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Does an orifice control mass or volume flow?
5

Does an orifice control mass or volume flow?

Does an orifice control mass or volume flow?

(OP)
I am trying to understand the effect of ambient pressure on venting a gas from a  pressurized vessel. I understand that conductance of an orifice has units of torr-litre/sec.  Does this mean that if there is a pressure difference of 1 psi, regardless of the absolute value of the input/output pressures, the same mass will flow?  That is, if I hypothetically pressurized a vessel to 14 psia and vented it through a 1/16 inch diameter hole into a 13 psia ambient, the mass flow rate would be the same as if I vented that same vessel from an initial internal pressure of 4 psi into a 3 psi ambient?  If this is the case, then an orifice controls mass flow.  I'm having a hard time visualizing this as I am not a fluids person.  I'm assuming that the gas is incompressible in this case, and flow is viscous.  Any insight much appreciated.  

RE: Does an orifice control mass or volume flow?

No, you don't quite have it right.  For a fixed upstream pressure there is a corresponding downstream pressure that represents the start of maximium (or "choked") flow.  Lowering the downstream pressure below this point with a constant upstream pressure will not change mass flow.

If you know the upstream density then you can calculate sonic velocity (choked flow is at 1.0 Mach) and determine volume flow rate from the orifice area.

In your example of 14 psia upstream and 13 psia downstream you would have flow, but determining the exact flow rate is a difficult exercise with questionable accuracy and poor repeatibility.

On the other hand, if your vessel was above about 25 psia (depending on your atmospheric pressure and the gas makeup) then the flow out your 1/16" hole (provided the material was suffeciently thin) would be very well known.  A hole that small my not actually be an "orifice", it may be a "pipe" and subject to friction losses.

David

RE: Does an orifice control mass or volume flow?

(OP)
Thank you very much for your replies.  I took a look at the links and they don't quite fit my question. I don't think that the gas flow in my case is necessarily choked, although I could be wrong.  I think it would be helpful to explain my application a bit more.  I am studying the venting behavior of space telescopes on launch.  The initial condition is that the telescope interior is at atmospheric pressure.  On launch, the ambient pressure drops to approximately 0 in 100-200 seconds.  I want to understand the flow through the orifices, which may be vents of complicated shapes.  I have made mock up vents and have tested their flow characteristics with ambient pressure being 1 atm.  I end up with a relationship of mass flow vs delta P (upstream - downstream pressure).  The question is whether I can apply that same relationship to the same delta P, but different ambient pressure.  e.g.  I measure 10 g/sec at 1 psi from 14.7 psi to 13.7 psi.  Will 10 g/sec also flow between 3 psi to 2 psi (assuming subsonic flow...unchoked).  That's what I had meant by my original question of whether an orifice/vent controls mass flow or volume flow...Thanks again for any insights.

RE: Does an orifice control mass or volume flow?

For an incompressible liquid flow through an orifice remains the same for a constant differential pressure across an orifice.  No difference in flow (either mass or volumetric) between 14psia let down to 13 psia and 4 psia let down to 3 psia because a given mass of the liquid will always have the same volume.

Your original post states that, “I'm assuming that the gas is incompressible in this case, and flow is viscous.”  This is not a valid assumption for any gas when there are significant changes in pressure.  Do not make that assumption.  

Does this mean that if there is a pressure difference of 1 psi, regardless of the absolute value of the input/output pressures, the same mass will flow?  No, not for a gas.  You have a transient analysis where the density of the gas changes with time.   

RE: Does an orifice control mass or volume flow?

jennyrabbit:

Read this website page: www.air-dispersion.com/source.html which leads you to either the metric or USA versions of an article entitled Source Terms For Accidental Discharge Flow Rates.

It discusses and explains the gas discharge mass flow rate for non-choked flow as well as for choked flow of gases.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

RE: Does an orifice control mass or volume flow?


Beside some factors which depend on the prevailing T,P conditions, and the orifice geometry, the mass flow rate is proportional to √(ρ×ΔP).

For the same orifice and the same ΔP it would be proportional to √ρ, where ρ is the upstream density.

The volumetric flow rate, would, of course, be proportional to √(1/ρ).

RE: Does an orifice control mass or volume flow?

(OP)
Thanks again to all of you.  I read more of Milton's link, and it looks like mass flow depends on the square root of the product of density and upstream pressure, not square root of product of density and delta P...This isn't what 25362 posted.  The full equation in Milton's post does have additional factors  in the square root which are ratios of upstream/downstream pressures, but not delta P's.  The point was made earlier that if I have a very small hole in a plate, it is probably more reasonably modeled was a tube.  I agree with this, but am trying to get the fundamentals figured out first.  I have a much better gut understanding if molecular flow than viscous flow.  With molecular flow, I can visualize the impingement of individual molecules at an orifice zipping through.  With viscous flow, I understand there is a boundary layer, but I don't  understand under what conditions I must consider compressibility.  In my application, I will never have more than 1 psi difference between upstream and downstream pressures.  Do I have to consider compressibility?  it seems the equations are more complicated!

RE: Does an orifice control mass or volume flow?

(OP)
Also, is the discharge coefficient of an orifice or vent independent of pressure or flow rate?  Is it just a geometric thing?  

RE: Does an orifice control mass or volume flow?

2
You may never have more than 1 psi ΔP, but as the telescopes approach 0 psia won't the mechanism change to molecular flow?

Good luck,
Latexman

RE: Does an orifice control mass or volume flow?

(OP)
You are correct, Latexman.  But at that point, I'm not worried about the telescope blowing up.  ;  )

RE: Does an orifice control mass or volume flow?

Fundamental Answer: Orifice as Choked Orifice, Volume Flow.  Orifice at ratio of pressures less than critical ratio: Not Choked, still Volume Flow device.  Area constant to a third order approximation; Integral of Velocity over Area is Volume Flow.

Orifice bleeding down a fixed volume to a varying pressure O/S is a Fixed Volume analysis, solve according to velocity as profiled across area.  Boundary layer effects can then be incorporated if length and diameter, viscoisty and density indicate boundary layer actually developed.  Solving as decay of starting mass contained is going to complicate the topic.

Develop equation as model for dV/dt and integrate toward V total....

RE: Does an orifice control mass or volume flow?

Based on the telescope construction, how many "resistances" does the air have to pass thru to get outside?  One?  Several?  Or, does it depend on which internal volume (compartment) the air originates from?  I.e., some have one resistance and some have several?

We've been putting telescopes and other satellites in space for several decades.  Have you searched NASA (1958 - present) and the old NACA (1917 - 1958) collections?  They are public records and are at:

http://ntrs.nasa.gov/search.jsp

Why struggle over "complicated shapes"?  Just assume all the compartments are air tight and add vent holes (circular) where they are needed.

Good luck,
Latexman

RE: Does an orifice control mass or volume flow?

As for the subject question, an orifice is simply a restriction to flow.  Mass flow and volumetric flow are related by the continuity equation and PVT relationship.

Have you considered CFD?

Good luck,
Latexman

RE: Does an orifice control mass or volume flow?

I've been reading this thread with interest and have learned allot, but have a question (related to the thread - no hijacking) - is mass measurement not a better alternative to volume measurement in many cases, custody transfer?  Am i correct that mass measurement does not require the correlation of temperature, pressure, and empirical data as would volume measurement?

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com

RE: Does an orifice control mass or volume flow?

Mass measurement correlates a signal to density and, since products are not sold by density, correlation of the product's density at the measured temperature to volume and/or weight is still necessary.

http://virtualpipeline.spaces.msn.com

RE: Does an orifice control mass or volume flow?

(OP)
I am also enjoying this thread.  More comments.  Amazingly, there is only one article in the literature that I have found on flow through orifices at reduced pressures, and the famous rule of thumb of venting 1 cubic foot through a 1/4 inch diameter vent.  The whole aerospace industry uses that.  Unfortunately, to your point, Latexman, we don't want to use circular orifices because we want to block light and particulates.  We also use various types of filters which inhibit flow.  I would love it if orifices were mass flow devices independent of upstream pressure because then I would just measure mass flow into atmospheric pressure and use that mass flow in my models. I know that is not the case, but I'm not convinced an orifice is only a volume control device either.  I'm not sure on that one yet, and have not followed all the links provided to me so far.  Being an experimentalist by heart, I believe that experimental results are essential in this case, but I still want to understand flow a bit better.  The continuity equation doesn't really satisfy my more molecular based thinking.  I'm a condensed matter physicist.  

RE: Does an orifice control mass or volume flow?

Jenny, I'm not sure what you mean with this "mass or volume controled" question.  An orifice, like a control valve, does not control either flow or pressure.  Both allow a certain passage of mass based on the pressure differential... nothing else.  A control valve's controls may be configured to change the orifice properties such that it attempts to maintain a certain pressure, upstream, downstream or differential across two points, or a set flowrate at some measurement point, if its configured for flowrate control.  How they are configured is simply a matter of what signal they read.

http://virtualpipeline.spaces.msn.com

RE: Does an orifice control mass or volume flow?

Greg,
People use Volume Flow Rate at Standard Conditions as a surrogate for mass flow rate and it has all the good features of mass flow rate (i.e., if velocity is not constrained then changes in mass flow rate are not a function of local density changes).  If density goes up with constant area, then local velocity will go down by a proportionate amount and mass flow rate will stay constant.  But, you must know both density and area at some point to be able to determine your mass flow rate and therefore your volume flow rate at standard conditions.

Volume flow rate at actual conditions is pretty much worthless for calculations with dramatically changing pressure or temperature.  The only place I ever use it is in empirical compressor hp calculations.

David

RE: Does an orifice control mass or volume flow?

(OP)
OK, what I'm hearing is that if I measure a mass flow of 1 g/sec flowing across my orifice for a fixed upstream pressure of 15.7 psi and a downstream pressure of 14.7 psi, THEN I expect to get 1 g/sec flowing across my orifice with an upstream pressure of 5.7 psi and a downstream pressure of 4.7 psi.  I seem to be hearing that a fixed orifice, like a hole, allows a certain mass flow depending on the pressure differential across it.  Period.  Any dissenters?  

RE: Does an orifice control mass or volume flow?

(OP)
Thank you, Latexman.  I had a copy of that and had been so frustrated with the poor copy quality, but I did take another gander at it.  Alot of great stuff in there....although "area" still has to be determined for any vent which is not a straight shot hole.  

RE: Does an orifice control mass or volume flow?

You are welcome.  When you did not say you had looked at or downloaded some material from NASA the first time I mentioned them, I just had to do a quick search.

If you got 1 g/sec from 15.7 psi to 14.7 psi (29.4/30.4 = 0.967), should you expect 1 g/sec from 5.7 psi to 4.7 psi (19.4/20.4 = 0.951).  Without doing any detailed calcs, I'll say no.  You are not near sonic velocity, so that's not it.  You'll get < 1 g/sec from 5.7 psi to 4.7 psi due to density differences (19.4/29.4 = 0.66).

Good luck,
Latexman

RE: Does an orifice control mass or volume flow?

(OP)
I agree with you, Latexman.  What makes me kind of crazy is that Scialdone in that reference you gave me gives a dependence of dP/dt on sqrt(deltaP * P).  Analytical equation for flow through a simple orifice shows a dependence of dP/dt on P and sqrt(expression of ratios of upstream and downstream pressures).  My empirical results give dP/dt depending linearly on P and a power law dependence on delta P (close to sqrt, not quite).  At least I'm getting clearer on what is not clear!  

RE: Does an orifice control mass or volume flow?

Did Scialdone assume isothermal expansions/conditions in that article?  If so, his sqrt(dP * P) is equivalent to sqrt(dP * rho) 25362 mentioned above for orifice mass flow rate equations.  If not, then I'm as lost as you.

Good luck,
Latexman

RE: Does an orifice control mass or volume flow?

So, if this thing launches, as you say; won't there be a temperature difference, and therefore a change in density?

Charlie
www.facsco.com

RE: Does an orifice control mass or volume flow?

(OP)
Scialdone assumes an isothermal process as the launch is pretty fast (order 100 seconds).  And I misspoke when I said that an orifice had a different dependence.  I was looking at dependence for a circular pipe.  

RE: Does an orifice control mass or volume flow?

Quote (jennyrabbit):

Does an orifice control mass or volume flow? I am trying to understand the effect ...on venting a gas ...

[highlisht]Massflow.[/highlight]

I'll keep it simple jennyrabbit.

eg if 3 kg/s of gas molecules were to hit a sealed orifice from the left and 2 kg/s were to hit the sealed orifice from the right, then what happens if the seal were to magically disappear?

3 kg/s of gas would pass from left to right, and 2 kg/s would pass from right to left. We would see that as net flow of 1 kg/s from left to right.

RE: Does an orifice control mass or volume flow?

Hiya,

Don't ever confuse mathematical theory with reality. The reality is what you are dealing with - the imperfect gas. If it is natural atmosphere it will contain moisture, as well as a mix of Nitrogen, Oxygen, and other gasses and/or pollutants. That will have pressure, density, and humidity (which can cause internal corrosion).

By some fluke of nature most of the gasses approximately follow Bernoulli's equation. If you assume the gas to be "incompressible" it will act more like a liquid. Then you can make more simplifying assumptions. That approach is usually used in the design of Air Conditioning and Ventilation applications.

When you start looking at viscous effects, then the gasses approximately follow Reynolds equation, and viscous effects are approximated by the effects of Reynold's number. In that case you can use the Darcy Weisbach formula, together with the Moody Diagram. By some fluke of nature this is applicable in a general way to both liquids and gases, provided that your assumption of "incompressability" is adequate. The tables published in the ASHRAE Guide, and other air handling text, are based on an interpretation of the Moody Chart.

The same approach is used in the published tables for the flow of compressed air and other gases. Steam tables, however, are used to allow for the effect of partial pressure of water vapour in the air, and hence a "humidity ratio" - i.e. what part of the volume of gas follows the steam table.

When you put a magnifying glass on the flow effects, you have to ask - "Does the gas, in the range I am interested in, follow Newton's Formula, or is it non-Newtonian?"

You see - the real world is much more amazing that your engineering Professors would have you believe. Everything in nature is calculable. But do your numbers make sense in the range you are interested in? Most suprises come from making assumptions which ain't necessarily so.

Enjoy life - that's what it is all about.

Regards - Sgt John.Rz

www.latviantourists.com.au

Johnp.Rz
http://www.mets.net.au

RE: Does an orifice control mass or volume flow?

johnp (Mechanical)
If as I understand the problem, the real gas has a max pressure of 1 atmosphere.  Most gases will follow perfect gas behavior at those conditions.  Wheter specific heats can be considered constant can be questionable, dependent upon temperature variation.  With regard to being a mixture of "gases", most gases that I am aware of will follow Dalton's Law of partial pressures.  Therefore the physical mathematics apply.  Whether the process is isothermal or quasi-static can be questioned.
My experience is that for a fast process, heat transfer is not fast enough to maintain an isothermal condition. Further for a fast process, a transient model may be more applicable than a quasi-steady one.

An engineering professor who fails to include real world effects in analyses should not be in the teaching profession.  

RE: Does an orifice control mass or volume flow?

BigInch (Petroleum) 3 Jun 07 2:21  
I thought that's why they were.

That has not been my experience.
Are you speaking up for your schooling?
 

RE: Does an orifice control mass or volume flow?

When you are trying to be humorus on the internet, due to the nature of this form of communication, it is generally accepted convention to use a smiley face (smile), otherwise somebody might think you are making a personal attack or something.  Do you know how to make a smiley face?  If not, I would suggest you learn how to make one... quick.

http://virtualpipeline.spaces.msn.com

RE: Does an orifice control mass or volume flow?

(OP)
My philosophy is to try to understand the theory and combine that understanding with experimental results specific to the problem I am trying to solve. The posts have provided me food for thought and I am now better able to understand the problem and have been getting experimental results which I am using in my model.  Heck, we have all had good profs and terrible profs...!  Thank you, everyone, for your help.

RE: Does an orifice control mass or volume flow?

Quote:

You may never have more than 1 psi ?P, but as the telescopes approach 0 psia won't the mechanism change to molecular flow?

jennyrabbit (Materials) 26 May 07 23:06  
You are correct, Latexman.  But at that point, I'm not worried about the telescope blowing up.  ;  )

Quote (jennyrabbit):


In my application, I will never have more than 1 psi difference between upstream and downstream pressures.  

So test the telescope with a bigger pressure difference than 1 psi and if it survives that without blowing up, or distorting (or whatever you suspect might be a problem), then it should be okay, shouldn't it?

Or am I missing something?

My understanding so far is that the flow rate is a worry because if air doesn't leak out fast enough there would be a pressure difference between the inside and outside which could cause problems.

Yet you also seem certain that the maximum pressure difference will not be more than 1 psi. (I don't know how you can say that, but I'll trust you.)

So if you test a 1 psi difference, you will have tested the worst case won't you?

RE: Does an orifice control mass or volume flow?

(OP)
It would be quite impractical to test the telescopes at 1 psi pressure.  Mechanical designers specify maximum pressure and provide that to me.  Reasonable suggestion but not really feasible or necessary.

RE: Does an orifice control mass or volume flow?

BigInch has stated
An orifice, like a control valve, does not control either flow or pressure.  Both allow a certain passage of mass based on the pressure differential... nothing else.

Not so for choked flow.

RE: Does an orifice control mass or volume flow?

(OP)
This post seems to have taken on a life of its own.  ; )
As the original poster, I actually don't need anymore input..I got all I need!  Thanks, and on to the next question!  ;  )

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