5 replies to this topic

[ayan_dg]

Can anyone give me an insight on Choked flows related to control valves and answer the following questions.

1) Is choked flow same as sonic flow (where the fluid velocity reaches the speed of sound)?
2) Should we always avoid choked flow?
3) Can choked flow occur in liquid flow ?
4) If choked flow happens what is the remedy?

[Art Montemayor]

Ayan:

Your exposure to Choked Flow (a.k.a., Sonic Flow) should be at the 3rd or 4th year of university engineering courses - practically when you take your first Fluid Mechanics course. I hope you are at that level now because I don't want to write over your head or confuse you more.

Answers to your questions are:

1) Is choked flow same as sonic flow (where the fluid velocity reaches the speed of sound)?
They are both the same. However, due to a lot of ignorance or lack of full understanding of Fluid Mechanics, some oil field workers call a throttling valve a choke valve and the resulting flow as "choked" flow - when it isn't necessarily that at all. But basically, both terms should be the same description of a gaseous or vapor fluid flowing at the speed of sound in that fluid. For the classical - and I regard it as the best - description, go to Milton Beychok's webpage for complete definitions and equations on Sonic Flow. Use the Search feature on the Forums and you will find Milton's signature and webpage URL.


2) Should we always avoid choked flow?
Not necessarily. Do you feel that choked flow is wicked or evil? Sometimes it simply can't be avoided, and does no harm.


3) Can choked flow occur in liquid flow?
I do not believe that choked flow - in the practical and useful sense - can be achieved in a liquid medium. Some theorists have argued that it is possible, but have failed to identify for what purpose or how.

4) If choked flow happens what is the remedy?
What is the problem here? Choked flow is actually designed for by me and many other engineers. That's how a Restriction Orifice - an RO - basically works! We can control the flow of a gaseous fluid at a pre-determined, CONSTANT, mass flow rate by applying the correct equations and orifices in compliance with the Fluid Mechanics laws that apply. I have done this countless times out in industry both for control purposes and for safety purposes. Please explain yourself with full details Ayan, because I don't want to confuse you. This is the bad part of keeping your personal background secret from us. If we don't know your level of expertise, we can confuse you and cause you harm in your learning. I am willing to bet that you never knew before that sonic flow is actually a design parameter and is useful in engineering process and control philosophy. You probably also never considered that every time you open a compressed gas valve to the atmosphere, you are conducting a sonic flow (choked flow) step. Just about every safety relief device out in industry - working on compressed gases or vapors - undergoes sonic flow the moment it relieves the over-pressure it is supposed to protect against.

So, what is the problem? Choked Flow can be beneficial.

[ayan_dg]

Thanks Art for clarifying my doubts.

As far as my education background goes, I am in 4th year. Prior to your answer my concept on "choked flow" was that of an evil which must be eliminated.

[Art Montemayor]

Ayan:

Thank you for being forthright and honest with all of us.

There is nothing that pleases us, old-timers on the Forum, more than the knowledge that we have succeeded in correctly and efficiently explaining an engineering truth that will help all students in the future become better and more knowledgeable professional engineers in their projects.

I hope that I succeeded in showing you that Choked Flow can be a very valuable tool to have in your "tool box" for solving future problems. The ability of an engineer to ensure that a gas flow is kept at a constant mass flow rate by simply throttling a control valve is a very valuable thing to know. Do not fail to go to Milton's website and download his lengthy explanation of what choked flow is and how it is predicted. You will never regret it.

[gvdlans]

Art,

It is not that easy to find, but I guess you refer to the following: http://www.air-dispe...m/feature2.html

[djack77494]

2) Should we always avoid choked flow?
Not necessarily. Do you feel that choked flow is wicked or evil? Sometimes it simply can't be avoided, and does no harm.

I would go a small step further and say that it is often desired and is designed to take place.

3) Can choked flow occur in liquid flow?
I do not believe that choked flow - in the practical and useful sense - can be achieved in a liquid medium. Some theorists have argued that it is possible, but have failed to identify for what purpose or how.

I concur that choked flow is not (normally) achievable in liquid flow. You may hear folks speak about chocked flow in a liquid stream. If they really know what they're talking about (i.e. not using the term in a colloquial sense), then they may be referring to a situation where the minimum pressure within the valve body is sufficiently low that some vapors are generated and do result in (vapor phase) choked flow.