Needle valve spreadsheet?

[rjack321]

Does anyone have a spreadsheet that illustrates the changes in choked flow through a needle valve as IP increases?

Like if I wanted to have 0.65 L/min flow at 75m what would the flow be at the surface? I have seen the formulas for this and I'm not sure I can make the not quite algebraic way excel reads a formula do what its supposed to do. Was hoping someone had already coded this.

 

[kotik]

Does anyone have a spreadsheet that illustrates the changes in choked flow through a needle valve as IP increases?

Like if I wanted to have 0.65 L/min flow at 75m what would the flow be at the surface? I have seen the formulas for this and I'm not sure I can make the not quite algebraic way excel reads a formula do what its supposed to do. Was hoping someone had already coded this.

What are the formulas?

 

[rjack321]

Here with the general formula for CMF orifices in a simplified fashion:
Dn= d 2×p0 ×A

Where:
Dn = flow in l/min from the nozzle
d = orifice diameter in mm
P0 = intermediate pressure in bar
A = gas constant = 8.65 for pure O2

Untitled 1

 

[runsongas]

High pressure drop flow is a nearly linear correlation to inlet pressure at low flow rates, so 0.37 liter/min at the surface should get you fairly close.

 

[rjack321]

So its...
set Dn = 0.65 L/min @ P0 = 17.45 bar then d = X
If d = X then Dn = Y

(145psi IP at the surface + 7.45 bar for 75m of water = 17.45 bar IP at depth)

 

[rjack321]

High pressure drop flow is a nearly linear correlation to inlet pressure at low flow rates, so 0.37 liter/min at the surface should get you fairly close.

Thanks, this is nice because its easily testable.

 

[runsongas]

Swagelok has a guide about their valves

https://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF

 

[kotik]

Swagelok has a guide about their valves

https://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF

Nice article. This chart looks helpful:

One form for a back-of-the-envelope calculation I use for Cv estimation is
Cv=23.6*D²
Where D is the orifice diameter in inches. That would get you a number to use for the 4th bullet in the example.

 

[rjack321]

That graph is waaaaay higher than the flow of O2 which is less than 1 L/min.
I appreciate the suggestions but if you don't know how a CMF works this isn't useful.

Swagelok has a guide about their valves

https://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF

Does the Cv of a needle valve change as you alter the opening size?

 

[kotik]

Does the Cv of a needle valve change as you alter the opening size?

Yes, very much so. The more you open it, the larger the orifice will get until it hits the maximum. Cv is a measure of flow capacity, so the bigger the orifice then the more flow that can pass.

The calculation I mentioned above is easy to use for a fixed round orifice. It can be manipulated around to work with an annular flow area but you would need details about the parts in the valve.

I don't think you need to get in the weeds on that though- if my guess is right you set the needle valve once and don't mess with it during the dive? If so then if you know your flow rate at the surface then given the delta P at the surface you can see what that Cv is using the chart, and you'll be able to predict the flow rate at the depth you're interested in, using the new smaller delta P.

Caveat-- I know flow and valves, but I'm not a rebreather guy so I could have misconceptions about how some things are used.