4 replies to this topic

Hi guys, how am i suppose to determine the flow rate of a 1/2 inches pipe, venting from a 4 bar vessel tank into atmosphere? The gas is oxygen gas. What formula should i used? thanks...

[Art Montemayor]

Sabah:

First and foremost, I have to presume you are a chemical engineering student who already has fluid mechanics under his belt – and I also presume that this homework assignment is related to that.

Therefore, I direct you to the following elementary – but very important details about this problem on which you should take special note:

• The fluid in question is a gas;
• The driving force (pressure drop) being applied is greater than 2:1;
• The fluid is obviously flowing at sonic velocity.

Knowing the above, you should be able to apply the basic equations related to the same conditions.

If you are having trouble understanding sonic flow of gases (or “choked flow”), I recommend you go to visit Milton Beychok’s webpage at: http://www.air-dispe...m/feature2.html

You can also find many sources of this same subject in our Forums by using the SEARCH feature. Try the terms sonic, choked flow, gas velocity.

[Krunal Shah]

Hi guys, how am i suppose to determine the flow rate of a 1/2 inches pipe, venting from a 4 bar vessel tank into atmosphere? The gas is oxygen gas. What formula should i used? thanks...

Hi,

Flow rate(cubic meter per second) = Cross sectional area of pipe(square meter) * Velocity of gas through pipe(meter/second)
Both cross sectional area and velocity you can easily measure and it will give you actual flow rate of gas.

Generally recommended velocity of gases for short distance pipe (length of pipe is less than 1000m) is around 20 m/s.

[Art Montemayor]

Krunal:

I am afraid that the answer is much more complex than your simplistic approach. You fail to either read my post or follow my recommendations:

• The fluid in question is a gas;
• The driving force (pressure drop) being applied is greater than 2:1;
• The fluid is obviously flowing at sonic velocity;
• Read Milton Beychok’s excellent treatise on sonic (choke) flow of compressible fluids.

How is it that you can “easily” measure the velocity of a gas in a pipe? And at which point in the pipe – and for how long? The OP hasn’t stated whether his venting is batch or steady state (like he hasn’t given much other basic data), but the fact is that if the pressure drop ratio is more than approximately 2:1, the flow of the gas will be sonic. Are you aware and knowledgeable about sonic flow and why/how it occurs?

[Krunal Shah]

Mr. Art,

Sir there are many instruments available by which we can measure air velocity but as you say it will only useful if the process is operated at steady state.
regards,
Shah Krunal

Edited by Krunal Shah, 26 May 2011 - 04:46 AM.