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Definition of Choking or Critcal Flow 3

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leongw

leongw

Industrial
Dec 31, 2004
7
Dear all,

I am looking for an explanation on the definition of choking or critical flow and what happen during choking or critical flow condition.

Appreciate if someone can assist.

Thank you.
 
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Take a look at my faq1203-1293. It may answer your question.

Good luck,
Latexman
 
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BigInch,
That is not quite right. It is when a further decrease in downstream pressure will not produce and increase in flow. If you raise upstream pressure the speed of sound will increase.

David

David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem
 
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Dear all,

Thanks for your comments and advice. I have searched the necessary info and it is very helpful plus informative.

Thanks again.

Regards,
 
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An interesting discussion, I think there are 2 answers to this particular question depending on who you are talking to.
The link gives the academic description which is great for the boffins there as the description from BigInch "Choked flow is when no further increase in differential pressure will produce an increase in flow", is a more useful description in day-to-day terms for normal pump application and discusions where you are trying to establish the interaction between pump and (pipe) friction losses.

Although there is really no argument that the wikipedia description is the correct one - it is probably not what the majority of application engineers etc mean.
 
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I didn't intend to discourage boffins, just to get to the heart of the matter quickly, which is... the segment is basically stuffed to the max (at any given mass density and dt).

BigInch[worm]-born in the trenches.
 
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If we are dealing with single phase compressible flow, there is 2 types of choked flow; acoustically choked flow in a single element, and frictionally choked flow in a distributed element. In both cases, the exit velocity is either at Mach 1 (soundspeed), or some fixed fraction of soudnspeed( in the case of oblique shock waves).

Acoustically choked flow typically happens across a fixed orifice plate, when the DP across the orifice plate is more than roughly 50% of the inlet pressure.( correction factors apply , as a function of Cp/Cv) Another common occurrence is across a single stage valve, but in this case the oblique shock waves have an effect. Refer to the ISA handbook of control valves, factor Xt is related to oblique shock waves. For acoustically choked cases, noise is usually an issue.

2 typical frictionally choked cases are (a) a multistage valve, such as a CCI Self drag valve , and (b) long pipes. For case B, the usual predictive method is using the Fanno flow relationships ( adiabatic perfect gas, constant diameter long pipe).

There are other cases that can be considered. One is when you have 2 phase flashing flow- ASME steam tables has a set of experimental curves that predict the choked flowrate thru long pipes in this service,as with drum blowdownlines. Another case is when the exit element is shaoped to allow supersonic flow. But these are the exceptions.

The normal cases have been discussed in other threads.
 
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In the interest of engineering accuracy and practical design application, it is important to get our facts straight regarding sonic or "choked" gas flow:

1) The main thrust of this topic is GAS (or VAPOR) FLOW – never liquid flow. Even David, who is precisely correct in his definition, fails to mention this important fact. But I firmly believe that he, like all of us, know that choked (or sonic, or “critical”) flow involves the gaseous phase;

2) Therefore, the comment that choked flow “is a more useful description in day-to-day terms for normal pump application and discusions where you are trying to establish the interaction between pump and (pipe) friction losses” is not only wrong, but totally inapplicable. Pumps have nothing whatsoever to do with (or related to) choked flow – as process engineers well know. Pumps pump liquids.

3) Those of us who have grown up and/or worked out in the oil patch and related hydrocarbon production areas also fully know about the colloquial (and mis-applied) term of choked flow as applied to the throttling of a liquid hydrocarbon stream. This is a term born out of ignorance vis-à-vis its employment as a description of truly gaseous sonic flow. Choked flow in a gas stream is popularly called that name simply because of the practical, maximum mass flow rate limitation that is attained at sonic velocity - i.e., the mass flow rate is "choked" (or throttled) and cannot be increased even when the downstream pressure is lowered. Liquids do not share that same characteristic in practice – nor can a Restriction Orifice (RO) be designed to control maximum liquid flow as they are amply & similarly used out in process plants for gas flow.

4) Milton Beychok’s description is certainly not an academic description. It is a pure, practical, engineering description that places the emphasis where it belongs in an engineering design: You can control the maximum mass flow rate of a gas stream by simply designing an RO such that the gas flow rate will be constant (& maximum) no matter how much you reduce the DOWNSTREAM PRESSURE. However, the mass flow rate will change if you change the upstream pressure. Consequently, I believe that BigInch knows that his phrase was simply not correct as stated - it was a mere over-simplification which, unfortunately is not correct engineering. It makes a BIG difference in the engineering results how the pressure difference is made. That’s why it’s important to stress David’s accurate and specific description. It’s not a matter of semantics or academics; it’s just common sense engineering to place the importance on the factors that change the outcome.

5) After 47 years in the process industries here and abroad, I know that Beychok’s description is exactly what the majority of applications engineers mean when they discuss the use or employment of choked, sonic, critical, (or whatever else you want to call it) flow. It is much too important a property of gaseous systems to confuse it with liquid flow characteristics.

 
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Montemayor (Chemical)
Comment 1 The original question relates to choking.
A liquid can choke.
Comment 2 I strongly agree with.
Comment 3 If a one component fluid is homogeneous, choking will occur at sonic velocity. Consider one component choking where the fluid goes two phase. If homogenous, the sound speed can be determined and conditions for choking thereby obtained. However, if the two phase fluid is under conditions of "slip flow", then one phase flows at a different condition than the other phase. Clearly choking with slip flow can and does occur and is dependent upon the flow model (real or unreal). This choking occurs when lowering of downsream pressure will no longer effect mass flow. Choked flow, of steam, liquid water, in layered conditions has been calculated and results in higher flow then homogeneous flow with the same upstream conditions.

Regards
 
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Artisi,
The Wikipedia definition is correct, clear, and precise. To say that it is only for "boffins" is just silly. Short cuts in language cause more harm in engineering than maliciousness ever will.

A few years ago we had a purge-related accident that resulted in a fatality. I was charged with re-writing the company's purge procedure. Relying on a definition of choked flow based on exceeding a fixed dP I put together the procedure and a training class. After about 10 presentations of the class and 3 months of people using the procedures, I went to the Mary Kay O'Connor process safety school class on relief valves. In that excellent class, the instructor went carefully through the arithmetic of choked flow and I was dumbfounded that I had based a purge-volume calculation on an incorrect assumption. I went back and recalculated, and found that I had been significantly under estimating the required purge time for a clearing purge (I had calculated sonic velocity at one pressure and temperature and my procedure was requiring the purge to be at a different pressure).

I have way too much mud on my boots and I'm far too profane to ever be called a "boffin", and had I been able to find a definition as clear as the Wikipedia one I would have saved myself a bunch of embarrassment, and (more importantly) I wouldn't have had a 3 month period where people were following a procedure that I had written with inadequate safety factors.

David
 
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Yes. I am guilty of oversimplification. As the vast "bulk" of the problem was already covered in quite sufficient technical detail in the thread that Latexman was kind enough to reference, I thought a lighter treatment was also warrented in the case that the OP might not have been looking for such a technical treatment.

Yes. Liquids do "choke". Reaching the speed of sound in a liquid has an equivalent effect on them just as it does with a gas.

BigInch[worm]-born in the trenches.
 
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Don't know if this would help, but there is a good website I found that has a choked flow spreadsheet that allows you to experiment with different input parameters, and look at how the mass flow rate changes in a chocked condition.


It seems like the site is still in an infant stage, but something to check out regardless.
 
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Leongw, one explanation of what is happening in critical (choked) flow would be that when the fluid reaches sound velocity, pressure signals can not be transmitted backwards from the decreasing downstream pressure, since these signals travel at the velocity of sound in the fluid.

Any comments ?
 
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to provide a conventional answer to the original question, refer to :

<
The above link will describe choked flow for single phase, compressible flow. There also exists choked flow in 2-phase flow conditions, and different correlations apply.

The understanding of compressible flow at high mach numbers was a principle part of research efforts from WWII thru to 1970, as it has fundamental importance to military objectives to improve the performance of planes, jets, missiles, rockets, and their propulsion systems. As a result, there is a tremendous amount of literature on the subject, and you could spend a lifetime reading it. (Most of it would seem unusual to today's students, as they had to rely on approximate analytic solution to diff eqn's as opposed to todays' practice of relying on numerical simulation)

In normal engineering practice for other civilian fields such as flow thru pipe and valves, only a small amount of compressible flow theory is needed to address industrial problems.
 
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.
I am going to repeat my January 10th response in this thread:

I would strongly suggest that you read this article for a full and complete answer to your question:


You should also read FAQ798-1196 in this forum.

Obviously, I agree with everything Art Montemayor said ... since I wrote the Wikipedia article as well as the above referenced FAQ798-1196. I really don't understand why this thread keeps going on ... we have all said just about everything that can be said on the subject, myself included.

Milton Beychok
(Visit me at www.air-dispersion.com)
.
 
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Why do we limit the back pressure on a relief valve to 25% of the set point when the downstream pressure has no control as to the capacity of the relief valve then?
 
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