Mutli-Hole Orifice Sizing 1

[Dressler]

I am assisting in the research of a modification dealing with Multi-Hole Orifice Sizing. Currently, we have a single-hole orifice in our system. Due to cavitation problems, though, we would like to replace it with a multi-hole orifice. I've been asked to research and find a theory or method for the sizing of an appropriate multi-hole orifice (but not to to size the orifice myself, though, or at least not yet). I've found a few threads that start pointing me in the right direction, but seem to come just short of what I'm ultimately trying to find. If anyone has any suggestions or information on weblinks or other places I can find information on how to size a multi-hole orifice, I would greatly appreciate your input. If you need more specific information on the orifice and piping itself let me know, though, again, all I really need is the method for the time being. Thanks.

Craig

 

[TrevorP]

I have not found any article or research on this. If possible in your application I would use a single orifice. I have used the following two approaches in the past with reasonable results, but it wasn't a critical application, and I have no idea of the validity. All I can say is that it worked in my case! The first approach is to assume that the holes act together as one larger hole, and so you sum the area of the individual holes and equate that to one larger hole, and carry out your calculations on that size hole. For example, 4 x 1" holes are equivalent to 1 x 2" hole. The other approach is to assume that each hole will act as independent orifices and are unaffected by one another, so that for a given pressure drop across them, they will all allow the same flow through each of them, which will be the same flow as if there was only one orifice multiplied by the number of holes. So, you calculate the flow one orifice and then multiply them by the number of holes.
Both these methods gave a very similar result in my case. If the orifices are to be used as restrication orifices rather than for flow measurement is very important to remember to take the pressure drop recovery into account. In my experience this is the most common cause of wrongly sized orifice plates.
Another word of caution, if you are drilling many holes into a plate, especially if there is a reasonable pressure drop across it, make sure that the plate thickness is sufficient to take the stresses applied to it.
Hope this helps.

 

[ecas]

To design multi-hole orifice plates look in POWER magazine September 1991 the paper named "Look at orifices plates to cut piping noise, cavitation".
The size and number of the holes have no much influence in the results when the ratio of equivalent pipe diameter to pipe internal diameter varies between 0.2 and 0.8.
Regards.

 

[25362]

If instead of a multiple-hole orifice you accept a multiple-orifice system,you may find help in an article titled: "Program gives design data for multiple-orifice system to avoid cavitation" by Goksem and Patel in the TECHNOLOGY chapter of the O&GJ, April 15, 1985.

 

[BRIS]

Have a look at SPX polyjet valve type 816 which is in fact a multi hole orifice -

http://www.cmb-ind.com/Products.asp?p=3

http://www.cmb-ind.com/files/products/English/PolyJet/Spec Sheets/b-816.pdf

 

[Dressler]

Thanks for the help.

ecas - I've contacted a library about the POWER article and hopefully they will be able to send a copy of the article my way. In the mean time, though, if you know where I can find an electronic copy of the article that would save me some time (I've looked for quite a while unsucessfully).

25362 - Thanks for the suggestion, but we are trying to avoid a multi-orifice system to keep down the costs. We currently have a single-hole orifice, and to simply replace it with a multi-hole orifice is, obviously, much more economical than installing more flanges and plates in the pipeline. If for whatever reason that does not work out, though, I will keep your suggestion in mind.

 

[ecas]

Dressler, tell me your e-mail address and I´ll send you a copy of the POWER paper.
Regards.

 

[virk]

Hi!

We had the same problem with caviation within an orifice.

We thought about the following solution:

Instead of an orifice, where in my opinion pressure can decrease beneath vapor pressure and cause cavitation at outlet, we wanted to replace the orifice by a defined length of small tube which causes same pressure loss.
So in my opinion you better can prevent pressure decreasing too much and probably avoid caviation.

But finally we also used a seven hole orifice instead of a one hole orifice because of routing (place-) problems. We calculated 7 seven holes with the same free area. One hole in the middle and six around it. I also tried to find some literature about it and even contacted one factory delivering orifices but even they could't help me.

It works now, but unfortunately I cannot say that it works because of this change because we also changed process conditions.

We throttled 10% ammonia with rest water down from roughly 19 bara to 0.2 bara. We had noise like a "starfighter", or F-16 or aeroplanes like this It was unbelievable.

Kind regards

 

[Dressler]

ecas, my email is dressler@purdue.edu. Thanks.