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?
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RE: Does an orifice control mass or volume 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?
Milton Beychok
(Visit me at www.air-dispersion.com)
.
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
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?
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?
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
Good luck,
Latexman
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or 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?
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?
Have you considered CFD?
Good luck,
Latexman
RE: Does an orifice control mass or volume flow?
Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website: www.oil-gas-consulting.com
RE: Does an orifice control mass or volume flow?
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
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?
h
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
This seems to be pertinent to your post:
http://nt
Good luck,
Latexman
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
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?
RE: Does an orifice control mass or volume flow?
Good luck,
Latexman
RE: Does an orifice control mass or volume flow?
Charlie
www.facsco.com
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
[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?
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?
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?
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
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?
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
RE: Does an orifice control mass or volume flow?
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?
RE: Does an orifice control mass or volume flow?
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?
http://virtualpipeline.spaces.msn.com
RE: Does an orifice control mass or volume flow?
As the original poster, I actually don't need anymore input..I got all I need! Thanks, and on to the next question! ; )