WHY do multiple orifice orifice plates reduce noise?

[DaiG]

Im currently working on a project regarding noise in orifice plates. I've seen it mentioned in several places that a multiple orifice orifice plate is quieter than a single orifice orifice plate of the same overall orifice area. Does anyone know WHY this is is the case?

 

[zdas04]

It's all about velocity. Say you start with 1000 psia and are trying to get to 200 psia using orifices. If you size the first orifice to limit the velocity to under 0.3 Mach then your noise level will be really low, and the downstream pressure will be around 900 psia. Size the second one for less than 0.3 Mach and the outlet pressure will be around 700 psia. And so forth. It will take about 6 steps to get a silent drop to 20% of upstream pressure. With three steps it will be pretty quiet, but not silent.

This all assumes an orifice of sufficient length to give you a permanent pressure drop--AGA 3 plates result in a dP that is partially recovered by interaction with downstream vortices. A choke nipple that is about 3 times the length of the main pipe ID, will have approximately zero pressure recovery.

David

 

[DaiG]

I don't think you've understodd the query. Your describing a multi-stage orifice meter. My query is with regards to a single plate with many orifices. My project aims to determine noise generated by orifice plates. My knowledge of acoustics is limited but I've heard it said repeatedly that these orifice plates are quieter than single holed ones of same overall orifice area. This is what I'm trying to understand. Thanks for your help anyway.

 

[zdas04]

There is a LOT of wishful thinking in statements that multiple holes in a plate will reduce the noise. The statements are based on an assumption that interference among the various standing waves will tend to cancel. It happens. Typically at a single Reynolds Number, or a small range of Reynolds Numbers. Mostly it doesn't work.

David

 

[stanier]

Check out Mitac they have software for predicting the noise and cavitation of a multiple port /plate orifice stack. These are used in severe service. Also CCI have tortous path devices for high pressure let down that avaoid noise.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[DaiG]

Thanks both. stanier, you wouldn't happen to have a link to the software your talking about? can't seem to find it..

 

[stanier]

Daig,

You have to ask Mitac for it. I will dig up a contact. Send me an email, address on website.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/