Orifice Design (noise, cavitation, number of holes)

[sajego]

I am doing research regarding an orifice plate that was originally designed with a single hole but was changed to about 150 small openings. About 5 inches in diameter, It looks like a honey comb and often gets confused with a strainer. The holes in the orifice clog up quite often (even though there are strainers in the system upstream of the orifice) and a larger or single hole would be more appropriate for this reason.

I don't have a lot of history about the change and why it was made (its been years, and nobody remembers... etc etc) but the drawing is labeled "Quiet Orifice Plate" so we believe that it was a noise issue. This seems flawed since the environment is already very very loud, but its possible that the testers wanted quieter operation and made the change.

If noise is the only issue then it makes sense for us to go back to the single hole and end the clogging issues.

Some system specs, 1200 gpm, 3 to 4 inch orifice, though the current design is 150 holes with a 0.221 diameter spread over an area of closer to 6 inches.

What I am looking for information on is:
-How to determine noise across orifices with different hole sizes and shapes
-Cavitation calculations dealing with orifices
-Other possible reasons this design might have been changed that I should look into?

Thanks!

 

[Shmulik]

Please try the following, I hope you will find what you are looking for:

http://www.optimal-systems.demon.co.uk/flow-orifice.htm

http://cav2001.library.caltech.edu/documents/disk0/00/00/02/22/00000222-00/Pan-Xiang-Wu.pdf

http://cav2001.library.caltech.edu/documents/disk0/00/00/02/21/00000221-00/Zhang-Liu-Jin.pdf

http://www.accutech2000.com.au/index.html

 

[hacksaw]

sajego,

you are gealing with a restriction plate (also called a Lo-db plate).

multiple holes are for control of noise.

what pressure drop are you dealing with and what sort of solids build-up?

 

[sajego]

Thanks for the links. I had already found a couple of papers.

The pressure drop is approx 70 psi over such an orifice. The clogging is gradual deposits from the salt water (I'm waiting for my co-worker to confirm that for me).

Sara

 

[vanstoja]

A critical factor for noise generation of a single circular orifice is the L/d ratio which should not be between 0.5 and 2.0 to avoid "lock-in" (of the entrance edge-separated flow reattachment instability at or near the trailing edge) to a structural resonance of the structure mounting the orifice. L/D of 4.0 or more is a safer bet. Your present perforated plate holes have an L/d about 15 times greater than a single hole orifice of 3.5 inch diameter. What's your plate thickness?

 

[sajego]

Vanstoja, thanks for that info. The plate thickness is .5 inch, but the diameter of the holes tapers at 22 degrees from the .221 inch smaller diameter. That gives L=.25/(tan 22) so L/d is about 2.8

For the single hole L/d is under 0.2

And I found your post in another thread about the tea-pots.

http://www.eng-tips.com/viewthread.cfm?spid=384&newpid=384&sqid=36566

Very useful. Thanks!

 

[hacksaw]

S~

Such a plate should only be used in clean, non-plugging services. Sea water applications have too much solids and biological activity for such a plate.


Suggest that use valve possibly with a pressure controller.

 

[sajego]

I agree with you. But they've been using this plate since 1975! It has to be checked and cleaned out every 3 months... hence we are hopefully moving away from it.

 

[davefitz]

The modern solution is not to use the downstream orifice if the sytem requires any turndown . The fixed orifice only works when the turndown is mild , perhaps 2:1- any more tundown and the cavitation shifts to the valve throat.

The modern solution is to use a multi stage anti-cavitation valve. The cadillac version is a Fisher turbo cascade , Cav IV, or equivalent from other vendors. Be sure the seat area is suitable for the salt water environment, though.

If there is only a mild turndown, maybe a better solution is a few "capillary tubes" in parrellel. Zero noise and zero erosion if the alloy is selected properly. If 3/4" tubing is used, each tube 10 ft long in a coil, fL/d of 160, you can quickly estimate how many are required for the 70 psi DP. Cleaning them may be an issue, though.

 

[wilbert95]

hello, I am dealing with the same problem as you were a year ago Sara. Did you find any answers??

Wilbert

 

[davefitz]

Another issue is that of cavitation . If you switch to a single large orifice then one can expect erosion of the downstream pipe wall if cavitation is occurring, but multiple small holes are sometimes resorted to to reduce damage to downstream piping.