one (that would work) in 47 years. But just because my efforts haven't had success doesn't mean others won't. However, I seriously doubt it, based on the principles and theory of choked flow.

The term "choke" is often battered about in the oil patch, and it can have at least two meanings. One of them is the choked flow of a a gaseous stream to yield a constant mass flow rate at sonic velocity. The other is a simple throttling of a 2-phase stream or of a liquid stream - with a throttling valve or with a fixed orifice. I believe you are looking to apply the former and not the latter. Am I correct? If so, then an oil field "choke" may not serve your purpose.

If you are trying to depressurize a vessel or system, then a standard control valve with instrumentation can achieve your goal - depending on your constraints and application (which you haven't mentioned)."

After we were given "the rest of the story", I added:
"Is this an academic project or is it a theoretical exercise? Please be frank and tell us all the facts so that we can accurately identify the concerns.

For example, you state that 'methanol is close to critical conditions'. I don’t know if you are an experienced process engineer, but methanol’s critical temperature and pressure are 1,174 psia and 463 ^oF respectively. You are not close to what you state. Perhaps you are referring to other “critical” conditions.

You cannot 'depressurize' a process vessel or reactor with efficient accuracy by draining the liquid – which is what you are proposing. If you want to depressurize the reactor you should vent the upper, vapor region. Once the vapor/gas is vented you have, in effect, depressurized your vessel. The pressure in a reactor (as in a boiler) is primarily generated by the generation of vapor or gas. This is the major source of pressure in a heated vessel. There is always an expansion of the contained liquid in any heated vessel, but the normal vapor space allotted makes the “swelling” of the liquid a minor contribution to the overall pressure rise. Therefore, the indicated manner of quickly and efficiently 'de-pressuring' a vessel is to vent the top gas/vapor phase – not the bottom liquid phase. Do you (or your process colleagues) have a special reason for draining the liquid in attempting to lower the pressure of the reactor?

You further state 'The depressurization will be done with a line connected to the bottom of the vessel (by process design), so I will get liquid and some vapor in my restriction device'. I maintain that you will get 100% liquid INTO your throttling device and probably a 2-phase mixture OUT of the device as this is probably an adiabatic flashing operation.

What kind or type of reactor do you have and what are the contents and their levels within the reactor?"

I still stand by my general comments because the basic data has been changing. Is this the same query as the one in Eng-Tips? The problem's pressure and description are almost identical. I await a reply.

[Luigi]

Thanks Art, it is me indeed, posting in the two forums. I already clarified the doubts and answer your questions in the other forum. If you can take a look and give me some more comments, would be great. See you there.