Estimation of Kv or flow rate.

Can anyone point me towards formulas or software to estinate the flow rate of air through an oriface/poppet valve.
Example:-
dP 200bar
hole 3mm dia by 10mm long.

Spent 2 weeks trawling the net so far so any advice welcome.

Vince

 

[235zzzo]

Hello Vince

Being clear, as possible!

Your say, that we have a strongly compressed air across the valve. As you know, the Kv, by the definition, is the the volumetric flowrate crossing a valve (at normal conditions, NPT, in [Nm3/h]),for a deltaP of 1 bar (or 100 kPa),in IS unities, for instance.
Thus, the role of Kv is to serve as a reference measure of flowrate, for the particular flow conditions of such valve. It is mainly concerning with the physical flow properties across that valve, (e.g., the flow behaviour/development inside it), not with any generic/wide software, unless from the same origin/manutacturer as that valve. Being the valve a specific one, you have to ask to the manufacturer for that formula, or the graphic representation for the whole working range conditions, which relates the flowrate with deltaP and parametrically with the diameter. This is common data in any valve catalogue. If impossible to get this data, try to compare with another very similiar/concurrence valve, of known references for the same hole/section/diameter.

As a second approach, we can measure it directly, however that means some extra lab work and equipment. Let me hear something from you, before we talk about this.
Good luck.
zzzo

 

[235zzzo]

Hi Vince again!

After a second reading, let me ask you, are you sure about the deltaP/pressure drop across your valve, being 200 bar?

For the value you mentioned, we have a compressible flow conditions, and these ones put another serious nature of problems. Look out about sonic flow velocity conditions. That may be very hazardous (and noisy!) for the components of that valve. May our discussion be opened to the expertise of our membership. Please, make clearance of this point.
Kind regards.
zzzo

 

[MortenA]

API 520 has all the formulars for calculating flow through a RV / orifice both at sonic and sub sonic conditions. Unless you have more the 400 bar upstream the valve the you can bet on the flow beeing sonic - but API 520 also contains a formula for checking that.

Best Regards

Morten Andersen

 

Cheers Guys.
Yes I am sure about the pressure -up to 200bar.
Some more details.
This is for a .22 target airgun (Pneumatic / precharged). I am trying to design a better system than is currently in standard use.
The system consists of a reservoir of about 0.5 litres at 300 bar that is regulated down to 100 - 200 bar (adjustable) and fed to a small (4cc?) firing chamber. The chamber, upon firing should not entirely exhaust, but allow about half the pressure out. (this is for efficiency as its no use continuing to blow when the pellet is 20mts away). There is nothing revolutionary in this system except the ‘self opening & closing’ firing chamber that I have in mind. Most current systems rely on a weight hitting a valve stem, opening the valve for a few milliseconds. This can be fairly inconsistent and most designs fire straight from the reservoir so every shot is slightly weaker than the last.
I am from a Mechanical Engineering background so am OK about sizing pressure vessels & fittings but just need to estimate how much air will rush out of a given size hole with these compressible / sonic conditions. Also current knowledge on valve seat materials / angles for these pressures would help.

Thanks for your time so far. Hope this makes things clearer. More details if interested.
Vince

vincehogg@yahoo.co.uk

 

Just found what I'm looking for I think.
Take a look.

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