Estimate Leakage Rates Low Pressure Air Seal

[trmills]

Hi,

I have an application with large (24" diameter) man-hole flanges sealed with a gasket or o-ring in a vessel containing air at 1 bar (14 psi) pressure. I would like to avoid machining the seating surfaces on the vessel. I have a limit on the allowed leakage rate for the air, so it would be helpful if I could somehow estimate leakage rate for a given sealing material, surface finish, etc.

Does anybody have any experience or know of any references which may be helpful? From what I have read, I gather it may not be as straightforward as a series of data tables for leak rate as a function of working fluid, pressure, etc (but it sure would be nice if those were available).

Thanks!

 

[Sparweb]

A leak test is practical in many applications.
A test is also valuable when demonstrating to the customer that you have met their requirement (if it is the customer who specified the leak rate, in your case)...or when showing your boss that your seal won't work without X product on the flange.
I can't guess what your limit is, but the rate of air leak from a small gap on a circumference of about 75 inches at 14 psi can easily be on the order of several pounds per minute.
I have successfully modeled air leaks in terms of air discharge through an orifice. In your case, you have described 14 psi GAUGE over or under standard atmosphere, so either nearly complete vacuum or 29 psi ABSOLUTE. In flow through an orifice, you are concerned with pressure ratio from upstream to downstream. This ratio determines the flow rate - up to a point. At high pressure ratios, the flow escaping through the leak gets close to sonic velocity and that limits the flow somewhat. This may be a factor for you to consider as you study this subject.
I believe that a member of Eng-Tips has written a FAQ on this topic... I think I know who, but I don't remember where the FAQ is... yet...

One more thing to note: leaks and discharge rates that you will be working with will be mostly worked out in MASS flow. However, it is a common tendency to discuss air flow in VOLUMETRIC terms. Since the pressure is changing through the path of travel of the air as it leaks, using volume (such as cubic feet per minute) is absolutely useless. Of course, you won't convince your boss that CFM is useless, so be prepared to convert your results back into CFM anyway.

STF

 

[Sparweb]

Here:

http://www.eng-tips.com/faq.cfm?pid=378

Notice that there are two FAQ's that might help you!

STF

 

[trmills]

Thanks SparWeb! I found some good orifice calculators and the FAQ was helpful. I think the next question is what kind of flow area to assume. Definitely a test would be best.

 

[Sparweb]

You should be able to back-calculate from the test data.

STF