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: Velocity of compressed air escaping a tank
: Velocity of compressed air escaping a tank -- Merc1136	Post Reply	Top of thread	Engineering Forum

Posted by: rnelson ® 09/23/2009, 11:17:34 Author Profile eMail author Edit

The K will vary depending on your orifice geometry. Choose the best fit and use that K as a constant and calculate the V. You can find common K values in the link below. How was your orifice manufactured? Drilled, punched, burned, etc. The manner will affect flow through it and the corresponding K value Also, use this link. It will probably be very helpful. https://www.mcnallyinstitute.com/13-html/13-12.htm Also, check if your velocity is choked or not. I doubt it based on the delta P, but it could be based on the very small diameter. Good luck.

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: : Velocity of compressed air escaping a tank
: : Velocity of compressed air escaping a tank -- rnelson	Post Reply	Top of thread	Engineering Forum

Posted by: Merc1136 ® 09/23/2009, 12:56:36 Author Profile eMail author Edit

Thank you for the response. I believe the hole to be burned through using a laser. The website you mentioned may have well been the source of the formula my coworker handed me, however, it says near the bottom: "All of these above numbers were generated assuming that you were moving water through the orifice. If you are making calculations for a liquid other than water you will have to factor in the viscosity of that liquid compared to water." I have a tank filled with compressed air, not water. Are the formulas the same?

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: : : Velocity of compressed air escaping a tank
: : : Velocity of compressed air escaping a tank -- Merc1136	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 09/23/2009, 23:59:04 Author Profile eMail author Edit

As long as the tank pressure (absolute) exceeds twice the ambient,(14.7psia) you will get choked flow meaning that the velocity at the orifice would be the speed of sound at the temperature there.You 35 psi, if maintained would meet this condition. The equation for that condition is V=sqrt[(2gK/(K+1))*(RT1)] K=1.4 (for air) g= gravitational constant 32 ft/sec-sec R= gas law constant= 53 ft/deg (for air) T absolute temperature deg Rankine

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: : : : Velocity of compressed air escaping a tank
: : : : Velocity of compressed air escaping a tank -- zekeman	Post Reply	Top of thread	Engineering Forum

Posted by: merc1136 ® 09/24/2009, 15:09:09 Author Profile eMail author Edit

Thank you very much for your reply. I plugged the velocity from your response in the equation Q=AVK, with K approximated as 1. My results are still off - I get 13 ccm versus observed 100 ccm. However I am thinking we need to question whether the hole truly is 30 micron in diameter, or look at the fact that the topology of the leak path (a 30 micron hole burnt through a bung plug installed into the sidewall of the tank) is throwing things off. This hole project is making me long for the comfort of voltage, current, and magnetic fields...

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: : : : : Velocity of compressed air escaping a tank
: : : : : Velocity of compressed air escaping a tank -- merc1136	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 09/24/2009, 16:19:55 Author Profile eMail author Edit

Let's see,the 30 micron hole is so small that it wouldn't take much like dirt to compromise the actual opening. I would test this with a hole (maybe more than one) to get some real data. I can't understand why you are "reinventing the wheel". Do you question Maxwell's equations? The equation I gave are extremely accurate for your needs and any empirical data you get that questions that formula is in error , most probably due to the the size or the method of collecting the ccm data.But you don't need it and it makes no sense to pursue this in this manner. The thing your company should do is to specify an acceptable pressure drop over some time period , use the equations and get a hole size analytically, and use k=1 conservatively. If you need more assistance on this I can shed additional light on it.

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: : : : : : Velocity of compressed air escaping a tank
: : : : : : Velocity of compressed air escaping a tank -- zekeman	Post Reply	Top of thread	Engineering Forum

Posted by: merc1136 ® 09/24/2009, 16:41:49 Author Profile eMail author Edit

I appreciate your feedback greatly. It isn't a case of me trying to reinvent the wheel - more so me being out of my realm of expertise and not knowing which wheel to use. :-) We collected by very crude methods ~100 ccs of air from the 30u hole in 1 minute. This involved submerging the tank in a few inches of water so that a cup could be placed over the hole to catch the bubbles. We collected data on two other leaking tanks with unknown size leaks. I know enough that if you cannot prove out the equation using all known values, then you shouldn't try to apply it to unknowns. I'm not even sure the equation I'm using is the correct one. Fluid dynamics is not my background. The only way I can resolve my data is if I have a k value of 10. Is this even feasible? Again, thank you so much for your input!

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: : : : : : : Velocity of compressed air escaping a tank
: : : : : : : Velocity of compressed air escaping a tank -- merc1136	Post Reply	Top of thread	Engineering Forum

Posted by: zekeman ® 09/25/2009, 18:04:19 Author Profile eMail author Edit

' I'm not even sure the equation I'm using is the correct one. Fluid dynamics is not my background.' You can be sure it is.And, you don't need a background in fluid mechanics; it's only pure logic. Q=V*A*K which is no mystery. It is the flow rate equation V is obtainable by known equations to within a few percent K is the vena contracta and must be between 0.5 and 1 So any significant error in your experiment vs real world must be between either/or both difference between perceived A and actual A difference between your measured Q and actual Q

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: : : : : : : : Velocity of compressed air escaping a tank
: : : : : : : : Velocity of compressed air escaping a tank -- zekeman	Post Reply	Top of thread	Engineering Forum

Posted by: mwick ® 09/28/2009, 16:03:06 Author Profile eMail author Edit

I currently work on containment isolators for the nuclear industry (Hot Cells) and commonly have different areas of containment with a pressure differential. One area that needs to be looked at is how much air is flowing and it's velocity from one space to the next via a penetration, i.e hole. The penetrations I deal with are ports and doorways, but I would think that the same equations should apply to get something fairly close to reality versus theoretical. V=4005*(Delta_P)^.5 where Delta_P is in Inches of Water. Q=VA(K) and you know the cross-sectional area of your penetration (hole) for variable "A". For my calcs I assume a K value of just 1...but for your case it might be something different.

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