Long Bore Orifice

[rjw57]

I've been given the task of calculating the flow of a compressible fluid (nitrogen in this particular case) through a long bore orifice (it WILL potentially go sonic). I suppose the best way to think of it would be a hole drilled thru a fitting in the end of a rigid vessel. The hole is about 3/8" in diameter while the fitting is about 2 to 3 inches in diameter. The length thru the bore of the fitting (to the interior of the tank) is about 1.5 to 2 inches. I am not certain what equations to use for this application since it seems to be somewhere between an orifice and a flush inlet to a short tube. Anyone have any recommended formulas for calculating compressible mass flow rates in this situation? I am leaning towards a tube long tube with a contaction coefficient factored in for the vena contracta I assume will be there. I am also not certain that the contraction coefficient is constant since it seems to me that it might vary with flow rate. Thanks for any input!

 

[Latexman]

The length thru the bore of the fitting (to the interior of the tank) is about 1.5 to 2 inches.

Is this the thickness of the metal the 3/8" hole goes through? If not, what is that metal thickness?

Good luck,
Latexman

 

[rjw57]

Yes, it is. It is basically a round slug containing the 3/8" hole which is welded into the end of a vessel. The slug is between 2 and 3 inches in diameter and 1.5 to 2 inches thick.

 

[whammett]

Is the slug flush with the vessel?


Will Chevron Corp.

 

[rjw57]

No, it projects beyond the crown of the vessel head. It is located in the center of the vessel head. If you are thinking Borda, notice how much bigger the slug diameter is than the hole diameter. Seems more like a flush inlet to me (if this is the way you are headed).

Bob

 

[Latexman]

Using 1.75", t/d = 4.67, which is quite large. I don't have my references at hand, but I believe the proper way to handle this is treat it as pipe with an inlet and possibly an outlet loss, not as an orifice. Omit the outlet loss and if it's sonic at the exit, you're good. If it's subsonic, add the exit loss in.

So, you were leaning the right way!

Good luck,
Latexman

 

[whammett]

I agree with Latexman. Are the only times it will be subsonic is at start up and shut down?


Will Chevron Corp.

 

[rjw57]

More complicated than that. It's like this...
This vessel I have described is in another larger vessel. Both are initially under vacuum. We need to repressurize both vessels so that the vessel I am asking about is not crushed in the process. It is a very thin wall vessel and cannot take a large pressure differential outside to inside during hte backfill process.

 

[dcasto]

just use a rotometer or other tecnology forget a dp measurement system

 

[casflo]

For high pressure water and high pressure drop (1500 psia and 1430 psi) I have designed these type of restriction orifices for minimum flow pump recirculation lines, safety injection lines in nuclear power plants, etc. considering an entrance and an exit for the ends and as a tube of a heat exchanger for the straight part and in the tests they run with an error less than 1% that the design values.